Targeting DNA Repair and Survival Signaling in Diffuse Intrinsic Pontine Gliomas to Prevent Tumor Recurrence

Therapeutic resistance remains a major obstacle to successful clinical management of diffuse intrinsic pontine glioma (DIPG), a high-grade pediatric tumor of the brain stem. In nearly all patients, available therapies fail to prevent progression. Innovative combinatorial therapies that penetrate the blood-brain barrier and lead to long-term control of tumor growth are desperately needed. We identified mechanisms of resistance to radiotherapy, the standard of care for DIPG. On the basis of these findings, we rationally designed a brain-penetrant small molecule, MTX-241F, that is a highly selective inhibitor of EGFR and PI3 kinase family members, including the DNA repair protein DNA-PK. Preliminary studies demonstrated that micromolar levels of this inhibitor can be achieved in murine brain tissue and that MTX-241F exhibits promising single-agent efficacy and radiosensitizing activity in patient-derived DIPG neurospheres. Its physiochemical properties include high exposure in the brain, indicating excellent brain penetrance. Because radiotherapy results in double-strand breaks that are repaired by homologous recombination (HR) and non-homologous DNA end joining (NHEJ), we have tested the combination of MTX-241F with an inhibitor of Ataxia Telangiectasia Mutated to achieve blockade of HR and NHEJ, respectively, with or without radiotherapy. When HR blockers were combined with MTX-241F and radiotherapy, synthetic lethality was observed, providing impetus to explore this combination in clinically relevant models of DIPG. Our data provide proof-of-concept evidence to support advanced development of MTX-241F for the treatment of DIPG. Future studies will be designed to inform rapid clinical translation to ultimately impact patients diagnosed with this devastating disease.

Loss of NF1 in Melanoma Confers Sensitivity to SYK Kinase Inhibition

Neurofibromin 1 (NF1) loss of function (LoF) mutations are frequent in melanoma and drive hyperactivated RAS and tumor growth. NF1LoF melanoma cells, however, do not show consistent sensitivity to individual MEK, ERK, or PI3K/mTOR inhibitors. To identify more effective therapeutic strategies for treating NF1LoF melanoma, we performed a targeted kinase inhibitor screen. A tool compound named MTX-216 was highly effective in blocking NF1LoF melanoma growth in vitro and in vivo. Single-cell analysis indicated that drug-induced cytotoxicity was linked to effective cosuppression of proliferation marker Ki-67 and ribosomal protein S6 phosphorylation. The antitumor efficacy of MTX-216 was dependent on its ability to inhibit not only PI3K, its nominal target, but also SYK. MTX-216 suppressed expression of a group of genes that regulate mitochondrial electron transport chain and are associated with poor survival in patients with NF1LoF melanoma. Furthermore, combinations of inhibitors targeting either MEK or PI3K/mTOR with an independent SYK kinase inhibitor or SYK knockdown reduced the growth of NF1LoF melanoma cells. These studies provide a path to exploit SYK dependency to selectively target NF1LoF melanoma cells.

SIGNIFICANCE

A kinase inhibitor screen identifies SYK as a targetable vulnerability in melanoma cells with NF1 loss of function.

Abstract 148: A dual and selective small molecule inhibitor of EGFR and PI3 kinase shows promising preclinical activity against KRAS and BRAF mutant colorectal tumors

Agents targeting epidermal growth factor receptor (EGFR) have met with limited success in the clinical management of colorectal cancer (CRC). Mutations in KRAS, BRAF, and PIK3CA are important drivers of resistance to EGFR-targeted therapy. Conversely, EGFR-mediated feedback mechanisms serve to mediate resistance to MEK inhibitor-based treatment of CRC by reactivating MAP kinase signaling. Our central hypothesis is that a dual small molecule inhibitor that potently and selectively targets only EGFR and PI3KA, when combined with a MEK inhibitor, will be highly efficacious against subpopulations of BRAF mutant or KRAS mutant colorectal cancers that are dependent upon these kinase molecules to drive tumor progression. Employing a computational modeling approach, we exploited the known binding modes of structurally related ATP binding site inhibitors of EGFR and PI3K to design small molecules that simultaneously inhibit both kinases in a selective manner. To the best of our knowledge, the lead compound MTX-211, whose binding mode is flipped in PI3K compared to EGFR, represents a first in class selective inhibitor of these two critical oncogenic kinases. MTX-211 exhibits a favorable pharmaceutical and selectivity profile, possessing sub- to low nanomolar potency against both targets, >70% oral bioavailability, strong pharmacodynamic modulation of both EGFR and PI3K signaling, and strong in vivo single agent efficacy against multiple BRAFmt and KRASmt colorectal cancer models, as evidenced by T/C values of 29 to 36% after 10-14 days of oral dosing of 50 mg/kg. A significantly higher degree of in vivo activity is seen when MTX-211 is co-administered with the MEK inhibitor trametinib (>400% increase in survival compared to single agent arms), lending support for this polypharmacology approach over triple drug combination strategies. Based on its promising therapeutic profile, MTX-211 is the focus of an ongoing mouse trial of a large panel of patient-derived xenograft BRAFmt and KRASmt CRC models to inform the design of future human clinical trials.

