Leveraging RB status to define therapy for castrate-resistant prostate cancer.

96 Background: Loss of retinoblastoma (RB) tumor suppressor is overrepresented in castrate-resistant prostate cancer (CRPC) compared to primary PCa. We previously showed using analyses of human tissue and in vitro and in vivo modeling that RB constrains androgen receptor (AR) function, and that loss of RB is sufficient promote resistance to castration and AR antagonists. Thus, novel strategies are needed to treat RB-deficient tumor. By contrast, in tumors retaining RB, suppressing enhancing RB activity would be of therapeutic advantage, and may be accomplished through next-generation Cdk4/6 inhibitors. Methods: Stable isogenic pairs of prostate cancer cell lines either retaining RB or RB depleted (by shRNA) were assessed in vitro and in xenografts for response to Cdk4/6 kinase inhibitors or the cabazitaxel. In addition, using an ex vivo explant assay, fresh tumor tissue samples from radical prostatectomy were exposed to the Cdk4/6 inhibitor or cabazitaxel for up to 7 days, and evaluated by IHC for Ki67, C...