Resistance to PRMT5-targeted therapy in mantle cell lymphoma

ABSTRACT

Mantle cell lymphoma (MCL) is an incurable B-cell non-Hodgkin lymphoma, and patients who relapse on targeted therapies have poor prognosis. Protein arginine methyltransferase 5 (PRMT5), an enzyme essential for B-cell transformation, drives multiple oncogenic pathways and is overexpressed in MCL. Despite the antitumor activity of PRMT5 inhibition (PRT-382/PRT-808), drug resistance was observed in a patient-derived xenograft (PDX) MCL model. Decreased survival of mice engrafted with these PRMT5 inhibitor-resistant cells vs treatment-naive cells was observed (P = .005). MCL cell lines showed variable sensitivity to PRMT5 inhibition. Using PRT-382, cell lines were classified as sensitive (n = 4; 50% inhibitory concentration [IC50], 20-140 nM) or primary resistant (n = 4; 340-1650 nM). Prolonged culture of sensitive MCL lines with drug escalation produced PRMT5 inhibitor-resistant cell lines (n = 4; 200-500 nM). This resistant phenotype persisted after prolonged culture in the absence of drug and was observed with PRT-808. In the resistant PDX and cell line models, symmetric dimethylarginine reduction was achieved at the original PRMT5 inhibitor IC50, suggesting activation of alternative resistance pathways. Bulk RNA sequencing of resistant cell lines and PDX relative to sensitive or short-term-treated cells, respectively, highlighted shared upregulation of multiple pathways including mechanistic target of rapamycin kinase [mTOR] signaling (P < 10-5 and z score > 0.3 or < 0.3). Single-cell RNA sequencing analysis demonstrated a strong shift in global gene expression, with upregulation of mTOR signaling in resistant PDX MCL samples. Targeted blockade of mTORC1 with temsirolimus overcame the PRMT5 inhibitor-resistant phenotype, displayed therapeutic synergy in resistant MCL cell lines, and improved survival of a resistant PDX.

PRMT5 inhibition drives therapeutic vulnerability to combination treatment with BCL-2 inhibition in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an incurable B-cell malignancy that comprises up to 6% of non-Hodgkin lymphomas diagnosed annually and is associated with a poor prognosis. The average overall survival of patients with MCL is 5 years, and for most patients who progress on targeted agents, survival remains at a dismal 3 to 8 months. There is a major unmet need to identify new therapeutic approaches that are well tolerated to improve treatment outcomes and quality of life. The protein arginine methyltransferase 5 (PRMT5) enzyme is overexpressed in MCL and promotes growth and survival. Inhibition of PRMT5 drives antitumor activity in MCL cell lines and preclinical murine models. PRMT5 inhibition reduced the activity of prosurvival AKT signaling, which led to the nuclear translocation of FOXO1 and modulation of its transcriptional activity. Chromatin immunoprecipitation and sequencing identified multiple proapoptotic BCL-2 family members as FOXO1-bound genomic loci. We identified BAX as a direct transcriptional target of FOXO1 and demonstrated its critical role in the synergy observed between the selective PRMT5 inhibitor, PRT382, and the BCL-2 inhibitor, venetoclax. Single-agent and combination treatments were performed in 9 MCL lines. Loewe synergy scores showed significant levels of synergy in most MCL lines tested. Preclinical, in vivo evaluation of this strategy in multiple MCL models showed therapeutic synergy with combination venetoclax/PRT382 treatment with an increased survival advantage in 2 patient-derived xenograft models (P ≤ .0001, P ≤ .0001). Our results provide mechanistic rationale for the combination of PRMT5 inhibition and venetoclax to treat patients with MCL.

PRMT5 supports multiple oncogenic pathways in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an incurable B-cell malignancy with an overall poor prognosis, particularly for patients that progress on targeted therapies. Novel, more durable treatment options are needed for patients with MCL. Protein arginine methyltransferase 5 (PRMT5) is overexpressed in MCL and plays an important oncogenic role in this disease via epigenetic and posttranslational modification of cell cycle regulators, DNA repair genes, components of prosurvival pathways, and RNA splicing regulators. The mechanism of targeting PRMT5 in MCL remains incompletely characterized. Here, we report on the antitumor activity of PRMT5 inhibition in MCL using integrated transcriptomics of in vitro and in vivo models of MCL. Treatment with a selective small-molecule inhibitor of PRMT5, PRT-382, led to growth arrest and cell death and provided a therapeutic benefit in xenografts derived from patients with MCL. Transcriptional reprograming upon PRMT5 inhibition led to restored regulatory activity of the cell cycle (p-RB/E2F), apoptotic cell death (p53-dependent/p53-independent), and activation of negative regulators of B-cell receptor-PI3K/AKT signaling (PHLDA3, PTPROt, and PIK3IP1). We propose pharmacologic inhibition of PRMT5 for patients with relapsed/refractory MCL and identify MTAP/CDKN2A deletion and wild-type TP53 as biomarkers that predict a favorable response. Selective targeting of PRMT5 has significant activity in preclinical models of MCL and warrants further investigation in clinical trials.

Abstract 1594: SMARCA2 (BRM) degraders promotes differentiation and inhibit proliferation in AML models

Dysregulated cellular differentiation is a major pathological feature of myeloid malignancies such as acute myeloid leukemia (AML). Targeting cellular differentiation programs has emerged as a novel therapeutic approach to treat patients with AML. Advantages of such differentiation therapy may include fewer systemic side-effects as well as opportunities to target leukemic stem cells (LSCs) and a broader range of clonal populations, likely resulting in lower frequencies of resistance and relapse in AML patients. The success of ATRA and decitabine in subsets of AML patients has proven that inducing differentiation can play a critical role in long-term durable responses. More agents targeting epigenetic regulators have been increasingly studied as differentiation inducers, including LSD1, DNMT1, Menin, and BET inhibitors. Recently, targeting SWI/SNF chromatin remodeling complexes has also been shown to regulate key leukemic gene expression signatures and induce AML differentiation. Small molecule inhibitors as well as gene knockdown for ATP-dependent SWI/SNF subunits SMARCA2 (BRM) and SMARCA4 (BRG1) are associated with re-direction of oncogenic transcriptional regulation to drive cellular differentiation and apoptosis in AML models. We have previously described the activity of highly potent, bispecific SMARCA2 degraders that efficiently promote SMARCA2 protein degradation in preclinical models. In the present study, we investigated the effects of our SMARCA2 selective degraders in AML models. Treatment with SMARCA2 degraders significantly inhibits AML cell line proliferation in vitro with IC50 ranging from 10 to 50 nM. In the SMARCA2 degrader treated cells, expression of PU.1 (SPI1), a key transcription factor in myeloid leukemias, was downregulated. In an in vivo OCI-AML3 xenograft model, treatment with a SMARCA2 selective degrader showed moderate tumor growth inhibition accompanied by robust increases in monocytic maturation markers CD11b and CD14. Further analyses of SMARCA2 degrader effects on global transcriptome and AML immunophenotypes as well as combination effects with other therapies are currently in progress. These findings highlight the potential of SMARCA2 degraders to target AML differentiation blocks and to improve the effectiveness of other therapeutic agents such as decitabine and venetoclax in AML patients.

Citation Format: Anjana Agarwal, Olusola Peace Osinubi, Komali Vykuntam, Norman Fultang, Neha Bhagwat, Diane Heiser, Kris Vaddi, Koichi Ito, Peggy Scherle. SMARCA2 (BRM) degraders promotes differentiation and inhibit proliferation in AML models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1594.

Abstract 5973: The brain penetrant CDK4/6 Inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of NSCLC, CRC, and HER2-positive breast cancer

Cell cycle deregulation is a hallmark of cancer and the hyperactivation and overexpression of CDKs are often drivers of cancer pathogenesis. Cyclin-dependent kinase 4 and 6 (CDK4)/(CDK6) are critical mediators of cellular transition into S phase and important for the initiation, growth, and survival of many cancers. Activated CDK4/CDK6 complexes phosphorylate Rb1, reduce their binding affinities and release Rb1-containing transcription repressor complexes from E2F transcription factors, resulting in activation of E2F controlled cell cycle genes and progression of the cell cycle. At present three CDK4/CDK6 inhibitors are approved for the treatment of ER+/HER2- breast cancer, and are being explored in other cancer indications as well. Previously we described a novel brain penetrant CDK4/CDK6 inhibitor, PRT3645, that exhibits single digit nanomolar biochemical potency against CDK4/CDK6 and &gt;2000-fold selectivity against CDK1, CDK2 and CDK9. PRT3645 inhibits cellular phosphorylation of Rb and exhibits a protein binding-adjusted cellular IC50 of &lt;300 nM. PRT3645 exhibits favorable in-vitro safety pharmacology and ADME properties, including increased brain penetration, and demonstrates oral bioavailability across rodents, dog, and non-human primates. In addition to robust monotherapy activity observed in preclinical models of ER+/HER2- breast cancer, we explored the activity of PRT3645 in other tumor types as well as in combination with other targeted therapies. In NSCLC, PRT3645 treatment resulted in significant inhibition of cell lines that harbor activation of the RAS/MEK/ERK pathway in proliferation assays and demonstrated comparable high synergy scores when combined with clinically approved covalent KRAS G12C inhibitors. In-vivo, oral PRT3645 was well tolerated and induced anti-tumor efficacy in two KRAS G12C mutant xenograft models that harbor the CDKN2A (p16) deletion. Anti-tumor efficacy was further improved when PRT3645 was combined with KRAS/MEK inhibitors in xenograft models and the combination therapy was well tolerated. In addition, we explored combinations of PRT3645 with a brain penetrant receptor tyrosine kinase inhibitor (TKI), an approved treatment for patients with advanced HER2+ breast cancer, including patients with brain metastases. In a HER2+ orthotopic human breast cancer brain metastasis model, PRT3645 was highly efficacious in combination with HER2 kinase inhibition and enhanced median survival significantly. In summary, PRT3645 demonstrates an excellent balance of potency, selectivity, PK parameters across species and brain penetrance. In preclinical studies, PRT3645 was highly efficacious when combined with KRAS/MEK inhibitors, and with a brain penetrant HER2 receptor TKI, both in-vitro and in-vivo. PRT3645 has advanced into Phase 1 clinical trials (NCT05538572).

Citation Format: Yue Zou, Srijita Dhar, Kirsten Gallagher, Andrew Buesking, Sarah Pawley, Ryan Holmes, Xiaowei Wu, Katarina Rohlfing, Min Wang, Joseph Rager, Tom Emm, Stefan Ruepp, Miles Cowart, Jing Ni, Jean Zhao, Bruce Ruggeri, Andrew Combs, Kris Vaddi, Sandy Geeganage, Ashish Juvekar, Sang Hyun Lee, Peggy Scherle. The brain penetrant CDK4/6 Inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of NSCLC, CRC, and HER2-positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5973.

Abstract 6147: MCL1 inhibitor PRT1419 demonstrates anti-tumor activity in PBRM1-altered clear cell renal cancer and synergizes with standard of care agents

Induced myeloid leukemia cell differentiation protein (MCL1) is a member of the B-cell lymphoma-2 (BCL2) family of apoptosis regulators, which plays a critical role in maintaining cellular homeostasis and promoting cancer cell survival. Increased expression of MCL1 in various cancers has been associated with poor prognosis and resistance to chemotherapeutic and targeted agents. We previously described PRT1419, a novel, potent, selective MCL1 inhibitor that demonstrates anti-tumor efficacy in various preclinical models of solid and hematologic malignancies. PRT1419 is currently under evaluation in Phase I clinical trials in patients with relapsed/refractory hematologic malignancies and advanced solid tumors. To identify novel biomarkers that might predict sensitivity to MCL1 inhibition, we conducted a gene essentiality analysis using publicly available human cancer cell line gene dependency data generated from genome-wide CRISPR/Cas9 cell viability screens. We observed that clear cell renal cancer (ccRCC) cell lines with deleterious alterations in PBRM1 (Polybromo 1) displayed a strong dependency on MCL1. PBRM1, also known as BAF180, is a chromatin-targeting subunit of mammalian pBAF (SWI/SNF-B) complexes. PBRM1 is frequently altered in various human cancers but it has a particularly high alteration rate in ccRCC with ~40% of tumors harboring damaging PBRM1 alterations. We had previously described alterations in other mammalian SWI/SNF factors as biomarkers of MCL1 inhibitor sensitivity.We observed potent inhibition of tumor growth as well as induction of apoptosis by PRT1419 in various preclinical models of PBRM1-mutant ccRCC but not in PBRM1-WT tumor models. Depletion of PBRM1 via RNAi in PBRM1-WT ccRCC induced sensitivity to PRT1419. Mechanistically, PBRM1 depletion coincided with increased expression of pro-apoptotic factors, priming PBRM1-loss cells for caspase-mediated cell death following MCL1 inhibition. Increased MCL1 activity has previously been described as a resistance mechanism to Sunitinib and Everolimus, two approved targeted agents for ccRCC. To investigate if MCL1 inhibition could potentiate the anti-tumor effects of these agents, we evaluated PRT1419 in combination with Sunitinib or Everolimus in PBRM1-loss ccRCC. PRT1419 synergized with both Sunitinib and Everolimus in inhibiting tumor growth in various models. Taken together, these findings suggest PBRM1 loss is associated with sensitivity to MCL1 inhibition in ccRCC and provide rationale for the evaluation of PRT1419 for the treatment for PBRM1-deficient ccRCC

Citation Format: Norman Fultang, Brian Vidal, Ashley M. Schwab, Alexander Grego, Diane Heiser, Kris Vaddi, Neha Bhagwat, Peggy Scherle. MCL1 inhibitor PRT1419 demonstrates anti-tumor activity in PBRM1-altered clear cell renal cancer and synergizes with standard of care agents. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6147.

