Keytruda (pembrolizumab) does not have a prostate cancer-specific FDA approval, but it can be used for prostate cancer under two broader, tissue-agnostic approvals that cover all solid tumors, including prostate. The catch: your tumor must have specific genetic features to qualify. Only a small percentage of prostate cancers do.
How Keytruda Qualifies for Prostate Cancer
The FDA has approved Keytruda for all solid tumors, regardless of where they started, as long as the tumor meets one of two biomarker criteria. The first approval, granted in May 2017, covers tumors that are microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR). These terms describe tumors whose DNA repair systems are broken, leading to a buildup of genetic errors that makes them more visible to the immune system. The second approval, from June 2020, covers tumors with a high tumor mutational burden (TMB-H), defined as 10 or more mutations per megabase of DNA.
Both approvals require that the cancer is unresectable or metastatic, has progressed after prior treatment, and that no satisfactory alternative treatment options remain. In practice, this means Keytruda is typically considered late in the treatment sequence for prostate cancer, after hormone therapies and chemotherapy have already been tried.
The challenge is that these biomarkers are uncommon in prostate cancer. MSI-H/dMMR occurs in roughly 2 to 5 percent of advanced prostate cancers, and high TMB is similarly rare. So while Keytruda is technically available for prostate cancer, most patients won’t have tumors that qualify.
Testing Required Before Treatment
To find out whether your prostate cancer qualifies, your oncologist will need to order genomic profiling of your tumor. The FDA has approved specific companion diagnostic tests for this purpose. FoundationOne CDx, a next-generation sequencing test, can identify both MSI-H status and TMB-H status from a tissue biopsy. A newer test, MI Cancer Seek from Caris Life Sciences, was approved in late 2024 specifically for detecting MSI-H in solid tumors.
These tests analyze a sample of your tumor tissue (typically from a biopsy you’ve already had). Results usually take a couple of weeks. If neither MSI-H/dMMR nor TMB-H is found, Keytruda would not be an approved option.
Where Guidelines Place Keytruda
The National Comprehensive Cancer Network (NCCN), which sets widely followed treatment guidelines, supports using Keytruda in patients with metastatic castration-resistant prostate cancer (mCRPC) whose tumors are MSI-H or dMMR, and whose disease has progressed through both docetaxel chemotherapy and a newer hormone therapy like enzalutamide or abiraterone. For patients with TMB of 10 or more mutations per megabase, the NCCN also includes Keytruda as an option, though this recommendation is based partly on extrapolation from results in other cancer types rather than large prostate-specific trials.
What Clinical Trials Show
The evidence for Keytruda in prostate cancer specifically, without biomarker selection, is modest. The KEYNOTE-199 trial tested Keytruda in men with heavily treated metastatic castration-resistant prostate cancer regardless of biomarker status. The objective response rate was about 5 percent in one group and 3 percent in another. Those numbers are low, but for the small number of patients who did respond, the results were durable: median response duration reached nearly 17 months in the combined group, and some responses lasted over 21 months.
Larger phase 3 trials attempted to show that adding Keytruda to standard treatments could benefit a broader prostate cancer population. None succeeded. KEYNOTE-921 combined Keytruda with docetaxel chemotherapy in mCRPC patients. Median overall survival was 19.6 months with the combination versus 19.0 months with chemotherapy alone, a difference that was not statistically meaningful. KEYNOTE-641 paired Keytruda with enzalutamide in mCRPC and was stopped for futility after the combination failed to improve either progression-free survival or overall survival. KEYNOTE-991 tested the same Keytruda-plus-enzalutamide approach in earlier-stage hormone-sensitive prostate cancer and was also stopped for futility, with the added concern that the combination caused more serious side effects, including higher rates of severe rash.
These negative results are a key reason Keytruda has not received a prostate cancer-specific approval. Other checkpoint inhibitors have fared similarly: a phase 3 trial of atezolizumab plus enzalutamide in mCRPC also showed no survival benefit. Prostate cancer, as a whole, tends to be less responsive to immunotherapy than cancers like melanoma or lung cancer, likely because most prostate tumors have fewer mutations and create an environment that suppresses immune activity.
Why Biomarker-Positive Tumors Respond Differently
Keytruda works by blocking a protein called PD-1 on immune cells. Normally, cancer cells exploit PD-1 to hide from the immune system. Keytruda removes that disguise, allowing your immune cells to recognize and attack the tumor. But this strategy works best when the tumor has lots of genetic mutations, because mutations produce abnormal proteins that the immune system can target. MSI-H and TMB-H tumors have exactly this feature: they’re riddled with mutations that make them stand out to immune cells. That’s why the small subset of prostate cancers with these biomarkers can respond well to Keytruda, even though the drug shows little benefit for the broader prostate cancer population.
Side Effects in Prostate Cancer Patients
Keytruda’s side effects stem from its immune-boosting mechanism. When the immune system becomes more active against cancer, it can also become more active against healthy tissue. In prostate cancer trials, the most notable immune-related side effects included thyroid problems (particularly an underactive thyroid) and skin reactions like rash and hives. In the combination trials with enzalutamide, rash was a particular concern, occurring in up to 22 percent of patients in one study, with some cases being severe.
Keytruda is given as a 30-minute intravenous infusion every three weeks for up to two years, assuming the cancer doesn’t progress and side effects remain manageable. If you’re one of the small number of prostate cancer patients whose tumor carries MSI-H, dMMR, or TMB-H biomarkers, this treatment represents a real option after other therapies have stopped working. For everyone else, the current evidence doesn’t support its use outside of a clinical trial.