Donor-Derived Anti-CD33 CAR T-Cells Show Safety in Relapsed AML Post-Transplant

The challenge of post-transplant relapse in myeloid malignancies

Relapse following allogeneic hematopoietic cell transplantation remains the primary driver of treatment failure for patients with acute myeloid leukemia, frequently resulting in a dismal prognosis [1, 2]. While standard salvage interventions, such as donor lymphocyte infusions or the combination of venetoclax and hypomethylating agents, are routinely employed, many patients fail to achieve durable complete remissions [1, 3]. The development of chimeric antigen receptor T-cell therapies for myeloid malignancies has been historically constrained by the scarcity of truly lineage-specific antigens and the significant risk of prolonged myelosuppression (a profound decrease in bone marrow activity resulting in low blood cell counts) [4, 5]. Although targeting antigens such as CD33 and CLL-1 has demonstrated potential in early-phase trials, identifying a safe and effective therapeutic window for cellular therapy in the post-transplant setting remains a critical clinical priority [6, 7]. A recent study evaluates a donor-derived cellular approach designed to enhance post-transplant surveillance and address these persistent therapeutic gaps.

Trial design and dose escalation in post-transplant AML

VCAR33 is a donor-derived chimeric antigen receptor T-cell product engineered to target CD33, a transmembrane protein expressed on the surface of most myeloid leukemia cells. The researchers developed this construct specifically to mitigate the risk of relapse in patients with high-risk acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) following allogeneic hematopoietic cell transplantation (alloHCT), the process of replacing a patient's hematopoietic system with donor-derived stem cells. During preclinical characterization, the VCAR33 construct was optimized for long-term antitumor surveillance using killing and persistence assays (laboratory tests that measure how effectively and for how long T-cells can eliminate cancer cells) to ensure sustained activity against malignant targets. The phase 1/2 clinical study (NCT05984199) evaluated the safety and efficacy of VCAR33 in 15 adult patients presenting with either relapsed disease or measurable residual disease (MRD), which refers to the presence of small numbers of cancer cells detectable only by highly sensitive laboratory methods rather than standard morphology. All participants had CD33-positive disease and had previously undergone an allogeneic transplant. The researchers stratified these 15 patients into two arms based on disease burden: Arm A included 7 patients with high disease burden (bone marrow blasts ≥5%), while Arm B included 8 patients with lower disease burden (bone marrow blasts <5%). Dosing followed a structured escalation protocol: in Arm A, all 7 patients received dose level 1 (DL1) at 1 × 10^6 CAR-positive T-cells per kg. In Arm B, 5 patients received DL1 and 3 patients were escalated to dose level 2 (DL2) at 3 × 10^6 CAR-positive T-cells per kg. Because the study concluded for non-safety reasons before reaching the planned dose level 3 (1 × 10^7 cells/kg), the maximum tolerated dose was not determined.

Safety profile and risk of graft-versus-host disease

The safety profile of VCAR33 was defined by frequent but manageable immune-mediated inflammatory responses. Cytokine release syndrome occurred in 93.3% of patients (14 of 15), a systemic inflammatory response triggered by the rapid activation and release of inflammatory mediators from infused T-cells. Notably, all cytokine release syndrome events were less than grade 3, indicating that the systemic inflammatory burden did not reach life-threatening severity at the dose levels evaluated. Neurological complications were also observed, though at a lower frequency. Four patients (26.7%) experienced immune cell–associated neurotoxicity syndrome (ICANS), a clinical syndrome involving confusion, encephalopathy, or other neurological deficits following CAR T-cell infusion. While most cases were low-grade, one patient (6.7%) experienced grade 3 or higher ICANS, necessitating intensive clinical management. A primary concern when utilizing donor-derived cells in the post-transplant setting is the risk of the infused cells attacking the recipient's healthy tissues, a complication known as graft-versus-host disease. In this study, the risk appeared limited during the immediate post-infusion period, as only one patient (6.7%) experienced grade 3 acute graft-versus-host disease within 28 days of VCAR33 infusion. For the practicing clinician, these data suggest that donor-derived CAR T-cells can be administered with a manageable safety profile, provided there is rigorous monitoring for both systemic inflammation and neurotoxicity.

Cellular expansion and preliminary antileukemic activity

The biological activity of VCAR33 was confirmed by the successful proliferation of the donor-derived cells within the recipients, a process known as cellular expansion. Fourteen patients (93.3%) demonstrated transient VCAR33 expansion, indicating that the cells successfully recognized their target and multiplied in the systemic circulation. This expansion is a critical pharmacokinetic marker, as the peak concentration of CAR T-cells often correlates with both the risk of toxicity and the potential for a therapeutic response. Although the expansion was transient, it provided a window of active surveillance against CD33-positive leukemic blasts. Clinical efficacy was observed across both treatment arms, with an overall response rate of 20% (3 of 15 patients). In Arm A, 2 patients achieved complete remission with incomplete count recovery (CRi), a state where leukemic blasts disappear from the marrow but peripheral blood counts have not yet returned to normal levels. In Arm B, 1 patient achieved MRD clearance, meaning that even highly sensitive molecular or flow cytometric testing could no longer detect markers of the malignancy. These findings suggest that donor-derived CD33-directed therapy is feasible and possesses antileukemic activity in the difficult-to-treat post-transplant setting. While the 20% response rate reflects the preliminary nature of this trial, the achievement of complete remissions and MRD clearance provides a foundation for future protocols aimed at matching high-risk patients to targeted cellular interventions.

References

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