Clinical Pharmacology for Raxibacumab
Mechanism Of Action
Raxibacumab is a monoclonal antibody that binds the PA of B. anthracis [see CLINICAL PHARMACOLOGY].
Pharmacokinetics
The PK of raxibacumab is linear over the dose range of 1 to 40 mg/kg following single intravenous dosing in humans; raxibacumab was not tested at doses higher than 40 mg/kg in humans. Following single intravenous administration of raxibacumab 40 mg/kg in healthy, male and female human subjects, the mean Cmax and AUCinf were 1,020.3 ± 140.6 mcg/mL and 15,845.8 ± 4,333.5 mcg•day/mL, respectively. Mean raxibacumab steady-state volume of distribution was greater than plasma volume, suggesting some tissue distribution. Clearance values were much smaller than the glomerular filtration rate indicating that there is virtually no renal clearance of raxibacumab.
Because the effectiveness of raxibacumab cannot be tested in humans, a comparison of raxibacumab exposures achieved in healthy human subjects to those observed in animal models of inhalational anthrax in therapeutic efficacy studies is necessary to support the dosage regimen of 40 mg/kg intravenously as a single dose for the treatment of inhalational anthrax in humans. Humans achieve similar or greater systemic exposure (Cmax and AUCinf) to raxibacumab following a single 40 mg/kg intravenous dose compared with New Zealand White rabbits and cynomolgus macaques receiving the same dosage regimen.
Effects Of Gender, Age, And Race
Raxibacumab PK was evaluated via a population PK analysis using serum samples from 322 healthy subjects who received a single 40 mg/kg intravenous dose across 3 clinical trials. Based on this analysis, gender (female versus male), race (non-white versus white), or age (elderly versus young) had no meaningful effects on the PK parameters for raxibacumab.
Raxibacumab PK has not been evaluated in children [see DOSAGE AND ADMINISTRATION, Use In Specific Populations].
Repeat Dosing
Although raxibacumab is intended for single-dose administration, the PK of raxibacumab following a second administration of 40 mg/kg given intravenously 14 days after the first 40-mg/kg intravenous dose was assessed in 23 healthy subjects (Study 3). The mean raxibacumab concentration at 28 days after the second dose was approximately twice the mean raxibacumab concentration at 14 days following the first dose. In the human trial assessing the immunogenicity of raxibacumab (Study 2), 20 healthy subjects who had initially received a single dose of raxibacumab 40 mg/kg intravenously received a second 40 mg/kg intravenous dose at ≥4 months following their first dose. No statistically significant differences in mean estimates of AUCinf, CL, or half-life of raxibacumab between the 2 doses administered ≥4 months apart were observed. The mean Cmax following the second dose was 15% lower than the Cmax following the first dose.
Ciprofloxacin Interaction Trial
In an open-label trial evaluating the effect of raxibacumab on ciprofloxacin PK in healthy adult male and female subjects (Study 1), the administration of 40 mg/kg raxibacumab intravenously following ciprofloxacin intravenous infusion or ciprofloxacin oral tablet ingestion did not alter the PK of ciprofloxacin administered orally and/or intravenously. Likewise, ciprofloxacin did not alter the PK of raxibacumab. [See DRUG INTERACTIONS]
Microbiology
Mechanism Of Action
Raxibacumab is a monoclonal antibody that binds free PA with an affinity equilibrium dissociation constant (Kd) of 2.78 ± 0.9 nM. Raxibacumab inhibits the binding of PA to its cellular receptors, preventing the intracellular entry of the anthrax lethal factor and edema factor, the enzymatic toxin components responsible for the pathogenic effects of anthrax toxin.
Activity In Vitro And In Vivo
Raxibacumab binds in vitro to PA from the Ames, Vollum, and Sterne strains of B. anthracis. Raxibacumab binds to an epitope on PA that is conserved across reported strains of B. anthracis.
