Research Appraisals

Blood

A phase 1/2 study of donor-derived anti-CD33 CAR T-cell therapy (VCAR33) for relapsed/refractory AML after allogeneic HCT

VCAR33, a donor-derived CD33-directed chimeric antigen receptor T-cell (CAR T) product, was developed to decrease relapse of high-risk acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) after allogeneic hematopoietic cell transplantation (alloHCT). We describe preclinical characterization of the VCAR33 construct, which was optimized for long-term antitumor surveillance based on killing and persistence assays. Prior to its use in post-alloHCT maintenance, we evaluated safety and efficacy of VCAR33 in a phase 1/2 clinical study for adults with relapsed or measurable residual disease (MRD)-positive CD33+ AML/MDS after alloHCT. Fifteen patients received VCAR33 across 2 arms stratified by disease burden: 7 patients in arm A (bone marrow blasts ≥5%) at dose level 1 (DL1; 1 × 106 CAR+ Ts per kg) and 8 patients in arm B (bone marrow blasts <5%) at DL1 (n = 5) and DL2 (3 × 106 CAR+ Ts per kg; n = 3). The study ended for nonsafety reasons before escalation to DL3 (1 × 107 CAR+ Ts per kg) and maximum tolerated dose was not determined. The most common treatment-related adverse event was cytokine release syndrome (93.3%; all <grade 3). Four patients (26.7%) experienced immune cell-associated neurotoxicity syndrome (1 ≥grade 3) and 1 patient (6.7%) had grade 3 acute graft-versus-host disease within 28 days of VCAR33 infusion. Fourteen patients (93.3%) had transient VCAR33 expansion. Overall response rate was 20%: 2 patients had complete remission with incomplete count recovery in arm A and 1 arm B patient achieved MRD clearance. This allogeneic CAR T product demonstrated acceptable safety and preliminary antileukemic activity. This trial was registered at www.clinicaltrials.gov as #NCT05984199.