Although antibiotics treat bacteremia in inhalational anthrax, pathogenesis is 
mainly driven by bacterial exotoxins. Raxibacumab, an IgG1 monoclonal antibody, 
binds the protective antigen (PA) of Bacillus anthracis, thus blocking toxin 
effects and leading to improved survival in the rabbit and monkey models of 
inhalational anthrax. To assess raxibacumab's added benefit over levofloxacin 
(LVX) alone, rabbits surviving to 84 h after a challenge with 200 times the 
median (50%) lethal dose of B. anthracis spores were randomized to receive 3 
daily intragastric LVX doses of 50 mg/kg of body weight, with the first LVX dose 
administered just prior to administration of a single intravenous dose of 
placebo or 40 mg/kg raxibacumab. The percentages of animals alive at 28 days 
following the last LVX dose were compared between the 2 treatment groups using a 
two-sided likelihood-ratio chi-square test. The 82% survival rate for the 
LVX-raxibacumab combination was higher than the 65% survival rate for LVX alone 
(P=0.0874). There were nearly 2-fold fewer deaths for the combination (7 deaths; 
n=39) than for LVX alone (13 deaths; n=37), and the survival time was prolonged 
for the combination (P=0.1016). Toxin-neutralizing-activity titers were similar 
for both treatment groups, suggesting that survivors in both groups were able to 
mount a toxin-neutralizing immune response. Microscopic findings considered 
consistent with anthrax were present in animals that died or became moribund on 
study in both treatment groups, and there were no anthrax-related findings in 
animals that survived. Overall, raxibacumab provided a meaningful benefit over 
antibiotic alone when administered late in the disease course.