A Non-Genotoxic Anti-CD117 Antibody Drug Conjugate (ADC) Designed for Patient Conditioning Prior to Stem Cell Transplant and HSC-Based Gene Therapy Has a Broad Therapeutic Window across Species

Current approaches to conditioning prior to transplant and gene therapy employ non-specific genotoxic agents that are associated with severe acute and long-term toxicities. Consequently, many transplant eligible patients either refuse transplant or receive reduced intensity regimens, which still have adverse toxicities with increased risk of relapse and graft failure. Importantly, patients with non-malignant disease may choose to forgo a potentially curative transplant due to risks of conditioning-associated toxicities, potentially limiting access of these patients to transplant and emerging HSC-based gene therapy. Thus, safe and effective targeted conditioning agents would have broad application to transplant in both malignant and non-malignant settings as well as gene therapy. We developed a CD117 antibody conjugated to amanitin (anti-CD117-AM) to specifically deplete hematopoietic stem and progenitor cells (HSPCs) as a novel approach to patient preparation for allogeneic and autologous transplant. ADCs have significant single agent anti-tumor activity in the clinic, however, achieving suitable tolerability at efficacious doses has presented a clinical challenge. We engineered the properties of the anti-CD117-AM to provide a broad therapeutic window across pre-clinical models. Through modification of the linker moiety, the maximum tolerated dose (MTD) of the ADC in C57Bl6 mice was improved 17-fold compared to previous versions. The engineered anti-CD117-AM maintained potent cytotoxicity on primary human CD34+ cells and a CD117+ AML cell line ( 95% of human HSPCs (Fig. 1A). Single doses of the anti-CD117-AM demonstrated robust tumor control in xenograft models of AML, despite its purposefully shortened half-life as a targeted agent for transplant indications (Fig. 1B). The tolerability of the engineered anti-CD117-AM was assessed in cynomolgus NHPs. The ADC was well-tolerated with an MTD at ≥ 2 mg/kg (Fig. 1C). A single dose of the ADC was able to drive marked depletion of HSPCs and downstream progeny. The pharmacokinetics of the ADC exhibited linear clearance supporting tolerability well above target saturation. In summary, optimization of the chemical linker between antibody and payload yielded a well-tolerated ADC with wide safety margins that could eliminate HSPCs and confer survival benefit in AML xenograft models. ADC-mediated conditioning through selective depletion of CD117+ cells may provide an improved approach to patient preparation for transplant in malignant and non-malignant disease including gene therapy, broadening patient access to potentially curative treatments.