Patients with relapsed acute myeloid leukemia (AML), particularly following allogeneic stem cell transplant (alloHCT), have extremely limited therapeutic options. CD123 is an AML-associated antigen and represents an attractive target for immunotherapy. We report outcomes of a phase 1 trial evaluating CD123-targeting chimeric antigen receptor (CAR) T cells in patients with relapsed/refractory (r/r) AML or blastic plasmacytoid dendritic cell neoplasm (BPDCN).

This was a single center, open-label, phase 1 dose escalation study enrolling patients with either CD123-positive r/r AML (Arm 1) or BPDCN (Arm 2). Patients received autologous or donor-derived allogeneic CD123 CAR T cells following lymphodepletion. The co-primary objectives were to examine safety and anti-tumor activity using an activity-constrained for toxicity design and to determine the recommended phase 2 dose. Secondary objectives included assessments of progression-free and overall survival.

We enrolled 41 patients, of whom 21 patients (n = 19 on Arm 1 and n = 2 on Arm 2) received CD123 CAR T-cell infusion. The median age of treated AML patients was 45 years (range: 20–71); the two BPDCN patients were aged 24 and 75 years. Among AML patients, 17 (89%) had undergone prior alloHCT. AML patients received 50 × 10⁶ (n = 2), 200 × 10⁶ (n = 6), or 500 × 10⁶ (n = 11) CAR T cells; BPDCN patients received 100 × 10⁶ CAR T cells. There was no dose-limiting toxicity, prolonged myelosuppression, or graft-versus-host disease. Fourteen (74%) AML patients developed grade ≤ 3 cytokine release syndrome (CRS) and 11 (58%) experienced grade ≤ 2 neurotoxicity. Fifteen of 19 treated AML patients were evaluable for disease response for overall best response, of whom 4 (26.7%) had best response of complete remission, including 2 with incomplete count recovery. Neither patient with BPDCN developed CRS but both experienced grade 1 neurotoxicity. One of 2 BPDCN patients achieved CR and relapsed at 3 months. CAR T cell expansion was dose-dependent, but persistence was limited.

CD123-directed CAR T-cell therapy is feasible and demonstrates a favorable safety profile in patients with r/r AML and BPDCN, including those with prior alloHCT. Although the estimated overall complete remission rate was modest, these findings provide a foundation for further optimization of CD123 CAR T-cell design, patient selection, and combination strategies.

This trial was registered at ClinicalTrials.gov as NCT02159495.