Successful Allogeneic Hematopoietic Stem Cell Transplantation in Mice Mediated By Well-Tolerated Conditioning Regimens Based on CD45-Targeted Antibody Drug Conjugate: Implications for Haplo Transplantation

Abstract

<h3>Introduction</h3> Allogeneic hematopoietic stem cell transplant (Allo-HSCT) is a potentially curative treatment for malignant and non-malignant blood disorders. However, current conditioning regimens limit its use due to regimen-related mortality and morbidities. We are developing novel antibody drug conjugates (ADCs) to provide the benefit of full-intensity conditioning to remove disease-causing cells while reducing the severity of treatment-related adverse events. These ADCs are designed to deliver agents specifically to CD45+ target cells as a sole conditioning agent or as the primary conditioning agent in a reduced intensity conditioning protocol for allo-HSCT. The aim of this study was to model this approach with a tool anti-mouse CD45 ADC to determine if targeted ADCs can be used to enable allo-HSCT in mice. <h3>Methods</h3> We developed a tool anti-mouse CD45 ADC engineered to have a short half-life (T_1/2 = 1.7hr) to enable HSCT. The optimal dose of tool CD45-ADC was established in a congenic autologous mouse transplant model. Next, the tool CD45-ADC was evaluated alone or in combination with low dose (0.5 Gy) total body irradiation (TBI) or T cell-depleting anti-mouse antibodies (CD4 and CD8, 0.25 mg/kg IP) in a full mismatch allo-HSCT model (Balb/c donors (H-2d, CD45.1+) into C57Bl/6 recipients (H-2b, CD45.2+)). 9 Gy TBI served as the conventional conditioning positive control. Conditioned mice were transplanted with 2 × 10⁷ whole bone marrow cells, and peripheral blood chimerism assessed over 16 weeks. <h3>Results</h3> A single 3 mg/kg dose of the tool CD45-ADC was an effective regimen in a congenic autologous mouse transplant model, resulting in full donor chimerism comparable to conditioning with 9 Gy TBI. In a full mismatch allo-HSCT model, a single dose of CD45-ADC enabled mixed myeloid chimerism out to 3 weeks as a single agent, but the chimerism was transient. In combination with low dose TBI or T cell depleting antibodies, the tool CD45-ADC enabled >90% peripheral donor chimerism by week 4 post-transplantation, which was maintained through week 16. Multilineage reconstitution of T-, B-, and myeloid cell compartments was observed (>90% donor chimerism) and was comparable to chimerism seen in the 9 Gy TBI positive control. Treatment with a non-targeting isotype ADC was not effective (Figure 1A, 1B). <h3>Conclusion</h3> A single dose of the tool CD45-ADC is fully myeloablative and enables complete chimerism in a full mismatch allo-HSCT model with low dose TBI or supplemental T cell depletion. Future experiments will examine the use of tool CD45-ADC conditioning in HSCT as a treatment in mouse autoimmune disease models. This targeted, readily translatable approach for safer conditioning could improve the risk-benefit profile for allogenic and haploidentical HSCT and may extend the curative potential of this therapeutic modality.