Keytruda has been FDA-Approved on a Limited Basis

Bestbird

KEYTRUDA (Pembrolizumab) has been approved for patients with metastatic, microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) solid tumors that have progressed following prior treatment and who have no satisfactory alternative treatment options.The FDA based the approval in part on data from five uncontrolled, multicohort, multicenter, single-arm clinical trials designed to evaluate KEYTRUDA in 149 patients — including 90 patients with colorectal cancer and 59 patients diagnosed with one of 14 other malignancies. Researchers reported an objective response rate of 39.6% (95% CI, 31.7-47.9) — including 11 complete responses and 48 partial responses — which appeared similar irrespective of malignancy (colorectal cancer, 36%; other cancer type, 46%). Seventy-eight percent of responding patients had responses that lasted for 6 months or longer.From: https://www.fda.gov/Drugs/InformationOnDrugs/ApprovedDrugs/ucm560040.htm

Whereas I do not know much (or anything!) about microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) tumors, it appears that some mbc patients have cancer that fits this profile. So for those who are running through their list of options, this may be worthwhile discussing with their oncologists.