Preclinical discovery and validation of high-dose androgen drug synergies for the treatment of prostate cancer

202 Background: Recent clinical studies have taken advantage of the dichotomous ability of androgen receptor signaling to elicit growth-suppressive and differentiating effects by giving hyper-physiological levels of testosterone. This approach has demonstrated safety, improved quality of life, and met primary endpoints for efficacy in clinical trials; however, novel strategies to enhance and optimize the anti-tumor efficacy of this approach are needed. Methods: We performed high-throughput drug screening in PCa cell lines to identify agents showing synergy with high-dose androgen treatment. Results: The Survivin inhibitor YM155, demonstrated a potent synergy with high-dose androgen. YM155 IC50 values shifted from 24 nM without R1881 to 3.85 nM with 160 pM R1881.Using qRT-PCT and ChIP-qPCR we establish the mechanism of this synergy was due to the direct upregulation of YM155 transporter SLC35F2 by androgen receptor transcriptional activity. Knockdown and overexpression of SLC35F2 dramatically modulated sensitivity to YM155. In two xenograft models SLC35F2 and tumor DHT levels are highly correlated and castration-induced decreases in tumor androgens cause an identical decline in levels of SLC35F2. In CRPC metastases SLC35F2 expression significantly correlated with AR activity score (r = 0.62, p < 0.001). However, ~1/3 of tumors examined had transcripts for AR and SLC35F2, 1/3 AR+ but SLC35F2 negative, and 1/3 are negative for both. Conclusions: We describe a novel synergy between YM155 and high-dose androgen therapy mediated by the direct upregulation of YM155 transporter SLC35F2 by androgen receptor and implicate SLC35F2 expression as a potential biomarker for YM155 sensitivity. Whether ligand independent AR variants (AR-Vs) can constitutively mediate SLC35F2 expression is unknown, but if so would also provide a means of stratifying identifying men likely to respond to YM155.This study demonstrates a novel paradigm for precision medicine in the pharmacologic induction of drug sensitivity in a tissue restricted manner. Hormonal induction of carrier-mediated drug transport is a completely novel and unexplored concept that is poised to exploit androgen-based treatment strategies.