Effect of Obexelimab Surrogate Monoclonal Antibody on Inhibition of Autoimmune Hemolytic Anemia (AIHA) Induction in Mice

Autoimmune hemolytic anemia (AIHA) is characterized by premature destruction of patient’s own red blood cells (RBCs) by autoantibodies and can be life-threatening with limited treatment options. Obexelimab is a bifunctional, non-depleting humanized monoclonal antibody that mimics the action of antigen-antibody complexes by binding CD19 and FcγRIIb to inhibit B cells, plasmablasts, and CD19-expressing plasma cells. Obexelimab is an investigational product and not approved for any indication. To determine whether obexelimab can inhibit anti-RBC autoantibodies in AIHA and improve markers of anemia, the pharmacological activity of obexelimab surrogate monoclonal antibody (mAb) XENP8206 was tested in an experimental murine model of AIHA. The Playfair-Marshall Clarke AIHA mouse model (Cox 1974) involves repeated immunization with rat RBCs resulting in the induction of anti-mouse RBC autoantibodies and development of anemia. Human FcgRIIb transgenic B6 mice were injected with leukoreduced rat RBCs (2 × 10⁸ per mouse) intraperitoneally (IP) on a weekly basis for 11 weeks to induce anemia. XENP8206 mAb, which binds mouse CD19 and human FcgRIIb, was administered twice weekly IP from week -1 through week 11 at a dose of 10 mg/kg (n=16 mice). Control groups included disease-induced mice dosed with PBS (n=16 mice), and non-diseased mice dosed with XENP8206 (n=3 mice) or PBS (n=3 mice). Immune cell subsets and levels of mouse IgG autoantibodies were measured by flow cytometry. Hemoglobin levels were determined by a hematology analyzer. Within the first 3 weeks, bi-weekly dosing with XENP8206 significantly reduced mean circulating B-cell counts in the blood (p<0.05) while no effect was observed on non-B lineage cell populations. XENP8206 decreased mean anti-mouse RBC autoantibody levels with significant differences in levels compared to the PBS dosed control diseased group at weeks 10 through 12 (p<0.05). Mean B cell frequencies and absolute counts of splenic transitional (B220+CD93+), marginal zone (B220+CD21/35+IgM+), germinal center (GL7+CD95+), plasmablast (B220+CD138+) and plasma cells (B220-CD138++) at termination (week 12) were also reduced in the XENP8206 treated disease mice (p<0.05). Unlike the spleen, inguinal lymph node B220+ B cell counts were not affected by treatment. In the bone marrow, a trend toward lower numbers of total B220+ B cells with an increase in pre-pro B cells (B220+CD43+BP-1- CD24-CD93+) and early pro-B cells (B220+CD43+BP-1-CD24+) but a decrease in immature (B220+CD43+/-IgM+ IgD-), transitional (B220+CD43+/- IgM hi IgD-) and recirculating B cells (B220+CD43+/-IgM+/hi IgD+) were observed. These data suggest a block in B cell development (or compensatory mechanism to increase B cell production). Most importantly, the mean levels of hemoglobin, a marker of anemia, were higher in XENP8206 treated disease mice when compared to levels in the PBS treated disease mice at weeks 11 and 12 (p<0.05). Taken together, the current study demonstrated in vivo pharmacological activities of obexelimab surrogate mAb XENP8206 in a relevant disease model and provide supportive scientific rationale for the development of obexelimab as a treatment for AIHA.