Development of 211Astatine-Conjugated Anti-CD45 Antibody-Based Conditioning for Hematopoietic Stem Cell Gene Therapy and Editing

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ABSTRACT The inherited disorders of hemoglobin (Hb) are the most common monogenic diseases worldwide and, even in developed countries, associated with substantial morbidity and shortened life expectancy. Allogeneic hematopoietic cell transplantation (HCT) is clinically pursued as a means to treat the underlying cause of these disorders – the genetic defect in the patients’ hematopoietic stem and progenitor cells (HSPCs). However, this approach is limited by the availability of HLA-matched donors in the majority of patients and associated immunological complications. Use of autologous HSPCs either transduced with a functional b-hemoglobin gene or modified with recently-developed genome-editing technologies would overcome the current limitations of allogeneic HCT. In particular, the recapitulation of naturally-occurring hereditary persistence of fetal hemoglobin (HPFH) mutations in HSPCs using gene editing can, in principle, reverse the clinical phenotype of these disorders. However, just like with allogeneic HCT, there is still need for conditioning to facilitate engraftment of these cells. To date, this is accomplished with g-beam total body irradiation (TBI) or alkylating agents such as busulfan which carry the risk of significant toxicities including infertility, growth retardation, and – as has already been reported – secondary malignancies. Thus, a critical remaining factor for next-generation transplant approaches and gene therapy/editing will be the development of nongenotoxic conditioning regimens that have minimal toxicity and allow robust engraftment of allogeneic or modified autologous HSPCs. One promising strategy is the use of radioimmunotherapy (RIT) with a-emitting radionuclides conjugated to antibodies targeting CD45, an antigen expressed on almost all hematopoietic cells except platelets and erythrocytes and some of their progenitors. Compared to b-emitters, a-emitters deliver a higher amount of energy over just a few cell diameters for potent, precise, and efficiently targeted cell kill and minimized toxicity to non-targeted surrounding cells. With a half-life of 7.2 hours, astatine-211 (211At) is ideal for patient application. Based on our previous studies in dogs demonstrating that 211At-anti-CD45 RIT can replace g-beam TBI as conditioning before allogeneic HCT, we are currently using 211At-anti-CD45 RIT in patients with active hematologic malignancies. We now plan to develop 211At-anti-CD45 RIT as conditioning before autologous transplantation of gene-modified HSPCs for people with hemoglobinopathies, exploiting Fc engineering of antibodies to further minimize non-specific toxicities associated with RIT. We hypothesize that optimized 211At-anti-CD45 RIT will enable engraftment of autologous HSPCs edited with CRISPR/Cas9 at the g-globin gene locus to reproduce HPFH mutations and have significantly less off-target toxicities and better tolerability than the standard conditioning with high-dose g-beam TBI. As we are interested in rapid clinical translation of our findings and have already collected substantial data demonstrating feasibility, we will test this hypothesis in our established nonhuman primate model of autologous HCT for hemoglobinopathies.