When is Keytruda used for prostate cancer?

Medically reviewed by Kristianne Hannemann, PharmD. Last updated on June 23, 2025.

Official Answer by Drugs.com

Keytruda (pembrolizumab) is used only for advanced prostate cancer with specific genetic features: microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) tumors. This immunotherapy is not a standard prostate cancer treatment but is reserved for biomarker-confirmed cases where other options have failed.

Biomarker-Driven Eligibility

Keytruda’s use in prostate cancer depends on the presence of specific genetic markers that make the tumor more likely to respond to immunotherapy. MSI-H/dMMR biomarkers are genetic alterations that impair DNA repair mechanisms, causing tumors to accumulate mutations. This makes them more visible to the immune system. Genetic testing (via tissue biopsy or liquid biopsy) is mandatory to confirm MSI-H/dMMR status before treatment. Only about 3–5% of metastatic prostate cancers exhibit these markers.

Efficacy and FDA Approval

While Keytruda has shown promise in some cancers, its effectiveness in prostate cancer is limited to a small group of patients with unique genetic profiles.

Limited patient subset: In small clinical studies, 34–65% of MSI-H/dMMR prostate cancer patients responded to Keytruda, with some achieving durable remissions. However, these biomarkers are rare in prostate cancer.
Tumor-agnostic approval: The FDA fully approved Keytruda in 2023 for any advanced MSI-H/dMMR solid tumor (including prostate cancer) progressing after prior therapy. This marked the first cancer drug approved based on genetics rather than tumor location.

Dosing and Administration

Keytruda is given as an intravenous (IV) infusion. The typical dose is 200 mg every 3 weeks or 400 mg every 6 weeks. Each infusion takes approximately 30 minutes to complete. Treatment is usually continued until disease progression, unacceptable side effects, or for up to 24 months.

Key Takeaway

Keytruda is not used for typical prostate cancer. It is exclusively for metastatic cases with confirmed MSI-H/dMMR biomarkers after chemotherapy or hormonal therapies fail. Genetic testing is essential to identify eligible patients.

This is not all the information you need to know about Keytruda (pembrolizumab) for safe and effective use and does not take the place of your doctor’s directions. Review the full patient medication guide and discuss this information and any questions you have with your doctor or other health care provider.

References

Altomare, N. J., Li, Y., Neill, C., Hussain, M., & VanderWeele, D. J. (2025). Response to pembrolizumab in advanced prostate cancer with predictive biomarkers. The oncologist, 30(3), oyaf025. https://doi.org/10.1093/oncolo/oyaf025
Barata, P., Agarwal, N., Nussenzveig, R., Gerendash, B., Jaeger, E., Hatton, W., Ledet, E., Lewis, B., Layton, J., Babiker, H., Bryce, A., Garje, R., Stein, C., Kiedrowski, L., Saylor, P., & Sartor, O. (2020). Clinical activity of pembrolizumab in metastatic prostate cancer with microsatellite instability high (MSI-H) detected by circulating tumor DNA. Journal for immunotherapy of cancer, 8(2), e001065. https://doi.org/10.1136/jitc-2020-001065
Hoch, D., Rabaglio, M., Grob, T., von Gunten, M., Beyer, J., & Akhoundova, D. (2023). Exceptional Response to Pembrolizumab in a Mismatch Repair-Deficient Aggressive Prostate Cancer with Somatic EPCAM, MSH2, and MSH6 Co-Deletion: A Case Report. Case reports in oncology, 16(1), 1280–1286. https://doi.org/10.1159/000534177
Keytruda [package insert]. Updated June 2025. Merck Sharp & Dohme LLC. Accessed on June 22, 2025 at https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=9333c79b-d487-4538-a9f0-71b91a02b287
Lenis, A. T., Ravichandran, V., Brown, S., Alam, S. M., Katims, A., Truong, H., Reisz, P. A., Vasselman, S., Nweji, B., Autio, K. A., Morris, M. J., Slovin, S. F., Rathkopf, D., Danila, D., Woo, S., Vargas, H. A., Laudone, V. P., Ehdaie, B., Reuter, V., Arcila, M., … Abida, W. (2024). Microsatellite Instability, Tumor Mutational Burden, and Response to Immune Checkpoint Blockade in Patients with Prostate Cancer. Clinical cancer research : an official journal of the American Association for Cancer Research, 30(17), 3894–3903. https://doi.org/10.1158/1078-0432.CCR-23-3403
Merck.com. FDA Converts to Full Approval Indication for KEYTRUDA® (pembrolizumab) for Certain Adult and Pediatric Patients With Advanced Microsatellite Instability-High (MSI-H) or Mismatch Repair Deficient (dMMR) Solid Tumors. (2023, March 29). Accessed on June 22, 2025 at https://www.merck.com/news/fda-converts-to-full-approval-indication-for-keytruda-pembrolizumab-for-certain-adult-and-pediatric-patients-with-advanced-microsatellite-instability-high-msi-h-or-mismatch-repair-deficient/
Tucker, M. D., Zhu, J., Marin, D., Gupta, R. T., Gupta, S., Berry, W. R., Ramalingam, S., Zhang, T., Harrison, M., Wu, Y., Healy, P., Lisi, S., George, D. J., & Armstrong, A. J. (2019). Pembrolizumab in men with heavily treated metastatic castrate-resistant prostate cancer. Cancer medicine, 8(10), 4644–4655. https://doi.org/10.1002/cam4.2375
Yamamoto, H., Watanabe, Y., Arai, H., Umemoto, K., Tateishi, K., & Sunakawa, Y. (2024). Microsatellite instability: A 2024 update. Cancer science, 115(6), 1738–1748. https://doi.org/10.1111/cas.16160
Zhao, P., Li, L., Jiang, X., & Li, Q. (2019). Mismatch repair deficiency/microsatellite instability-high as a predictor for anti-PD-1/PD-L1 immunotherapy efficacy. Journal of hematology & oncology, 12(1), 54. https://doi.org/10.1186/s13045-019-0738-1

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When is Keytruda used for prostate cancer?

Biomarker-Driven Eligibility

Efficacy and FDA Approval

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Dosing and Administration

Key Takeaway

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