Immunotherapy in prostate cancer: review of the current evidence

Prostate cancer is the most common male malignancy in the Western world. Once it metastasizes, it is incurable. The current gold standard for metastatic disease is the combined docetaxel/prednisone regimen. Prostate cancer shows several characteristics that make it a suitable candidate for immunotherapy, as recently exemplified by the approval of sipuleucel-T, the first vaccine to treat any malignancy. Here, we review different tumor-associated antigen immunotherapy strategies currently being investigated, from a humanized radiolabeled monoclonal antibody (J-591) that targets radiation into tumor cells, moving on to vaccines and through to immunomodulator agents such as anti-CPLA-4 and anti-PD-1 monoclonal antibodies that activate T-cell responses via immune checkpoint inhibition. We explore different opinions on the best approach to integrate immunotherapy into existing standard therapies, such as androgen-deprivation therapy, radiotherapy or chemotherapy, and review different combination sequences, patient types and time points during the course of the disease to achieve a lasting immune response. We present data from recent phase III clinical trials that call for a change in trial endpoint design with immunotherapy agents, from the traditional tumor progression to overall survival and how such trials should include immune response measurements as secondary or intermediate endpoints to help identify patient clinical benefit in the earlier phases of treatment. Finally, we join in the recent questioning on the validity of RECIST criteria to measure response to immunotherapeutic agents, as initial increases in the size of tumors/lymph nodes, which are part of a normal immune response, could be categorized as disease progression under RECIST.