Dysregulation of PRMT5 in chronic lymphocytic leukemia promotes progression with high risk of Richter's transformation

Richter's Transformation (RT) is a poorly understood and fatal progression of chronic lymphocytic leukemia (CLL) manifesting histologically as diffuse large B-cell lymphoma. Protein arginine methyltransferase 5 (PRMT5) is implicated in lymphomagenesis, but its role in CLL or RT progression is unknown. We demonstrate herein that tumors uniformly overexpress PRMT5 in patients with progression to RT. Furthermore, mice with B-specific overexpression of hPRMT5 develop a B-lymphoid expansion with increased risk of death, and Eµ-PRMT5/TCL1 double transgenic mice develop a highly aggressive disease with transformation that histologically resembles RT; where large-scale transcriptional profiling identifies oncogenic pathways mediating PRMT5-driven disease progression. Lastly, we report the development of a SAM-competitive PRMT5 inhibitor, PRT382, with exclusive selectivity and optimal in vitro and in vivo activity compared to available PRMT5 inhibitors. Taken together, the discovery that PRMT5 drives oncogenic pathways promoting RT provides a compelling rationale for clinical investigation of PRMT5 inhibitors such as PRT382 in aggressive CLL/RT cases.

Targeting OXPHOS de novo purine synthesis as the nexus of FLT3 inhibitor-mediated synergistic antileukemic actions

Using a genome-wide CRISPR screen, we identified CDK9, DHODH, and PRMT5 as synthetic lethal partners with gilteritinib treatment in fms-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD) acute myeloid leukemia (AML) and genetically and pharmacologically validated their roles in gilteritinib sensitivity. The presence of FLT3-ITD is associated with an increase in anaerobic glycolysis, rendering leukemia cells highly sensitive to inhibition of glycolysis. Supportive of this, our data show the enrichment of single guide RNAs targeting 28 glycolysis-related genes upon gilteritinib treatment, suggesting that switching from glycolysis to oxidative phosphorylation (OXPHOS) may represent a metabolic adaption of AML in gilteritinib resistance. CDK9i/FLT3i, DHODHi/FLT3i, and PRMT5i/FLT3i pairs mechanistically converge on OXPHOS and purine biosynthesis blockade, implying that targeting the metabolic functions of these three genes and/or proteins may represent attractive strategies to sensitize AML to gilteritinib treatment. Our findings provide the basis for maximizing therapeutic impact of FLT3-ITD inhibitors and a rationale for a clinical trial of these novel combinations.

Discovery of 1'-(1-phenylcyclopropane-carbonyl)-3H-spiro[isobenzofuran-1,3'-pyrrolidin]-3-one as a novel steroid mimetic scaffold for the potent and tissue-specific inhibition of 11β-HSD1 using a scaffold-hopping approach

11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) has been identified as the primary enzyme responsible for the activation of hepatic cortisone to cortisol in specific peripheral tissues resulting in the concomitant antagonism of insulin action within these tissues. Dysregulation of 11β-HSD1, particularly in adipose tissues, has been associated with metabolic syndrome and type 2 diabetes mellitus. Therefore, inhibition of 11β-HSD1 with a small nonsteroidal molecule is therapeutically desirable. Implementation of a scaffold-hopping approach revealed a three-point pharmacophore for 11β-HSD1 that was utilized to design a steroid mimetic scaffold. Reiterative optimization provided valuable insight into the bioactive conformation of our novel scaffold and led to the discovery of INCB13739. Clinical evaluation of INCB13739 confirmed for the first time that tissue-specific inhibition of 11β-HSD1 in patients with type 2 diabetes mellitus was efficacious in controlling glucose levels and reducing cardiovascular risk factors.

Abstract 2159: PRMT5 inhibitor PRT543 displays potent antitumor activity in U2AF1S34F and RBM10LOF spliceosome-mutant non-small cell lung cancer in vitro and in vivo

PRMT5 (protein arginine methyltransferase 5) is a major Type II PRMT, which catalyzes the symmetric dimethylation of protein arginine residues (sDMA). As an epigenetic regulator, PRMT5 plays essential roles in promoting cancer growth and survival, including through mechanisms that control alternative splicing and RNA processing, and the expression of DNA damage repair genes. Spliceosome mutations have been suggested to represent a potential biomarker for PRMT5 inhibitors, and our previous work highlighted this sensitivity in SF3B1R625C/G expressing uveal melanoma cells. Here, we highlight in vitro and in vivo activity of PRT543, a potent, selective, and orally available PRMT5 inhibitor, in cancer cells harboring mutations in other spliceosome factors such as U2 small nuclear RNA auxiliary factor 1 (U2AF1) and RNA binding motif protein 10 (RBM10). Mutations in U2AF1 (including S34F hotspot mutations) and RBM10 (primarily loss of function (LOF) mutations) occur in 5-10% of all non-small cell lung cancers (NSCLC). Cell proliferation (10-day assay) was assessed in a panel of NSCLC cell lines treated with PRT543, either wild-type or harboring U2AF1S34F or RBM10LOF mutations. Strikingly, both U2AF1S34F and RBM10LOF cell lines were significantly more sensitive to PRT543 compared to wild-type cell lines. Furthermore, PRT543 induced significant dose-related tumor growth inhibition at well-tolerated doses in cell-line derived xenograft (CDX) models harboring the U2AF1S34F or RBM10LOF mutation. Consistent with our previous findings in other tumor types, PRT543 decreased expression of DNA damage repair-associated genes (e.g. BRCA1, RAD51AP1, FANCA, and FANCL) in U2AF1S34F or RBM10LOF mutant NSCLC cells. Combination with PRT543 increased the effectiveness of specific chemotherapeutic agents in both in vitro and in vivo (CDX) models of U2AF1S34F and RBM10LOF NSCLC. Efficacy studies in patient-derived xenograft (PDX) models, as well as genomic profiling of spliceosome-mutant cellular models in response to PRT543 are ongoing. PRT543 is currently under evaluation in a Phase I clinical trial in patients with advanced solid tumors and hematological malignancies (NCT03886831).

Citation Format: Jack Carter, Koichi Ito, Venkat Thodima, Monisha Sivakumar, Michael Hulse, Joseph Rager, Komali Vykuntam, Neha Bhagwat, Kris Vaddi, Bruce Ruggeri, Peggy Scherle. PRMT5 inhibitor PRT543 displays potent antitumor activity in U2AF1S34F and RBM10LOF spliceosome-mutant non-small cell lung cancer in vitro and in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2159.

Abstract 2300: Brain penetrant CDK4/6 inhibitor PRT3645 demonstrates anti-tumor activity and enhances survival in glioblastoma and breast cancer brain metastasis models

Cell cycle deregulation is a hallmark of cancer and CDK inhibitors, specifically inhibiting CDK4/6 and blocking cells transition from the G1 to the S phase of the cell cycle are the first and only class of highly specific CDK inhibitors approved for cancer treatment to date. CDK4/6 inhibitors have transformed the treatment paradigm of estrogen receptor-positive (ER+), HER2- breast cancer with three CDK4/6 inhibitors currently FDA approved.

Brain metastasis commonly arises in patients with breast, lung, melanoma and other cancer types, is associated with poor survival outcomes and poses distinct challenges in clinical management. Due to advances in imaging technologies, the detection of brain metastases is increasing and there is a dearth of novel therapies to combat brain metastatic cancers and impact patient survival.

Here, we describe a novel brain penetrant CDK4/6 inhibitor, PRT3645 exhibiting single digit nanomolar biochemical potency against CDK 4/6 and &gt;2000-fold selectivity against other CDK family members (CDK1, CDK2, and CDK9). In cellular assays, PRT3645 inhibits cellular phosphorylation of RB with low nanomolar activity. Consistent with this, PRT3645 treatment resulted in concentration-dependent inhibition of cell proliferation in glioblastoma (GBM) cell lines and in HER2- and HER2+ breast cancer lines (EC50 values &lt; 125 nM). Furthermore, PRT3645 demonstrated additive in vitro activity with fulvestrant or tucatinib in ER+ and HER2+ breast cancer lines.

PRT3645 exhibits favorable in vitro safety pharmacology and ADME profiles, including brain exposure in rodents at steady state, and demonstrates oral bioavailability across rodents, dog and nonhuman primates. In vivo, oral PRT3645 was well tolerated and highly efficacious in a dose-dependent manner in subcutaneous xenograft models of GBM and breast cancer and in orthotopic human breast cancer brain metastasis (BCBM) and GBM models in mice as a monotherapy. PRT3645 showed tumor regression as single agent in the MCF7 ER+ breast cancer model and a combinatorial benefit with the estrogen receptor blocker, fulvestrant. In a HER2+ BT474-luc orthotopic model, similarly efficacious single agent activity of PRT3645 was achieved, as well as a significant combinatorial benefit on tumor growth and median survival when administered with the brain penetrant HER2 kinase inhibitor, tucatinib. PRT3645 was highly efficacious in a U87-luc GBM orthotopic model and demonstrated enhanced median survival benefit when combined with an orally active brain penetrant PRMT5 inhibitor.

In summary, PRT3645 demonstrates an excellent balance of potency, selectivity, PK parameters across species, brain penetrance and favorable tissue distribution relative to brain exposure, and currently has advanced into IND-enabling preclinical studies.

Citation Format: Ashish Juvekar, Yang Zhang, Andrew Buesking, Min Wang, Dave Rominger, Joseph Rager, Stefan Ruepp, Kirsten Gallagher, Yue Zou, Miles Cowart, Xiaowei Wu, Sarah Pawley, Ryan Holmes, William Gowen-MacDonald, Kris Vaddi, Andrew Combs, Bruce Ruggeri, Peggy Scherle. Brain penetrant CDK4/6 inhibitor PRT3645 demonstrates anti-tumor activity and enhances survival in glioblastoma and breast cancer brain metastasis models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2300.

Abstract 5471: PRT2527, a novel highly selective cyclin-dependent kinase 9 (CDK9) inhibitor, is active in preclinical models of prostate cancer

CDK9 is a serine/threonine kinase belonging to the subclass of the transcription associated CDKs. CDK9 complexes with cyclin T and cyclin K, the positive transcription elongation factor b (P-TEFb), and phosphorylates Serine 2 of RNA polymerase II (pSer2 RNAPII) to activate transcription. Consequently, targeting CDK9 could effectively interfere with epigenetic and transcriptional reprogramming and prevent disease progression and treatment resistance in human cancers. Androgen receptor (AR)-dependence in prostate cancer is linked to CDK9 function. CDK9 stabilizes AR-associated proteins, and pharmacological inhibition of CDK9 can inhibit AR, AR variants, and their downstream transcription programs. We evaluated the novel CDK9 inhibitor, PRT2527, in prostate cancer models to evaluate its effects on cell proliferation, stem-like tumor cells, and tumor growth. PRT2527 is a potent inhibitor of CDK9/CyclinT1 complex, and when evaluated at concentration 200 times the biochemical IC50, PRT2527 was highly selective for CDK9 inhibition. We verified in biochemical assays the ability of PRT2527 to suppress pSer2 RNAPII and reduce expression of c-Myc, a common target of CDK9, in a concentration-dependent manner in multiple human prostate cancer cell lines. PRT2527 also inhibited c-Myc-dependent transcription in vitro in luciferase reporter assays. Furthermore, RNA sequencing showed altered expression of several genes with significant enrichment of c-Myc and E2F targets and RNAPII dependent transcription among downregulated genes in PRT2527-treated VCaP cells. In vitro, PRT2527 inhibited the proliferation of androgen-dependent and androgen-independent prostate cancer cell lines (IC50, ≤50 nM). PRT2527 was highly effective (IC50, ≤10 nM) in tumor-spheroid assays in blocking the growth of stem-like tumor cells and significantly suppressed the in vitro growth of tumor organoids from both human cell lines and patient-derived xenografts (PDXs). In mice, PRT2527 IV administration reduced pSer2 RNAPII in tumor xenografts and c-Myc-dependent transcriptional activity in a DU145 luminescence reporter model. PRT2527 administration in mice significantly reduced growth of PDX LuCaP 35 (castration-sensitive, adenocarcinoma) and LuCaP 145.2 (castration-resistant, neuroendocrine) along with the fraction of tumor-initiating stem-like cells in ex vivo assays. PRT2527 reduced pSer2 RNAPII in both PDXs, whereas c-Myc decreased in LuCaP 35 and Sox2 in LuCaP 145.2, relative to basal expression levels. Collectively, our data demonstrate that PRT2527 has potent pharmacodynamic and antitumor activity in multiple models of castration-sensitive and castration-resistant prostate cancer. PRT2527 is advancing into phase 1 studies in solid tumors.

