Abstract 3415: Preclinical characterization of a brain penetrant RAF inhibitor, BDTX-4933, targeting oncogenic BRAF Class I/II/III and RAS mutations

Mutations in BRAF and RAS are often oncogenic and lead to a constitutively active MAPK pathway that promotes aberrant cell proliferation and tumor growth. Currently approved BRAF inhibitors are selective against monomeric BRAF V600 mutants. These drugs are largely inactive against non-V600 dimeric BRAF mutants and have poor brain penetration. Although there is an FDA-approved KRAS G12C mutant-selective inhibitor, there are no approved inhibitors for cancer patients who harbor other (non-G12C) KRAS and NRAS mutations which promote tumor growth likely through constitutively active RAF dimers. There remains a high unmet clinical need for a highly CNS penetrant oral RAF inhibitor that targets a broad spectrum of BRAF mutations and constitutively active RAF dimers without paradoxical activation of the MAPK signaling pathway. BDTX-4933 is a potent, reversible, CNS penetrant RAF MasterKey inhibitor designed to target a large family of oncogenic BRAF mutations including BRAF monomers and RAF dimers. The compound inhibits not only all classes (I, II, and III) of BRAF mutations but also targets constitutively active RAF dimers promoted by upstream oncogenic MAPK pathway alterations, such as RAS mutations. In a panel of cancer cell lines that endogenously express BRAF or RAS mutations, BDTX-4933 demonstrates inhibition of the MAPK pathway signaling without paradoxical activation, resulting in potent inhibition of cellular proliferation. BDTX-4933 shows target engagement, inhibiting ERK phosphorylation, in tumor models in vivo, achieving strong anti-tumor efficacy and tumor regression across tumor models driven by either BRAF or RAS mutations. Furthermore, BDTX-4933 exhibits high CNS exposure leading to dose-dependent tumor growth inhibition, and survival benefit in mice implanted intracranially with xenograft BRAF mutant tumors. BDTX-4933 has a best-in-class profile to treat cancer patients harboring BRAF mutations or RAF dimer-promoting upstream genetic alterations. IND-enabling studies for BDTX-4933 are on-going.

Citation Format: Yoon-Chi Han, Pui-Yee Ng, Luisa Shin Ogawa, Shao Ning Yang, Miao Chen, Noboru Ishiyama, Tai-An Lin, Elizabeth Buck. Preclinical characterization of a brain penetrant RAF inhibitor, BDTX-4933, targeting oncogenic BRAF Class I/II/III and RAS mutations [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 3415.

Abstract P246: Discovery and characterization of selective, FGFR1 sparing, inhibitors of FGFR2/3 oncogenic mutations for the treatment of cancers

Background: Targeting FGFR genetic alterations using small molecule inhibitors is a validated therapeutic strategy for urothelial carcinoma and cholangiocarcinoma. However, the current FDA-approved pan-FGFR inhibitors, erdafitinib and pemigatinib, are subject to FGFR1-mediated dose-limiting toxicities (e.g., hyperphosphatemia). These treatments necessitate a high rate of dose reductions, interruptions, and discontinuations, thereby potentially limiting efficacy. In addition, drug-resistant mutations (e.g., gatekeeper) in FGFR2 and FGFR3 genes rapidly emerge in patients treated with these drugs. Our research goals are to reveal the full spectrum of oncogenic FGFR2 and FGFR3 mutations that drive tumor growth and to discover an inhibitor that selectively targets these mutations together with FGFR2 and FGFR3 gene fusion and drug-resistance mutations, while minimizing FGFR1 activity and associated toxicities. We hypothesize that this will deliver an FGFR precision medicine with enhanced anti-tumor activity, an improved drug resistance profile, and broader mutational coverage. Methods: Applying the Mutation-Allostery-Pharmacology (MAP) platform technology developed by Black Diamond Therapeutics, we defined a spectrum of 34 allosteric FGFR2/3 oncogenic mutations, including over 28 previously uncharacterized mutations that we now show to be oncogenic. The MAP platform allowed us to further classify those mutations into functional clusters or families of mutations that can be targeted using a single compound. While located throughout the extracellular and kinase domains, we demonstrated how these functional clusters activate FGFR2 or FGFR3. Among the mechanisms identified, a functional cluster of mutations is activated due to disulfide-bond mediated dimerization. Results: Herein, we report the discovery of a series of orally available, selective FGFR2/3 inhibitors that 1) shows antiproliferative potency across all 34 mutations; 2) spares FGFR1-wild-type; 3) is active against gatekeeper mutations and 4) shows favorable selectivity versus a subset of closely related kinases in the human kinome. In addition to being potent against FGFR2 and FGFR3 primary mutations, we demonstrated that our FGFR1 sparing inhibitors retain potency against the most prevalent FGFR2 resistant mutations. When dosed orally, one example was well tolerated and exhibited dose-dependent PK/PD and anti-tumor efficacy and regression in several FGFR2 and FGFR3 driven xenograft models in mice. In addition, when dosed at efficacious doses, no FGFR1-mediated hyperphosphatemia was observed in these animals instead of the animals that were treated with pan FGFR inhibitors. Conclusion: Our data support the development of rationally designed selective inhibitors targeting a spectrum of FGFR2/3 mutations while sparing dose limiting FGFR1 activity.

