A CD33-Deleted Allograft (Trem-cel) Enables Post-Hematopoietic Cell Transplant (HCT) Maintenance Dosing of Gemtuzumab Ozogamicin (GO) with Therapeutic Levels of Drug Exposure and Low Hematologic and Hepatic Toxicity in Patients with High-Risk Acute Myeloid Leukemia (AML)

Background: Relapse is the leading cause of death for patients undergoing allogeneic HCT for AML. Strategies to reduce relapse including leukemia-directed maintenance therapies post-HCT have been largely ineffective. GO (Mylotarg^TM) is an anti-CD33 antibody-drug conjugate approved for use in AML, but its use has been limited by hepatotoxicity and on-target, off-tumor hematopoietic toxicity toward CD33+ normal blood cells leading to severe cytopenias, especially post-HCT. Tremtelectogene empogeditemcel (trem-cel; formerly VOR33) is a hematopoietic stem and progenitor cell product manufactured from CD34+ cells from a matched donor and CRISPR/Cas9 gene-edited to delete CD33. Trem-cel was developed to shield normal hematopoietic cells from CD33-directed therapies and allow exclusive targeting of residual CD33+ leukemia.

Methods: VBP101 (NCT04849910) is a Phase 1/2 multicenter trial to establish the safety of using trem-cel as an allograft followed by GO maintenance therapy for patients with CD33+ AML or MDS who are at high risk of relapse and undergoing HCT. Patients (18-70 y) must have CD33+ AML/MDS with high-risk features for relapse, such as adverse-risk cytogenetics or measurable residual disease, and an 8/8 HLA-matched related or unrelated donor. Trem-cel is manufactured from donor CD34+ cells isolated from G-CSF/Plerixafor-mobilized peripheral blood. Patients undergo busulfan- or TBI-based myeloablative conditioning with rATG prior to HCT with trem-cel. After ~60 days post-HCT patients begin maintenance therapy with GO in a 3+3 dose escalation strategy with cohorts of 0.5, 1 and 2 mg/m². GO is dosed at a planned 28 days between cycles for 4-8 cycles. Relapsed patients could receive subsequent therapies including VCAR33 (a donor-derived CD33 CAR-T, NCT05984199).

Results: 18 patients have received trem-cel with a median cell dose of 8.71 x 10⁶ CD34+ cells/kg (2.62-12.44) and CD33 editing efficiency of 89% (71-94%). All patients achieved primary neutrophil engraftment at a median of 9 days (8-12) and platelet recovery at a median of 16 days (13-22) excluding one patient with anti-platelet antibodies. At the D28 assessment, peripheral blood showed full donor myeloid chimerism in all evaluable patients and flow cytometry demonstrated a mean of 93.1% neutrophils (73-99%) and 90.5% monocytes (73-96%) lacked CD33 expression. Ten of the 18 patients received maintenance GO. Median no. of cycles received for the 0.5, 1 and 2 mg/m² doses were 4 (3-8), 4.5 (3-6) and 2 (2-3), respectively, with dosing ongoing for the 1 and 2 mg/m² cohorts.

Pharmacokinetic (PK) analysis demonstrated proportional exposures in trem-cel patients with the 0.5, 1 and 2 mg/m² doses of GO similar to efficacious AUC exposures in relapsed/refractory (R/R) AML patients after 1-4, 2-5 and 9 mg/m² doses, respectively. C_max, which correlates with hepatotoxicity, after 0.5, 1 and 2 mg/m² doses in trem-cel patients, was comparable to R/R AML patients receiving 1-2, 1-2 and 4-5 mg/m² doses respectively, suggesting a reduced risk of hepatotoxicity.

No episodes of Gr 4 neutropenia were observed and only a single episode of Gr 4 thrombocytopenia was reported in 40 GO cycles administered. No elevation of transaminases or bilirubin beyond transient Gr 1 was observed during GO cycles. CD33 negative myeloid cells increased from a mean 93.2% (76-99%) pre-GO to 98.2% (95-99.9%) after the first GO cycle suggesting enrichment for CD33-edited cells which was increased and sustained through subsequent cycles. With a median follow-up of ~6 months, 4 relapses (2 pre-GO dosing) were observed and 1 death related to infection. Enrollment is ongoing at the 2 mg/m² dose.

Conclusions: Preliminary results of VBP101 show the CD33-deleted allograft (trem-cel) rapidly engrafts and sustains hematopoiesis with persistent absence of CD33 in the myeloid compartment. CD33-deleted hematopoietic cells show protection from the prolonged deep cytopenias associated with GO. GO exposures correlate with higher doses in the standard R/R AML population, likely due to the reduction in clearance by virtue of less CD33 antigen present in trem-cel engrafted patients. Thus, trem-cel supports an increased therapeutic window for GO as lower doses have higher AUC with lower C_max than corresponding doses in R/R AML patients. These data support safe and effective administration of GO after trem-cel HCT, enabling repeated maintenance dosing intended to reduce risk of relapse.