Phase 2 study of pembrolizumab for measurable residual disease in adults with acute lymphoblastic leukemia.

The presence of measurable residual disease (MRD) in acute lymphoblastic leukemia (ALL) confers a poor prognosis. CD19-targeted immunotherapy is effective against MRD but is logistically challenging, potentially toxic, and not applicable to T-cell ALL. We thus hypothesized that inhibition of PD-1 with pembrolizumab could also be effective for MRD, but without lineage restriction. The primary objective of this phase 2 study was to evaluate the efficacy of pembrolizumab in patients with ALL and MRD. Key eligibility criteria included adults with B- or T-cell ALL and MRD detectable by multiparameter flow cytometry or quantitative polymerase chain reaction from bone marrow aspirate (BMA) despite chemotherapy (plus ABL kinase inhibitor if Philadelphia chromosome positive). Pembrolizumab 200 mg IV was given every 3 weeks. Response was assessed by BMA using methods that previously detected MRD. The primary end point was complete MRD response rate. We stopped enrollment early; only 1 of 12 (8%) experienced a complete MRD response, which lasted 3 weeks. Interestingly, this patient had previously received hematopoietic cell transplantation and CD19-targeted chimeric antigen receptor-modified T-cell therapy and was the only patient to experience an immune-related adverse event from pembrolizumab (grade 3 Stevens-Johnson syndrome). Median overall survival from enrollment was 12.7 months. In summary, pembrolizumab had minimal activity against MRD but was generally well tolerated. These data can be compared with ongoing anti-PD-1 combination studies in ALL, and they further establish the role of trials specifically for patients with MRD. This trial was registered at www.clinicaltrials.gov as #NCT02767934.