Citation Format: Etienne Dardenne, Fernando Padilla, Sara Rasmussen, Shao Ning Yang, Ahmet Mentes, Luisa Shin Ogawa, Anthony Trombino, Darlene Romashko, Maria Chevtsova, Shalabh Thakur, Elisabeth Buck, Christopher Roberts, Matthew Lucas, Tai-An Lin. Discovery and characterization of selective, FGFR1 sparing, inhibitors of FGFR2/3 oncogenic mutations for the treatment of cancers [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 P246.

Consecutive Day HSP90 Inhibitor Administration Improves Efficacy in Murine Models of KIT-Driven Malignancies and Canine Mast Cell Tumors

PURPOSE

STA-1474, prodrug of the heat shock protein 90 inhibitor (HSP90i) ganetespib, previously demonstrated activity in canine preclinical models of cancer; interestingly, prolonged infusions were associated with improved biologic activity. The purpose of this study was to identify the ideal treatment schedule for HSP90i in preclinical models of KIT-driven malignancies and in dogs with spontaneous mast cell tumors (MCT), where KIT is a known driver.

EXPERIMENTAL DESIGN

In vitro and murine xenograft experiments and clinical studies in dogs with MCTs were used to define the effects of HSP90i-dosing regimen on client protein downregulation and antitumor activity.

RESULTS

Continuous HSP90 inhibition led to durable destabilization of client proteins in vitro; however, transient exposure required >10× drug for comparable effects. In vivo, KIT was rapidly degraded following a single dose of HSP90i but returned to baseline levels within a day. HSP90 levels increased and stabilized 16 hours after HSP90i and were not elevated following a subsequent near-term exposure, providing a functional pool of chaperone to stabilize proteins and a means for greater therapeutic activity upon HSP90i reexposure. HSP90i administered on days 1 and 2 (D1/D2) demonstrated increased biologic activity compared with D1 treatment in KIT or EGFR-driven murine tumor models. In a trial of dogs with MCT, D1/D2 dosing of HSP90i was associated with sustained KIT downregulation, 50% objective response rate and 100% clinical benefit rate compared with D1 and D1/D4 schedules.

CONCLUSIONS

These data provide further evidence that prolonged HSP90i exposure improves biologic activity through sustained downregulation of client proteins.

