Abstract 334: Novel selective glucocorticoid receptor modulators (SGRMs) in castration-resistant prostate cancer

(A) Introduction: Because of the central role of androgen receptor (AR) signaling in prostate cancer (PC), AR inhibition is the primary modality of initial PC treatment. While AR-targeted therapies are often initially effective in castration-resistant prostate cancer (CRPC), disease progression is common. AR and glucocorticoid receptor (GR) are structurally similar nuclear receptors, bind the same DNA response elements, and regulate common genes. Previous work from our lab demonstrated that GR expression is upregulated following AR antagonism, GR expression decreased the effectiveness of AR blockade, and inhibiting GR activity reversed these effects. However, the only FDA-approved GR antagonist, mifepristone, is not GR-specific and has significant drug-drug interactions; therefore, we report our initial data with a new generation of selective GR-antagonists (SGRMs) for PC treatment using models of CRPC. (B) Experimental Procedures: The LAPC4, CWR-22Rv1 and VCAP PC cell lines were utilized. GR target gene expression analysis was performed at various time-points by qRT-PCR and protein immunoblots subsequent to treatment with AR agonist (R1881, 1nM), AR antagonist (enzalutamide, 10 μM), GR agonist (dexamethasone, 100nM) and the new SGRMs CORT108297 and CORT118335(1μM). Quantification of cell numbers in vitro in response to these treatments was determined by trypan blue exclusion/live cell counting and the CellTiter Glo 2.0 ATP-luciferase assay. For in vivo studies, PC cells were injected subcutaneously into male nude mice and grown to a predetermined tumor size, mice were then castrated, and fifteen days later treated with mifepristone (12mg/kg), SGRM (16mg/kg, 20mg/kg), or vehicle by daily intra-peritoneal injection to assess time to CRPC. (C) Results: AR antagonism with concurrent GR activation resulted in increased expression of canonical AR/GR-target genes and SGRMs (1μM) effectively inhibited expression of glucocorticoid-mediated genes at the RNA and protein levels. SGRMs did not affect AR-mediated gene expression. When AR was antagonized, activated GR promoted cell survival, which was inhibited by SGRM treatment to varying degrees. Preliminary studies support the hypothesis that these novel SGRMs delay CRPC tumor xenograft progression in vivo. (D) Conclusion: In vitro, new generation SGRMs inhibit GR-mediated, but not AR-selective gene expression and result in decreased tumor cell number. SGRM administration to nude mice bearing PC xenografts can delay castration-resistant tumor growth. Therefore, new SGRMs may be an effective therapy for CRPC treatment worthy of further exploration. Studies are ongoing to determine how SGRMs modulate GR target gene expression globally and whether they decrease expression of tumorigenic pro-survival genes in vivo. Citation Format: Jacob Kach, Phillip Selman, Diana C. West, Donald J. Vander Griend, Suzanne D. Conzen, Russell Z. Szmulewitz. Novel selective glucocorticoid receptor modulators (SGRMs) in castration-resistant prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 334.