Vitamin D Receptor Agonists, Cancer and the Immune System: An Intricate Relationship

Vitamin D receptor (VDR) agonists can inhibit cell growth, promote apoptosis, and induce differentiation of many cell types, in addition to inhibiting metastasis and angiogenesis, all desirable properties for a drug to control cancer. However, from an immunological perspective, the immunomodulatory properties of VDR agonists are apparently just opposite to the main aims of cancer immunotherapy: boosting the immune response and breaking tumor-related tolerance. While it may be possible to identify VDR agonists with enhanced anti-proliferative/pro-differentiative and reduced immunomodulatory activities as anti-cancer agents, a complementary approach could rely on identifying clinical indications where their systemic immunomodulatory properties could be minimized. Superficial bladder cancer, where treatments are usually administered by vesical instillation, may represent such an indication. We have observed a strong synergism in vitro between calcitriol and doxorubicin or epirubicin in the inhibition of bladder cancer cell proliferation. Thus, calcitriol and doxorubicin or epirubicin in combination may have clinical value in the management of superficial bladder cancer.