The HSP90 Inhibitor Ganetespib Alleviates Disease Progression and Augments Intermittent Cyclophosphamide Therapy in the MRL/lpr Mouse Model of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a complex, systemic autoimmune disease with a diverse range of immunological and clinical manifestations. The introduction of broad spectrum immunosuppressive therapies and better management of acute disease exacerbations have improved outcomes for lupus patients over recent years. However, these regimens are burdened by substantial toxicities and confer significantly higher risks of infection, thus there remains a significant and unmet medical need for alternative treatment options, particularly those with improved safety profiles. Heat shock protein 90 (HSP90) is a ubiquitously expressed molecular chaperone that acts as an important modulator of multiple innate and adaptive inflammatory processes. Of note, accumulating clinical and experimental evidence has implicated a role for HSP90 in the pathogenesis of SLE. Here we evaluated the potential of HSP90 as a therapeutic target for this disease using the selective small molecule inhibitor ganetespib in the well-characterized MRL/lpr autoimmune mouse model. In both the prophylactic and therapeutic dosing settings, ganetespib treatment promoted dramatic symptomatic improvements in multiple disease parameters, including suppression of autoantibody production and the preservation of renal tissue integrity and function. In addition, ganetespib exerted profound inhibitory effects on disease-related lymphadenopathy and splenomegaly, and reduced pathogenic T and B cell lineage populations in the spleen. Ganetespib monotherapy was found to be equally efficacious and tolerable when compared to an effective weekly dosing regimen of the standard-of-care immunosuppressive agent cyclophosphamide. Importantly, co-treatment of ganetespib with a sub-optimal, intermittent dosing schedule of cyclophosphamide resulted in superior therapeutic indices and maximal disease control. These findings highlight the potential of HSP90 inhibition as an alternative, and potentially complementary, strategy for therapeutic intervention in SLE. Such approaches may have important implications for disease management, particularly for limiting or preventing treatment-related toxicities, a major confounding factor in current SLE therapy.

Abstract 1619: Hsp90 inhibitor drug conjugates (HDCs): Construct design and preliminary evaluation

Background: Most cytotoxic agents are often broadly active but non-selective, and have the disadvantage of high toxicity due to collateral damage to normal tissues. Drugs that target specific protein drivers of cancer cell growth are more tumor selective, yet often lead to tumor resistance via point mutations in their target or activation of alternative signaling pathways. Targeted delivery strategies, such as Antibody-Drug Conjugates (ADCs), offer a solution to these limitations by delivering potent anti-cancer payloads more directly to tumors. Hsp90 is a chaperone protein required by many cancer cells to maintain the stability and function of numerous proteins that drive cancer cell growth, survival, and metastasis. Small molecule inhibitors of Hsp90 such as ganetespib, PU-H71 and 17-AAG are found to be retained in tumors with half- time up to 65 hours in mouse xenografts. These properties are believed to be due to overexpression of an active form of Hsp90 in cancer cells as compared to normal tissues, and have recently been applied for tumor imaging in patients.

Results: We have developed a small molecule drug conjugate platform technology using the unique properties of Hsp90 proteins and Hsp90 inhibitors. We have synthesized HDCs with various Hsp90 inhibitor scaffolds including resorcinol, purine, geldanamycin, etc. A cell-based client protein degradation assay is carried out to confirm the intracellular uptake and the Hsp90 binding of the conjugates. While different Hsp90 inhibitor scaffolds offer different DMPK and toxicology profiles, cellular uptake and Hsp90 binding are not greatly affected by the different chemical classes of Hsp90 inhibitors. Over 30 payloads have been conjugated with Hsp90 inhibitors so far. We have selected the payloads based on the hypothesis that HDC can improve the safety profile of a cytotoxic drug, expand the application of chemotherapeutic agents to different tumor types, combat drug resistance, and enable novel anticancer approach. Examples of payloads include topoisomerase inhibitors (camptothecin), microtubule modulators (taxanes), proteasome inhibitors (carfilzomib), CDK inhibitors (flavopiridol) and others. Unlike most other conjugate technologies, HDC does not require lengthy spacing between the anchor Hsp90 inhibitor and the payload. Linker cleavage mechanism is considered a key feature in the HDC design. We have been able to incorporate several linkers such as disulfide, hydrazone, peptide, carbamate, carboxylate, etc. into our HDC designs.

Conclusion: In this HDC platform, we provide a method which can be applied to many well-studied mechanisms for modulating cancer pathways and stopping tumor cell growth. We have created a promising platform technology which can result in many anticancer agents in the near future.

