Trem-Cel, a CRISPR/Cas9 Gene-Edited Allograft Lacking CD33, Shows Rapid Primary Engraftment with CD33-Negative Hematopoiesis in Patients with High-Risk Acute Myeloid Leukemia (AML) and Avoids Hematopoietic Toxicity during Gemtuzumab Ozogamicin (GO) Maintenance Post-Hematopoietic Cell Transplant (HCT)

Background: Allogeneic HCT is the standard of care for eligible patients with AML harboring high-risk features such as adverse-risk cytogenetics or measurable residual disease (MRD), however, the majority of these patients still relapse post-HCT and have very poor outcomes. The use of maintenance therapies intended to control post-HCT relapse is often limited by hematopoietic toxicity towards normal engrafted cells. GO (tradename: Mylotarg ^TM) is an anti-CD33 antibody-drug conjugate targeting CD33+ AML, but its use is limited by on-target, off-tumor hematopoietic toxicity toward CD33+ myeloid and progenitor cells. Tremtelectogene empogeditemcel (trem-cel; formerly VOR33) is a hematopoietic stem and progenitor cell product, manufactured from CD34+ cells isolated from a patient-matched donor, that has been modified by CRISPR/Cas9 gene-editing to lack CD33. Trem-cel was developed to protect normal hematopoietic cells from post-HCT CD33-directed therapies and allow exclusive targeting of residual leukemia.

Methods: VBP101(NCT04849910) is a first-in-human Phase 1/2 open-label multicenter trial to establish the safety of using trem-cel as a donor allograft for CD33+ AML patients who are at high risk of relapse and undergoing myeloablative HCT followed by treatment with GO. Eligible patients (18-70 y) must have CD33+ AML with high-risk features for relapse, such as adverse-risk cytogenetics or MRD, and a 10/10 human leukocyte antigen (HLA)-matched related or unrelated donor. Donors undergo mobilization with G-CSF and plerixafor prior to apheresis. Trem-cel is manufactured from isolated CD34+ cells and cryopreserved. Patients undergo either a busulfan- or TBI-based myeloablative conditioning regimen prior to transplantation with trem-cel. After engraftment and recovery of a sufficient CD33-negative neutrophil count, patients begin maintenance therapy with GO in a 3+3 dose escalation strategy starting at 0.5 mg/m ² every 28d for 4-8 cycles.

Results: To date, 6 patients between 32-68 y (median 63.5 y) have been treated with trem-cel at a median dose of 5.2 x 10 ⁶ CD34+ cells/kg (2.6 - 7.6) and CD33 editing efficiency of 88% (80 - 91). Primary neutrophil engraftment occurred in all patients after a median of 10 days (9 - 11) and platelet recovery occurred after a median of 16 days (15-22) excluding one patient with documented anti-platelet antibody (Fig 1). At the day 28 assessment, full peripheral blood (PB) myeloid chimerism was achieved in all patients and CD33 expression by flow was absent in a median of 94% of neutrophils (86-99) and 92% of monocytes (82 - 94), respectively. Comparable CD33 edits were detected in donor cells across myeloid and lymphoid lineages. GO dosing is ongoing at 0.5 mg/m ² dose and neutrophil and platelet counts remain stable, suggesting protection from GO-mediated hematotoxicity. Following the 0.5 mg/ m ² dose, the exposures of GO were predictively higher in the context of CD33-negative hematopoiesis relative to GO doses of 1-2 mg/m ² (C _max) and 4-5 mg/m ² (AUC), respectively, in CD33+ relapsed/refractory AML patients. During GO dosing, the percent of CD33-negative cells increased in the PB and bone marrow (BM) across cell lineages consistent with enrichment of edited donor cells. Patients are followed for safety, hematologic protection from GO, disease status, and persistence of CD33 editing.