Added benefit of raxibacumab to antibiotic treatment of inhalational anthrax.

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.

Bacillus anthracis protective antigen kinetics in inhalation spore-challenged untreated or levofloxacin/ raxibacumab-treated New Zealand white rabbits.

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.

Protective antigen antibody augments hemodynamic support in anthrax lethal toxin shock in canines.

BACKGROUND: Anthrax-associated shock is closely linked to lethal toxin (LT) release and is highly lethal despite conventional hemodynamic support. We investigated whether protective antigen-directed monoclonal antibody (PA-mAb) treatment further augments titrated hemodynamic support. METHODS AND RESULTS: Forty sedated, mechanically ventilated, instrumented canines challenged with anthrax LT were assigned to no treatment (controls), hemodynamic support alone (protocol-titrated fluids and norepinephrine), PA-mAb alone (administered at start of LT infusion [0 hours] or 9 or 12 hours later), or both, and observed for 96 hours. Although all 8 controls died, 2 of 8 animals receiving hemodynamic support alone survived (median survival times 65 vs 85 hours, respectively; P = .03). PA-mAb alone at 0 hour improved survival (5 of 5 animals survived), but efficacy decreased progressively with delayed treatment (9 hours, 2 of 3 survived; 12 hours, 0 of 4 survived) (P = .004 comparing survival across treatment times). However, combined treatment increased survival irrespective of PA-mAb administration time (0 hours, 4 of 5 animals; 9 hours, 3 of 3 animals; and 12 hours, 4 of 5 animals survived) (P = .95 comparing treatment times). Compared to hemodynamic support alone, when combined over PA-mAb treatment times (0, 9, and 12 hours), combination therapy produced higher survival (P = .008), central venous pressures, and left ventricular ejection fractions, and lower heart rates, norepinephrine requirements and fluid retention (P ≤ .03). CONCLUSIONS: PA-mAb may augment conventional hemodynamic support during anthrax LT-associated shock.

Raxibacumab for the treatment of inhalational anthrax.

BACKGROUND: Inhalational anthrax caused by Bacillus anthracis is associated with high mortality primarily due to toxin-mediated injury. Raxibacumab is a human IgG1lambda monoclonal antibody directed against protective antigen, a component of the anthrax toxin. METHODS: We evaluated the efficacy of raxibacumab as a prophylactic agent and after disease onset in a total of four randomized, placebo-controlled studies conducted in rabbits and monkeys. Animals were exposed to an aerosolized target exposure of B. anthracis spores that was approximately 100 times (in the prophylactic studies) and 200 times (in the therapeutic-intervention studies) the median lethal dose. In the therapeutic-intervention studies, animals were monitored for the onset of symptoms. Animals with detectable protective antigen in serum, a significant increase in temperature, or both received a single intravenous bolus of placebo or raxibacumab at a dose of either 20 mg per kilogram of body weight or 40 mg per kilogram. The primary end point was survival at day 14 (in rabbits) or at day 28 (in monkeys). Safety studies were conducted with intravenous raxibacumab (40 mg per kilogram) in 333 healthy human volunteers. RESULTS: In both rabbits and monkeys, the time to detection of protective antigen correlated with the time to bacteremia (r=0.9, P<0.001). In the therapeutic-intervention studies, the survival rate was significantly higher among rabbits that received raxibacumab at a dose of 40 mg per kilogram (44% [8 of 18]) than among rabbits that received placebo (0% [0 of 18]; P=0.003). Raxibacumab treatment also significantly increased survival in monkeys (64% [9 of 14], vs. 0% [0 of 12] with placebo; P<0.001). In human subjects, intravenous raxibacumab at a dose of 40 mg per kilogram had a half-life of 20 to 22 days and provided a maximum concentration of the drug in excess of levels that are protective in animals. Concentrations of raxibacumab provide a surrogate end point that should be predictive of clinical benefit. CONCLUSIONS: A single dose of raxibacumab improved survival in rabbits and monkeys with symptomatic inhalational anthrax. (ClinicalTrials.gov number, NCT00639678.)