Synergistic potential of sipuleucel-T in enhancing immunotherapy for metastatic castration-resistant prostate cancer

Clinical trials of immunotherapy in metastatic castration-resistant prostate cancer (mCRPC) have largely been unsuccessful despite promising preclinical studies and proven efficacy in other solid tumors. These disappointing clinical outcomes have been attributed to an immunosuppressive tumor microenvironment, a relative lack of infiltrating immune effector cells, and tumor-related and host-related factors, which collectively render prostate cancer a relatively immunologically "cold" tumor. Sipuleucel-T (Provenge), an autologous cellular immunotherapy, induces an immune response targeted against prostatic acid phosphatase. It received approval from the US Food and Drug Administration in 2010, marking the first immunotherapy to show an overall survival benefit in patients with mCRPC in large phase III randomized trials. Unfortunately, subsequent immunotherapy-based strategies have been less efficacious in mCRPC relative to other tumor types. Given the use of sipuleucel-T as a standard of care backbone, there is emerging interest in combining it with other immunotherapies, hormonal therapies, or chemotherapies to improve its clinical efficacy. This review summarizes past experiences and current knowledge of combining sipuleucel-T with other treatments and explores future approaches to enhance such combinatorial strategies.