Inhaled Bacillus anthracis spores germinate and the subsequent vegetative growth 
results in bacteremia and toxin production. Anthrax toxin is tripartite: the 
lethal factor and edema factor are enzymatic moieties, while the protective 
antigen (PA) binds to cell receptors and the enzymatic moieties. Antibiotics can 
control B. anthracis bacteremia, whereas raxibacumab binds PA and blocks lethal 
toxin effects. This study assessed plasma PA kinetics in rabbits following an 
inhaled B. anthracis spore challenge. Additionally, at 84 h post-challenge, 42% 
of challenged rabbits that had survived were treated with either 
levofloxacin/placebo or levofloxacin/raxibacumab. The profiles were modeled 
using a modified Gompertz/second exponential growth phase model in untreated 
rabbits, with added monoexponential PA elimination in treated rabbits. Shorter 
survival times were related to a higher plateau and a faster increase in PA 
levels. PA elimination half-lives were 10 and 19 h for the levofloxacin/placebo 
and levofloxacin/raxibacumab groups, respectively, with the difference 
attributable to persistent circulating PA-raxibacumab complex. PA kinetics were 
similar between untreated and treated rabbits, with one exception: treated 
rabbits had a plateau phase nearly twice as long as that for untreated rabbits. 
Treated rabbits that succumbed to disease had higher plateau PA levels and 
shorter plateau duration than surviving treated rabbits.