Complete Response to Pembrolizumab in a Metastatic, Castration‑Resistant Prostate Cancer Survivor

A remarkable case published in Annals of Internal Medicine: Clinical Cases highlights a man with metastatic, castration‑resistant prostate cancer who achieved a complete and durable response to pembrolizumab after exhausting multiple standard therapies. The patient had previously received abiraterone‑based treatment followed by chemotherapy, and then olaparib, all of which he had progressed on, leaving him with few established options. At that point, however, his tumor was found to harbor a particularly sensitive molecular profile: microsatellite instability–high (MSI‑H), mismatch‑repair deficient (dMMR), high tumor mutational burden (TMB‑H), and a BRCA2 alteration. These features are known to predict responsiveness to PD‑1 inhibitors such as pembrolizumab, but complete responses in prostate cancer remain rare.

Pembrolizumab was started as monotherapy, and the result was striking. The patient achieved a complete response by both imaging and PSA, with no detectable disease on follow‑up scans and an undetectable PSA that remained suppressed for more than four years. This prolonged remission occurred despite the fact that he began treatment with clear metastatic disease in the bone and lymph nodes, underscoring that even advanced metastatic prostate cancer can, in selected molecular subgroups, behave more like a highly immunoresponsive cancer. The case did not report a formal overall survival number as the patient was described as a long‑term responder still alive and progression‑free well over four years from the start of pembrolizumab.

From a clinical perspective, this case reinforces the importance of broad molecular profiling in men with advanced prostate cancer, especially once they have progressed on standard androgen‑axis therapies, chemotherapy, and PARP inhibitors. While MSI‑H/dMMR and high TMB are uncommon in prostate cancer, identifying them can open the door to immunotherapy that may lead to deep, durable responses in a small subset of patients.

NOTE: To be intellectually correct, in practical terms, the probability that all those features line up in a single prostate‑cancer patient is well under 1% of all advanced prostate cancers, making this case a rare but biologically informative outlier rather than a common scenario.

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