A Single Dose of CD117 Antibody Drug Conjugate Enables Hematopoietic Stem Cell Based Gene Therapy in Nonhuman Primates

Abstract

Autologous hematopoietic stem cell transplantation (Auto-HSCT) with gene-modification represents a potential cure for multiple genetic diseases, but its broad curative potential, is limited because of morbidity/mortality from cytotoxic chemotherapy-based conditioning. To overcome these limitations, we developed antibody drug conjugates (ADC) targeting CD117 (C-KIT) to specifically deplete the hematopoietic stem and progenitor cells (HSPC). To validate CD117 ADC-mediated depletion prior to HSCT, we developed an optimized non-human primate (NHP) tool anti-CD117 ADC and evaluated it in an auto-gene modified HSCT in a rhesus model. The CD117-ADC is potent on primary human and NHP CD34+ cells <i>in vitro</i> (Figure 1A). Humanized NSG mice treated with a single dose had full depletion of human HSPCs in the bone marrow, while maintaining peripheral immune cells. In rhesus a single administration was fully myeloablative (>99% HSPC depletion) and comparable to HSPC depletion observed following busulfan conditioning (6 mg/kg/day x4). There was no effect of the ADC on the peripheral and bone marrow lymphocytes and the ADC was well tolerated compared to busulfan where the animals had diarrhea, loss of appetite and weight loss. To facilitate use in HSCT, the CD117-ADC was engineered to have a fast clearance and the half-life was < 10 h in NHP. We next explored whether the tool CD117-ADC could enable auto gene modified HSCT in the rhesus model. Two rhesus NHP were mobilized with GCSF and plerixafor. The selected CD34+ cells were transduced with β-globin encoded lentivirus and cryopreserved. The tool CD117-ADC was dosed on day -6 and the CD34+ cells were thawed and infused on day 0. Bone marrow aspirates analyzed on the day of infusion (day 0) demonstrated >99% depletion of the HSPCs and maintenance of the bone marrow lymphocytes (Figure 1B). The primates engrafted neutrophils (day 8 and 10) and platelets (day 10 and 11), and the peripheral lymphocytes were maintained throughout the transplant (Figure 1C-D). The gene marking in the granulocytes is comparable to busulfan conditioned animals previously reported (Tisdale, Molecular Therapy 2019). Longer follow up and data from additional animals will be presented. In summary, we have developed a tool CD117 ADC that shows potent activity on NHP CD34+ cells, is fully myeloablative, has a favorable safety profile, spares the immune system and is cleared rapidly as designed. In a rhesus model of auto-gene modified HSCT, a single dose of the ADC enables engraftment of gene modified HSC. These proof of concept studies validate the use of CD117-ADC for targeted HSPC depletion prior to transplant and support its use as a new conditioning agent for auto-gene modified HSCT. This targeted approach for safer conditioning could improve the risk benefit profile for patients undergoing HSCT and enable more patients to benefit from these potentially curative therapies.