In vivo studies in rats suggest that raxibacumab neutralizes the toxicity due to lethal toxin, as animals slowly infused with lethal toxin (a combination of PA + lethal factor) survived 7 days following administration. The median time to death in control rats was 16 hours. Similar observations were noted in animal efficacy studies in rabbits and monkeys challenged with B. anthracis spores by the inhalational route. PA was detected in animals following exposure to B. anthracis spores. PA levels rose and then fell to undetectable levels in animals that responded to treatment and survived, whereas levels continued to rise in animals that failed treatment and died or were euthanized because of poor clinical condition. [See Clinical Studies]
Clinical Studies
Because it is not feasible or ethical to conduct controlled clinical trials in humans with inhalational anthrax, the effectiveness of raxibacumab for therapeutic treatment of inhalational anthrax is based on efficacy studies in rabbits and monkeys. Raxibacumab effectiveness has not been studied in humans. Because the animal efficacy studies are conducted under widely varying conditions, the survival rates observed in the animal studies cannot be directly compared between studies and may not reflect the rates observed in clinical practice.
The efficacy of raxibacumab for treatment of inhalational anthrax was studied in a monkey model (Study 2) and a rabbit model (Studies 3 and 4) of inhalational anthrax disease. These 3 studies tested raxibacumab efficacy compared with placebo. Another study in a rabbit model (Study 1) evaluated the efficacy of raxibacumab in combination with an antibacterial drug relative to the antibacterial drug alone. Studies were randomized and blinded.
The animals were challenged with aerosolized B. anthracis spores (Ames strain) at 200 x LD50 to achieve 100% mortality if untreated. In rabbit Study 1, treatment was delayed until 84 hours after spore challenge. In monkey Study 2, study treatment commenced at the time of a positive serum electrochemiluminescence (ECL) assay for B. anthracis PA. The mean time between spore challenge and initiation of study treatment was 42 hours. In rabbit Studies 3 and 4, sustained elevation of body temperature above baseline for 2 hours or a positive result on serum ECL assay for PA served as the trigger for initiation of study treatment. The mean time between spore challenge and initiation of study treatment was 28 hours post-exposure. Efficacy in all therapeutic studies in animals was determined based on survival at the end of the study. Most study animals (88% to 100%) were bacteremic and had a positive ECL assay for PA prior to treatment in all 4 studies.
Treatment Of Inhalational Anthrax In Combination With Antibacterial Drug
The efficacy of raxibacumab administered with levofloxacin as treatment of animals with systemic anthrax disease (84 hours after spore challenge) was evaluated in New Zealand White rabbits (Study 1). The dose of levofloxacin was chosen to yield a comparable exposure to that achieved by the recommended doses in humans. Levofloxacin and raxibacumab PK in this study were unaffected by product co-administration. Forty-two percent of challenged animals survived to treatment. Treatment with antibacterial drug plus raxibacumab resulted in 82% survival compared with 65% survival in rabbits treated with antibacterial drug alone, P = 0.0874 (Table 4).
Table 4: Survival Rates in NZW Rabbits in Combination Therapy Study, All Treated Animals
| NZW Rabbits (35 days)a Study 1 |
| Number (%) Survivors | P Valueb | 95% CIc Levofloxacin versus Levofloxacin + Raxibacumab |
| Levofloxacin alone | 24/37 (65%) | - | - |
| Levofloxacin + Raxibacumab 40 mg/kg intravenous single dose | 32/39 (82%) | 0.0874 | (-2.4, 36.7) |
aSurvival assessed 28 days after last dose of levofloxacin. bP value based on a two-sided likelihood ratio chi-square test. c95% confidence interval based on normal approximation. |
Post-Exposure Prophylaxis/Early Treatment Of Inhalational Anthrax
Monkey Study 2 and rabbit Studies 3 and 4 evaluated treatment with raxibacumab alone at an earlier time point after exposure than rabbit Study 1. Treatment with raxibacumab alone resulted in a statistically significant dose-dependent improvement in survival relative to placebo when administered at the time of initial manifestations of anthrax disease in the rabbit and monkey infection models (Table 5). Raxibacumab at 40-mg/kg intravenous single dose was superior to placebo in the rabbit and monkey studies in the all treated and the bacteremic animal analysis populations. All surviving animals developed toxin-neutralizing antibodies.
