Selective activity of deguelin identifies therapeutic targets for androgen receptor-positive breast cancer

Triple-negative breast cancers (TNBC) are aggressive malignancies with no effective targeted therapies. Recent gene expression profiling of these heterogeneous cancers and the classification of cell line models now allows for the identification of compounds with selective activities against molecular subtypes of TNBC. The natural product deguelin was found to have selective activity against MDA-MB-453 and SUM-185PE cell lines, which both model the luminal androgen receptor (LAR) subtype of TNBC. Deguelin potently inhibited proliferation of these cells with GI(50) values of 30 and 61 nM, in MDA-MB-453 and SUM-185PE cells, respectively. Deguelin had exceptionally high selectivity, 197 to 566-fold, for these cell lines compared to cell lines representing other TNBC subtypes. Deguelin's mechanisms of action were investigated to determine how it produced these potent and selective effects. Our results show that deguelin has dual activities, inhibiting PI3K/Akt/mTOR signaling, and decreasing androgen receptor levels and nuclear localization. Based on these data, we hypothesized that the combination of the mTOR inhibitor rapamycin and the antiandrogen enzalutamide would have efficacy in LAR models. Rapamycin and enzalutamide showed additive effects in MDA-MB-453 cells, and both drugs had potent antitumor efficacy in a LAR xenograft model. These results suggest that the combination of antiandrogens and mTOR inhibitors might be an effective strategy for the treatment of androgen receptor-expressing TNBC.