A Non-Genotoxic Antibody Drug Conjugate Targeting C-Kit for Hematopoietic Stem Cell Transplant Conditioning

Hematopoietic stem cell transplantation (HSCT) has the potential to cure a wide variety of diseases, but patient access to this life-saving therapy is limited due to the morbidity and mortality associated with current conditioning regimens. To address this significant unmet medical need, a conditioning regimen using a non-genotoxic antibody drug conjugate (ADC) targeting C-kit (CD117) was developed to specifically deplete hematopoietic stem cells (HSCs). CD117 is highly expressed on HSCs and ligand binding promotes hematopoiesis, self-renewal, and rapid internalization of the receptor. The restricted expression and rapid internalization of CD117 make it an attractive target for an ADC-based approach to deplete HSCs and limit systemic toxicity. This approach preserves the adaptive immune system making it ideal for autologous gene modified transplants. In addition, conditioning with an anti-CD117 ADC may decrease relapse in the allogeneic transplant setting since many hematologic malignancies overexpress CD117. To advance this technology into the clinic, we developed a short half-life, fully human, anti-CD117-antibody conjugated to a non-genotoxic small molecule toxin (CD117-ADC), that kills >95% of primary human HSCs in vitro. Humanized NSG mice treated with a single dose of CD117-ADC displayed >98% depletion of human HSCs in the bone marrow, while sparing T and B cells. Preliminary tolerability data suggest a wide therapeutic index, greater than 20-fold. An unconjugated CD117 antagonist antibody was less effective with 100-fold more antibody unable to achieve similar levels of HSC depletion. The CD117-ADC is engineered to have a half-life of less than 12 hours, to facilitate clearance prior to donor cell infusion without the need for real-time pharmacokinetic monitoring. Finally, preliminary data shows that CD117-ADC may reduce tumor burden in mice challenged with CD117 expressing leukemic cells. Hematopoietic stem cell depletion with a targeted ADC has the promise to enable donor HSC engraftment, while preventing systemic toxicity associated with irradiation and chemotherapy. We have demonstrated that a short half-life ADC targeting CD117 depletes human HSCs in vitro and in vivo and has the potential to reduce tumor burden in mice. This novel conditioning approach could reduce the morbidity associated with current conditioning protocols, increase the number of patients eligible for HSCT, and enable autologous HSCT of gene modified stem cells.