Citation Format: Elisa Federici, Gianluca Civenni, Aleksandra Kokanovic, Giada Sandrini, Luca Guarrera, Simone Mosole, Alessia Cacciatore, Valeria Uboldi, Manuel Lessi, Giovanni Papa, Domenico Albino, Elisa Storelli, Jessica Merulla, Andrea Rinaldi, Marco Bolis, Yang Zhang, Kris Vaddi, Peggy Scherle, Bruce Ruggeri, Giuseppina M. Carbone, Carlo V. Catapano. PRT2527, a novel highly selective cyclin-dependent kinase 9 (CDK9) inhibitor, is active in preclinical models of prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5471.

Abstract 3263: Preclinical characterization of PRT3789, a potent and selective SMARCA2 targeted degrader

SWI/SNF complexes play an important role in controlling gene expression by remodeling chromatin. SMARCA2 (BRM) and SMARCA4 (BRG1) are the core catalytic subunits of the SWI/SNF complexes, containing an ATPase domain and a DNA binding bromodomain. SMARCA4 protein expression is lost in some cancers due to nonsense mutations, and SMARCA4-deleted cancer cells are highly dependent on its paralog gene SMARCA2 for their survival. Therefore, targeting SMARCA2 in SMARCA4-deleted cancers using selective SMARCA2 degraders induces synthetic lethality while sparing SMARCA4 wild type (WT) normal cells. We have identified PRT3789, a potent and selective SMARCA2 targeted degrader, that selectively inhibits proliferation of SMARCA4-deleted cancer cells. Here, we describe the potential mechanism of action for PRT3789 at the molecular level and the in vitro and in vivo anti-tumor activity in SMARCA4-deleted cancer cells. To further elucidate the SMARCA2 degradation selectivity of PRT3789, we performed mass spectrometry to identify the selective SMARCA2 lysine residues ubiquitinated following treatment with PRT3789. This data, in combination with site-directed mutagenesis against these SMARCA2-specific ubiquitinated residues, has revealed important insights into the mechanism of action of PRT3789. In addition, to further understand the specific vulnerability of SMARCA2 in SMARCA4-deleted cells, we investigated whether PRT3789 affected the integrity of the residual SWI/SNF complex. Coimmunoprecipitation of SMARCC1 revealed that PRT3789 disrupts specific SWI/SNF complex subunits, including ACTL6A (BAF53). Functional genome-wide experiments are ongoing to evaluate the impact of this finding and the residual activity of the SWI/SNF complex. Furthermore, treatment with PRT3789 demonstrated robust inhibition of cell proliferation of SMARCA4-deleted non-small cell lung cancer (NSCLC) cells in vitro and NSCLC PDX tumors ex vivo, but not SMARCA4 WT cancer cells, in a concentration-dependent manner. Lastly, PRT3789 shows favorable pharmacokinetic properties in vivo, which correlate to its pharmacodynamics effects as evidenced by reduced SMARCA2 protein and KRT80 mRNA levels in tumor tissues. In subcutaneous cell-line derived xenograft (CDX) models of NSCLC, administration of PRT3789 demonstrated significant dose-related inhibition of SMARCA4-deleted NSCLC growth at tolerated doses, but no effect on the growth of SMARCA4 WT cancers. In summary, consistent with our previous validation studies and genomic perturbation analyses, our potent and selective SMARCA2 targeted degrader PRT3789 induces strong synthetic lethality in SMARCA4-deleted cancers in vitro and in vivo.

Citation Format: Michael Hulse, Anjana Agarwal, Min Wang, Jack Carter, Monisha Sivakumar, Brian Vidal, Justin Brown, Andrew Moore, Alexander Grego, Neha Bhagwat, Joseph Rager, Liang Lu, Corey Basch, Klare Bersch, Chaofeng Dai, Philip Pitis, Andrew Combs, Bruce Ruggeri, Kris Vaddi, Peggy Scherle, Koichi Ito. Preclinical characterization of PRT3789, a potent and selective SMARCA2 targeted degrader [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3263.

Abstract 420: Combination of the MCL1 inhibitor PRT1419 and SMARCA2 degrader PRT3789 shows combinatorial benefit in SMARCA4 deleted lung cancer

MCL1 is a member of the anti-apoptotic BCL2 family of proteins and plays a critical role in maintaining cellular homeostasis and promoting cell survival. MCL1 amplifications occur frequently in multiple tumor types. It has also been implicated in mediating resistance to chemotherapeutic agents and targeted therapies. We have previously described a novel, potent and orally bioavailable MCL1 inhibitor, PRT1419, that demonstrates anti-tumor efficacy in various preclinical models of cancer and is currently under evaluation in a Phase I clinical trial in patients with relapsed/refractory hematologic malignancies and advanced solid tumors. In an effort to identify novel biomarkers that might predict sensitivity to MCL1 inhibition, we conducted a gene dependency analysis using publicly available human cancer cell line data generated from genome-wide CRISPR/Cas9-mediated cell viability screens. We observed that mutations in the SWI/SNF complex, particularly in lung and ovarian cancer cell lines, conferred a strong functional dependency on MCL1. The mammalian SWI/SNF complex functions as a tumor suppressor in a number of cancers and regulates gene expression via chromatin-remodeling. It is comprised of multiple subunits, including one of two catalytic ATPases (SMARCA2 or SMARAC4), DNA-binding proteins ARID1A, ARID1B and ARID2, and other chromatin-binding subunits. Gene mutations in members of this complex occur in &gt;20% of human cancers, and therapeutic agents targeting its function are under active clinical investigation. We and others have shown potent synthetic lethality with the use of SMARCA2 targeted protein degraders in SMARCA4 deleted lung cancer models. A previously published genome-wide CRISPR screen in SMARCA4-mut lung cancer cell lines demonstrated that loss of MCL1 could sensitize these cells to SMARCA2 degradation. Therefore, we evaluated PRT1419 in combination with a novel and selective SMARCA2 degrader, PRT3789, in SMARCA4 deleted lung cancer models. We observed a potent synergistic interaction in SMARCA4 deleted cell lines in vitro, whereas no additive benefit was seen in SMARCA4 WT lines. Further, combining PRT1419 and PRT3789 in vivo in cell line-derived xenograft models resulted in significant tumor growth inhibition, including tumor regressions. Additionally, we profiled PRT1419 ex vivo in a panel of lung cancer PDX models and observed significant, dose-dependent effects on cell viability in SMARCA4 deleted models with low SMARCA2 expression. In a broader lung cancer cell line viability screen conducted with PRT1419, we observed that the presence of multiple, co-occurring alterations in SWI/SNF family members such as SMARCA4, ARID1A/B mutations and loss of SMARCA2 protein were associated with sensitivity to PRT1419. Based on these findings, preclinical evaluation of PRT1419 in other tumor types with recurrent SWI/SNF mutations is ongoing.

Citation Format: Norman Fultang, Neha Bhagwat, Diane Heiser, Alexander Grego, Michael Hulse, Venkat Thodima, Koichi Ito, Kris Vaddi, Bruce Ruggeri, Peggy Scherle. Combination of the MCL1 inhibitor PRT1419 and SMARCA2 degrader PRT3789 shows combinatorial benefit in SMARCA4 deleted lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 420.

Abstract 1031: PRMT5 inhibition alters mitochondrial dynamics in mantle cell lymphoma, creating vulnerability to BH3 mimetic compounds

Mantle cell lymphoma (MCL) is an aggressive and incurable blood cancer comprising 5% of all non-Hodgkin lymphomas diagnosed annually. The median age of diagnosis is 68yo, and while many patients initially respond to frontline treatment, relapse is common. There is an unmet need to develop novel therapeutic strategies for the treatment of MCL. Our group has identified protein arginine methyltransferase 5 (PRMT5) as a key driver of MCL pathogenesis. PRMT5 symmetrically dimethylates arginine residues on a number of proteins (P53, E2F1, P65) and histones (H4R3, H3R8, H2AR3), which support tumorigenesis. Selectively inhibiting PRMT5 has shown significant anti-tumor activity in preclinical MCL models, and a Phase 1 clinical trial with PRT543 (Prelude), a novel PRMT5 inhibitor, is underway. While exploring pathways that converge with PRMT5 activity as potential avenues for combination treatment, we identified the intrinsic apoptotic pathway as an attractive target in MCL. BCL2 family proteins either promote or inhibit intrinsic apoptosis at the outer mitochondrial membrane through a dynamic set of binding interactions. Prior work has shown PRMT5 inhibition to drive the expression of multiple pro-death BCL2 family gene products (BAX, BAK, and BBC3/PUMA) in MCL. We hypothesized that combining PRMT5 inhibition with BH3 mimetics, compounds that target pro-survival BCL2 proteins, would induce synergistic cell death in MCL. Selective PRMT5 inhibition with PRT382 inhibits the viability of MCL cell lines with an IC50 below 1uM in eight of nine lines (IC50 44.8nM - 1905.5nM). BH3 mimetics navitoclax (BCL2, BCLXL, and BCLw inhibitor), A852 (BCLXL inhibitor), and AMG176 (MCL1 inhibitor) were found to have IC50s below 1uM in five, two, and four of the nine cell lines respectively. We chose six cell lines to test combination treatment ranging in sensitivity to BH3 mimetics and expression of BCL2 family proteins (CCMCL1, Z-138, UPN1, Granta 519, Mino, and Maver1). Synergistic decreases in viability were tested via MTS assay and analyzed with the Loewe model of synergy. Mino, which showed sensitivity to all three mimetics, exhibited a synergistic reduction in viability with combination PRT382 treatment. Granta 519 and Z-138 exhibited a similar effect with the combination of PRT382 and navitoclax or A852. These observations were confirmed through BH3 profiling, supporting MCL cell line dependence on BCL2, BCLXL, BCLw and MCL1, and increased sensitivity to BCL2 family protein targeting with PRMT5 inhibition. Two patient derived xenograft models were tested ex vivo after treatment with 10mg/kg of the PRMT5 inhibitor PRT382 or vehicle. iBH3 flow-based analysis revealed increased sensitivity of ex vivo PDX cells to pan BCL2, BCLXL, and MCL1 inhibition. These results provide rationale for combining BH3 mimetics and PRMT5 inhibition in clinical trials as a novel treatment strategy for MCL.

Citation Format: Claire Hinterschied, Fiona Brown, Janani Ravikrishnan, JoBeth Helmig-Mason, Kris Vaddi, Peggy Scherle, Jennifer Woyach, Selina Chen-Kiang, Oliver Elemento, Jihye Paik, Robert Baiocchi. PRMT5 inhibition alters mitochondrial dynamics in mantle cell lymphoma, creating vulnerability to BH3 mimetic compounds [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1031.

Abstract P237: PRT2527 is a potent and selective CDK9 inhibitor that demonstrates anti-cancer activity in preclinical models of hematological malignancies and solid tumors with MYC amplification

Cyclin-dependent kinase 9 (CDK9) is a master regulator of transcription that controls paused RNA polymerase II (RNAP2) release through phosphorylation of its carboxy-terminal domain, resulting in productive transcription elongation. CDK9 has been extensively studied as a potential target for cancer therapy in “transcriptionally addicted” tumors as transient inhibition of CDK9 primarily depletes proteins with short half-lives, such as the oncogenes MCL1 and MYC, making CDK9 a promising target in cancer. Here we show that PRT2527 is a potent and highly selective CDK9 inhibitor with moderate to high clearance that achieves optimal temporal target engagement and exhibits potent in vitro and in vivo activities. PRT2527 inhibited CDK9 enzymatic activity with an IC50 of 0.98 nM in a biochemical assay and showed high selectivity in a panel of kinases when tested at physiologically relevant 1 mM ATP concentration. In vitro, PRT2527 inhibited phosphorylation of Ser2RNAP2 in NCI-H929 cells with an IC50 of 54 nM, and an IC50 of 198 nM in a plasma assay to adjust for human plasma protein binding. Transient treatment of cells with PRT2527 inhibited pSer2RNAP2, depleted MCL1 and MYC proteins, and activated cleaved caspase-3 (CC3) in a concentration-dependent manner. In a proteomic profiling study, MCL1 was identified as one of the major down-regulated proteins following PRT2527 treatment. In a panel of hematological cancer cell lines representing B- and T-ALL, AML, and non-Hodgkin’s lymphoma (NHL), as well as subsets of sarcoma, prostate, adenoid cystic carcinoma (ACC), and non-small cell lung cancer (NSCLC) cell lines, PRT2527 treatment consistently led to a potent, concentration-dependent inhibition of proliferation. In a pharmacokinetic/pharmacodynamic (PK/PD) study, intravenous (IV) administration of PRT2527 achieved transient target engagement, depletion of MCL1 and MYC proteins, and induction of apoptosis in tumor tissue. This PK/PD correlation was successfully translated into in vivo efficacy in multiple models. Once weekly dosing of PRT2527 was well-tolerated and significantly inhibited tumor growth in various AML CDX models and induced tumor regressions in double-hit and triple-hit diffuse large B-cell lymphoma (DLBCL) CDX and PDX models carrying the MYC translocation. Combining PRT2527 with venetoclax achieved complete tumor regressions in a venetoclax resistant OCI-AML3 model. PRT2527 demonstrated potent ex vivo activity in PDX models of B-ALL and T-ALL, as well as various solid tumor PDX models with high levels of MYC amplification and overexpression, including pancreatic carcinoma, gastric and gastroesophageal carcinomas, NSCLC, and sarcoma. In vivo efficacy studies with once weekly IV administration of PRT2527 confirmed significant tumor growth inhibition in select MYC-amplified solid tumor PDX models. Taken together, this preclinical characterization supports the advancement of PRT2527 into clinical studies for transcriptionally addicted hematological malignancies and solid tumors with MYC amplification and/or dysregulation.

