Abstract 2820: Effects of a CDK 4/6 inhibitor, G1T38, in androgen receptor sensitive and resistant models of castrate resistant prostate cancer

Introduction: Resistance to endocrine therapies, via expression of mutations or variants of the androgen receptor (AR), remains an impediment to enduring therapeutic responses in advanced castration resistant prostate cancer (CRPC). AR-dependent upregulation of cyclins leads to activation of the Cyclin D1-cyclin dependent kinase 4/6 (CDK 4/6) complex and cell proliferation, suggesting that targeting of this axis may be effective in CRPC. We have developed a series of potent and selective CDK 4/6 inhibitors, of which G1T38 has an IC50 in the low nanomolar range, and >3000 fold selectivity for CDK4/6 over CDK 2/cyclin A/E complexes. The efficacy of G1T38 was evaluated using models of CRPC; the results of which have significant clinical implications. Methods: For proliferation assays, cells were treated for 5-10 days with G1T38 or standard of care comparators prior to quantitation of cell number/viability. Cell cycle progression and apoptosis were assayed by flow cytometry using propidium iodide or Annexin V/Sytox red staining, respectively. Orchiectomized Nu/Nu or NOD SCID Gamma mice bearing 22rV1 or LnCAP-AR-F876L xenograft tumors, respectively, were treated with G1T38 or clinically relevant comparators. Results: G1T38 inhibited the growth of prostate cancer cell lines expressing wild type AR (LnCAP and VCAP), resistance associated AR variant AR v7 (22rV1 and LnCAP95), and LnCAP cells overexpressing the MDV3100 resistant AR mutation F876L. Corresponding decreases in cell cycle G0/G1 progression and in Rb phosphorylation (S807/811) were observed in all cell lines tested, whereas no changes were observed in Cyclin D1, E or Cdk2, 4, or 6 expression. Growth inhibition by G1T38 was dependent on Rb, and not AR, status as DU145 (AR-/Rb-) cells were not growth inhibited by G1T38, while PC3 (AR-/Rb+) were growth arrested. Treatment of G1T38 in combination with the anti-androgen MDV3100 increased the sensitivity of VCAP and 22RV1 cells to growth inhibition. Interestingly, MDV3100 in combination with G1T38 at high doses produced a synergistic apoptotic effect in LnCAP, VCAP, and 22rV1 cells, which could be attenuated by the caspase inhibitor Q-VD-OPH. The growth of 22rV1 cells and LnCAP AR F876L cells were assessed when propagated as xenografts. In 22RV1 cells, pharmacologic CDK 4/6 inhibition, significantly resulted in tumor regression compared to vehicle or docetaxel. In the LnCAP-AR-F876L xenograft model, tumor growth and doubling time was significantly decreased b