Abstract 1817: A novel non-secosteroidal VDR agonist, VDRM2, inhibits prostate tumor growth

The role of vitamin D signaling in prostate cancer is controversial. Some studies find an inverse correlation between levels of vitamin D metabolites and PCa risk, but others do not. The active metabolite 1α,25-dihydroxyvitamin D3 (1,25D), a ligand for the vitamin D receptor (VDR), inhibits the growth of PCa cells in vitro. 1,25D or less calcemic analogs also inhibit growth in some pre-clinical models, but clinical trials have been disappointing. A majority of prostate cancers contain a genomic rearrangement that links the androgen and 1,25D regulated TMPRSS2 promoter to the coding region of an ETS transcription factor, typically ERG, termed T/E. VCaP cells are the only commonly used cell line that contains the T/E fusion. We have shown previously that VDR agonists inhibit VCaP cell growth in vitro despite inducing T/E. However, EB1089, a 1,25D analog, which inhibited growth of LNCaP xenografts, had no effect on VCaP xenografts. VDR and ERG cooperate to hyper-induce the vitamin D metabolizing enzyme, CYP24A1. Thus, T/E may limit VDR signaling through inactivation of 1,25D. To counteract the induction of CYP24A1 in VCaP cells, we tested a novel non-secosteroidal VDR agonist, VDRM2, developed by Eli Lilly & Co. VDRM2 inhibits LNCaP and VCaP cell growth and is resistant to metabolism. Using a subcutaneous VCaP xenograft model, we saw a significant reduction in tumor volume and tumor mass in mice treated with 3 ug/kg of VDRM2 compared to vehicle treated animals. We have shown for the first time a VDR agonist inhibits growth of a T/E expressing prostate cancer cell line in vivo and we achieved this effect without causing hypercalcemia as indicated through serum calcium levels. To gain a better understanding of the differences in biology of 1,25D and VDRM2 as well as VDR actions in T/E and non-T/E expressing cells, we used next generation sequencing (RNA-Seq). We compared gene expression in proliferating LNCaP and VCaP cells treated with concentrations of 1,25D and VDRM2 that resulted in similar levels of CYP24A1 gene induction. We had previously found that downregulation of c-Myc was a key factor in VDR mediated growth inhibition. As predicted, we observed significant downregulation of genes enriched in both the Myc and E2F pathways indicating our dataset recapitulates our current understanding of VDR actions in prostate cancer. There was substantial overlap between the 1,25D and VDRM2 mediated genes in each cell line, but substantial differences between the two cell lines, some of which is likely due to induction of T/E and its downstream effects in VCaP cells. Our study demonstrates a positive role for the use of VDR agonists in the treatment of T/E containing prostate cancer. Citation Format: Justin Roberts, James MacKrell, D B. Piyarathna, Gary Krishnan, Cristian Coarfa, David Rowley, Nancy L. Weigel. A novel non-secosteroidal VDR agonist, VDRM2, inhibits prostate tumor growth. [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 1817.