Therapeutic Targeting of CD163 on Hematopoietic Progenitor Cells By Novel Monoclonal Antibody TBI 304H Demonstrates an Increase in Erythropoiesis in Humanized Mice (2015)

Abstract Hemoglobin (Hb) is known to stimulate erythropoiesis, a process that may be mediated by CD163. CD163 is a receptor for the hemoglobin-haptoglobin (Hb-Hp) complex expressed on monocyte/macrophages as well as a subpopulation of human CD34+ hematopoietic progenitor cells (HPCs). We have demonstrated that administration of ligands to the CD163 receptor can measurably stimulate erythropoiesis in human CD34+ cell-engrafted severe-combined immunodeficiency (SCID) mice. To better elucidate the role of CD163 in hematopoiesis, we investigated the effects of the natural ligand to CD163 (Hb-Hp) as well as a stimulatory antibody, TBI 304H, on HPCs in vivo. SCID mice engrafted with human CD34+ cells were used as a model system were used to investigate the effect of Hb and anti-CD163 monoclonal antibodies (TBI 304 and TBI 304H) on human hematopoiesis in vivo. In an initial study, NOD-SCID IL2R gammanull (NSG) mice were engrafted with human CD34+ cells and animals with < 30% human CD45+ cells in the peripheral blood were administered 2 mg Hb/mouse, or 100 or 500 µg/mouse TBI 304 every 4 days for a total of four doses. At study termination on day 14, bone marrow cells (BMCs) were examined by flow cytometry and CD34+ cells were recovered from the BMCs for enumeration in colony-forming assays. Hemoglobin administration resulted in an increase of human CD34+ cells ranging from 4% to 7% of BMCs and a corresponding 57% increase in colony-forming cells (CFCs) over control animals. In contrast, the monoclonal antibody (mAb), TBI 304, produced a dose-dependent decrease in CD34+ and colonies, possibly reflecting a depletion of CD34+/CD163+ cells as a result of overstimulation due to the much longer circulating half-life of the mAb compared to Hb.. To confirm this hypothesis, human CD34+ cell engrafted animals were given only a single dose of 10 or 100 µg/mouse of TBI 304 and BMCs were examined earlier on day 7. TBI 304 provided a 3.5-fold increase in human CD34+ cells as well as a 1.8 to 6.7-fold increase in bone marrow erythroid lineage engraftment (huGlyA+, huCD36+ and huCD71+) and a 2-fold increase in colony-forming cells. The ability of TBI 304 to stimulate erythropoiesis in preclinical models led to the creation of an anti-CD163 mAb suitable for human clinical use. TBI 304H was generated by grafting the complementarity-determining regions derived from TBI 304 onto a humanized IgG4 framework without altering antigen specificity. An IgG4 framework, as an antibody without Fc effector function, was deemed the most suitable for an agonistic mAb. In the single dose, 7 day Hu-SCID model human CD34+ cells were mobilized from the mouse bone marrow by TBI 304H, as reflected by dose dependent decreases in huCD34+, huCD71+, and huGlyA+ cells in the mouse marrow. At the highest dose tested (500 µg/mouse) the decrease in human HPCs was similar to that found in animals administered Hb (2 mg/mouse). In this model, human hematopoiesis derived from the engrafted human CD34+ cells is not sustained and these date may reflect a mobilization of human HPCs through stimulation by an anti-CD163 antibody. Therapure has received U.S. FDA approval to conduct a Phase I trial of the novel therapeutic antibody TBI 304H. The Phase I clinical trial is a single-center, open-label, intra-subject escalating dose study, which will evaluate the safety, tolerability and pharmacokinetics of TBI 304H following administration to subjects experiencing chemotherapy-induced anemia. Disclosures Matthews: Therapure Biopharma: Employment. Eberle-Ayres:Therapure Biopharma: Employment. Lu:Therapure Biopharma: Employment. Singh:Therapure Biopharma: Employment. Cutler:Therapure Biopharma: Employment. Bell:Therapure Biopharma: Employment.