A pilot trial of neoantigen DNA vaccine in combination with nivolumab/ipilimumab and prostvac in metastatic hormone-sensitive prostate cancer (mHSPC).

Abstract

TPS192 Background: Despite robust advances in the use of immune checkpoint blockade (ICB) across multiple cancer types, responses in prostate cancer remain suboptimal. Overall response rates for single-agent ICB in prostate cancer are low, but recent modest gains have been made with ipilimumab and nivolumab combination therapy in metastatic castrate-resistant prostate cancer. Prostvac-VF Tricom is a therapeutic vaccine that incorporates DNA for the shared self-antigen PSA into the vaccinia (or fowlpox) virus strain. A large randomized phase III clinical trial recently showed no improvement in overall survival (OS) with Prostvac compared with placebo, suggesting that a combinatorial approach is warranted. Personalized neoantigen vaccines based on specific mutated epitopes may have the ability to overcome immunoresistance seen with self antigens. Even in low mutational burden tumors like prostate cancer, T cell responses against neoantigens are observed in patients with favorable clinical outcomes, supporting neoantigen vaccination as a promising therapeutic strategy. Improvements in computational genomics and predictive algorithms have allowed the incorporation of MHC class II neoepitopes and those from gene fusion events relevant in prostate cancer. We thus hypothesized that a strategy utilizing both shared antigen (Prostvac) and personalized MHC-I and MHC-II neoantigen vaccines combined with dual ICB would induce robust immune responses and improve clinical outcomes. To maximize tumor burden reduction and minimize tumor-mediated immunoresistence, we are evaluating this novel strategy in patients with metastatic hormone-sensitive prostate cancer (mHSPC) who have completed frontline docetaxel chemotherapy. Methods: This ongoing trial (NCT03532217) began accrual in September 2018. Eighteen of 20 planned patients have been enrolled to date. Eligible patients have histologically confirmed high risk/volume mHSPC defined as 4 or more sites of metastatic disease or visceral involvement. Patients receive continuous androgen deprivation therapy (ADT) and have completed upfront docetaxel chemotherapy. Patients begin treatment with Prostvac-VF in combination with ipilimumab (1 mg/kg every 3 weeks for 2 doses), and nivolumab (3 mg/kg every 3 weeks for 6 doses) within 60 days of the last dose of docetaxel. Subsequently, patients receive nivolumab 480 mg IV every 4 weeks in conjunction with a personalized neoantigen DNA vaccine administered monthly via intramuscular electroporation. The primary objective of this exploratory study is to assess the feasibility, safety/tolerability, and immune responses of this combination strategy. Key secondary objectives include failure-free survival and milestone survival at 2 years, PSA response, and radiographic progression-free survival. Clinical trial information: NCT03532217.