Non-Genotoxic Conditioning for Hematopoietic Stem Cell Transplant Using a Human Antibody Drug Conjugate Targeting C-KIT

For patients undergoing hematopoietic stem cell transplantation (HSCT), conditioning remains a primary source of transplant related morbidity and mortality and limits the broader application of this life-saving therapy. Current conditioning regimens, including total body irradiation and high dose chemotherapy, cause acute toxicity, cytokine storm, increased graft-versus-host disease, and organ damage. In addition, these regimens are also associated with the development of secondary malignancies, autoimmunity, and infertility.

To address this significant unmet medical need, we have developed a conditioning regimen using an antibody drug conjugate (ADC) targeting C-KIT (CD117) to specifically deplete host hematopoietic stem cells. CD117 is highly expressed on hematopoietic stem and progenitor cells (HSPCs) and binding to its ligand, stem cell factor, promotes hematopoiesis, self-renewal, and rapid internalization of the receptor. The rapid internalization and selective expression of CD117 on HSPCs make it an ideal target for an ADC-based approach to depleting HSPCs, while limiting systemic toxicity. In addition, CD117 is frequently overexpressed on malignant cells in patients with acute myelogenous leukemia. Thus, an ADC targeting CD117 may provide an opportunity for disease control prior to or concomitant with an HSCT. Previous studies have demonstrated that an antagonistic antibody against CD117 lead to high levels of HSPC depletion and donor engraftment in immunocompromised mice, but failed to facilitate this effect in immunocompetent hosts. To overcome the engraftment barrier, additional studies were conducted with an anti-CD117 antibody conjugated to the ribosome inactivating toxin, saporin, which allowed for robust HSPC depletion and high levels of congenic donor chimerism in immunocompetent mice (Czechowicz et.al., Blood 2016, 128:493).

To advance this technology toward the clinic, we have developed a fully human anti-CD117 antibody conjugated to a small molecule toxin capable of depleting proliferating and quiescent cells (CD117-ADC), that demonstrated greater than 90% killing of the leukemia cell line Kasumi-1, and was equally effective at killing primary human CD34+ bone marrow cells during in vitro culture. Humanized NSG mice treated with CD117-ADC had greater than 90% depletion of human HSPCs in the bone marrow, 21 days following a single administration of the ADC. The specificity of CD117-ADC for HSPCs was confirmed by the presence of stable peripheral human lymphocyte populations, and absence of human myeloid cells; indicating a lack of stem and progenitor cells capable of replenishing these short-lived cells. In addition to the ability of CD117-ADC to deplete HSPCs in humanized mice, preliminary data suggests that CD117-ADC is also effective at reducing tumor burden in mice challenged with CD117 expressing tumor cells.

Eliminating HSPCs from the stem cell niche with a targeted ADC has the promise to enable the engraftment of donor HSCs, while preventing the systemic toxicity associated with irradiation and chemotherapy. We have demonstrated that an ADC targeting CD117 is capable of depleting human HSPCs in vitro and in vivo and has the potential to reduce tumor burden in mice. Ultimately, this novel approach for safer conditioning prior to HSCT could greatly increase the number of patients eligible for transplant and significantly reduce the damaging side effects associated with current protocols and may provide effective non-genotoxic conditioning for all patients suffering from malignant or non-malignant diseases who require a HSCT.

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