Vitamin D receptor-dependent antitumour effects of 1,25-dihydroxyvitamin D3 and two synthetic analogues in three in vivo models of prostate cancer

Objective To determine the in vitro and in vivo effects of 1,25-dihydroxyvitamin D-3 (calcitriol) and two newer less hypercalcaemic analogues, EB1089 and CB1093 (as the use of calcitriol as a therapeutic agent in humans has been limited by hypercalcaemia) in three rodent models of prostate cancer. Materials and methods The highly metastatic MAT LyLu Dunning prostate model, PAIII tumours in Lobund-Wistar rats and LNCaP xenografts in nude mice were used. Vitamin D receptor (VDR) expression and binding were assessed in all cell lines. The effects of calcitriol, EB1089 and CB1093 on tumour growth, cell cycle and angiogenesis in vitro, and growth and serum calcium levels in vivo, were assessed. Results The growth of prostate adenocarcinoma was inhibited by calcitriol, EB1089 and CB1093 in the Dunning prostate model. Although both analogues increased serum calcium levels, the levels were significantly less than in rats treated with calcitriol. Tumour growth was also inhibited in male athymic nu/nu mice with LNCaP tumour xenografts. PAIII cells failed to express functional VDR and were insensitive to calcitriol and its analogues, either in vitro or in vivo. The analogues of calcitriol did not inhibit angiogenesis in a rat aorta assay. Conclusion This is the first report comparing the actions of calcitriol and its analogues in different in vivo models. The results suggest that the newer less hypercalcaemic analogues of calcitriol may offer a novel therapeutic option for treating prostate cancer. VDR-dependent growth inhibition and not the inhibition of angiogenesis is the main mechanism of action of these compounds in vivo.