PRIMARY OBJECTIVE:~I. To evaluate the overall response rate (ORR) of pembrolizumab in patients with relapsed or refractory multiple myeloma (MM) (RRMM) after anti-B-cell receptor maturation antigen (BCMA) chimeric antigen receptor T-cell (CAR-T) therapies.~SECONDARY OBJECTIVES:~I. To evaluate the safety and tolerability of pembrolizumab in participants who relapsed or were refractory to anti-BCMA CAR-T therapies.~II. To evaluate the depth of response for patients treated with pembrolizumab who had previously relapsed after or were refractory to anti-BCMA CAR-T therapies.~III. To evaluate time-to-event outcomes, including time-to-next treatment (TNT), progression-free survival (PFS), and overall survival (OS) after initiation of pembrolizumab therapy.~EXPLORATORY OBJECTIVE:~I. To explore the immune profile after pembrolizumab treatment, including changes in absolute lymphocyte count (ALC) and lymphocyte subsets by flow-cytometry.~OUTLINE:~Patients receive pembrolizumab intravenously (IV) over 30 minutes on day 1. Cycles repeat every 21 days for up to 2 years in the absence of disease progression or unacceptable toxicity.~Participants will be followed for at least 30 days after last treatment or removal from study, or until death. Participants who have completed 35 cycles of therapy or discontinued treatment for reasons other than disease progression will be followed for up to 1-year, until removal from study, start of new anti-cancer therapy, or death.
This phase II trial tests whether pembrolizumab works to shrink tumors in patients with multiple myeloma whose cancer has come back (relapsed) or did not respond to previous treatment (refractory) with anti-BCMA CAR-T therapies. Immunotherapy with pembrolizumab, may induce changes in body's immune system and may interfere with the ability of cancer cells to grow and spread.