Toward Maximizing Immunotherapy in Metastatic Castration-Resistant Prostate Cancer – Rationale for Combinatorial Approaches Using Chemotherapy

Prostate cancer is particularly suited for active immunotherapy because of the expression of a distinctive number of antigens which are overexpressed on prostate cancer cells and cell lines. There is evidence in this disease that tumors promote immune tolerance starting early in the disease course. As such, chemotherapy, by suppressing tumors and activating immune system homeostatic mechanisms, may help overcome this tumor-induced immune tolerance. Sipuleucel-T which has recently been approved in the US, is an autologous cellular product immunotherapy that induces immune activity likely through activation of dendritic cells. This was associated with a survival benefit in the absence of significant toxicity. However, a post hoc analysis of phase III trial participants found a substantial survival benefit to receiving docetaxel some months after sipuleucel-T. However, another phase III immunotherapy trial combining a prostate cancer therapeutic vaccine GVAX plus docetaxel versus standard docetaxel therapy in advanced prostate cancer, observed a lower overall survival with the vaccine regimen. These trials highlight major unresolved questions concerning the optimum choice, dosing, and timing of chemotherapy relative to active immunotherapy and the overall merits of considering this approach. The ideal treatment approach remains unclear; advances in biomarker validation and trial design may likely improve our ability to assess biologic benefit irrespective of the development of true antitumor immunity.