Safety Evaluation of an Fully Human Antagonist Anti-CD40 Antibody, CHIR-12.12, in a Dose Range-Finding Study in Cynomolgus Monkeys.

Abstract

CD40 is highly expressed in B-cell malignancies and its activation is a growth and survival signal for these cells. We have generated a novel, highly potent, fully human anti-CD40 IgG1 monoclonal antibody, CHIR-12.12 using XenoMouse® mice (Abgenix, Inc), a strain of transgenic mice expressing fully-human IgG antibodies. CHIR-12.12 has at least two mechanisms of cytotoxicity in vitro: it blocks CD40-ligand mediated CD40 activation and mediates B-cell killing by ADCC. To select an appropriate animal species to conduct toxicology studies, a variety of animal species including mouse, rat, rabbit, cynomolgus monkey, rhesus monkey and marmoset monkey were screened for CHIR-12.12 cross-reactivity. The antibody showed binding to rabbit and non-human primate CD40, but not to rodent CD40. Based on functional assays, the cynomolgus monkey was then identified as the appropriate species for toxicity studies: Similar to its activity in human lymphocytes, CHIR-12.12 binds to cynomolgus cell surface CD40 and inhibits CD40-ligand induced proliferation of cynomolgus lymphocytes. A range-finding toxicology study was conducted with CHIR-12.12 doses of 0, 1, 3, 10 and 30 mg/kg once a week for 3 weeks in two cynomolgus monkeys per group per sex. One week after the last dose (Day 22), the animals were humanely euthanized for pathological and histopathological evaluation of all tissues. During the study flow cytometric analysis were performed to monitor changes in B-cell, T-cell and NK-cell numbers. In addition, the antigen saturation on B-cells was monitored over time. Samples for clinical pathology, hematology and urinalysis were collected as well as samples for pharmacokinetics and anti-human antibody determination. The results of the study showed that CHIR-12.12 was well tolerated and did not elicit any adverse clinical signs or effect on body weights, clinical pathology and hematology, including differential blood counts. The expected reduction in B-cell counts was observed as early as 4 hours after the first dosing and at all subsequent time points throughout the study. T-cell counts (CD3+, CD4+ and CD8+ cells) showed no changes. Dose dependent antigen saturation was observed through the whole study. Necropsy and histopathology showed minimal findings in the pancreas of single animals. No significant treatment related adverse events were seen in any organs. The pharmacokinetics of CHIR-12.12 was characterized by small volume of distribution limited to blood volume and slow clearance. The elimination half-life after single dose increased with dose and tended to increase further after multiple dose administration. The half-life ranged between 1.8 days at the 1mg/kg dose to about 7.5 days in the 30 mg/kg group. In conclusion, there were no dose-limiting toxicities in any organ following 3 weekly doses of CHIR-12.12 up to 30 mg/kg. The observed B-cell reduction is the therapeutic target of CHIR-12.12 and an indication for the bioactivity of CHIR-12.12 and is therefore not considered adverse. These results support the clinical development of CHIR-12.12 antibody for treatment of B-cell malignancies.