Compared to total body irradiation or chemotherapy, antibody-drug conjugates (ADCs) offer a more targeted and potentially less toxic method for transplant conditioning. CD45-ADC targets a pan-leukocyte antigen expressed on HSPCs and immune cells, offering a promising strategy for gene therapy or allogeneic transplantation. We evaluated reconstitution, clonal dynamics, immune tolerance, and toxicity following conditioning with a DGN549-C-conjugated CD45-ADC followed by autologous transplantation in rhesus macaques. Two doses (0.2 and 0.3 mg/kg) were tested, with HSPC infusion 10 days post-conditioning. All animals received barcoded, CopGFP-expressing lentivirally transduced HSPCs. Both doses resulted in profound depletion of HSPCs and expected cytopenias, with incomplete lymphocyte depletion. With 0.2 mg/kg CD45-ADC conditioning, two animals showed robust multilineage engraftment with gene-modified cells and high clonal diversity, comparable to TBI. However, CopGFP+ cell levels and clonal diversity declined at 4-8 months, accompanied by development of anti-CopGFP antibodies, suggesting immune rejection. Incomplete T-cell depletion may have contributed. Notably, this rejection was slower and less complete than following busulfan conditioning, suggesting partial immune tolerance. Dexamethasone treatment in one animal reversed rejection and stabilized CopGFP+ levels for over 2 years. At 0.3mg/kg CD45-ADC, two animals developed severe respiratory distress 4-6 days post-transplantation, requiring humane euthanasia, accompanied by elevated inflammatory cytokines. This severe syndrome was not seen in 9 additional animals conditioned with CD45-ADC. These findings highlight the importance of pre-clinical evaluation of experimental therapeutics. Combining lower-dose CD45-ADC with immune suppression may enable durable engraftment in settings of alloantigen or neoantigen expression.
Hayal et al. (Tue,) studied this question.