BIOTHRAX Suspension for injection Ref.[9978] Active ingredients: Anthrax antigen

The effect of BioThrax on embryo-fetal and pre-weaning development was evaluated in a developmental toxicity study using pregnant rabbits. One group of rabbits was administered BioThrax twice prior to gestation and during the period of organogenesis (gestation day 7). A second group of rabbits was administered BioThrax twice prior to gestation and on gestation day 17. BioThrax was administered at 0.5 ml/rabbit/occasion, by intramuscular injection. No adverse effects on mating, fertility, pregnancy, parturition, lactation, embryo-fetal or pre-weaning development were observed. There were no vaccine-related fetal malformations or other evidence of teratogenesis noted in this study.

Since it is not feasible or ethical to conduct controlled clinical trials with anthrax, the efficacy of BioThrax in a post-exposure setting is based on studies in animals. Pre-exposure prophylaxis animal models were used to derive protective antibody thresholds to bridge animal efficacy and human immunogenicity data and predict efficacy in humans.

Pivotal efficacy animal studies were conducted in rabbits and nonhuman primates (NHPs). Animals received two IM vaccinations four weeks apart with serial dilutions of BioThrax and were subjected to lethal challenge on study day 70 with aerosolized B. anthracis spores at a target dose exceeding the 50% lethal dose by 200-fold. Serum samples were collected at various time points prior to challenge for immune response analysis via anthrax lethal toxin neutralizing antibody (TNA) assay. The relationship between pre-challenge serum TNA levels and survival was evaluated. Logistic regression analysis demonstrated that a 70% probability of survival was associated with a TNA NF₅₀ (50% neutralization factor) level of 0.56 in rabbits and 0.29 in NHPs.

The ability of BioThrax to increase survival after the cessation of the post-exposure antimicrobial treatment, as compared with antimicrobial treatment alone, was investigated in two post-exposure animal model studies. In these studies, rabbits were challenged via inhalation with aerosolized B. anthracis spores and subsequently treated with levofloxacin administered via oral gavage once daily for 7 days starting at 6-12 hours post-exposure, with or without two intramuscular injections of BioThrax one week apart. Survival among animals that received both antimicrobial treatment and vaccination was between 70-100% and increased in a vaccine dose-dependent manner. In contrast, only 44% and 23% survival was observed among animals that received antimicrobial treatment only in the first and the second study, respectively (p <0.0006 and p <0.004, respectively) [See Clinical Studies (14.2)].

14.1 Pre-Exposure Prophylaxis

A controlled field study using an earlier version of a protective antigen-based anthrax vaccine developed in the 1950’s and supplied by G. G. Wright and associates of the U.S. Army Chemical Corps, Fort Detrick, Frederick, MD, that consisted of an aluminum potassium sulfate-precipitated cell-free filtrate from an aerobic culture, was conducted from 1955-1959⁴. This study included 1,249 workers [379 received anthrax vaccine, 414 received placebo, 116 received incomplete inoculations (with either vaccine or placebo) and 340 were in the observational group (no treatment)] in four mills in the northeastern United States that processed imported animal hides. The anthrax vaccine was administered subcutaneously at 0, 2, 4 weeks, 6, 12, 18 months. Prior to vaccination, the yearly average number of human anthrax cases (both cutaneous and inhalational) was 1.2 cases per 100 employees in these mills. During the trial, 26 cases of anthrax were reported across the four mills – 5 inhalation and 21 cutaneous. Of the five inhalation cases (four of which were fatal), two received placebo and three were in the observational group. Of the 21 cutaneous cases, 15 received placebo, three were in the observational group, and three received anthrax vaccine. Of those three cases in the vaccine group, one case occurred just prior to administration of the scheduled third dose, one case occurred 13 months after an individual received the third of the scheduled 6 doses (but no subsequent doses), and one case occurred prior to receiving the scheduled fourth dose of vaccine. The calculated efficacy of the vaccine to prevent all types of anthrax disease, regardless of the route of exposure or clinical manifestations, was 92.5% (lower 95% Confidence Interval (CI) = 65%).

