Beyond Sipuleucel-T: Immune Approaches to Treating Prostate Cancer

At present, sipuleucel-T represents the only approved immunotherapy for prostate cancer. Sipuleucel-T is an autologous cellular therapy, which primes autologous antigen-presenting cells against the prostatic acid phosphatase (PAP) antigen. For patients with metastatic castrate-resistant prostate cancer (CRPC) who are asymptomatic or minimally symptomatic, sipuleucel-T monotherapy is one of the standard of care treatment options pre- or postdocetaxel. With the approval of new treatments, including abiraterone and enzatutamide, sequencing and combination of these treatments with sipuleucel-T represent unanswered questions facing the field. Whereas steroids that are coadministered with abiraterone and chemotherapy have long been thought to be immunosuppressive, early results show that concurrent abiraterone and prednisone does not significantly impact the ability to develop immune responses to this treatment. Additional clinical data are needed to elucidate optimal sequencing of therapeutic agents in CRPC. Several novel immunotherapies are currently in development, and enrollment in clinical trials should be considered. These include PROSTVAC-VF, a viral vaccine that encodes PSA and T-cell costimulatory molecules, which is currently undergoing phase III clinical trials. DNA plasmid-based vaccines targeting different antigens, including PAP, also are under investigation. Immune checkpoint blockade with ipilimumab, a monoclonal antibody against CTLA-4, which is approved for metastatic melanoma, also is being evaluated. Whereas this treatment failed to show significant improvement in overall survival in CRPC patients treated with docetaxel, results from a phase III trial in the predocetaxel setting are pending. Conventional therapies for prostate cancer, such as radiation and hormonal therapy, may have immunomodulatory effects. Future areas for research include the sequencing and combination of immunotherapies as well as other conventional therapies.