Citation Format: Weiwen Ying, Dinesh Chimmanamada, Junyi Zhang, Teresa Przewloka, Jun Jiang, Genliang Lu, Sami Osman, James Loch, Dharma Vutukuri, Shoujun Chen, Robert Stein, John Chu, David Proia, Pat Rao, Takayo Inoue, Luisa Shin Ogawa, Ritu Singh, Noriaki Tatsuta. Hsp90 inhibitor drug conjugates (HDCs): Construct design and preliminary evaluation. [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 1619. doi:10.1158/1538-7445.AM2014-1619

UDP glucuronosyltransferase 1A expression levels determine the response of colorectal cancer cells to the heat shock protein 90 inhibitor ganetespib

HSP90 inhibition represents a promising route to cancer therapy, taking advantage of cancer cell-inherent proteotoxic stress. The HSP90-inhibitor ganetespib showed benefit in advanced clinical trials. This raises the need to identify the molecular determinants of treatment response. We tested the efficacy of ganetespib on a series of colorectal cancer (CRC)-derived cell lines and correlated their sensitivities with comprehensive gene expression analysis. Notably, the drug concentration required for 50% growth inhibition (IC₅₀) varied up to 70-fold (from 36 to 2500 nM) between different cell lines. Correlating cell line-specific IC₅₀s with the corresponding gene expression patterns revealed a strong association between ganetespib resistance (IC₅₀>500 nM) and high expression of the UDP glucuronosyltransferase 1A (UGT1A) gene cluster. Moreover, CRC tumor samples showed a comparable distribution of UGT1A expression levels. The members of the UGT1A gene family are known as drug-conjugating liver enzymes involved in drug excretion, but their function in tumor cells is hardly understood. Chemically unrelated HSP90 inhibitors, for example, 17-N-allylamino-17-demethoxygeldanamycin (17-AAG), did not show correlation of drug sensitivities with UGT1A levels, whereas the ganetespib-related compound NVP-AUY922 did. When the most ganetespib-resistant cell line, HT29, was treated with ganetespib, the levels of HSP90 clients were unaffected. However, HT29 cells became sensitized to the drug, and HSP90 client proteins were destabilized by ganetespib upon siRNA-mediated UGT1A knockdown. Conversely, the most ganetespib-sensitive cell lines HCT116 and SW480 became more tolerant toward ganetespib upon UGT1A overexpression. Mechanistically, ganetespib was rapidly glucuronidated and excreted in resistant but not in sensitive CRC lines. We conclude that CRC cell-expressed UGT1A inactivates ganetespib and other resorcinolic Hsp90 inhibitors by glucuronidation, which renders the drugs unable to inhibit Hsp90 and thereby abrogates their biological activity. UGT1A levels in tumor tissues may be a suitable predictive biomarker to stratify CRC patients for ganetespib treatment.

FGFR3 translocations in bladder cancer: differential sensitivity to HSP90 inhibition based on drug metabolism

Activating mutations and/or overexpression of FGFR3 are common in bladder cancer, making FGFR3 an attractive therapeutic target in this disease. In addition, FGFR3 gene rearrangements have recently been described that define a unique subset of bladder tumors. Here, a selective HSP90 inhibitor, ganetespib, induced loss of FGFR3-TACC3 fusion protein expression and depletion of multiple oncogenic signaling proteins in RT112 bladder cells, resulting in potent cytotoxicity comparable with the pan-FGFR tyrosine kinase inhibitor BGJ398. However, in contrast to BGJ398, ganetespib exerted pleiotropic effects on additional mitogenic and survival pathways and could overcome the FGFR inhibitor-resistant phenotype of FGFR3 mutant-expressing 97-7 and MHG-U3 cells. Combinatorial benefit was observed when ganetespib was used with BGJ398 both in vitro and in vivo. Interestingly, two additional FGFR3 fusion-positive lines (RT4 and SW480) retained sensitivity to HSP90 inhibitor treatment by the ansamycins 17-AAG and 17-DMAG yet displayed intrinsic resistance to ganetespib or AUY922, both second-generation resorcinol-based compounds. Both cell lines, compared with RT112, expressed considerably higher levels of endogenous UGT1A enzyme; this phenotype resulted in a rapid glucuronidation-dependent metabolism and subsequent efflux of ganetespib from SW780 cells, thus providing a mechanism to account for the lack of bioactivity.