Citation Format: Yang W. Zhang, Liang Lu, Min Wang, Dave Rominger, Stefan Ruepp, Kirsten Gallagher, William Gowen-MacDonald, Chaofeng Dai, Miles Cowart, Andrew Combs, Bruce Ruggeri, Peggy Scherle, Kris Vaddi. PRT2527 is a potent and selective CDK9 inhibitor that demonstrates anti-cancer activity in preclinical models of hematological malignancies and solid tumors with MYC amplification [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P237.

Abstract 983: Preclinical characterization of PRT1419, a potent, selective and orally available inhibitor of MCL1

MCL1 is a member of the anti-apoptotic BCL2 family of proteins and plays a critical role in maintaining cellular homeostasis and promoting cell survival. It is frequently amplified in cancer and increased expression of MCL1 is associated with a higher grade and poor prognosis in multiple tumor types. Importantly, MCL1 has been implicated in mediating resistance to chemotherapy as well as targeted therapies, including the BCL2 inhibitor, venetoclax. Here, we describe the in vitro and in vivo activity of PRT1419, a potent and selective inhibitor of human MCL1, that can induce tumor cell death by apoptosis. PRT1419 inhibits the binding of MCL1 to its physiological ligand, BIM, with low nanomolar potency. PRT1419 also demonstrated &gt;200-fold selectivity against other BCL2 family members, including BCL2 and BCL-XL. In vitro, PRT1419 treatment resulted in robust activation of apoptotic markers such as cleaved caspase-3 in a concentration-dependent manner in several cancer cell lines. Consistent with its pro-apoptotic effects, PRT1419 treatment led to robust inhibition of cell proliferation in a concentration-dependent manner in a panel of cancer cell lines. Cell lines representing hematologic cancers as well as a subset of breast and non-small cell lung cancer lines were sensitive to PRT1419, and this response was associated with a significantly higher MCL1/BCL-XL mRNA ratio. Also, PRT1419 treatment resulted in potent, concentration-dependent cytotoxic activity ex vivo in patient-derived xenograft (PDX) models of various subtypes of human sarcoma, breast and esophageal cancer. PRT1419 demonstrated good oral bioavailability and favorable pharmacokinetic properties in vivo. In subcutaneous cell-line derived xenograft (CDX) models of multiple myeloma, acute myeloid leukemia (AML) and diffuse large B-cell lymphoma, oral administration of PRT1419 demonstrated potent anti-tumor activity with complete tumor regressions observed at tolerable doses. This response correlated with a dose-dependent induction of cleaved caspase-3 and cleaved-PARP in tumor tissue. Significant in vivo activity, including complete responses, was also observed in PDX models of lymphoma. In preclinical models of solid tumors, PRT1419 demonstrated significant tumor growth inhibition in a PDX model of human soft tissue sarcoma and a CDX model of breast cancer. PRT1419 was also tested in combination with other approved targeted therapies in vitro and in vivo. In AML, combining PRT1419 with a BCL2 inhibitor revealed a synergistic interaction in cell lines, ex vivo PDX models as well as a CDX model in vivo. Further, PRT1419 demonstrated synergistic activity with tyrosine kinase inhibitors to inhibit the proliferation of breast, melanoma, and non-small cell lung cancer cell lines. PRT1419 is currently under evaluation in a Phase I clinical trial in patients with relapsed/refractory hematologic malignancies (NCT04543305).

Citation Format: Neha Bhagwat, Alexander Grego, William Gowen-MacDonald, Min Wang, Miles Cowart, Xiaowei Wu, Jincong Zhuo, Andrew Combs, Bruce Ruggeri, Peggy Scherle, Kris Vaddi. Preclinical characterization of PRT1419, a potent, selective and orally available inhibitor of MCL1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 983.

Abstract 1138: PRMT5 inhibition downregulates MYB and NOTCH1 signaling, key molecular drivers of adenoid cystic carcinoma

Adenoid cystic carcinoma (ACC) is a rare cancer of secretory glands, characterized by slow and unpredictable growth, unrelenting relapse and high rates of metastasis. Poor response to chemotherapy and targeted drugs has resulted in there being no approved therapies to date, which contributes to poor prognosis with less than a 20% 5-year survival rate in patients with high-risk ACC. A vast majority of ACC tumors are known to have recurrent chromosomal translocation t(6;9) or t(8;9) resulting in MYB/MYBL1-NFIB fusions, and high levels of MYB protein expression. Furthermore, a subset of ACC tumors expresses NOTCH1 activating mutations (20-25%) and hyperactive NOTCH signaling in both primary and recurrent/metastatic tumors. A previous report has demonstrated that an inhibitor of PRMT5 (protein arginine methyltransferase) shows favorable responsiveness in patients with advanced ACC in a phase I clinical trial. Here, we investigate mechanistic aspects as to how PRMT5 inhibition potentially regulates the unique molecular drivers of ACC tumor growth. Due to a limited number of validated in vitro cellular models, we selected a group of head and neck cancer cell lines, including those originating from salivary gland, along with MYB and NOTCH1-driven leukemia models, to evaluate the effects of PRMT5 inhibition on MYB and NOTCH regulated genes. Treatment with PRT543, a potent and selective PRMT5 inhibitor, decreased expression of MYB and its associated downstream gene GATA3, as well as ACC-related genes such as SOX4 and POU3F2. Furthermore, PRT543 downregulated MYB signaling (MYB, FOXM1, NIK and SKI) as well as MYB alternative isoforms MYB-9A and MYB-10A in MYB expressing leukemia cells. Intriguingly, treatment with PRT543 also downregulated the transcription factor, HSF4, that is uniquely linked to regulation by N-terminal truncated MYB, a variant driven by an alternative promoter (TSS2) and reported as being active in ACC tumors. In addition to regulation of MYB, we also show that PRT543 downregulates NOTCH1 expression as well as multiple NOTCH1 target genes in leukemia cells which harbor a NOTCH1 active mutation. Given these results, we investigated the ability of PRT543 to inhibit the growth of ACC PDX tumor models in vivo and demonstrate that PRT543 significantly inhibits the growth of ACC PDX tumors in vivo. We confirmed that PRT543 downregulates levels of global symmetric dimethylarginine (sDMA, a substrate of PRMT5) in the treated tumors. Further studies with PRT543 in ex vivo and in vivo ACC models are ongoing. Collectively, these findings provide a strong molecular rationale for exploring PRT543 as a potential therapeutic option for ACC tumors. PRT543 is currently under evaluation in a Phase I clinical trial in patients with advanced solid tumors and hematological malignancies (NCT03886831).

Citation Format: Jack Carter, Koichi Ito, Venkat Thodima, Neha Bhagwat, Joseph Rager, Nicole Spardy Burr, Jeffrey Kaufman, Bruce Ruggeri, Peggy Scherle, Kris Vaddi. PRMT5 inhibition downregulates MYB and NOTCH1 signaling, key molecular drivers of adenoid cystic carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1138.

Abstract 1185: PRMT5 inhibition epigenetically regulates DNA damage response pathways in cancer cells and sensitizes to chemotherapy and PARP inhibition

Genetic defects of DNA damage response (DDR) pathways in cancer cells induce synthetic lethality with chemotherapy and PARP inhibitors. However, many patients with advanced cancers develop resistance to these therapies mainly due to circumvention and/or restoration of the inactivated DDR genes. In the present study, we demonstrate that pharmacological inhibition of PRMT5, the major type II protein arginine methyltransferase, epigenetically downregulates multiple genes involved in the DDR/DNA replication pathways, resulting in the sensitization of cancer cells to chemotherapy and PARP inhibition. Treatment with C220 or PRT543, potent and selective PRMT5 inhibitors, downregulates expression of DDR genes and/or proteins including BRCA1/2, ATM/ATR, CHK1/2, RAD51, POLD1/3, and PNKP in multiple types of cancer cells in vitro. Mechanistically, PRT543 promotes global alternative splicing (ΔPSI) changes, including intron retention and exon skipping. In particular, PRT543 significantly increases retained intron of POLD1 and PNKP genes, which are downstream of the spliceosome subunit SRSF1, a major protein substrate of PRMT5. Additionally, PRT543 increases intron retention of ATM and ATR genes. For BRCA1/2, CHK1/2 and RAD51, no changes in alternative splicing were observed, suggesting the involvement of other mechanisms such as the CLNS1A/H4R3me2s chromatin regulation pathway. The regulation of the DDR pathway by PRT543 is associated with increased DNA damage as determined by COMET and gamma H2AX analyses. Combining PRT543 with PARP inhibitors or DNA-alkylating agents demonstrates a potent synergistic interaction in both HR proficient and HR deficient cancer cell lines in vitro. Similar combination effects are observed in primary cultures of patient-derived breast cancer and high-grade serous ovarian cancer. Moreover, combination of PRT543 with PARP inhibition effectively inhibits the growth of HCC1569 CDX breast cancer in vivo. Further ex vivo and in vivo combination studies with PRT543 and PARP inhibitors or chemotherapy in genetically defined CDX and PDX models are ongoing. These studies not only reveal a novel mechanism of PRMT5 inhibition, but also suggest beneficial combination effects with other therapies, particularly in patients with tumors that are resistant to therapies that are dependent on DNA damage as their mechanism of action. PRT543 is currently under evaluation in a Phase I clinical trial in patients with advanced solid tumors and hematological malignancies (NCT03886831).

Citation Format: Koichi Ito, Jack Carter, Venkat Thodima, Youyou Zhang, Monisha Sivakumar, Mu Xu, Neha Bhagwat, Joseph Rager, Jacob Spruance, Lin Zhang, Bruce Ruggeri, Peggy Scherle, Kris Vaddi. PRMT5 inhibition epigenetically regulates DNA damage response pathways in cancer cells and sensitizes to chemotherapy and PARP inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1185.

Abstract 1137: PRMT5 inhibition regulates alternative splicing and DNA damage repair pathways in SF3B1 R625G expressing uveal melanoma cells

PRMT5 (protein arginine methyltransferase 5) is a predominant Type II PRMT that catalyzes symmetric dimethylation of protein arginine residues (sDMA). PRMT5 is overexpressed in many types of cancer and plays roles in multiple essential biological processes to promote cancer growth. Previous studies have shown that PRMT5 is a critical molecule for RNA processing and pre-mRNA splicing. Mechanistically, PRMT5 directly methylates arginine residues of several splicing factors such as Small nuclear ribonucleoprotein (SNRPB and SNRPD3) and Serine and arginine rich splicing factor 1 (SRSF1), which contributes to spliceosome assembly and promotes canonical splicing of many essential genes in cancer cells. In the present study, we examined the effects of PRT543, a potent and selective PRMT5 inhibitor, on alternative splicing in uveal melanoma which frequently express hotspot mutations on Splicing factor 3b subunit 1 (SF3B1). We first confirmed that PRT543-treated MEL202 (SF3B1R625G active mutant) and MEL270 (SF3B1WT) cells show significantly increased global alternative splicing, such as increased retained intron (RI) and skipping exon (SE), determined by delta-PSI (percentage of splice-in) analysis. PRT543 downregulates SF3B1 target genes such as FBXW5, MAP3K7, MBD4 and BRD9 that are associated with increased retention of specific intron sites. Interestingly, downregulation of the SF3B1 target genes are more significant in MEL202 (SF3B1R625G) than MEL270 (SF3B1WT), indicating that PRT543 can regulate the activity of the SF3B1 gain of function mutant. Consistent with previously reported PRMT5 knockout studies in hematological cancer cells, PRT543 also downregulates expression of SRSF1 target genes such as POLD1 and PNKP through increased intron retention in primary and metastatic uveal melanoma cell lines. Furthermore, we uncover that PRT543 strikingly increases retention of a specific intron site of ATM (ex33-34), resulting in a significant reduction of ATM protein levels in PRT543-treated MEL202 and MEL270 cells. Gene set enrichment analysis (GSEA) further reveals that PRT543 significantly and specifically regulates DNA replication and repair pathways in MEL202 cells. Importantly, combining PRT543 with DNA-alkylating agents or PARP inhibitors yields a synergistic reduction in cell viability. In summary, our results suggest that PRMT5 inhibition regulates cancer-associated RNA splicing machinery and the DNA damage response, resulting in synergistic antitumor activity when combined with chemotherapy and/or PARP inhibitors, particularly in cancers with spliceosomal mutations. PRT543 is currently under evaluation in a Phase I clinical trial in patients with advanced solid tumors and hematological malignancies (NCT03886831).

Citation Format: Koichi Ito, Venkat Thodima, Jack Carter, Neha Bhagwat, Monisha Sivakumar, Alexander Grego, Joseph Rager, Mizue Terai, Takami Sato, Omar Abdel-Wahab, Bruce Ruggeri, Peggy Scherle, Kris Vaddi. PRMT5 inhibition regulates alternative splicing and DNA damage repair pathways in SF3B1 R625G expressing uveal melanoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1137.

