A Phase 1 Study of PAmAb, a Fully Human Monoclonal Antibody against Bacillus anthracis Protective Antigen, in Healthy Volunteers

Abstract

BACKGROUND: Inhibition of the binding of Bacillus anthracis protective antigen (PA) to its cellular receptor can abrogate the downstream toxin-mediated deleterious effects of the anthrax toxin. A fully human monoclonal antibody against B. anthracis PA, PAmAb, was previously shown to provide a survival advantage in rabbit and monkey models of inhalational anthrax. METHODS: A randomized, single-blind, placebo-controlled, dose-escalation study with 105 healthy volunteers was conducted to evaluate the safety, pharmacokinetics, and biological activity of PAmAb. Subjects received PAmAb or placebo as a single intramuscular injection (11 subjects/cohort) or intravenous infusion (10 subjects/cohort). Three intramuscular dose levels (0.3, 1.0, and 3.0 mg/kg) and 5 intravenous dose levels (1.0, 3.0, 10, 20, and 40 mg/kg) were studied. Two separate intramuscular injection sites (gluteus maximus and vastus lateralis) were evaluated in the cohorts (hereafter, the "IM-GM" and "IM-VL" cohorts, respectively). RESULTS: PAmAb was well tolerated, with no dose-limiting adverse events. All adverse events were transient and mild to moderate in incidence and/or severity. The pharmacokinetics of PAmAb were linear within each route and site of administration but were significantly different between the IM-GM and IM-VL cohorts. The mean terminal elimination half-life ranged from 15 to 19 days. The bioavailability of PAmAb is approximately 50% for IM-GM injection and 71%-85% for IM-VL injection. The biological activity of PAmAb in serum, assessed using a cyclic adenosine monophosphate assay, correlated with serum concentrations. CONCLUSIONS: PAmAb is safe, well tolerated, and bioavailable after a single intramuscular or intravenous dose, which supports further clinical development of PAmAb as a novel therapeutic agent for inhalational anthrax.