In this extended follow up of a single arm phase 2 study, patients who progressed on enzalutamide were given four cycles of standard dose pembrolizumab every three weeks in addition to enzalutamide. Inclusion criteria included any patient with biochemical or radiographic progression on enzalutamide. Prior therapy with sipuleucel-T and abiraterone was permitted. Patients were excluded if they had prior PD-1, PD-L1, or CTLA-4 antibodies or chemotherapy for mCRPC.
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The primary endpoint was a PSA reduction of 50%, and secondary endpoints were objective response, PSA PFS, time to subsequent treatment, and time to death.
As of this abstract submission, 28 patients have been treated on protocol. 5/28 (18%) have had a PSA response ≥ 50% and 3/12 (25%) have had a partial radiographic response. The median overall survival of patients was 22.2 months and median PSA progression-free survival was 28 months. Two patients developed grade 3 colitis, and a number of other grade 3 adverse events occurred in single patients (myelitis, femoral fracture, humeral fracture, fatigue, anemia, hyponatremia, UTI).
Of the 5 patients who responded to pembrolizumab, 4 had prior abiraterone, and 3 had a response to enzalutamide. Only one of three responders who had genomic testing had a DNA repair defect (ATM). Of the non-responders, there were 4 patients with DNA repair defects (ATM, FANCC, CHK2, FANCA).
In conclusion, pembrolizumab in combination with enzalutamide after progression on enzalutamide may produce a durable response in certain patients. The number of patients in this trial was small so it is hard to generalize this data - it may be helpful to conduct a larger trial with more biomarker analysis to hopefully ascertain which factors are predictive of a durable response. Based on the available data, in an unselected population, most patients with mCRPC will not benefit from combination enzalutamide and pembrolizumab after progression on enzalutamide.
1. Bishop JL, Sio A, Angeles A, et al. PD-L1 is highly expressed in Enzalutamide resistant prostate cancer. Oncotarget 2015; 6:234.
Presented By: Julie Nicole Graff, MD, Oregon Health & Science University.
Written By: Jason Zhu, MD. Fellow, Division of Hematology and Oncology, Duke University, Twitter: @TheRealJasonZhu at the 2018 ASCO Annual Meeting - June 1-5, 2018 – Chicago, IL USA