Retinoids and retinoic acid metabolism blocking agents in combination with histone deacetylase inhibitors for prostate cancer therapy

Abstract

All-trans-retinoic acid (ATRA), a metabolite of vitamin A, is 5 to 8 times lower in prostate carcinoma tissue when compared to a normal prostate and benign prostatic hyperplasia. ATRA plays a major role in a number of biological processes and is metabolized by cytochrome P450 enzymes into inactive polar metabolites. The anticancer effects of a novel class of agents called atypical retinoic acid metabolism blocking agents (RAMBAs) were examined in combination with histone deacetylase inhibitors (HDACIs). The purpose of this study was to select combinations of agents which would have potent anti-cancer activities in a hormone insensitive prostate cancer model. One particular RAMBA, VN/66-1 which displays favorable pharmacokinetics and minimal toxicity was found to be highly potent in inhibiting PC-3 cell growth. Two HDACIs, SAHA and MS-275 were also found to be potent in inhibiting PC-3 cell viability. The combinations of VN/66-1 + MS-275 and VN/66-1 + SAHA synergistically inhibited PC-3 cell growth caused cytotoxicity/cytostaticity. Treatment of PC-3 xenografts with VN/66-1 (10 mg/kg/day) + MS-275 (2.5 mg/kg/day) for 18 days resulted in an 85% reduction in final mean tumor volume compared with control. The combination of VN/66-1 + SAHA, however, did not reduce tumor growth as effectively as the two agents individually most likely due to too low doses. The data obtained suggest that the mechanism of action of the combination of VN/66-1 + MS-275 is through apoptosis and DNA damage-induced p21 WAF1/CIP1 activation. Active p21 is involved in the inhibition of the cdc2-cyclin B1 complex resulting in eventual G2/M phase arrest. This induction of p21 is also in part due to the enhancement of acetylation of histones H3 and H4 which is also responsible for the activation of tumor suppressor gene RARbeta2, the latter which also plays a role in mediating cell death. Up-regulation of Bad and down-regulation of Bcl-2 as well as PARP cleavage indicated that apoptosis via the intrinsic pathway is also contributing to cell death. Together, these results suggest that VN/66-1 or its combination with MS-275 may be a novel therapy for the treatment of hormone refractory prostate carcinoma.