IMPLICATIONS

Pharmacologic blockade of the molecular chaperone HSP90 represents a promising approach for treating bladder tumors driven by oncogenic gene rearrangements of FGFR3. Furthermore, UDP-glucuronosyltransferase enzyme expression may serve as a predictive factor for clinical response to resorcinol-based HSP90 inhibitors.

Associating retinal drug exposure and retention with the ocular toxicity profiles of Hsp90 inhibitors

3086

Background: In clinical trials certain Hsp90 inhibitors, including AUY922, SNX-5422, and 17-DMAG have caused visual symptoms suggesting retinal dysfunction; others, including ganetespib and 17-AAG, have not. Previous animal toxicology experiments have suggested that retinal changes may be linked to photoreceptor degeneration or cell death. Here histopathologic changes and/or exposure profiles of Hsp90 inhibitors with or without clinical reports of ocular toxicity were evaluated to understand the observed differences in toxicity profile between agents in this class. Methods: We reviewed visual symptoms among subjects administered ganetespib, a potent Hsp90 inhibitor currently in phase II trials; evaluated retinal morphology in rats and cyno monkeys given ganetespib; and compared TUNEL-positive photoreceptors and drug exposure profiles in the retina of rats treated with AUY922, 17-DMAG, and 17-AAG. Studies of ganetespib’s retinal pharmacokinetics and photoreceptor toxicity in rats are ongoing. Results: AUY922 and 17-DMAG induce visual disorders in the clinic. Both drugs induced marked photoreceptor cell death (increased TUNEL-positive cells) in rats with a high retina/plasma (R/P) exposure ratio, and a slow elimination rate (at 6 h post-dose, over 54% of AUY922 present at 30 min remained in the retina). In contrast, and consistent with an absence of clinical visual changes, 17-AAG at the maximum tolerated dose did not elicit photoreceptor injury. Further, retinal elimination was rapid (at 6 h post-dose, 94% of 17-AAG had been eliminated from the retina, resulting in a low R/P ratio). Ganetespib given by 1‑hour IV infusion on days 1 and 15 of four 21-day cycles did not cause histopathological changes in the retina of rats or cynos. This finding is consistent with very low rate (~3%) of drug-related visual symptoms observed among over 300 subjects to date that received ganetespib in the clinic. Conclusions: Unlike AUY922 and 17-DMAG, ganetespib is not associated with ocular toxicity in humans, primates, or rodents. The findings suggest that Hsp90 inhibitors may elicit visual disorders when associated with high retina/plasma exposure ratio and lower retinal elimination rate.

The oncology drug elesclomol selectively transports copper to the mitochondria to induce oxidative stress in cancer cells

Elesclomol is an investigational drug that exerts potent anticancer activity through the elevation of reactive oxygen species (ROS) levels and is currently under clinical evaluation as a novel anticancer therapeutic. Here we report the first description of selective mitochondrial ROS induction by elesclomol in cancer cells based on the unique physicochemical properties of the compound. Elesclomol preferentially chelates copper (Cu) outside of cells and enters as elesclomol-Cu(II). The elesclomol-Cu(II) complex then rapidly and selectively transports the copper to mitochondria. In this organelle Cu(II) is reduced to Cu(I), followed by subsequent ROS generation. Upon dissociation from the complex, elesclomol is effluxed from cells and repeats shuttling elesclomol-Cu complexes from the extracellular to the intracellular compartments, leading to continued copper accumulation within mitochondria. An optimal range of redox potentials exhibited by copper chelates of elesclomol and its analogs correlated with the elevation of mitochondrial Cu(I) levels and cytotoxic activity, suggesting that redox reduction of the copper triggers mitochondrial ROS induction. Importantly the mitochondrial selectivity exhibited by elesclomol is a distinct characteristic of the compound that is not shared by other chelators, including disulfiram. Together these findings highlight a unique mechanism of action with important implications for cancer therapy.

