Modeling Targeted Lymphodepletion with Radiolabeled CD45 Antibody As a Promising Preparative Regimen Prior to Adoptive Cell Therapy or CAR-T

Abstract Prior to a patient receiving a dose of an adoptive cell transfer such as engineered autologous or allogeneic CAR-T cells, it is common to perform a lymphodepletion step often using high dose chemotherapy. This process is considered important to create sufficient space in the immune microenvironment, e.g. bone marrow, to allow the transferred cells to engraft. Further, it appears to elicit a favorable cytokine profile for establishment and proliferation of the donor lymphocytes. Anti-CD45 radioimmunotherapy (RIT) is being investigated in a Phase III clinical trial as a myeloablative targeted conditioning regimen prior to allogeneic hematopoietic cell transplantation in AML patients. In this study, a low 131I-CD45 RIT dosimetric dose is administered to each patient and imaging performed to define a personalized therapeutic dose. Results from studies performed post-dosimetric dose demonstrate that lower doses of 131I-anti-CD45 RIT may be suitable for use as a preparative conditioning or lymphodepleting regimen prior to cell therapy such as CAR-T. Significantly, targeted conditioning with pan-CD45 antibody, which selectively targets all nucleated immune cells, is anticipated to deplete not only lymphocytes, but also macrophages, as well as immune suppressive regulatory T cells (T-regs) and myeloid-derived suppressor cells in the immune microenvironment. It may also exert a direct anti-tumor effect on CD45+ hematopoietic cancers. We hypothesized that targeted lymphodepletion may result in a more suitable immune homeostatic environment for the reception of adoptive cell therapies, and possibly reduce the incidence of cytokine release syndrome. We have performed preclinical studies using a 131I- labeled surrogate anti-mouse pan-CD45 antibody (30F11) to investigate in a mouse model the response of targeted RIT lymphodepletion on particular immune cell types and resulting changes in immune cytokine expression. Following single dose administration of non-myeloablative doses of CD45-RIT, peripheral blood, bone marrow and spleen samples were collected from 8-12 week C57Bl/6 mice at 48 and 96 hours post-treatment for immunophenotyping to evaluate lymphoid and myeloid subsets for lymphodepletion, and serum for cytokine profiling. CD45-RIT was shown to effect a considerable reduction in both lymphocyte and myeloid cell counts, inclusive of immune suppressive T regs and MDSCs. Further, the cytoreduction by CD45-RIT was shown to induce the expression of immune homeostatic cytokines including IL-15. Studies are in progress to evaluate CD45-RIT as a targeted lymphodepletion regimen in E.G7 lymphoma tumor bearing mice prior to adoptive cell transfer with OVA-specific CD8+ T cells. Disclosures Ludwig: Actinium Pharmaceuticals: Employment, Equity Ownership. Berger:Actinium Pharmaceuticals: Employment, Equity Ownership. Ekaterina:Actinium Pharmaceuticals: Consultancy, Research Funding; Radimmune Therapeutics: Consultancy, Research Funding.