Table 5: Survival Rates in Animals Treated with Raxibacumab, All Treated Animals
| Study Drug | Cynomolgus Macaques at 28 Daysa Study 2 | NZW Rabbits at 14 Daysb Study 3 | NZW Rabbits at 28 Daysa Study 4 |
| Number (%) Survivors | P Valuec | 95% CId | Number (%) Survivors | P Valuec | 95% CId | Number (%) Survivors | P Valuec | 95% CId |
| Placebo | 0/12 | | | 0/17 | | | 0/24 | | |
| 20 mg/kg raxibacumab | 7/14 (50%) | 0.0064 | (19.3, 73.7) | 5/18 (28%) | 0.0455 | (6.6, 52.5) | - | - | - |
| 40 mg/kg raxibacumab | 9/14 (64%) | 0.0007 | (31.6, 84.7) | 8/18 (44%) | 0.0029 | (21.3, 66.7) | 11/24 (46%) | 0.0002 | (27.0, 66.1) |
aSurvival measured at 28 days after spore challenge. bSurvival measured at 14 days after spore challenge. cP value based on two-sided Fisher's exact test for comparisons between raxibacumab and placebo. d95% CIs are exact confidence intervals for the difference between raxibacumab and placebo. |
In other animal studies evaluating antibacterial drug alone and raxibacumab-antibacterial drug combination, the efficacy of an antibacterial drug alone (levofloxacin in rabbits and ciprofloxacin in monkeys) was very high (95% to 100%) when given at the initial manifestations of inhalational anthrax disease. The timing of treatment was similar to that reported for Studies 2, 3, and 4 above.
In another study, rabbits were exposed to 100 times LD B. anthracis spores and administered raxibacumab at a single dose of 40 mg/kg at the time of exposure, 12 hours, 24 hours, or 36 hours after exposure. Survival was 12/12 (100%) in animals treated at time of exposure or 12 hours, but decreased to 6/12 (50%) and 5/12 (42%) at 24 hours and 36 hours, respectively.
Animal Toxicology
Healthy cynomolgus macaques administered 3 intravenous doses or 3 subcutaneous doses of 40 mg/kg raxibacumab once every 12 days, or a single intramuscular dose (40 mg/kg) of raxibacumab, showed no adverse effects, including no effects up to 120 days post-dosing.
Studies with raxibacumab in rabbit, cynomolgus macaque, and human donor tissues showed no cross reactivity with brain.
Anthrax-infected rabbits and monkeys administered an intravenous injection of raxibacumab (40 mg/kg) at time of PA toxemia reproducibly showed greater severity of central nervous system (CNS) lesions (bacteria, inflammation, hemorrhage, and necrosis) in non-surviving animals compared with dead placebo-control animals, with no difference in mean time to death from spore challenge. The raxibacumab monoclonal antibody appears unable to penetrate the CNS until compromise of the bloodbrain barrier (BBB) during the later stages of anthrax infection. The most severe brain lesions in rabbits were associated with bacteria and raxibacumab tissue binding in a similar pattern as endogenous IgG antibody that leaked across the compromised BBB. No dose/exposure-response relationship for brain histopathology was identified. Surviving rabbits and monkeys at the end of the 28-day studies showed no microscopic evidence of CNS lesions. CNS toxicity was not observed in healthy monkeys administered raxibacumab (40 mg/kg) or in GLP combination treatment studies with antibacterials in rabbits (levofloxacin) or in monkeys (ciprofloxacin) at any time.