Potent anti-inflammatory activity of a novel of class Hsp90 inhibitors for multiple sclerosis (51.1)

Current therapies for multiple sclerosis (MS) generally reduce the frequency, severity and duration of relapses, and may slow disease progression, but are not curative. Clinical benefit also comes with side effects that range from mild to life-threatening. Heat shock protein 90 (Hsp90) is a molecular chaperone required for the stability and function of many proteins that modulate cellular response to environmental stresses. Due to its central role in multiple immune-mediated mechanisms, Hsp90 has recently emerged as a potential target for MS therapy. A number of the inflammatory mediators underlying MS are known Hsp90 client proteins, and Hsp90 blockade represents an attractive multi-targeted therapeutic approach given its alternative mechanisms of neuroprotection and inhibition of inflammation. Here we describe a novel class of small molecule inhibitors that preferentially target Hsp90 associated with immune-mediated pathways and which demonstrate potent inhibitory activity, high oral bioavailability and prolonged CNS exposure. Further, the lead compound displays broad activity in a number of relevant in vitro and in vivo models; including inhibition of cytokine production by human PBMCs, decreased nitric oxide production by primary rat astrocytes, and efficacy in an adoptive transfer model of MS. Taken together, this class of Hsp90 inhibitors may potentially provide new avenues to counter immune overstimulation in MS patients.

Abstract C168: Cancer-selective mitochondrial copper transport by elesclomol results in potent single agent efficacy in multiple tumor types

Elesclomol is a first-in-class investigational drug that exerts potent anticancer activity through the elevation of reactive oxygen species (ROS) levels and is currently under clinical evaluation as a novel anticancer therapeutic. We recently demonstrated that elesclomol preferentially binds extracellular copper (Cu) and selectively transports this metal ion to the mitochondria of tumor cells to promote mitochondrial ROS generation and subsequent apoptosis. Here we report that elesclomol-induced copper transport and apoptosis is tumor selective. Comparative analysis using human PBMCs and the promyelocytic tumor cell line HL-60 demonstrated increased Cu levels in the mitochondrial fraction of HL-60 cells following elesclomol-Cu treatment, but not in donor PBMCs. Further, elesclomol-Cu induces ROS in HL-60-derived mitochondria but not in those isolated from PBMCs. These results suggest that elesclomol-Cu selectively targets cancer cell mitochondria to ultimately produce critical elevations in oxidative stress. To evaluate increased exposure to elesclomol-Cu in vivo, elesclomol was administered continuously to tumor-bearing mice using an Alzet pump at a clinically relevant dose. Upon release, elesclomol immediately chelates copper from the blood to form an elesclomol-Cu complex, achieving 10-fold higher levels of elesclomol-Cu compared to those following bolus injection. Even with this increased formation of elesclomol-Cu in situ, no signs of toxicity have been observed. In five different tumor xenograft models tested, elesclomol consistently demonstrated marked single agent activity with significant tumor growth suppression, indicating that increased elesclomol-Cu levels result in selective and enhanced antitumor efficacy. These findings highlight a unique mechanism of action of elesclomol and support potential single agent activity of this compound in a variety of tumor types.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C168.

Abstract B105: Ganetespib, a unique resorcinolic Hsp90 inhibitor, exhibits potent antitumor activity and a superior safety profile in preclinical models

Targeted inhibition of the molecular chaperone heat shock protein 90 (Hsp90) results in the simultaneous blockade of multiple oncogenic signaling pathways and has thus emerged as an attractive strategy for the development of novel cancer therapeutics. Ganetespib (STA-9090) is a unique resorcinolic inhibitor of Hsp90 currently in clinical trials for a number of human cancers. Here we describe the key interaction of ganetespib with a number of amino acid residues in the ATP binding pocket of Hsp90 which results in high affinity binding. Ganetespib exhibits potent in vitro cytotoxicity in a range of solid and hematological tumor cell lines. By using a novel isotope-labeling scheme and LC-MS/MS detection technique, we have determined that Hsp90 occupancy by ganetespib in cancer cells is relatively fast under saturating conditions, reaching equilibrium within 5 minutes of ganetespib exposure. In vivo, ganetespib demonstrated strong single-agent activity in solid and hematological xenograft models, as evidenced by significant tumor growth inhibition and/or regression. Of note, evaluation of the microregional activity of ganetespib in tumor xenografts showed that ganetespib efficiently distributed throughout tumor tissue, including hypoxic regions >150 m from the microvasculature, to inhibit proliferation and induce apoptosis. Most importantly, ganetespib showed no evidence of cardiac or liver toxicity and exhibited minimal potential risk for CNS or ocular toxicities. Taken together, this preclinical activity profile suggests that ganetespib may have broad application for a variety of human malignancies and mechanistic and safety advantages over other first- and second-generation Hsp90 inhibitors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B105.

Abstract C91: Pharmacodynamic analysis of the Hsp90 inhibitor STA-9090 in a lung cancer xenograft model supports an infrequent dosing schedule in the clinic

Background: Heat shock protein 90 (Hsp90) is a molecular chaperone that is required for the stability and function of many important signal transduction proteins that regulate the growth of cancer cells. Hsp90 inhibition results in ubiquitination and proteasomal degradation of these client proteins, which include clinically validated drug targets such as BCR-ABL, mutant EGFR, HER2, KIT and VEGFR. STA-9090 is a novel small molecule Hsp90 inhibitor that is currently in multiple Phase 1/2 clinical trials in solid tumor and hematological malignancies. STA-9090 is structurally unrelated to the first-generation anasamycin Hsp90 inhibitors 17-AAG and IPI-504 and inhibits Hsp90 by binding to its N-terminal ATP-binding pocket. Although Hsp90 inhibitors such as STA-9090 induce rapid client protein degradation, cell cycle arrest and apoptosis of cancer cells, it is possible that frequent drug dosing in the clinic may be needed to continuously maintain decreased client protein expression and avoid renewed tumor growth. To investigate this possibility, we conducted in vitro and in vivo studies using the human NCI-H1975 non-small cell lung cancer cell line, which expresses the Hsp90 client protein EGFRL858R/T790M, a mutationally activated and erlotinib-resistant form of the epidermal growth factor receptor.

Results: In an in vitro cytotoxicity assay using this cell line, STA-9090 and 17-AAG displayed IC50 values of 10 and 40 nM after 72 hr drug exposure, respectively. These results closely correlated with decreased expression of EGFRL858R/T790M and other Hsp90 client proteins. Unexpectedly, exposure to STA-9090 for only 1 hr still resulted in an IC50 of 670 nM, suggesting that even brief drug exposure in vivo may be sufficient to affect tumor growth. Consistent with this, intravenous dosing of 125 mg/kg STA-9090 on a 1X/week × 3 week schedule (∼80–100% of the highest non-severely toxic dose) induced stable disease in a NCI-H1975 xenograft model, whereas 175 mg/kg 17-AAG resulted in progressive disease, with %T/C values of 15 and 50, respectively. Inhibition of tumor growth was correlated with decreased expression of EGFRL858R/T790M and other client proteins, and importantly, these effects persisted in tumors for 3–6 days after a single drug dose. Similarly, histological analysis of tumors indicated that STA-9090 inhibited cell proliferation by 7-fold and induced apoptosis by 9-fold, with maximal effects being observed at 1–3 days after treatment. Consistent with these observations, STA-9090 accumulated in tumors relative to normal tissues, with a tumor half-life of 58 hr versus 3–5 hr in liver, lung and plasma, and the tumor concentration remained 140-fold higher than the in vitro IC50 (72 hr) even 6 days after a single drug dose.

Conclusions: Taken together, these results demonstrate that STA-9090 is a highly potent Hsp90 inhibitor that selectively accumulates in tumors and induces long-lasting client protein degradation, cell cycle arrest, increased apoptosis and tumor growth inhibition in a lung cancer xenograft model. Our results suggest that an infrequent dosing schedule may have clinical activity in cancer patients.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C91.