Validation of protein arginine methyltransferase 5 (PRMT5) as a candidate therapeutic target in the spontaneous canine model of non-Hodgkin lymphoma

Non-Hodgkin lymphoma (NHL) is a heterogeneous group of blood cancers arising in lymphoid tissues that commonly effects both humans and dogs. Protein arginine methyltransferase 5 (PRMT5), an enzyme that catalyzes the symmetric di-methylation of arginine residues, is frequently overexpressed and dysregulated in both human solid and hematologic malignancies. In human lymphoma, PRMT5 is a known driver of malignant transformation and oncogenesis, however, the expression and role of PRMT5 in canine lymphoma has not been explored. To explore canine lymphoma as a useful comparison to human lymphoma while validating PRMT5 as a rational therapeutic target in both, we characterized expression patterns of PRMT5 in canine lymphoma tissue microarrays, primary lymphoid biopsies, and canine lymphoma-derived cell lines. The inhibition of PRMT5 led to growth suppression and induction of apoptosis, while selectively decreasing global marks of symmetric dimethylarginine (SDMA) and histone H4 arginine 3 symmetric dimethylation. We performed ATAC-sequencing and gene expression microarrays with pathway enrichment analysis to characterize genome-wide changes in chromatin accessibility and whole-transcriptome changes in canine lymphoma cells lines upon PRMT5 inhibition. This work validates PRMT5 as a promising therapeutic target for canine lymphoma and supports the continued use of the spontaneously occurring canine lymphoma model for the preclinical development of PRMT5 inhibitors for the treatment of human NHL.

Human Plasma In-Cell Western Assays-An In vitro Predictor for In vivo Pharmacology in Oncology Drug Discovery

Evaluation of in vivo potencies plays an important role in drug discovery. Traditionally, the cellular activity and percent of plasma protein binding of a test agent are evaluated separately, with the plasma protein binding-adjusted cellular potency computation used to estimate in vivo potency. This process is costly, takes weeks to complete, and is increasingly unreliable for compounds that bind extensively to plasma proteins. Described in this article is a simple, high-throughput human plasma in-cell Western (ICW) assay that directly incorporates plasma protein binding into a cellular pharmacodynamic assay to provide a rapid and accurate estimate of in vivo potencies. The assay is versatile and can be readily employed for various targets that require short treatment periods for displaying maximal biological responses. © 2021 Wiley Periodicals LLC. Basic Protocol: Concentration-dependent human plasma ICW assay to determine test compound IC₅₀ against the target of interest.

PRMT5 Inhibition Modulates E2F1 Methylation and Gene-Regulatory Networks Leading to Therapeutic Efficacy in JAK2V617F-Mutant MPN

We investigated the role of PRMT5 in myeloproliferative neoplasm (MPN) pathogenesis and aimed to elucidate key PRMT5 targets contributing to MPN maintenance. PRMT5 is overexpressed in primary MPN cells, and PRMT5 inhibition potently reduced MPN cell proliferation ex vivo. PRMT5 inhibition was efficacious at reversing elevated hematocrit, leukocytosis, and splenomegaly in a model of JAK2V617F+ polycythemia vera and leukocyte and platelet counts, hepatosplenomegaly, and fibrosis in the MPLW515L model of myelofibrosis. Dual targeting of JAK and PRMT5 was superior to JAK or PRMT5 inhibitor monotherapy, further decreasing elevated counts and extramedullary hematopoiesis in vivo. PRMT5 inhibition reduced expression of E2F targets and altered the methylation status of E2F1 leading to attenuated DNA damage repair, cell-cycle arrest, and increased apoptosis. Our data link PRMT5 to E2F1 regulatory function and MPN cell survival and provide a strong mechanistic rationale for clinical trials of PRMT5 inhibitors in MPN. SIGNIFICANCE: Expression of PRMT5 and E2F targets is increased in JAK2V617F+ MPN. Pharmacologic inhibition of PRMT5 alters the methylation status of E2F1 and shows efficacy in JAK2V617F/MPLW515L MPN models and primary samples. PRMT5 represents a potential novel therapeutic target for MPN, which is now being clinically evaluated.This article is highlighted in the In This Issue feature, p. 1611.

Abstract 2919: Discovery of PRT811, a potent, selective, and orally bioavailable brain penetrant PRMT5 Inhibitor for the treatment of brain tumors

Protein arginine methyltransferase 5 (PRMT5) is the major type II PRMT that catalyzes the formation of symmetrical dimethyl arginine (SDMA) on protein substrates and plays important roles in regulating RNA splicing as well as transcription and activity of critical oncogenic signaling pathways. Recent studies have revealed PRMT5 as a potential therapeutic target for various cancers including Central Nervous System (CNS) tumors such as glioblastoma (GBM), the most common and aggressive primary brain tumor in adults. Several PMRT5 inhibitors have entered into the clinic for hematological and solid tumors, but none have been reported to be brain penetrant, a potentially desirable feature of a small molecule designed for treating CNS cancers and brain metastases.

PRT811 is a potent, selective, and brain penetrant PRMT5 inhibitor, and exhibits potent in vitro and in vivo activities in preclinical models of brain tumors. PRT811 inhibited the methyltransferase activity of the PRMT5/MEP50 complex with an IC50 of 3.9 nM, but showed minimal inhibition against a panel of 37 human methyltransferases at 10 µM. In a panel of brain cancer cell lines of varied histology, PRT811 reduced SDMA levels and inhibited cell proliferation with IC50 values in the range of 7-40 nM and 29-134 nM, respectively, regardless of expression levels of methylguanine methyltransferase (MGMT), the expression of which is associated with temozolomide (TMZ) resistance. Furthermore, PRT811 demonstrated broad antiproliferative activity in a panel of 87 cell lines consisting of brain, breast, lung, and skin cancers, representing primary and the most common cancer types that metastasize to the brain. Finally, PRT811 effectively inhibited cell growth in a panel of patient derived xenograft models of GBM in ex vivo 3D cultures.

PRT811 is a highly permeable molecule with high brain penetrance in rodents, with an efflux ratio &lt;1 and brain/plasma ratio ranging from 2 to 5 in mice. In vivo, oral administration of PRT811 in nude mice bearing U-87 MG human GBM orthotopic xenografts reduced tumor SDMA levels by ~50% but had no effect on SDMA in normal brain cells, as assessed by immunohistochemistry (IHC). Moreover, in a U-87 MG subcutaneous xenograft model in nude rats, once daily oral dosing of PRT811 at 20 and 30 mg/kg resulted in 91% and 100% tumor growth inhibition, respectively. PRT811 and related analogues are being studied across a panel of primary human and murine brain tumors ex vivo, and in vivo in primary orthotopic models of CNS lymphoma, a rare and aggressive subset of non-Hodgkins lymphomas.

PRT811 is currently under evaluation in a Phase I clinical trial in patients with advanced solid tumors, gliomas, and myelofibrosis (NCT04089449).

Citation Format: Yang Zhang, Hong Lin, Min Wang, Dimitrios Angelis, Michael Hawkins, Dave Rominger, Tom Emm, Juan Luengo, Bruce Ruggeri, Peggy Scherle, Kris Vaddi. Discovery of PRT811, a potent, selective, and orally bioavailable brain penetrant PRMT5 Inhibitor for the treatment of brain tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2919.

PRMT5 promotes STAT-1 mediated T cell inflammatory response and is a therapeutic target for acute graft-versus-host disease

Introduction

T cell mediated acute graft-versus-host disease (aGVHD) is the main cause of non-relapse mortality in allogeneic hematopoietic cell transplant recipients. Protein arginine methyltransferase 5 (PRMT5) is a post-translational modifier and promotes activation of memory TH cells. We investigate mechanisms by which PRMT5 regulates T cell function and propose PRMT5 inhibition as a therapeutic strategy for aGVHD.

Materials and Methods

PRMT5 expression and function was evaluated in T cells of healthy and aGVHD mice and humans. We assessed T cell proliferation and effector function, using C220, a novel PRMT5 inhibitor. We tested effects of PRMT5 inhibition using in vivo mouse models of aGVHD, where mice received T cell depleted bone marrow + allogeneic splenocytes. In graft-versus-leukemia (GVL) experiments, P815 tumor cells were administered. Mice were treated with C220 or vehicle and monitored for survival and clinical aGVHD scores.

Results

PRMT5 expression and function is upregulated in T cells of mice and humans with aGVHD. Inhibition of PRMT5 reduces T cell proliferation, perturbs cell cycle and downregulates STAT-1 mediated interferon response. Administration of C220 improved survival in aGVHD mouse models and maintained tumor specific CTL response contributing to retention of Graft versus leukemia (GVL) effect.

Conclusions

Inhibition of PRMT5 using C220, down-regulates T cell proliferative and effector response, induces cell-cycle arrest, reduces interferon response and perturbs signaling pathways. C220 shows potent biological activity in vivo by reducing aGVHD clinical severity and prolonging survival in mouse models without compromising GVL. Therefore, PRMT5 is a novel and druggable target for aGVHD.

PRMT5 regulates T cell interferon response and is a target for acute graft-versus-host disease

Acute graft-versus-host disease (aGVHD) is a T cell-mediated immunological disorder and the leading cause of nonrelapse mortality in patients who receive allogeneic hematopoietic cell transplants. Based on recent observations that protein arginine methyltransferase 5 (PRMT5) and arginine methylation are upregulated in activated memory T cells, we hypothesized that PRMT5 is involved in the pathogenesis of aGVHD. Here, we show that PRMT5 expression and enzymatic activity were upregulated in activated T cells in vitro and in T cells from mice developing aGVHD after allogeneic transplant. PRMT5 expression was also upregulated in T cells of patients who developed aGVHD after allogeneic hematopoietic cell transplant compared with those who did not develop aGVHD. PRMT5 inhibition using a selective small-molecule inhibitor (C220) substantially reduced mouse and human allogeneic T cell proliferation and inflammatory IFN-γ and IL-17 cytokine production. Administration of PRMT5 small-molecule inhibitors substantially improves survival, reducing disease incidence and clinical severity in mouse models of aGVHD without adversely affecting engraftment. Importantly, we show that PRMT5 inhibition retained the beneficial graft-versus-leukemia effect by maintaining cytotoxic CD8+ T cell responses. Mechanistically, we show that PRMT5 inhibition potently reduced STAT1 phosphorylation as well as transcription of proinflammatory genes, including interferon-stimulated genes and IL-17. Additionally, PRMT5 inhibition deregulates the cell cycle in activated T cells and disrupts signaling by affecting ERK1/2 phosphorylation. Thus, we have identified PRMT5 as a regulator of T cell responses and as a therapeutic target in aGVHD.

Oral arsenic trioxide ORH-2014 pharmacokinetic and safety profile in patients with advanced hematologic disorders

Daily intravenous arsenic trioxide administered with all-trans retinoid acid, the standard-of-care for acute promyelocytic leukemia, is costly and challenging to administer. ORH-2014 is a novel, oral arsenic trioxide formulation, consisting of micron-size drug particles with rapid dissolution and high bioavailability. We conducted a multicenter phase 1 dose-escalating study in patients with advanced hematologic malignancies. Twelve patients received ORH-2014 at 5 mg (n=3), 10 mg (n=6), or 15 mg (n=3) orally once a day (fasted state). Objectives were to assess the safety, tolerability and pharmacokinetics of ORH-2014 to support a dose recommendation for future trials. The median age of the patients was 77 years (range: 45-81) and they had received a median of two (range: 1-5) prior therapies. There were no dose limiting toxicities and no drug-related severe adverse events, except one grade III QT prolongation occurring beyond the dose limiting toxicity assessment period and resolving after treatment interruption. ORH-2014 steady-state plasma concentration was reached on day 15. ORH-2014, 15 mg C_max was comparable to the calculated approved dose of intravenous arsenic trioxide (mean [% coefficient of variation]: 114 [21%] vs. 124 [60%] ng/mL) and area under the curve from 0 to 24 hours was 2,140 (36%) versus 1,302 (30%) h*ng/mL. These results indicate that ORH-2014 at 15 mg is safe, bioavailable, and provides the required arsenic exposure compared to intravenous arsenic trioxide at the approved dose (0.15 mg/kg); this ORH-2014 dose is recommended for future trials. (NCT03048344; www.clin-icaltrials.gov).

Discovery of Potent and Selective Covalent Protein Arginine Methyltransferase 5 (PRMT5) Inhibitors

Protein arginine methyltransferase 5 (PRMT5) is known to symmetrically dimethylate numerous cytosolic and nuclear proteins that are involved in a variety of cellular processes. Recent findings have revealed its potential as a cancer therapeutic target. PRMT5 possesses a cysteine (C449) in the active site, unique to PRMT5. Therefore, covalent PRMT5 inhibition is an attractive chemical approach. Herein, we report an exciting discovery of a series of novel hemiaminals that under physiological conditions can be converted to aldehydes and react with C449 to form covalent adducts, which presumably undergo an unprecedented elimination to form the thiol-vinyl ethers, as indicated by electron density in the co-crystal structure of the PRMT5/MEP50 complex.