Between 1962 and 1974, the Centers for Disease Control and Prevention (CDC) collected surveillance data on the occurrence of anthrax disease in mill workers or those living near mills in the United States⁵^,⁶. In that time period, individuals received either BioThrax or the earlier protective antigen-based anthrax vaccine used in the field trial described above. Of the 27 reported cases of anthrax, 24 cases occurred in unvaccinated individuals. In vaccinated individuals one case occurred after the person had been given one dose of anthrax vaccine and two cases occurred after individuals had been given two doses of anthrax vaccine. No documented cases of anthrax were reported for individuals who had received at least three doses of the originally licensed six-dose series of anthrax vaccine.

Between 2002 and 2007, a prospective double-blinded, randomized, placebo-controlled and active-controlled study was conducted to evaluate the impact on safety and immunogenicity on changing the administration route from SC to IM, and reducing the number of doses. This study enrolled 1,564 healthy civilian men and women between the ages of 18 and 61. A total of 1,563 subjects received at least one dose (one subject withdrew consent prior to the first injection). Subjects were randomized to one of six groups. See Table 1.

Using an Enzyme-Linked Immunosorbent Assay (ELISA), Immunoglobulin G (IgG) antibodies directed against anthrax protective antigen (PA) were measured at the Week 8 and Months 7, 13, 19, 31, and 43 time points. The three primary immunogenicity endpoints were: (1) Geometric Mean Concentration (GMC) (mcg/mL), (2) Geometric Mean Titer (GMT), and (3) percentage with 4-fold rise in anti-PA antibody titer from baseline.

The criteria for non-inferiority of comparisons based on ratios of GMCs and GMTs and differences in the rates of 4-fold rise in antibody titer were defined as follows:

• Mean antibody concentration ratio: non-inferiority was achieved when the upper bound of the 95% confidence limit was <1.5
• Mean antibody titer ratio: non-inferiority was achieved when the upper bound of the 95% confidence limit was <1.5
• 4-fold rise in antibody titer: non-inferiority was achieved when the upper bound of the 95% confidence limit was <0.10

To compare the originally licensed 6-dose SC schedule (0, 2, 4 weeks and 6, 12, and 18 months) versus a 3-dose IM primary series (at 0, 1, and 6 months), non-inferiority analyses were performed for all three primary immunogenicity endpoints. This evaluation compared the immune response at Month 7 for Group C (COM, where COM is Combined, as described in 6.1) to Month 19 for Group A (TRT-8SC, where TRT is Treatment) and Group B (TRT-8IM). Non-inferiority was demonstrated for all analyses (See Table 3). These results support a 3 dose primary series of BioThrax administered IM at 0, 1 and 6 months, followed by booster doses at 12 and 18 months and at 1-year intervals thereafter to maintain protective immunity.

The Month 7 antibody levels of Group A (TRT-8SC) were non-inferior to Month 13 and 19 antibody levels after a 0, 2, 4 week and 6 month primary SC series followed by SC booster injections at 12 and 18 months (see Table 3). These results support a 4 dose SC primary series of BioThrax administered at weeks 0, 2, 4, and at 6 months followed by booster doses at 12 and 18 months after initiation of the series, and at 1-year intervals thereafter to maintain protective immunity.

Catch-up Administration for Delayed or Missed Doses

In subjects who did not receive booster doses at 12, 18, and 30 months, PA antibody levels decline over time following the third dose of BioThrax administered intramuscularly at 6 months (Group F; 4IM; 0, 1, 6, and 42 months). In the absence of booster doses it is not known whether these individuals are adequately protected between 12 months and receipt of a booster dose at 42 months. One month following a dose of BioThrax at 42 months the immune response for Group F met the criteria for non-inferiority relative to Group A (8SC) for all three primary immunogenicity endpoints (see Table 3). The optimal schedule for further intramuscular booster doses among persons administered a single booster dose at 42 months following completion of a three-dose primary series at 0, 1, and 6 months is not known.

