Development of a guinea pig inhalational anthrax model for evaluation of post-exposure prophylaxis efficacy of anthrax vaccines.

A next-generation anthrax vaccine candidate, AV7909, is being developed for post-exposure prophylaxis (PEP) of inhalational anthrax in combination with the recommended course of antimicrobial therapy. Clinical efficacy studies of anthrax countermeasures in humans are not ethical or feasible, therefore, licensure of AV7909 for PEP is being pursued under the US Food and Drug Administration (FDA) Animal Rule, which requires that evidence of effectiveness be demonstrated in an animal model of anthrax, where results of studies in such a model can establish reasonable likelihood of AV7909 to produce clinical benefit in humans. Initial development of a PEP model for inhalational anthrax included evaluation of post-exposure ciprofloxacin pharmacokinetics (PK), tolerability and survival in guinea pigs treated with various ciprofloxacin dosing regimens. Three times per day (TID) intraperitoneal (IP) dosing with 7.5 mg/kg of ciprofloxacin initiated 1 day following inhalational anthrax challenge and continued for 14 days was identified as a well tolerated partially curative ciprofloxacin treatment regimen. The added benefit of AV7909 vaccination was evaluated in guinea pigs given the partially curative ciprofloxacin treatment regimen. Groups of ciprofloxacin-treated guinea pigs were vaccinated. 1 and 8 days post-challenge with serial dilutions of AV7909, a 1:16 dilution of AVA, or normal saline. A group of untreated guinea pigs was included as a positive control to confirm lethal B. anthracis exposure. Post-exposure vaccination with the AV7909 anthrax vaccine candidate administered in combination with the partially curative ciprofloxacin treatment significantly increased survival of guinea pigs compared to ciprofloxacin treatment alone. These results suggest that the developed model can be useful in demonstrating added value of the vaccine for PEP.

Biochemical and aerosol characterization of ricin for use in non-clinical efficacy studies.

Ricin toxin may be used as a biological warfare agent and no medical countermeasures are currently available. Here, a well-characterized lot of ricin was aerosolized to determine the delivered dose for future pre-clinical efficacy studies.  Mouse intraperitoneal (IP) median lethal dose (LD50 ) bioassay measured potency at 5.62 and 7.35 µg/kg on Days 0 and 365, respectively. Additional analyses included total protein, sodium dodecyl sulfate polyacrylamide gel electrophoresis, Western blotting, and rabbit reticulocyte lysate activity assay. The nebulizer aerosol produced consistent concentrations (2.5 × 103 , 5.0 × 103 , 1.0 × 104 , and 1.5 × 104  µg/mL) and spray factor values. The aerosol particle size distribution was of sufficient size to deposit in lung alveoli (1.12-1.43 µm). Ricinus communis Agglutinin II (RCA 60), prepared at 19 mg/mL in phosphate-buffered saline, pH 7.8, and stored at -70°C, maintained attributes for toxicity following 1-year storage and aerosolized consistently.

Botulinum neurotoxin neutralizing activity of immune globulin (IG) purified from clinical volunteers vaccinated with recombinant botulinum vaccine (rBV A/B).

The basis for efficacy of the recombinant botulinum vaccine, serotypes A and B (rBV A/B) is that neutralizing antibodies induced by vaccination bind to botulinum neurotoxin complex serotype A, subtype A1 (BoNT/A1) and serotype B, subtype B1 (BoNT/B1) and prevent their actions at cholinergic neurons. The protective capacity of BoNT/A1 and BoNT/B1 neutralizing antibodies derived from the serum of clinical volunteers vaccinated with rBV A/B was evaluated in a guinea pig passive transfer model and a mouse bioassay. Guinea pigs passively immunized to achieve circulating neutralizing antibody concentration (NAC) levels representing the lowest measurable concentrations for BoNT/A1 and BoNT/B1 were protected against an intramuscular (IM) challenge more than 10 times the guinea pig IM median lethal dosage for BoNT/A1 and BoNT/B1. The passively immunized guinea pigs were asymptomatic during the 14-day post-challenge observation period. Control guinea pigs died within 48 h after challenge. Calculation of neutralizing efficiency of antibodies using results from a mouse bioassay indicated that a simple linearly proportional relationship does not exist between NAC level and the amount of BoNT neutralized. Based on this finding, estimates of level of protection must consider variability in BoNT neutralizing efficiency at different NAC levels. The protective capacity of human BoNT/A1 and BoNT/B1 neutralizing antibodies induced by rBV A/B vaccination was verified in a guinea pig passive immunization model. Additionally, estimates of the neutralizing efficiency have been established for BoNT/A1 and BoNT/B1 neutralizing antibodies obtained from clinical volunteers vaccinated with the rBV A/B.

Inhalational botulism in rhesus macaques exposed to botulinum neurotoxin complex serotypes A1 and B1.

A recombinant botulinum vaccine (rBV A/B) is being developed for protection against inhalational intoxication with botulinum neurotoxin (BoNT) complex serotype A, subtype A1 (BoNT/A1), and BoNT serotype B, subtype B1 (BoNT/B1). A critical component for evaluating rBV A/B efficacy will be the use of animal models in which the pathophysiology and dose-response relationships following aerosol exposure to well-characterized BoNT are thoroughly understood and documented. This study was designed to estimate inhaled 50% lethal doses (LD(50)) and to estimate 50% lethal exposure concentrations relative to time (LCt(50)) in rhesus macaques exposed to well-characterized BoNT/A1 and BoNT/B1. During the course of this study, clinical observations, body weights, clinical hematology results, clinical chemistry results, circulating neurotoxin levels, and telemetric parameters were documented to aid in the understanding of disease progression. The inhaled LD(50) and LCt(50) for BoNT/A1 and BoNT/B1 in rhesus macaques were determined using well-characterized challenge material. Clinical observations were consistent with the recognized pattern of botulism disease progression. A dose response was demonstrated with regard to the onset of these clinical signs for both BoNT/A1 and BoNT/B1. Dose-related changes in physiologic parameters measured by telemetry were also observed. In contrast, notable changes in body weight, hematology, and clinical chemistry parameters were not observed. Circulating levels of BoNT/B1 were detected in animals exposed to the highest levels of BoNT/B1; however, BoNT/A1 was not detected in the circulation at any aerosol exposure level. The rhesus macaque aerosol challenge model will be used for future evaluations of rBV A/B efficacy against inhalational BoNT/A1 and BoNT/B1 intoxication.