Preclinical characterization of INCB053914, a novel pan-PIM kinase inhibitor, alone and in combination with anticancer agents, in models of hematologic malignancies

The Proviral Integration site of Moloney murine leukemia virus (PIM) serine/threonine protein kinases are overexpressed in many hematologic and solid tumor malignancies and play central roles in intracellular signaling networks important in tumorigenesis, including the Janus kinase-signal transducer and activator of transcription (JAK/STAT) and phosphatidylinositol 3-kinase (PI3K)/AKT pathways. The three PIM kinase isozymes (PIM1, PIM2, and PIM3) share similar downstream substrates with other key oncogenic kinases and have differing but mutually compensatory functions across tumors. This supports the therapeutic potential of pan-PIM kinase inhibitors, especially in combination with other anticancer agents chosen based on their role in overlapping signaling networks. Reported here is a preclinical characterization of INCB053914, a novel, potent, and selective adenosine triphosphate-competitive pan-PIM kinase inhibitor. In vitro, INCB053914 inhibited proliferation and the phosphorylation of downstream substrates in cell lines from multiple hematologic malignancies. Effects were confirmed in primary bone marrow blasts from patients with acute myeloid leukemia treated ex vivo and in blood samples from patients receiving INCB053914 in an ongoing phase 1 dose-escalation study. In vivo, single-agent INCB053914 inhibited Bcl-2-associated death promoter protein phosphorylation and dose-dependently inhibited tumor growth in acute myeloid leukemia and multiple myeloma xenografts. Additive or synergistic inhibition of tumor growth was observed when INCB053914 was combined with selective PI3Kδ inhibition, selective JAK1 or JAK1/2 inhibition, or cytarabine. Based on these data, pan-PIM kinase inhibitors, including INCB053914, may have therapeutic utility in hematologic malignancies when combined with other inhibitors of oncogenic kinases or standard chemotherapeutics.

Ruxolitinib: a targeted treatment option for patients with polycythemia vera

Polycythemia vera (PV) is a chronic myeloproliferative neoplasm characterized by erythrocytosis and the presence of Janus kinase (JAK) 2V617F or similar mutations. This review summarizes the pathophysiology of PV, the challenges associated with traditional treatment options, and the scientific rationale and supportive clinical evidence for targeted therapy with ruxolitinib. Accumulating evidence indicates that activating mutations in JAK2 drive the PV disease state. Traditional PV treatment strategies, including aspirin, phlebotomy, and cytoreduc­tive agents such as hydroxyurea, provide clinical benefits for some but not all patients and may not adequately treat PV-related symptoms. Furthermore, traditional treatment approaches are associated with potential side effects that may limit their usage and lead some patients to discon­tinue the treatment. Ruxolitinib is an orally available small-molecule tyrosine kinase inhibitor that is a potent and selective inhibitor of JAK1/JAK2. Ruxolitinib is approved in the US for patients with PV with an inadequate response or intolerance to hydroxyurea and in Europe for adults with PV who are resistant to or intolerant of hydroxyurea. In the Phase III RESPONSE registration trial, ruxolitinib was superior to the best available therapy in patients with PV who were resistant to or intolerant of hydroxyurea in controlling hematocrit levels, reducing spleen volume, and improving PV-related symptoms and quality-of-life measures. The most common nonhematologic adverse events in ruxolitinib-treated patients were headache, diarrhea, pruritus, and fatigue in the RESPONSE trial; hematologic adverse events were primarily grade 1 or 2. In the Phase IIIb nonregistration RELIEF trial, there were nonsignificant trends toward an improved symptom control in patients with PV on a stable hydroxyurea dose who were generally well controlled but reported disease-associated symptoms and switched to ruxolitinib vs those who continued hydroxyurea therapy. Updated treatment guidelines will be important for educating physicians about the role of ruxolitinib in the treatment of patients with PV.

A Multi-Institution Phase I Trial of Ruxolitinib in Patients with Chronic Myelomonocytic Leukemia (CMML)

PURPOSE

To conduct a phase I clinical trial exploring the safety and efficacy of ruxolitinib, a JAK1/2 inhibitor, for chronic myelomonocytic leukemia (CMML).

EXPERIMENTAL DESIGN

Patients with CMML-1 were included without regard to previous therapy. Key exclusion criteria included an absolute neutrophil count (ANC) <0.25 × 10(3) cells/dL and a platelet count <35 × 10(3) cells/dL. Four cohorts were enrolled using a "rolling six" study design, with doses ranging from 5 to 20 mg twice daily of ruxolitinib in 5-mg dose escalations.

RESULTS

Between March 2013 and January 2015, 20 patients were enrolled and treated with ruxolitinib. Seventy percent of patients had the proliferative subtype and 47% had higher risk disease by the Global MD Anderson Scoring System. Eight patients (42%) received a prior hypomethylating agent. No dose-limiting toxicities for ruxolitinib were identified. One subject had grade (G)3 thrombocytopenia with no other drug-associated G3 or G4 adverse events. The mean duration of therapy was 122 days (range, 28-409 days). Four had hematologic improvement and one patient had a partial response per 2006 International Working Group (IWG) criteria. Five of 9 patients with splenomegaly had a reduction in spleen size. Ten of 11 patients with reported disease-related symptoms had clinically meaningful or complete resolution. When combining IWG and spleen responses, a total response rate of 35% (n = 7) was identified. Correlative analysis demonstrated a reduction in inflammatory cytokines and GM-CSF-dependent STAT5 phosphorylation.

CONCLUSIONS

The recommended phase II dose of ruxolitinib is 20 mg twice daily. We demonstrate that ruxolitinib has promising activity in CMML with particular benefit in those with disease-related B symptoms that warrants further study. Clin Cancer Res; 22(15); 3746-54. ©2016 AACRSee related commentary by Solary, p. 3707.

Downmodulation of key inflammatory cell markers with a topical Janus kinase 1/2 inhibitor

BACKGROUND

INCB018424 is a novel, potent Janus kinase (JAK)1/JAK2 inhibitor that blocks signal transduction of multiple proinflammatory cytokines.

OBJECTIVES

To evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics and preliminary efficacy of topical INCB018424 phosphate cream in patients with plaque psoriasis.

METHODS

Topical INCB018424 phosphate 1·0% or 1·5% cream was applied once daily (QD) or twice daily (BID) for 4 weeks to 2-20% body surface area in five sequential cohorts of five patients aged 18-65 years. Target lesions were scored on a scale of 0-4 for erythema, scaling and thickness. Additionally, the overall disease activity in each patient was measured using Physician's Global Assessment. INCB018424 concentrations were measured in plasma, and cytokine stimulated phosphorylated signal transducer and activator of transcription 3 phosphorylation (pSTAT3) levels in peripheral blood cells were evaluated. Pretreatment and post-treatment skin biopsies were compared with healthy skin, including evaluation of histopathology, immunohistochemistry and mRNA expression.

RESULTS

Treatment with INCB018424 phosphate cream either 1·0% QD or 1·5% BID resulted in improvements in lesion scores. No significant inhibition of pSTAT3 in peripheral blood cells was observed following topical application, consistent with the generally low steady-state plasma concentrations of INCB018424 measured. Transcriptional markers of immune cell lineage/activation in lesional skin were reduced by topical INCB018424, with correlations observed between clinical improvement and decreases in markers of T helper 17 lymphocyte activation, dendritic-cell activation and epidermal hyperplasia. INCB018424 treatment reduced epidermal hyperplasia and dermal inflammation in most patient samples, with reductions in CD3, CD11c, Ki67 and keratin 16 observed by immunohistochemical analysis.

CONCLUSIONS

Topical INCB018424 dosed for 28 days QD or BID is pharmacologically active in patients with active psoriasis and modulates proinflammatory cytokines in the pathogenesis of psoriatic lesions.

Abstract 5416: Activity of the pan-PIM kinase inhibitor INCB053914 in models of acute myelogenous leukemia

The PIM family of serine-threonine protein kinases (PIM1, PIM2 and PIM3) was initially identified as preferential integration sites of the Moloney murine leukemia virus in Eμ-myc mice, resulting in perinatal lymphomagenesis. Molecular characterization has revealed that PIM kinases drive cell proliferation and survival in a number of hematological malignancies beyond lymphomas by mediating responses to cytokines, growth factors and cellular stress. Overexpression of various PIM kinase family members in these malignancies has been associated with poor overall survival and with resistance to chemotherapeutic agents. Therefore, development of a pan-PIM inhibitor may be useful in the treatment of hematological malignancies, both as a single agent and in combination with chemotherapy or targeted agents. The in vitro and in vivo activity of INCB053914, a pan-PIM kinase inhibitor, was determined in a panel of acute myelogenous leukemia (AML) cell lines. Greater than half of all AML cell lines tested were sensitive to single agent INCB053914, with anti-proliferative IC50 potencies &lt;100 nM. Consistent with recent findings, the majority of these sensitive cell lines expressed higher levels of CD25, which is co-regulated with PIM through the JAK/STAT signaling pathway and which may serve as a biomarker for PIM sensitivity in AML. INCB053914 suppressed the phosphorylation of multiple PIM kinase substrates and, in a compensatory manner, increased PIM kinase expression in both cell lines and primary AML patient samples. The potency of INCB053914 to suppress BAD phosphorylation in vivo in MOLM16 tumors is consistent with its potency in blocking BAD phosphorylation in MOLM16 cells spiked into human whole blood in vitro. Dose dependent tumor growth inhibition (TGI) was seen in mice bearing MOLM16 tumors, with maximal TGI achieved with 12-20 hours of MOLM16 WB IC50 coverage. Synergistic activity was seen in vivo in the KG1 model when INCB053914 was administered with cytarabine, a standard of care therapy for AML. Further studies were undertaken to assess the potential for combined PIM inhibition with other targeted agents. Because the PIM kinases have been shown to regulate c-myc expression levels, agents whose activity has been shown to be influenced by c-myc levels or to regulate c-myc expression were examined. The combination of INCB53914 with the pan-BET inhibitor INCB54329 in MOLM16 xenografts synergistically decreased pBAD and c-myc levels in tumors, resulting in enhanced efficacy. Synergy was also seen when INCB53914 was combined with the pan-FGFR inhibitor INCB54828 in KG1 xenografts harboring the FOP-FGFR1 translocation, suggesting that this combination might be useful in this rare patient population as well as other diseases driven by oncogenic FGFR activation. Taken together, these data support the utility of PIM inhibition in AML patients as monotherapy and in combination with other targeted agents.

Citation Format: Holly Koblish, Niu Shin, Leslie Hall, Sybil O'Connor, Qian Wang, Kathy Wang, Lynn Leffet, Maryanne Covington, Krista Burke, Jason Boer, Kevin Bowman, Ke Zhang, Hao Feng, Chu-Biao Xue, Yun-Long Li, Wenqing Yao, Reid Huber, Kris Vaddi, Peggy Scherle. Activity of the pan-PIM kinase inhibitor INCB053914 in models of acute myelogenous leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5416. doi:10.1158/1538-7445.AM2015-5416

Abstract 771: Preclinical characterization of the selective FGFR inhibitor INCB054828

Aberrant signaling through Fibroblast Growth Factor Receptors (FGFR) has been reported in multiple types of human cancers. Genomic analyses of squamous cell lung, gastric and urothelial tumors have revealed recurrent genetic alterations in FGFR1, FGFR2 and FGFR3 genes, respectively. FGFR proteins contribute to the development of malignancies by promoting tumor cell proliferation, survival, and migration and supporting angiogenesis. Therefore targeting FGFR kinases may provide therapeutic benefit to patients with cancers that have genetic alterations in genes encoding components of the FGF-FGFR axis. INCB054828 is a potent inhibitor of FGFR1, FGFR2, and FGFR3 that has selective pharmacological activity against cancer cells with FGFR alterations.

In vitro, INCB054828 potently inhibited the kinase activity of recombinant FGFR1, FGFR2 and FGFR3 enzymes and was highly selective against a panel of kinases including VEGFR2. In cellular assays, INCB054828 inhibited the autophosphorylation of FGFR proteins with low nanomolar IC50 values and blocked signal transduction by FGFR to downstream markers of pathway activation. Cancer cell lines that have genetic alterations in FGFR1, FGFR2 and FGFR3 were uniquely sensitive to growth inhibition by INCB054828, with IC50 values generally in the range of 3-50 nM, compared with cancer cell lines or normal cells without FGFR dependence (IC50 &gt; 1500 nM). In vivo, once-daily oral administration of INCB054828 inhibited the growth of tumors that are dependent upon FGFR1, FGFR2 and FGFR3 activity at tolerated doses. Suppression of tumor growth was dose-dependent and correlated with pharmacodynamic inhibition of FGFR. Collectively, these preclinical studies demonstrate that INCB054828 potently and selectively inhibits models of FGFR-dependent cancers in vitro and in vivo, supporting the compound's clinical evaluation in patients harboring oncogenic FGFR activation.

