CAR T-cell depletion in human AML xenograft models

CD123-redirected Chimeric Antigen Receptor (CAR) T-cells have been shown in previous studies to exert potent anti-leukemia activities in Acute Myeloid Leukemia (AML) preclinical studies. However, it is associated with severe hematologic toxicities which can impact Hematopoietic Stem Cell Transplantation (HSCT).¹ Developing an effective CAR T-cell depletion strategy after induction of leukemia remission is crucial to halt potentially life-threatening toxicities and to enable subsequent HSCT.

In an article published on 28^th February 2017 in Blood, Sarah K. Tasian from the Children’s Hospital of Philadelphia and colleagues compared the efficacy of three approaches for T-cell termination after CD123-redirected T-cell-induced eradication of AML in human AML xenograft models.²

Three approaches for T-cell termination were compared including treatment with shorter-persisting mRNA-modified CD123-redirected CAR T-cells (RNA-CART123), T-cell ablation with alemtuzumab (anti-CD52 monoclonal antibody) after treatment with lentivirally-transduced CD123-redirected CAR T-cells (CART123), and treatment with CART123 co-expressing surface CD20 protein (CART123-CD20) subsequently depleted with rituximab.

The key results of the study were:

• Compared to controls, CART123 treatment of MOLM-14 engrafted mice induced leukemia eradication; P < 0.0001
• In MOLM-14-engrafted mice, one dose of alemtuzumab at 4 weeks post-CART123 treatment depleted T-cells and sustained remission
• Re-challenge of alemtuzumab-ablated MOLM-14 engrafted mice with MOLM-14 cells led to rapid leukemia progression compared to CART123 only-treated mice
• Treatment of MOLM-14 engrafted mice with CART123-CD20 induced rapid leukemia clearance comparable to CART123
• In MOLM-14-engrafted mice, one dose of rituximab at 4 weeks post-CART123-CD20 treatment depleted T-cells and sustained remission
• Re-challenge of rituximab-ablated MOLM-14 engrafted mice with MOLM-14 cells led to leukemia progression
• Treatment of MOLM-14 engrafted mice with RNA-CART123 lead to AML clearance and long term remission
• Compared with CART123 and CART123-CD2, RNA-CART123 had a shorter persistence in vivo
• Human myeloid engraftment occurred in rituximab-ablated mice treated with CART123-CD20 but not in mice treated with CART123

In summation, in murine xenograft models of human AML, depletion of CD123-redirected CAR-T-cells with monoclonal antibodies can eliminate CAR T-cells and preserve leukemia remission. Furthermore, it was observed that RNA-CART123 has a potent anti-AML efficacy with a potential to limit hematological toxicity due to the shorter persistence in vivo. Additionally, CAR T-cell depletion enhanced the feasibility of subsequent allogenic stem cell transplantation.

The authors noted that their study had some limitations. They highlighted that the xenograft models used in their study could not predict the potential for concurrent on-target/on-tumor leukemia cytotoxicity and bystander toxicity, and also could not prove whether subsequent HSCT would be compromised by prior CAR-T cell therapy even with T-cell depletion. Furthermore, adverse effects associated with T-cell depletion were unknown.

The authors concluded by stating that their results may identify promising T-cell termination strategies that will augment efficacy of CAR T-cell therapy, particularly prior to HSCT. Based on this preclinical data, an early phase I trial (NCT02623582) is currently underway. In this study, the feasibility, safety, and efficacy of RNA-CART123 is being evaluated in patients with relapsed or refractory AML.

Abstract

Others and we previously reported potent anti-leukemia efficacy of CD123-redirected chimeric antigen receptor (CAR) T-cells in preclinical human acute myeloid leukemia (AML) models at cost of severe hematologic toxicity. This observation raises concern for potential myeloablation in patients with AML treated with CD123-redirected CAR T-cells and mandates novel approaches for toxicity mitigation. We hypothesized that CAR T-cell depletion with optimal timing after AML eradication would preserve leukemia remission and allow subsequent hematopoietic stem cell transplantation. To test this hypothesis, we compared three CAR T-cell termination strategies: (1) transiently-active anti-CD123 mRNA-electroporated CART (RNA-CART123), (2) T-cell ablation with alemtuzumab after treatment with lentivirally-transduced anti-CD123-4-1BB-CD3ζ T-cells (CART123), and (3) T-cell ablation with rituximab after treatment with CD20-coexpressing CART123 (CART123-CD20). All approaches led to rapid leukemia elimination in murine xenograft models of human AML. Subsequent antibody-mediated depletion of CART123 or CART123-CD20 did not impair leukemia remission. Time-course studies demonstrated that durable leukemia remission required CAR T-cell persistence for 4 weeks prior to ablation. Upon CAR T-cell termination, we further demonstrated successful hematopoietic engraftment with a normal human donor to model allogeneic stem cell rescue. Results from these studies will facilitate development of T-cell depletion strategies to augment the feasibility of CAR T-cell therapy for patients with AML.