PRIMARY OBJECTIVES:~I. To evaluate the efficacy of pembrolizumab in patients who have received B- cell maturation antigen (BCMA)-directed adoptive cell therapy (ACT) and have clinical evidence of progression.~II. To obtain anti-tumor activity (best response rates: objective response rate [ORR], very good partial response, [VGPR], complete response [CR], stringent complete remission [sCR], minimal response disease [MRD] negativity) in patients treated with pembrolizumab.~SECONDARY OBJECTIVES:~I. To evaluate the expansion of engrafted T cells following pembrolizumab administration in the peripheral blood and within the tumor microenvironment.~II. To evaluate the phenotype and function of engrafted T cells following pembrolizumab administration.~III. Progression free survival (PFS) and overall survival (OS) among patients progressing after ACT that received pembrolizumab.~IV. To determine immunogenicity of the salvage regimen.~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.~After completion of study treatment, patients are followed up every 3 months.
This phase II trial studies the effect of pembrolizumab in treating patients with multiple myeloma that is growing, spreading, or getting worse (progressing) on chimeric antigen receptor (CAR)-T cell therapy. Immunotherapy with monoclonal antibodies, such as pembrolizumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread.