Introduction: Phase II trials have shown activity with pembrolizumab against prostate cancer. However, the clinical factors predictive of a response to pembrolizumab in men with prostate cancer are unknown.
Patients and methods: A total of 54 consecutive men with progressive, recurrent, or advanced prostate cancer were treated with 1 to 12 cycles of pembrolizumab 200 mg every 3 weeks with or without stereotactic body radiotherapy (SBRT).
Results: For the 31 men evaluable for response, the median age, prostate-specific antigen (PSA) level, and Gleason score were 75 years, 30 ng/mL, and 8 (4 + 4), respectively, which were similar to those for the 23 nonevaluable patients. The treatments received before pembrolizumab were enzalutamide in 26, abiraterone in 18, and sipuleucel-T in 23. All but 4 men had had castrate-resistant disease. Of the 54 men, 31 had completed ≥ 4 cycles of pembrolizumab and were evaluable for the response. Ten men had undergone SBRT to an isolated metastasis shortly before or during pembrolizumab treatment, with the goal of inducing an abscopal effect. The clinical characteristics of the 17 men with a response or stable disease were compared with those of the 14 men with progressive disease. Grade ≥ 2 toxicity occurred in 16 men (30%). PSA stabilization or a response occurred in slightly more than one half (55%) of the men treated with ≥ 4 cycles of pembrolizumab. Five patients had a notable PSA decline of > 50%, which were sustained as long as they had continued receiving pembrolizumab. A PSA response or stabilization was more common for men who had begun taking pembrolizumab with a lower PSA level, fewer bone metastases, and fewer mutations and without previous chemotherapy. A statistically nonsignificant trend toward stabilization or a response was observed in men who had undergone concomitant SBRT.
Conclusion: Pembrolizumab showed modest anticancer activity against metastatic castrate-resistant prostate cancer. A PSA response or stabilization occurred more frequently in men with less-advanced disease.
Keywords: Castrate resistance; Keytruda; Prognostic factors; Response criteria; Toxicity.
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