Enzalutamide is a potent second-generation androgen receptor (AR) antagonist with activity in metastatic castrate-resistant prostate cancer (CRPC). Although enzalutamide is initially effective, disease progression inevitably ensues with the emergence of resistance. Intratumoral hypoxia is also associated with CRPC progression and treatment resistance. Given that both AR and hypoxia inducible factor-1 a (HIF-1a) are key regulators of these processes, dual targeting of both signaling axes represents an attractive therapeutic approach. Crosstalk of the AR and HIF-1a signaling pathways were examined in prostate cancer cell lines (LNCaP, 22Rv1) with assays measuring the effect of androgen and hypoxia on AR-dependent and hypoxia-inducible gene transcription, protein expression, cell proliferation, and apoptosis. HIF-1a inhibition was achieved by siRNA silencing HIF-1a or via chetomin, a disruptor of HIF-1a-p300 interactions. In prostate cancer cells, the gene expression of AR targets (KLK3, FKBP5, TMPRSS2) was repressed by HIFsignaling; conversely, specific HIF-1a target expression was induced by dihydrotestosterone-mediated AR signaling. Treatment of CRPC cells with enzalutamide or HIF-1a inhibition attenuated AR-regulated and HIF-1a-mediated gene transcription. The combination of enzalutamide and HIF-1a inhibition was more effective than either treatment alone. Similarly, the combination also reduced vascular endothelial growth factor protein levels. HIF-1a siRNA synergistically enhanced the inhibitory effect of enzalutamide on cell growth in LNCaP and enzalutamide-resistant 22Rv1 cells via increased enzalutamide-induced apoptosis. In conclusion, the combination of enzalutamide with HIF-1a inhibition resulted in synergistic inhibition of AR-dependent and genespecific HIF-dependent expression and prostate cancer cell growth.

Castration resistant prostate cancer (CRPC) has greater intratumoral testosterone concentrations than similar tumors from eugonadal men; simple diffusion does not account for this observation. The present study was undertaken to ascertain the androgen uptake kinetics, functional, and clinical relevance of de novo expression of the steroid hormone transporter OATP1B3 (SLCO1B3). Experiments testing the cellular uptake of androgens suggest that testosterone is an excellent substrate of OATP1B3 (KM=23.2μM; VMAX=321.6pmol/mg/min), and cells expressing a doxycycline-inducible SLCO1B3 construct had greater uptake of a clinically relevant concentration of 3H-testosterone (50nM; 1.6-fold, P=0.0027). When compared to Slco1b2 (−/−) mice, Slco1b2 (−/−)/hSLCO1B3 knockins had greater hepatic uptake (15% greater AUC, P=0.0040) and lower plasma exposure to 3H-testosterone (17% lower AUC, P=0.0030). Of 82 transporters genes, SLCO1B3 is the second-most differentially-expressed transporter in CRPC cell lines (116-fold vs androgen sensitive cells), with a differentially-spliced cancer-type ct-SLCO1B3 making up the majority of SLCO1B3 expression. Overexpression of SLCO1B3 in androgen responsive cells results in 1.5- to 2-fold greater testosterone uptake whereas siRNA knockdown of SLCO1B3 in CRPC cells did not change intracellular testosterone concentration. Primary human prostate tumors express SLCO1B3 to a greater extent than ct-SLCO1B3 (26% of total SLCO1B3 expression vs 0.08%), suggesting that androgen uptake in these tumor cells also is greater. Non-liver tumors do not differentially express SLCO1B3.

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270 Background: Enzalutamide is a potent second-generation androgen receptor (AR) antagonist with activity in metastatic castrate-resistant prostate cancer (CRPC). While enzalutamide is initially effective, disease progression inevitably ensues with the emergence of resistance. Intratumoral hypoxia is also associated with CRPC progression and treatment resistance. Given that both AR and HIF-1α are key regulators of these processes, dual targeting of both signaling axis represents an attractive therapeutic approach. Methods: Crosstalk of the AR and HIF-1α signaling pathways were examined in prostate cancer cell lines (LNCaP, 22Rv1) with assays measuring the effect of androgen and hypoxia on AR-dependent and hypoxia-inducible gene transcription, protein expression, cell proliferation, and apoptosis. Cells were stimulated with dihydrotestosterone (DHT) or the hypoxia mimetic cobalt chloride. HIF-1α inhibition was achieved by siRNA silencing HIF-1α or via chetomin, a disruptor of HIF-1α-p300 interactions. Results: In prostate cancer cells, gene expressions of AR targets (KLK3, FKBP5, TMPRSS2) were repressed by HIF-signaling; conversely, HIF-1α target expressions (VEGF, ENO1, LDHA) were induced by DHT-mediated AR signaling. Treatment of CRPC cells with enzalutamide and chetomin or HIF-1α siRNA attenuated AR-regulated and HIF-1α-mediated gene transcription. The combination of enzalutamide and HIF-1α inhibition was more effective than either treatment alone. Similarly, the combination also reduced VEGF protein levels. HIF-1α siRNA synergistically enhanced the inhibitory effect of enzalutamide on cell growth in LNCaP and enzalutamide-resistant 22Rv1 cells. Additionally, HIF-1α siRNA synergistically increased enzalutamide-induced apoptosis. Conclusions: Combination of enzalutamide with HIF-1α inhibition resulted in synergistic inhibition of AR-dependent and gene specific HIF-dependent expression, prostate cancer cell growth, and apoptosis.