The effect of pembrolizumab in combination with CD19-targeted chimeric antigen receptor (CAR) T cells in relapsed acute lymphoblastic leukemia (ALL).

103Background: CD19-targeted CAR T cells show CR rates of 70-95% in B-ALL. Yet a subset of patients do not respond or relapse due to poor CAR T cell expansion and persistence. We hypothesized that PD-1 checkpoint pathway inhibition may improve CAR T cell expansion, function and persistence. Methods: Four children with relapsed B-ALL treated with murine (CTL019) or humanized (CTL119) anti- CD19CAR T cells received 1-3 doses of the PD-1 inhibitor pembrolizumab(PEM) for partial/no response or prior history of poor CAR T cell persistence starting 14d-2mo post CAR T cell infusion. Results: PEM increased and/or prolonged detection of circulating CAR T cells in all 4 children, with objective responses in 2/4. It was well tolerated, with fever in 2 pts and no autoimmune toxicity. Pts 1-3 received CTL119 for CD19+ relapse after prior murine CD19CAR T cells. Pt 1 had 1.2% CD19+ residual disease despite expansion with detectable CTL119 by D28 and received PEM at 2mo for progressive diseasewith decreasing circulat...