Abstract B199: In vitro and in vivo efficacy of the novel Hsp90 inhibitor STA-9090 and its synergy with paclitaxel

Heat shock protein 90 (Hsp90) is a molecular chaperone that regulates the post-translational folding of its protein substrates (“client proteins”). Cancer cells contain elevated levels of active Hsp90 and, because many client proteins play critical oncogenic roles, cancer cells are especially sensitive to Hsp90 inhibition. Here we report on the initial characterization of STA-9090, a highly potent Hsp90 inhibitor that is currently in multiple Phase 1/2 clinical trials in solid tumor and hematological malignancies. STA-9090 is a small molecule drug that is structurally unrelated to the ansamycin Hsp90 inhibitor 17-AAG and binds the N-terminal ATP-binding pocket of the chaperone. In vitro, treatment with STA-9090 rapidly induced the degradation of known Hsp90 client proteins, such as HER2 and KIT, and growth inhibition IC50 values typically ranged from 1 to 100 nM. STA-9090 demonstrated, on average, ∼30-fold greater potency than 17-AAG for the ∼60 hematological and solid tumor cell lines tested. STA-9090 also retained its potency against cell lines expressing mutated kinases that confer resistance to kinase inhibitors such as erlotinib and imatinib.

In vivo, STA-9090 demonstrated single-agent activity in a wide variety of human tumor cell line subcutaneous xenograft models in mice, including those representing solid tumor malignancies such as gastric carcinoma, non-small cell lung cancer, prostate carcinoma and melanoma, and hematological malignancies such as acute myeloid leukemia, B-cell lymphoma, chronic myeloid leukemia and multiple myeloma. In a mouse leukemia model in which wild type BCR-ABL or imatinib/dasatinib-resistant BCR-ABLT315I was introduced into mouse bone marrow cells and then transplanted into host mice to induce the development of B-cell acute lymphoblastic leukemia, STA- 9090 prolonged average survival from 27 to 37 days for BCR-ABL and from 29 to 57 days for BCR-ABLT315I. STA-9090 also accumulated in tumors, with a half-life of 58 hr in tumors versus 3–5 hr in plasma and non-tumor tissues. To examine the potential for therapeutic synergy, STA-9090 was combined with the microtubule stabilizer paclitaxel. STA-9090 dramatically synergized with paclitaxel in in vitro cytotoxicity assays, particularly when paclitaxel treatment preceded treatment with STA-9090. Similarly, STA-9090 enhanced the activity of paclitaxel in the erlotinib-resistant NCI-H1975 lung cancer xenograft model, although this enhancement depended less on the order of dosing than was observed in vitro. No significant pharmacokinetic interactions were observed between the two agents.

In conclusion, STA-9090 is a highly potent Hsp90 inhibitor that in vitro and in vivo rapidly induces the degradation of Hsp90 client proteins and apoptosis in a broad range of cancer types, including those resistant to targeted kinase inhibitors. It also demonstrates synergy with paclitaxel.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B199.

Posterior Synechia of the Iris After Combined Pars Plana Vitrectomy, Phacoemulsification, and Intraocular Lens Implantation

PURPOSE

Combined pars plana vitrectomy, phacoemulsification, and intraocular lens implantation has become a widely accepted treatment for cataracts in patients with vitreoretinal diseases. We examined factors influencing the development of posterior synechia after this triple procedure.

METHODS

One hundred and three patients (107 eyes) were evaluated. The frequency of postoperative posterior synechia, the preoperative diagnosis, whether gas tamponade was used, and the type of lens implanted were reviewed.

RESULTS

Twenty-one eyes (19.6%) developed posterior synechia, and the highest rate (12/39 eyes, 30.8%) was in patients with proliferative diabetic retinopathy. Posterior synechia was more frequent after gas tamponade (28.1%) than in eyes without tamponade (10.1%). In proliferative diabetic retinopathy (PDR) patients, fibrin deposition and the amount of retinal photocoagulation were causative factors for posterior synechia.

CONCLUSIONS

Factors promoting postoperative synechia after the triple procedure included (1) the existence of PDR, (2) expanding gas tamponade, (3) fibrin deposition in PDR, and (4) the amount of photocoagulation in PDR.