CRISPR−Cas9 CD33-deleted allogeneic hematopoietic cell transplantation with gemtuzumab ozogamicin maintenance in AML: a phase 1/2 trial

Patients with high-risk acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) are likely to relapse despite allogenic hematopoietic cell transplantation (HCT). Post-HCT preventative maintenance can be limited by toxicity toward the normal donor cells. Tremtelectogene empogeditemcel (trem-cel) is a CRISPR–Cas9 gene-edited allogeneic HCT product lacking CD33, designed to shield the donor graft from cytotoxicity of subsequent CD33-targeted therapies such as gemtuzumab ozogamicin (GO). In this multicenter, phase 1/2a, open-label study, adult patients with AML/MDS with high relapse risk received trem-cel after myeloablative conditioning followed by GO maintenance (0.5–2.0 mg m⁻² day 1 per 28-day cycles). Patients receiving trem-cel were assessed for the primary safety endpoint of neutrophil engraftment by day 28 and secondary endpoints including time to neutrophil engraftment, incidence of graft-versus-host disease and graft failure, transplant-related mortality, percentage of CD33-negative myeloid cells and survival. Patients receiving trem-cel and GO were assessed for the additional secondary endpoints of safety of maintenance GO with trem-cel and pharmacokinetics of GO after trem-cel transplant. All 30 patients receiving trem-cel achieved the primary safety endpoint of neutrophil engraftment by day 28 with a median engraftment time of 10 days (95% confidence interval: 9–10). Nineteen patients received GO maintenance in phase 1 dose escalation (n = 15) and in phase 2 dose expansion (n = 4). The trial was stopped early, and this is the final report on the trial including the completed phase 1 portion. GO treatment was safely tolerated up to the recommended phase 2 dose of 2 mg m⁻², and no prolonged high-grade cytopenias were observed. The most common adverse events were cytopenias and infections. Three cases of transplant-related mortality were observed due to renal failure, sepsis and sinusoidal obstruction syndrome, respectively. In summary, trem-cel demonstrated safe, rapid, robust engraftment, and GO maintenance was administered without prolonged hematologic toxicity. ClinicalTrials.gov identifier: NCT04849910.