Engraftment of gene-edited hematopoietic stem cells after antibody-drug conjugate conditioning in nonhuman primates

Hematopoietic stem cell (HSC) gene therapies provide lifelong benefit in numerous hematological diseases and disorders, but safety and toxicity remain a critical barrier for routine application. In the setting of immunodeficiency syndromes and infectious diseases such as HIV infection, conditioning regimens may exacerbate immune dysfunction, blunting, or impairing overall efficacy. Here, we conduct a head-to-head comparison of 2 novel antibody-drug conjugates (ADCs) with a pyrrolobenzodiazepine payload for autologous transplant in rhesus macaques. These ADCs target either CD117 or CD45 and are benchmarked against the clinical standard busulfan. We quantified the extent of myeloablation and immunosuppression, time to hematopoietic recovery, long-term engraftment of CCR5 CRISPR-edited autologous HSCs, and resistance to infection when challenged with increasing concentrations of an HIV-like virus. Both ADCs enabled engraftment of CRISPR-edited HSCs, although with lower levels of long-term editing compared with busulfan. We observed myeloablation with similar times to hematopoietic recovery and preserved lymphocyte counts with all 3 conditioning regimens, but neither ADC conditioning nor busulfan enabled sufficient CCR5 editing for viral immunity. Although these results only apply to the specific ADC-conditioning protocols tested here, they are a step toward developing targeted strategies to engraft cells with therapeutic edits and highlight the need for further refinement of antibody-based selection.