Abstract 2671: 17beta-hydroxywithanolides inhibit the proliferation of castration-resistant prostate cancer cells by reducing levels of cFLIP

Using a high throughput gene expression profiling assay directly targeting genes of the androgen receptor pathway, we previously identified the 17beta-hydroxywithanolide (17-BHW), physachenolide C (PCC), as a potent inhibitor of prostate cancer cell growth. Using a panel of prostate cancer cell lines, PCC at low concentrations (IC 50 20-50 nM) reduced prostate cancer cell numbers under both standard and 3D in vitro growth conditions. This reduction in cell number was due to growth inhibition and not apoptosis, with PCC causing reductions in the levels of cyclin D, an increase in p21 and arrest of cells in the G1 phase of the cell cycle. Subsequently over 150 natural and semi-synthetic withanolides were evaluated for their ability to inhibit prostate cancer cell growth. All highly active withanolides were 17-BHWs, and some were up to 4-fold more active than PCC. Preliminary structure-activity relationship (SAR) studies suggested that the enone moiety in ring A was essential for activity. In addition, acetoxylation at C-18, an alpha orientation of the side-chain lactone group and the double bond at C-24(25) of the lactone ring played important roles in determining the activity of 17-BHWs as inhibitors of prostate cancer cell growth. Interestingly highly active 17-BHWs rapidly reduced levels of both antiapoptotic cFLIP L and cFLIP S proteins in 22Rv1 prostate cancer cells. However, this reduction did not sensitize 22Rv1 cells to apoptosis. Nonetheless specific reduction of cFLIP L in the castration-resistant 22Rv1 cells using siRNA significantly inhibited their proliferation. This suggests that cFLIP L plays a critical role in the proliferation of some castration-resistant prostate cancer cells. Since the 17-BHW scaffold is amenable to optimization by a medicinal chemistry approach, this could lead to the identification of highly active natural product-based inhibitors of castration-resistant prostate cancer cell proliferation. The cellular molecular target(s) of active 17-BHWs that promote cFLIP reduction are currently under further investigation. Funded by FNLCR Contract HHSN261200800001E and the University of Arizona. Citation Format: Alan D. Brooks, Nicola E. Wright, Ya-ming Xu, Kithsiri Wijeratne, Poonam Tewary, Neil Cross, A. A. Leslie Gunatilaka, Thomas J. Sayers. 17beta-hydroxywithanolides inhibit the proliferation of castration-resistant prostate cancer cells by reducing levels of cFLIP [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2671.