Table 3. Primary Immunogenicity Endpoints (According to Protocol^a):

CI: Confidence Interval;
^a According to Protocol (ATP): [NCT00119067] To be included in the ATP population at a particular timepoint, a participant must have: (a) received all injections up through that timepoint, (b) received these injections within the windows defined by protocol, © received the correct agent administered by the correct route according to subject’s assigned study arm, (d) received the correct injection volume. A shot of 0.3 mL or greater is considered valid.
^b Groups TRT-7IM, -5IM, and -4IM combined as group TRT-COM (combined) through Month 7 of the study, GMC: geometric mean concentration. GMT: geometric mean titer. IM: Intramuscular; SC: Subcutaneous, TRT: treatment.

14.2 Post-Exposure Prophylaxis

Based on the rabbit model-derived TNA threshold [See Nonclinical Toxicology (13.2)], a pivotal clinical study was conducted to evaluate the immunogenicity and safety of a post-exposure SC administration schedule of BioThrax in healthy adults following 3 doses at 0, 2, and 4 weeks. Two hundred subjects were enrolled and followed for 128 days. The primary objective was to assess immunogenicity using TNA following the completion of three SC doses of BioThrax. The primary immunogenicity endpoint was the proportion of subjects achieving a threshold TNA NF₅₀ value ≥0.56 at Day 63, 5 weeks after the third vaccination. Success was concluded if the lower bound of the 2-sided 95% CI of the proportion of human subjects achieving the TNA NF₅₀ threshold was ≥ 40%.

Overall, 71.2% of subjects achieved an NF₅₀ value ≥0.56 on Day 63 in the pivotal study. The lower bound of the 95% CI was 64.1% (See Table 4).

In a separate analysis of the pivotal clinical study using the threshold associated with a 70% probability of survival in NHPs, 93.5% of subjects achieved an NF₅₀ value ≥0.29 on Day 63 (Table 4). The lower bound of the 95% CI was 88.9% (Table 4). The bridging of human immunogenicity data to the NHP study was supportive of the primary analysis comparing human threshold data with rabbit survival. [See Nonclinical Toxicology (13.2)]

Table 4. Proportion of Subjects Achieving TNA NF₅₀ Threshold^a in the Pivotal Clinical Study (PP Population^b):

CI = confidence interval; NF₅₀ = 50% neutralization factor; PP = per protocol; SD = standard deviation; TNA = toxin-neutralizing antibody.
Note: Sample size (N) and denominators used for percentages are based on the number of subjects meeting the PP criteria at specified day(s).
^a TNA NF₅₀ threshold is defined as the TNA NF₅₀ value associated with 70% survival in the animal challenge studies.
^b Human data are from the pivotal clinical study (NCT01491607).
^c A logistic regression model with log10-transformed TNA NF₅₀ values as the predictor and survival as the response is used to derive the TNA NF₅₀ threshold associated with 70% probability of survival in rabbits and non-human primates, respectively.
^d 95% CI is calculated with the exact (Clopper-Pearson) method.
^e The proportion of subjects achieving a TNA NF₅₀ response at Day 63 that met or exceeded the TNA NF₅₀ threshold in the rabbit model at Day 69 comprised the primary immunogenicity endpoint.
^fComparison of the human TNA NF ⁵⁰ response at Day 63 with the NHP TNA NF ⁵⁰ threshold at Day 70 was defined as an immunogenicity endpoint and was supportive of the bridging of human immunogenicity data to rabbit survival.

14.3 Non-Interference of Post-Exposure Prophylaxis Vaccination and Antimicrobials When Used Concurrently

An open-label study was conducted to evaluate the potential impact 0.5 mL BioThrax administered SC at 0, 2 and 4 weeks had on the pharmacokinetics of ciprofloxacin in healthy adult male and female subjects (N=154). It also evaluated the potential impact of ciprofloxacin on immunogenicity of BioThrax two weeks following the last BioThrax dose.

Co-administration of 0.5 mL BioThrax SC with oral ciprofloxacin in human subjects did not alter the pharmacokinetics of ciprofloxacin or the immunogenicity of BioThrax as measured by the anthrax lethal toxin neutralization assay.