Citation Format: Phillip CC Liu, Liangxing Wu, Holly Koblish, Kevin Bowman, Yue Zhang, Ronald Klabe, Lynn Leffet, Darlise DiMatteo, Mark Rupar, Karen Gallagher, Michael Hansbury, Colin Zhang, Chunhong He, Paul Collier, Maryanne Covington, Richard Wynn, Swamy Yeleswaram, Kris Vaddi, Timothy Burn, Wenqing Yao, Reid Huber, Peggy Scherle, Gregory Hollis. Preclinical characterization of the selective FGFR inhibitor INCB054828. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 771. doi:10.1158/1538-7445.AM2015-771

Abstract 691: Activity of the BET inhibitor INCB054329 in models of multiple myeloma

Multiple myeloma (MM) is a disease of plasma cell transformation. Current therapy for MM is initially effective, but nearly all tumors relapse, making new therapeutic options a necessity. Due to their efficacy in MM models, small molecule inhibitors of Bromodomain and Extra Terminal (BET) proteins have generated much interest as potential therapeutic agents for MM. Efficacy from BET inhibitors in MM is thought to be driven by their ability to reduce transcript levels of the c-myc oncogene. Current MM standard of care (SoC) therapeutics include lenalidomide (Revlimid, Celgene), bortezomib (Velcade, Takeda), and Melphalan (Alkeran, GlaxoSmithKline). In preparation for potential clinical studies, these MM SoC agents were each combined with a potent pan-BET inhibitor, INCB054329, for in vitro and in vivo studies. In vitro, treatment of MM cell lines with INCB054329 inhibited expression of c-MYC, induced HEXIM1 levels and inhibited cell growth with potencies less than 200 nM. Combination of INCB054329 with SoC therapeutics showed synergistic effects in blocking MM cell proliferation. INA-6 and MM1.S mouse xenograft models of MM were utilized to study the effects of the combination of INCB054329 with the MM SoC agents in vivo. In each instance (INCB054329/lenalidomide, INCB054329/bortezomib, INCB054329/melphalan), additive to synergistic effects, as measured by inhibition of subcutaneous tumor growth, were seen in both models of MM. Since lenalidomide has recently been shown to bind the ubiquitin ligase cereblon (CRBN), leading to the degradation of two Ikaros family members, IKZF1 and IKZF3, that are essential to B cell fate and survival, we examined whether there could be functional overlap between BET inhibition and the downstream effects of lenalidomide. From MM1.S xenograft pharmacodynamic assays, we found a synergistic repression of c-MYC protein levels at 3hr post dose for the combination of INCB054329 and lenalidomide. Interestingly, the sharp decline in c-MYC protein levels in the MM1.S model appears to be independent of the IRF4 pathway, as IRF4 protein levels do not decrease until 24 hrs post dose of lenalidomide. Overall, our data indicate that INCB054329 may provide a novel combination partner with current standard of care therapies for MM, and support the clinical evaluation of the compound within the anti-myeloma treatment landscape.

Citation Format: Matthew Stubbs, Xiaoming Wen, Valerie Dostalik, Sybil O'Connor, Eian Caulder, Alla Vogina, Thomas Maduskuie, Richard Sparks, Taisheng Huang, Nikoo Falahatpisheh, Padmaja Polam, Chu-Biao Xue, Xuesong M. Liu, Timothy Burn, Kris Vaddi, Andrew P. Combs, Reid Huber, Gregory Hollis, Peggy Scherle, Phillip CC Liu. Activity of the BET inhibitor INCB054329 in models of multiple myeloma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 691. doi:10.1158/1538-7445.AM2015-691

Abstract 779: Blockade of the IL-6/JAK/STAT3 signaling pathway inhibits pancreatic tumor cell growth in 3D spheroid cultures and in xenograft models

Emerging evidence has demonstrated the importance of tumor-associated inflammation in the development and progression of pancreatic ductal adenocarcinoma. Specifically, constitutive activation of the inflammation-related IL-6/JAK/STAT3 signaling pathway has been reported in pancreatic tumors and has been suggested to be a poor prognostic factor for overall survival in patients with advanced disease. The aim of this study was to assess the effects of inhibition of IL-6/JAK/STAT3 signaling on pancreatic cell growth in vitro and tumor growth in vivo. INCB039110, a JAK1 selective inhibitor currently in multiple Phase 2 clinical trials, was used to block the IL-6/JAK/STAT3 signaling pathway in cells. We show that while inhibition of IL-6/JAK/STAT3 signaling by INCB039110 showed no effects on the proliferation of pancreatic cell lines grown under conventional 2D monolayer cell culture conditions, this inhibitor demonstrated growth inhibitory activity against the pancreatic tumor cell lines, Hs700T and BxPC-3, in a 3D-spheroid culture system. Importantly, addition of cytokines that stimulated JAK/STAT3 signaling in these cells significantly promoted the growth of the spheroids, and this could be completely reversed by INCB039110. Furthermore, JAK1 inhibition enhanced the cytotoxicity induced by gemcitabine in both Hs700T and BxPC-3 spheroids in a combination study. The results were confirmed with another novel Jak1 selective inhibitor, INCB052793 which is currently in a Phase 1 clinical trial in patients with advanced malignancies. Since elevated serum IL-6 and its correlation with cachexia has been observed in pancreatic cancer, we tested and found that INCB039110 prevented weight loss in an IL-6 induced cachexia model in mice. Finally, we show that INCB039110 blocked tumor growth in human pancreatic xenograft models in mice at clinically relevant doses, both as monotherapy and in combination with cytotoxic agents such as gemcitabine. Thus, we demonstrate that pharmacological blockade of the IL-6/JAK/STAT3 signaling pathway with a JAK1 selective inhibitor can have both tumor intrinsic and extrinsic effects resulting in the inhibition of pancreatic tumor cell growth and the modulation of inflammation-associated cachexia. These data suggest that pancreatic cancer patients may potentially benefit from JAK1 inhibitors in the clinic.

Citation Format: Jun Li, Eian Caulder, Margaret Favata, Melody Diamond, Beth Rumberger, Holly Koblish, Taisheng Huang, Chu-Biao Xue, Wenqing Yao, Jordan Fridman, Peggy Scherle, Mike Liu, Reid Huber, Kris Vaddi. Blockade of the IL-6/JAK/STAT3 signaling pathway inhibits pancreatic tumor cell growth in 3D spheroid cultures and in xenograft models. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 779. doi:10.1158/1538-7445.AM2015-779

Abstract 5414: Activity of the pan-PIM kinase inhibitor INCB053914 in models of multiple myeloma

The PIM family of serine-threonine protein kinases (PIM1, PIM2 and PIM3) mediates responses to cytokines and growth factors and drives cell proliferation and survival in a number of hematologic malignancies. Overexpression of PIM kinases in these malignancies, including multiple myeloma (MM), has been associated with poor overall survival. Given the overlapping functions of these kinases, the ability of one family member to compensate for the loss of another as well as the relatively benign phenotype of mice deficient in all three PIM isoforms, discovery of pan-PIM kinase inhibitors is warranted. The in vitro and in vivo activity of the pan-PIM kinase inhibitor, INCB53914, was determined in MM cell lines. The antiproliferative potencies for INCB053914 were &lt;200 nM in the majority of MM cell lines tested. INCB053914 potently suppressed the phosphorylation of multiple PIM substrates in MM cell lines, however in contrast, a PIM2-sparing compound, INCB050646, was unable to impact signaling in the KMS12 MM cell line, suggesting the importance of the PIM2 isoform in myeloma growth and survival. An assay was established to measure the inhibition of the phosphorylation of the PIM substrate, BAD, in KMS12 cells when spiked into whole blood (WB) to assess the shift in compound potency due to protein binding. The IC50 for INCB053914 in this assay was similar to its potency in suppressing BAD phosphorylation in KMS12 tumors in vivo. Dose dependent tumor growth inhibition (TGI) was seen in mice bearing KMS12 tumors, with maximal TGI achieved with 24 hours of KMS12 WB IC50 coverage. Similar data were obtained in a second MM model, OPM2.

To understand the impact of inhibiting the PIM pathway in combination with other pathways dysregulated in hematological malignancies, an unbiased in vitro screen was performed and the potential synergy of INCB053914 in combination with 65 cytotoxic or targeted agents was determined. This screen identified several agents active against the PI3K pathway or which impacted cell cycle progression. In addition, the combinatorial activity of selected targeted agents hypothesized to exhibit significant interactions with the PIM pathway was assessed in vivo. Since PIM family members are STAT regulated genes, enhanced activity may be expected upon combined PIM and JAK inhibition. In fact, synergistic activity was seen with this combination in the INA6 multiple myeloma model, and pharmacodynamic analyses revealed enhanced suppression of both pBAD and c-myc levels in tumors from treated mice. Additionally, c-myc levels are regulated both by PIM and the BET family member, BRD4. The expected synergistic efficacy of PIM and BET inhibitors was also observed in the KMS12 model, again with enhanced reduction in pBAD and c-myc levels in the tumors of treated mice. Taken together, these data support the utility of PIM inhibition in MM patients, both as monotherapy and in combination with other targeted agents.

Citation Format: Holly Koblish, Niu Shin, Leslie Hall, Xiaoming Wen, Sybil O'Connor, Valerie Dostalik, Qian Wang, Kathy Wang, Maryanne Covington, Cindy Marando, Kevin Bowman, Jason Boer, Krista Burke, Ke Zhang, Hao Feng, Chu-Biao Xue, Yun-Long Li, Wenqing Yao, Reid Huber, Kris Vaddi, Peggy Scherle. Activity of the pan-PIM kinase inhibitor INCB053914 in models of multiple myeloma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5414. doi:10.1158/1538-7445.AM2015-5414

Efficacy, safety, and survival with ruxolitinib in patients with myelofibrosis: results of a median 3-year follow-up of COMFORT-I

In the phase III COMFORT-I study, the Janus kinase 1 (JAK1)/JAK2 inhibitor ruxolitinib provided significant improvements in splenomegaly, key symptoms, and quality-of-life measures and was associated with an overall survival benefit relative to placebo in patients with intermediate-2 or high-risk myelofibrosis. This planned analysis assessed the long-term efficacy and safety of ruxolitinib at a median follow-up of 149 weeks. At data cutoff, approximately 50% of patients originally randomized to ruxolitinib remained on treatment whereas all patients originally assigned to placebo had discontinued or crossed over to ruxolitinib. At week 144, mean spleen volume reduction was 34% with ruxolitinib. Previously observed improvements in quality-of-life measures were sustained with longer-term ruxolitinib therapy. Overall survival continued to favor ruxolitinib despite the majority of placebo patients crossing over to ruxolitinib [hazard ratio 0.69 (95% confidence interval: 0.46-1.03); P = 0.067]. Exploratory analyses suggest that crossover may have contributed to an underestimation of the true survival difference between the treatment groups. Ruxolitinib continued to be generally well tolerated; there was no pattern of worsening grade ≥ 3 anemia or thrombocytopenia with longer-term ruxolitinib exposure. These longer-term data continue to support the efficacy and safety of ruxolitinib in patients with myelofibrosis. The study is registered at clinicaltrials.gov: NCT00952289.

Selective inhibition of ADAM metalloproteases blocks HER-2 extracellular domain (ECD) cleavage and potentiates the anti-tumor effects of trastuzumab

The HER-2 receptor tyrosine kinase is an important regulator of cell proliferation and survival, and it is a clinically validated target of therapeutic intervention for HER-2 positive breast cancer patients. Its extracellular domain (ECD) is frequently cleaved by protease(s) in HER-2 overexpressing breast cancer patients, rendering the remaining membrane-bound portion (p95) a constitutively activated kinase. The presence of both serum ECD and cellular p95 protein has been linked to poor clinical outcome as well as reduced effectiveness of some therapeutic treatments. We have identified a series of potent, selective small molecule inhibitors of ADAM proteases, exemplified here by INCB003619, and demonstrate that these inhibitors effectively block HER-2 cleavage in HER-2 overexpressing human breast cancer cell lines. Intriguingly, when used in combination, INCB003619 dramatically enhances the antiproliferative activity of suboptimal doses of the anti-HER-2 antibody, trastuzumab, in HER-2 overexpressing/shedding breast cancer cell lines, accompanied by reduced ERK and AKT phosphorylation. Furthermore, INCB003619, in combination with trastuzumab, augments the pro-apoptotic and antiproliferative effects of the chemotherapeutic agent paclitaxel. Consistent with these in vitro data, INCB003619 reduces serum ECD levels and enhances the antitumor effect of trastuzumab in a xenograft tumor model derived from the HER-2 overexpressing BT-474 breast cancer cell line. Collectively, these findings suggest that blocking HER-2 cleavage with selective ADAM inhibitors may represent a novel therapeutic approach for treating HER-2 overexpressing breast cancer patients.

