Androgen ablation is the primary therapy for metastatic prostate cancer (PC). However, within 3 years progression towards castration resistant PC (CRPC) will occur. Despite low serum levels of testosterone (T) in patients under hormonal therapy, the androgen receptor (AR) remains active, indicating its remaining role in CPRC growth regulation. Previously, we demonstrated that conversion of adrenal androgens (AA) into T, rather than intratumoral de novo steroidogenesis, is the major source for AR activation in CRPC patients. AA are synthesized from precursor hormones, progesterone (Prog) and pregnenolone (Preg) by the enzyme CYP17A1. The CYP17A1 inhibitor Abiraterone (Abi) has shown to increase survival in CRPC patients even post-docetaxel. Blocking CYP17A1, however, may lead to the accumulation of precursor hormones such as Preg and Prog that have the potential to activate (over expressed) AR. In this preclinical study, we tested if Abi is able to block growth of CRPC cell lines and whether resulting accumulation of Preg and Prog can sustain cell growth.

Methods. CRPC clones were generated by long term culture of VCaP and DuCaP cell lines in steroid-stripped medium (DCC), with or without anti-androgens. Experiments using a subset of AR-overexpressing CRPC clones were done in DCC with the addition of Preg, Prog, DHT and/or Abi. Cell proliferation was assessed by MTT-assay. mRNA levels of AR target genes (PSA, FKB5) were assessed by RT-PCR. To study AR translocation HEP3B cells with GFP-tagged AR were used.

Results. Prog (100 nM) but not Preg (100 nM) induced cell growth in VCaP and DuCaP CRPC clones, which could not be blocked by levels of Abi (0.1 μM) known to inhibit CYP17A1 activity. Higher levels of Abi (>5 μM) inhibited cell growth. This finding and the observation that DHT-induced growth was inhibited by high levels of Abi indicate that Abi also acts as an anti-androgen. Expression of AR regulated genes was induced by 100 nM Prog, although not to the extent of DHT, and gene expression was again inhibited by higher levels of Abi (>5 μM). Finally, high levels of Prog (100 nM) induced AR translocation in eGFP-AR-HEP3B cells, suggesting that these levels of Prog can drive AR-regulated growth of VCaP and DuCaP CRPC clones. High levels of Abi (5> μM) slowed down R1881-induced AR translocation in eGFP-AR-HEP3B cells, further validating its additional property as an anti-androgen.

Conclusion. These data show that Prog, at 100 nM can activate AR driven cell growth in AR overexpressing CRPC in vitro. However, the direct anti androgen activity of abi at exposures achieved in patients may counter the precursor hormone levels induced by its CYP17 inhibitory activity. Thus Abi may interfere with prostate cancer growth at two levels CYP17 inhibition as well as direct AR inhibition.

Citation Format: Matthijs J. Moll, Robert J. van Soest, Jinpei Kumagai, Ralph Graeser, Ian Hickson, Guido W. Jenster, Wytske M. van Weerden. Dual activity of abiraterone: CYP17 mediated androgen ablation and direct AR inhibition. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4770. doi:10.1158/1538-7445.AM2013-4770