Non-genotoxic conditioning to overcome transplant-related toxicity in oc/oc osteopetrotic mouse model

Background/Introduction: Autosomal recessive osteopetrosis (ARO) is a rare disease, affecting osteoclast differentiation or function. Most patients present mutations in TCIRG1 gene, encoding a proton pump necessary for bone resorption. Symptoms include dense and brittle bones, limited bone marrow cavity, anaemia and progressive nerve compression, leading to death in the first decade of life. Standard treatment with allogeneic hematopoietic stem cell transplantation is hampered by HLA-matched donor availability, conditioning toxicity and significant morbidity Purpose: Our aim is to test the efficacy of non-genotoxic conditioning in the osteopetrotic oc/oc murine model, to reduce transplant-related complications and mortality while ensuring efficient engraftment of donor cells. Methods: Saporin (SAP) toxin was conjugated to anti-CD45 or anti-cKit antibodies to obtain CD45-SAP or cKit-SAP antibody-drug conjugates (ADCs). Due to the limited lifespan of oc/oc mice, we set up experimental conditions in WT newborn mice, that were conditioned with ADCs and transplanted with WT Lin- cells after 2 days. Survival and engraftment were monitored overtime. Animal procedures were approved by Institutional Animal Care and Use Committee. Results: We observed partial HSPC (hematopoietic stem and progenitor cell) depletion with both CD45-SAP and cKit-SAP (p<0.05 and p<0.01 vs untransplanted controls, respectively) (Figure 1A). Accordingly, peripheral blood showed a modest increase of engraftment in SAP-conditioned and transplanted mice compared to untreated controls. Similar results were obtained in hematopoietic organs at sacrifice 4 months after transplant (Figure 1B). Conclusion(s): ADCs can achieve sufficient HSPC depletion in newborn mice. However, further work to optimize engraftment efficacy in osteopetrotic setting is required.