Human IgG1λ Monoclonal Antibody Intravenous Infusion

Raxibacumab is a human IgG1λ monoclonal antibody that binds the PA component of B. anthracis toxin. Raxibacumab has a molecular weight of approximately 146 kilodaltons. Raxibacumab is produced by recombinant DNA technology in a murine cell expression system.

Raxibacumab is supplied as a sterile, liquid formulation in single-dose vials for intravenous infusion. Each vial contains 1,700 mg/34 mL (50 mg/mL) raxibacumab in citric acid (0.13 mg/mL), glycine (18 mg/mL), polysorbate 80 [0.2 mg/mL (w/v)], sodium citrate (2.8 mg/mL), and sucrose (10 mg/mL), with a pH of 6.5. Raxibacumab is a clear to opalescent, colorless to pale yellow, liquid.

Dosage Forms And Strengths

Raxibacumab is available as a single-use vial which contains 1,700 mg/34 mL (50 mg/mL) raxibacumab injection [see DESCRIPTION].

Storage And Handling

Raxibacumab is supplied in single-use vials containing 1,700 mg/34 mL (50 mg/mL) raxibacumab and is available in the following packaging configuration:

Single Unit Carton: Contains one (1) single-use vial of raxibacumab 1,700 mg/34 mL (deliverable) (NDC 49401-103-01).

Raxibacumab must be refrigerated at 2° to 8°C (36° to 46°F). DO NOT FREEZE. Protect the vial from exposure to light, prior to use. Brief exposure to light, as with normal use, is acceptable. Store vial in original carton until time of use.

Manufactured by : Human Genome Sciences, Inc. (a subsidiary of GlaxoSmithKline), Rockville, MD 20850. Marketed by GlaxoSmithKline, Research Triangle Park, NC 27709. Revised: November 2015.

Included as part of the PRECAUTIONS section.

Mechanism Of Action

Raxibacumab is a monoclonal antibody that binds the PA of B. anthracis [see Microbiology].

Pharmacokinetics

The PK of raxibacumab are linear over the dose range of 1 to 40 mg/kg following single IV dosing in humans; raxibacumab was not tested at doses higher than 40 mg/kg in humans. Following single IV administration of raxibacumab 40 mg/kg in healthy, male and female human subjects, the mean Cmax and AUCinfwere 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 IV 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 IV dose compared with New Zealand White rabbits and cynomolgus macaques receiving the same dosage regimen.

Raxibacumab PK were evaluated via a population PK analysis using serum samples from 322 healthy subjects who received a single 40 mg/kg IV 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 have not been evaluated in children [see DOSAGE AND ADMINISTRATION, Use In Specific Populations].

Although raxibacumab is intended for single dose administration, the PK of raxibacumab following a second administration of 40 mg/kg IV given 14 days after the first 40 mg/kg IV 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 IV received a second 40 mg/kg IV 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.

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 IV following ciprofloxacin IV 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

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.

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]

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 to 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 blood-brain 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.

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 to 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 200xLD50 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 to 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

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 IV 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

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 (95-100%) 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 LD50 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.

Clinical Trials Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared with rates in the clinical trials of another drug and may not reflect the rates observed in practice. The safety of raxibacumab has been studied only in healthy volunteers. It has not been studied in patients with inhalational anthrax.

The safety of raxibacumab has been evaluated in 326 healthy subjects treated with a dose of 40 mg/kg in 3 clinical trials: a drug interaction trial with ciprofloxacin (Study 1), a repeat-dose trial of 20 subjects with the second raxibacumab dose administered ≥ 4 months after the first dose (Study 2), and a placebo-controlled trial evaluating single doses with a subset of subjects receiving 2 raxibacumab doses 14 days apart (Study 3). Raxibacumab was administered to 86 healthy subjects in Study 1. In Study 3, 240 healthy subjects received raxibacumab (217 received 1 dose and 23 received 2 doses) and 80 subjects received placebo.

The overall safety of raxibacumab was evaluated as an integrated summary of these 3 clinical trials. Of 326 raxibacumab subjects, 283 received single doses, 23 received 2 doses 14 days apart, and 20 received 2 doses more than 4 months apart. The subjects were 18 to 88 years of age, 53% female, 74% white, 17% black/African American, 6% Asian, and 15% Hispanic.

Four subjects (1.2%) had their infusion of raxibacumab discontinued for adverse reactions: 2 subjects (neither of whom received diphenhydramine premedication) due to urticaria (mild), and 1 subject each discontinued for clonus (mild) and dyspnea (moderate).

The most frequently reported adverse reactions were rash, pain in extremity, pruritus, and somnolence.

Table 3: Adverse Reactions Reported in ≥ 1.5% of Healthy Adult Subjects Exposed to Raxibacumab 40 mg/kg IV

For all subjects exposed to raxibacumab in clinical trials, the rate of rash was 2.8% (9/326) compared with 1.3% (1/80) of placebo subjects. Mild to moderate infusion-related rashes were reported in 22.2% (6/27) of subjects who did not receive diphenhydramine premedication compared to 3.3% (2/61) of subjects who were premedicated with diphenhydramine in the ciprofloxacin/raxibacumab combination trial (Study 1). In the placebo-controlled raxibacumab study where all subjects received diphenhydramine (Study 3), the rate of rash was 2.5% in both placebo- and raxibacumab-treated subjects.

Clinically significant adverse reactions that were reported in < 1.5% of subjects exposed to raxibacumab and at rates higher than in placebo subjects are listed below:

• Blood and lymphatic system: anemia, leukopenia, lymphadenopathy
• Cardiac disorders: palpitations
• Ear and labyrinth: vertigo
• General disorders and administration site: fatigue, infusion site pain, peripheral edema
• Investigations: blood amylase increased, blood creatine phosphokinase increased, prothrombin time prolonged
• Musculoskeletal and connective tissue: back pain, muscle spasms
• Nervous system: syncope vasovagal
• Psychiatric: insomnia
• Vascular: flushing, hypertension

The development of anti-raxibacumab antibodies was evaluated in all subjects receiving single and double doses of raxibacumab in Studies 1, 2, and 3. Immunogenic responses against raxibacumab were not detected in any raxibacumab-treated human subjects following single or repeat doses of raxibacumab.

The incidence of antibody formation is highly dependent on the sensitivity and specificity of the immunogenicity assay. Additionally, the observed incidence of any antibody positivity in an assay is highly dependent on several factors, including assay sensitivity and specificity, assay methodology, sample handling, timing of sample collection, concomitant medications, and underlying disease. For these reasons, comparison of the incidence of antibodies to raxibacumab with the incidence of antibodies to other products may be misleading.

Read the entire FDA prescribing information for Raxibacumab (Human IgG1λ Monoclonal Antibody Intravenous Infusion)