Calcitriol, the principal active metabolite of vitamin D and a naturally occurring hormone, showed significant antineoplastic activity in pre-clinical models of prostate cancer and many other tumor types. These antineoplastic effects were observed at calcitriol concentrations substantially above the physiological range. While a number of mechanisms of action have been postulated, the induction of apoptosis and inhibition of proliferation have been most extensively reported. These pre-clinical findings motivated several investigators to pursue a series of clinical trials to examine the potential of targeting the vitamin D receptor for cancer treatment using calcitriol. Initial studies tested daily dosing of calcitriol and showed that substantial dose escalation was not feasible due to hypercalciuria and/or hypercalcemia. In contrast, weekly dosing of calcitriol allowed substantial dose escalation without dose-limiting toxicities. Notably, however, the commercially available formulation of calcitriol exhibited nonlinear pharmacokinetics at the highest doses tested. While substantially higher concentrations were achieved, the maximum tolerated dose was not established due to this pharmacological limitation. Intermittently-dosed calcitriol was then combined with several antineoplastic agents, including steroids, bisphosphonates and chemotherapeutic agents. The activity seen in a phase II study of weekly calcitriol plus docetaxel was particularly encouraging and led to the development of DN-101, a proprietary formulation designed for cancer treatment. DN-101 in combination with docetaxel is being evaluated in a placebo-controlled randomized clinical trial that has completed accrual.