Abstract 4207: JAK inhibition reverses IL10-mediated resistance to B cell receptor (BCR) pathway inhibition in DLBCL

Diffuse large B cell lymphoma (DLBCL) is the most common form of non-Hodgkin's lymphoma, and can be classified into activated B-cell like (ABC) and germinal center B-cell like (GCB) DLBCL according to gene expression profiling. In both subtypes, dysregulation of the B cell receptor (BCR) signaling pathway plays a critical role in tumorigenesis and survival. BCR activation induces phosphorylation of CD79A/B by Src family tyrosine kinases, which leads to the activation of the BTK and PI3Kδ/Akt pathways. Oncogenic mutations have been identified in DLBCL impacting CD79A/B (BCR complex), PTEN (negative regulator of PI3K), CARD11 and MYD88 (NF-kB activating factors) function. The influence of these mutations on the DLBCL phenotype appears to be further modified by aberrant cytokine signaling and JAK/Stat pathway activation. High levels of JAK/Stat-activating cytokines such as IL6 and IL10 are present in DLBCL, and patients with high serum IL10 have been shown to have shorter event-free survival and a higher international Prognostic Index score. Therefore, concurrent inhibition of both the BCR and JAK/Stat pathways may provide an attractive therapeutic approach in DLBCL. We have developed and characterized a potent and selective PI3Kδ inhibitor INCB040093 and have established that DLBCL cells are sensitive to INCB040093 treatment. We demonstrate that IL10 activates the JAK/Stat signaling pathway in DLBCL cells, which renders the cells resistant to INCB040093 in cell proliferation assays. This resistance can be reversed by co-treatment with a JAK1/JAK2 inhibitor. Although INCB040093 induces apoptosis in DLBCL cells, concurrent activation of the JAK/Stat pathway by IL10 prevents INCB040093-induced apoptosis, an effect that can be abrogated by a JAK1/JAK2 inhibitor. Similar resistance to IL-10 is observed with the BTK inhibitor, ibrutinib, which can also be reversed by a JAK1/JAK2 inhibitor. Interestingly, a JAK1 selective inhibitor is as effective as a JAK1/JAK2 inhibitor in reversing the IL10-mediated resistance to PI3Kδ and BTK inhibition. Combination treatment blocks both PI3K/Akt and JAK/Stat signaling pathways and results in the down-regulation of pro-survival proteins, including Pim2 and c-myc. These data illustrate the functional cross-talk that exists in DLBCL between the BCR and JAK/Stat signaling pathways, and support the clinical evaluation of combined BCR and JAK inhibition as a novel approach to targeted therapy.

Citation Format: Xuesong (Mike) Liu, Margaret Favata, Jun Li, Niu Shin, Kathy He Wang, Qian Wang, Yun-Long Li, Andrew Combs, Chu-Biao Xue, Robert Newton, Kris Vaddi, Peggy Scherle. JAK inhibition reverses IL10-mediated resistance to B cell receptor (BCR) pathway inhibition in DLBCL. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4207. doi:10.1158/1538-7445.AM2014-4207

Myelofibrosis-associated complications: pathogenesis, clinical manifestations, and effects on outcomes

Myelofibrosis (MF) is a rare chronic BCR-ABL1 (breakpoint cluster region-Abelson murine leukemia viral oncogene homologue 1)-negative myeloproliferative neoplasm characterized by progressive bone marrow fibrosis, inefficient hematopoiesis, and shortened survival. The clinical manifestations of MF include splenomegaly, consequent to extramedullary hematopoiesis, cytopenias, and an array of potentially debilitating abdominal and constitutional symptoms. Dysregulated Janus kinase (JAK)-signal transducer and activator of transcription signaling underlies secondary disease-associated effects in MF, such as myeloproliferation, bone marrow fibrosis, constitutional symptoms, and cachexia. Common fatal complications of MF include transformation to acute leukemia, thrombohemorrhagic events, organ failure, and infections. Potential complications from hepatosplenomegaly include portal hypertension and variceal bleeding, whereas extramedullary hematopoiesis outside the spleen and liver - depending on the affected organ - may result in intracranial hypertension, spinal cord compression, pulmonary hypertension, pleural effusions, lymphadenopathy, skin lesions, and/or exacerbation of abdominal symptoms. Although allogeneic stem cell transplantation is the only potentially curative therapy, it is suitable for few patients. The JAK1/JAK2 inhibitor ruxolitinib is effective in improving splenomegaly, MF-related symptoms, and quality-of-life measures. Emerging evidence that ruxolitinib may be associated with a survival benefit in intermediate- or high-risk MF suggests the possibility of a disease-modifying effect. Consequently, ruxolitinib could provide a treatment backbone to which other (conventional and novel) therapies may be added for the prevention and effective management of specific MF-associated complications.

Myelofibrosis-associated complications: pathogenesis, clinical manifestations, and effects on outcomes

Myelofibrosis (MF) is a rare chronic BCR-ABL1 (breakpoint cluster region-Abelson murine leukemia viral oncogene homologue 1)-negative myeloproliferative neoplasm characterized by progressive bone marrow fibrosis, inefficient hematopoiesis, and shortened survival. The clinical manifestations of MF include splenomegaly, consequent to extramedullary hematopoiesis, cytopenias, and an array of potentially debilitating abdominal and constitutional symptoms. Dysregulated Janus kinase (JAK)-signal transducer and activator of transcription signaling underlies secondary disease-associated effects in MF, such as myeloproliferation, bone marrow fibrosis, constitutional symptoms, and cachexia. Common fatal complications of MF include transformation to acute leukemia, thrombohemorrhagic events, organ failure, and infections. Potential complications from hepatosplenomegaly include portal hypertension and variceal bleeding, whereas extramedullary hematopoiesis outside the spleen and liver - depending on the affected organ - may result in intracranial hypertension, spinal cord compression, pulmonary hypertension, pleural effusions, lymphadenopathy, skin lesions, and/or exacerbation of abdominal symptoms. Although allogeneic stem cell transplantation is the only potentially curative therapy, it is suitable for few patients. The JAK1/JAK2 inhibitor ruxolitinib is effective in improving splenomegaly, MF-related symptoms, and quality-of-life measures. Emerging evidence that ruxolitinib may be associated with a survival benefit in intermediate- or high-risk MF suggests the possibility of a disease-modifying effect. Consequently, ruxolitinib could provide a treatment backbone to which other (conventional and novel) therapies may be added for the prevention and effective management of specific MF-associated complications.

Efficacy, safety and survival with ruxolitinib in patients with myelofibrosis: results of a median 2-year follow-up of COMFORT-I

COMFORT-I is a randomized, double-blind, placebo-controlled trial of the Janus kinase 1/Janus kinase 2 inhibitor ruxolitinib in 309 patients with intermediate-2 or high-risk myelofibrosis. This analysis of COMFORT-I describes the long-term efficacy and safety of ruxolitinib (median follow-up, 2 years). Spleen volume was measured by magnetic resonance imaging, and quality of life was evaluated using the EORTC QLQ-C30. Overall survival was determined according to randomized treatment group. At the time of this analysis, 100 of 155 patients randomized to ruxolitinib were still receiving treatment. All patients randomized to placebo crossed over to ruxolitinib or discontinued within 3 months of the primary analysis (median time to crossover, 41 weeks). Mean spleen volume reductions in the ruxolitinib group were 31.6% at week 24 and 34.9% at week 96; improvements in quality of life measures were also maintained. Improved survival was observed for ruxolitinib (n=27 deaths) versus placebo (n=41 deaths) (hazard ratio=0.58; 95% confidence interval: 0.36, 0.95; P=0.03). The incidence of new-onset grade 3 or 4 anemia and thrombocytopenia decreased over time to levels observed in patients receiving placebo. These data indicate that ruxolitinib treatment provides durable reductions in spleen volume and improvements in quality of life and suggest a continued survival advantage for ruxolitinib over placebo.

The clinical benefit of ruxolitinib across patient subgroups: analysis of a placebo-controlled, Phase III study in patients with myelofibrosis

Myelofibrosis (MF) patients can present with a wide spectrum of disease characteristics. We analysed the consistency of ruxolitinib efficacy across patient subgroups in the COntrolled MyeloFibrosis Study With ORal JAK Inhibitor Treatment (COMFORT-I,) a double-blind trial, where patients with intermediate-2 or high-risk MF were randomized to twice-daily oral ruxolitinib (n = 155) or placebo (n = 154). Subgroups analysed included MF subtype (primary, post-polycythaemia vera, post-essential thrombocythaemia), age (≤65, > 65 years), International Prognostic Scoring System risk group, baseline Eastern Cooperative Oncology Group performance status (0, 1, ≥2), JAK2 V617F mutation (positive, negative), baseline haemoglobin level (≥100, <100 g/l), baseline platelet count (100-200 × 10(9)/l, >200 × 10(9)/l), baseline palpable spleen size (≤10, >10 cm), and baseline quartile of spleen volume and Total Symptom Score (TSS; Q1 = lowest, Q4 = highest). Mean percentage change from baseline to week 24 in spleen volume and TSS were calculated for ruxolitinib and placebo in each subgroup. Overall survival was estimated by Kaplan-Meier method according to original randomization group. In ruxolitinib-treated patients, reductions in spleen volume and TSS and evidence of improved survival relative to placebo across subgroups were consistent with those seen in the COMFORT-I population, confirming that ruxolitinib is an effective therapy for the spectrum of MF patients studied in COMFORT-I.

Ruxolitinib, an oral JAK1 and JAK2 inhibitor, in myelofibrosis

INTRODUCTION

Myelofibrosis (MF) is a debilitating hematologic malignancy characterized by progressive splenomegaly, burdensome symptoms, cytopenias and shortened survival. Chronic alterations in Janus-associated kinase-signal transducer and activator of transcription (JAK-STAT) signaling have been identified in the pathogenesis of MF, making this pathway a target for drug development. Ruxolitinib is the first JAK1 and JAK2 inhibitor to be approved by the US Food and Drug Administration.

AREAS COVERED

This review describes the characteristics of MF, the current therapeutic options and need for effective therapies, the contribution of aberrant JAK-STAT signaling to various disease-specific manifestations and the pharmacodynamics, pharmacokinetics, efficacy and tolerability of ruxolitinib. Articles describing MF disease burden and results of ruxolitinib pre-clinical and clinical trials were identified and summarized.

EXPERT OPINION

Conventional MF treatments alleviate some MF symptoms but have limited efficacy, do not modify the natural history of the disease and are not approved for MF. The JAK1 and JAK2 inhibitor ruxolitinib has shown promising results in pre-clinical and clinical trials. In Phase III trials, ruxolitinib was shown to reduce splenomegaly and improve MF-related symptoms. Recent evidence also suggests that ruxolitinib may improve survival. The most common adverse events were anemia and thrombocytopenia, which were managed with dose adjustments (or red blood cell transfusions for anemia).

JAK inhibition for the treatment of rheumatoid arthritis: a new era in oral DMARD therapy

INTRODUCTION

In rheumatoid arthritis (RA) there is a significant medical need for safe and effective oral disease-modifying anti-rheumatic drugs (DMARDs) for patients who respond inadequately to methotrexate, the first-line therapy in RA. Oral agents targeting Janus-associated kinases (JAKs) are the most promising new agents in clinical development. This review describes the preclinical and clinical activities of the most advanced JAK inhibitors with different JAK selectivity profiles.

AREAS COVERED

This review first describes the current treatment landscape and the pathophysiology of RA. Role for cytokines in the disease pathogenesis followed by significance of JAK/STAT pathway in cytokine signaling are discussed. Available chemical description and enzymatic data on the most advanced JAK inhibitors in clinical development are provided. Preclinical and clinical results that are publicly available are summarized. Review of literature was conducted using National Library of Medicine (NLM) database, 'PubMed'. In addition, all publicly disclosed data from companies that are developing the JAK inhibitors was researched to obtain the most up-to-date information of the compounds discussed in this report.

EXPERT OPINION

Emerging clinical results demonstrate that JAK inhibition is a validated new mechanism for the development of oral DMARD agents that is likely to join the armamentarium against RA in the near future.

A double-blind, placebo-controlled trial of ruxolitinib for myelofibrosis

BACKGROUND

Ruxolitinib, a selective inhibitor of Janus kinase (JAK) 1 and 2, has clinically significant activity in myelofibrosis.

METHODS

In this double-blind trial, we randomly assigned patients with intermediate-2 or high-risk myelofibrosis to twice-daily oral ruxolitinib (155 patients) or placebo (154 patients). The primary end point was the proportion of patients with a reduction in spleen volume of 35% or more at 24 weeks, assessed by means of magnetic resonance imaging. Secondary end points included the durability of response, changes in symptom burden (assessed by the total symptom score), and overall survival.

RESULTS

The primary end point was reached in 41.9% of patients in the ruxolitinib group as compared with 0.7% in the placebo group (P<0.001). A reduction in spleen volume was maintained in patients who received ruxolitinib; 67.0% of the patients with a response had the response for 48 weeks or more. There was an improvement of 50% or more in the total symptom score at 24 weeks in 45.9% of patients who received ruxolitinib as compared with 5.3% of patients who received placebo (P<0.001). Thirteen deaths occurred in the ruxolitinib group as compared with 24 deaths in the placebo group (hazard ratio, 0.50; 95% confidence interval, 0.25 to 0.98; P=0.04). The rate of discontinuation of the study drug because of adverse events was 11.0% in the ruxolitinib group and 10.6% in the placebo group. Among patients who received ruxolitinib, anemia and thrombocytopenia were the most common adverse events, but they rarely led to discontinuation of the drug (in one patient for each event). Two patients had transformation to acute myeloid leukemia; both were in the ruxolitinib group.

CONCLUSIONS

Ruxolitinib, as compared with placebo, provided significant clinical benefits in patients with myelofibrosis by reducing spleen size, ameliorating debilitating myelofibrosis-related symptoms, and improving overall survival. These benefits came at the cost of more frequent anemia and thrombocytopenia in the early part of the treatment period. (Funded by Incyte; COMFORT-I ClinicalTrials.gov number, NCT00952289.).