Effect of Obexelimab Surrogate Monoclonal Antibody on Inhibition of Autoimmune Hemolytic Anemia (AIHA) Induction in Mice
Authors: Bao Weili , Su Shan , Liu Yunfeng , Yazdanbakhsh Karina , Feng Xiao , Youd Michele , Matijevic Mark 
Published: December 2024

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&lt;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&lt;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&lt;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&lt;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.