Citation Format: Joel D. Maust, Elizabeth K. Ziemke, Christy L. Frankowski-McGregor, Jun Beom Ku, Rachel Mumby, Karin M. Hardiman, Christopher E. Whitehead, Judith S. Sebolt-Leopold. A dual and selective small molecule inhibitor of EGFR and PI3 kinase shows promising preclinical activity against KRAS and BRAF mutant colorectal tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 148. doi:10.1158/1538-7445.AM2017-148

Sensitivity of KRAS-Mutant Colorectal Cancers to Combination Therapy That Cotargets MEK and CDK4/6

PURPOSE

The emerging need for rational combination treatment approaches led us to test the concept that cotargeting MEK and CDK4/6 would prove efficacious in KRAS-mutant (KRAS(mt)) colorectal cancers, where upregulated CDK4 and hyperphosphorylated retinoblastoma (RB) typify the vast majority of tumors.

EXPERIMENTAL DESIGN

Initial testing was carried out in the HCT-116 tumor model, which is known to harbor a KRAS mutation. Efficacy studies were then performed with five RB(+) patient-derived colorectal xenograft models, genomically diverse with respect to KRAS, BRAF, and PIK3CA mutational status. Tolerance, efficacy, and pharmacodynamic evaluation of target modulation were evaluated in response to daily dosing with either agent alone or concurrent coadministration.

RESULTS

Synergy was observed in vitro when HCT-116 cells were treated over a broad range of doses of trametinib and palbociclib. Subsequent in vivo evaluation of this model showed a higher degree of antitumor activity resulting from the combination compared to that achievable with single-agent treatment. Testing of colorectal patient-derived xenograft (PDX) models further showed that combination of trametinib and palbociclib was well tolerated and resulted in objective responses in all KRAS(mt) models tested. Stasis was observed in a KRAS/BRAF wild-type and a BRAF(mt) model.

CONCLUSIONS

Combination of trametinib and palbociclib was well tolerated and highly efficacious in all three KRAS-mutant colorectal cancer PDX models tested. Promising preclinical activity seen here supports clinical evaluation of this treatment approach to improve therapeutic outcome for patients with metastatic colorectal cancer.

Cancer stem cell marker phenotypes are reversible and functionally homogeneous in a preclinical model of pancreatic cancer

Survival rates associated with pancreatic cancer remain dismal despite advancements in detection and experimental treatment strategies. Genetically engineered mouse models of pancreatic tumorigenesis have gained considerable attention based on their ability to recapitulate key clinical features of human disease including chemotherapeutic resistance and fibrosis. However, it is unclear if transgenic systems exemplified by the Kras(G12D)/Trp53(R172H)/Pdx-1-Cre (KPC) mouse model recapitulate the functional heterogeneity of human pancreatic tumors harboring distinct cells with tumorigenic properties. To facilitate tracking of heterogeneous tumor cell populations, we incorporated a luciferase-based tag into the genetic background of the KPC mouse model. We isolated pancreatic cancer cells from multiple independent tumor lines and found that roughly 1 out of 87 cells exhibited tumorigenic capability. Notably, this frequency is significantly higher than reported for human pancreatic adenocarcinomas. Cancer stem cell (CSC) markers, including CD133, CD24, Sca-1, and functional Aldefluor activity, were unable to discriminate tumorigenic from nontumorigenic cells in syngeneic transplants. Furthermore, three-dimensional spheroid cultures originating from KPC tumors did not enrich for cells with stem-like characteristics and were not significantly more tumorigenic than cells cultured as monolayers. Additionally, we did not observe significant differences in response to gemcitabine or salinomycin in several isolated subpopulations. Taken together, these studies show that the hierarchical organization of CSCs in human disease is not recapitulated in a commonly used mouse model of pancreatic cancer and therefore provide a new view of the phenotypic and functional heterogeneity of tumor cells.

Abstract 1400: Evaluation of tumor-initiating cell populations in the KPC mouse model of pancreatic cancer

There continue to be many questions about the frequency and phenotype of tumor-initiating cells or cancer stem cells (CSCs) which may play an important role in cancer metastasis and drug resistance in several tumor types, including pancreatic cancer. The KrasG12D, Trp53R172H, and Pdx-1-Cre (KPC) induced model of pancreatic cancer has gained considerable attention for its ability to replicate the clinical features of the disease. Here, we report the generation of a novel syngeneic xenograft model of luciferase-tagged pancreatic tumor cells (KPCL) to facilitate the investigation of the tumorigenic capacity of this cancer model. Limiting dilution analysis of bulk KPCL tumor cells revealed that 1/87 cells were capable of forming new tumors, a number considerably higher than reported for human pancreatic adenocarcinomas. Cell surface markers CD44, CD24, CD133, c-Met, and functional ALDEFLUOR activity, previously shown to mark CSC populations in human and mouse pancreatic tumors, were heterogeneous across a large panel of spontaneous KPCL tumors. Additional testing of CD133 and ALDEFLUOR populations were unable to enrich for tumorigenic populations when transplanted into syngeneic mice. Additionally, spheroid cultures initiated from KPCL tumors did not enrich for cells with stem-like characteristics and were not significantly more tumorigenic than KPCL cells cultured as monolayers. These studies demonstrate that the tumor-forming potential of KPC cells is common, and may not fit the hierarchically organized CSC model that is reported for the human disease.

Citation Format: Joseph S. Dosch, Elizabeth K. Ziemke, Amrith Shettigar, Alnawaz Rehemtulla, Judith S. Sebolt-Leopold. Evaluation of tumor-initiating cell populations in the KPC mouse model of pancreatic cancer. [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 1400. doi:10.1158/1538-7445.AM2015-1400