Anthrax infected New Zealand white rabbits and cynomolgus macaques administered an intravenous injection of ANTHRASIL (15 units TNA per kg) that did not survive their infection showed an increase in the severity and/or incidence of central nervous system lesions (bacteria, hemorrhage and necrosis) as compared to intravenous immune globulin (“placebo”) treated animals who also did not survive the infection. The mean time to death between non-surviving ANTHRASIL and placebo treated animals was comparable.
Surviving rabbits had no evidence of central nervous system lesions at the end of the study. No surviving cynomolgus macaques in monotherapeutic studies were tested for central nervous system lesions.
Monotherapeutic Studies in Animal Models
In a monotherapeutic efficacy study, rabbits were exposed to a target dose of 200 x LD50 aerosolized anthrax spores and then administered 15 units per kg of ANTHRASIL at the onset of toxemia, as determined by the presence of PA in serum samples. Detection of PA was used as the trigger for initiation of treatment, while bacteremia status provided a retrospective confirmation of disease. Ninety-eight (98) percent of the treated animals were bacteremic prior to treatment. Of the animals that were toxemic and bacteremic prior to treatment, ANTHRASIL treatment resulted in a 26% survival in comparison to a 2% survival with IGIV placebo treatment (Table 4) over the 36 day duration of the study. ANTHRASIL treatment resulted in a significant decrease in the proportion of rabbits that were toxemic or bacteremic. The time to resolution of toxemia (p=0.0006) or bacteremia (p=0.0074) was also significantly reduced in rabbits that received ANTHRASIL.
Efficacy of ANTHRASIL was also assessed in cynomolgus macaques exposed to a target dose of 200 x LD50 aerosolized anthrax spores. Treatment with placebo or one of three dose levels of ANTHRASIL was initiated after animals became toxemic (positive for PA detection in serum samples), and bacteremia status provided a retrospective confirmation of disease. Survival was assessed over a period of 88 days in toxemic animals that were confirmed to be bacteremic at the time of treatment. Survival was 0% in placebo treated animals. Animals treated with 7.5 units per kg exhibited 36% survival, those treated with 15 units per kg exhibited 43% survival, and those treated with 30 units per kg exhibited 70% survival (Table 4). Compared to placebo, these survival rates were statistically significant at p=0.0451, 0.0339, and 0.0031, respectively The differences in survival between the 7.5, 15, and 30 unit per kg doses of ANTHRASIL were not statistically significant. ANTHRASIL treated animals showed a statistically significant reduction in anthrax toxin when compared to placebo treated animals.
Table 4 Survival Rates in NZW Rabbits and Cynomolgus Macaques Treated with ANTHRASILa Survival among animals that were bacteremic and toxemic prior to treatment
b Two-sided Fisher’s exact test
c Bonferroni-Holm adjusted one-sided Fisher’s exact test
d Dose not evaluated in rabbits in this study PI = Post-infection |
| NZW Rabbits at 36 Days PI | Cynomolgus Macaques at 28 Days PI |
No. Survivors (%)a
| p-Valueb
| No. Survivors (%)a
| p-Valuec
|
Placebo | 1/48 (2) | – | 0/11 (0) | – |
ANTHRASIL 7.5 U/kgd
| – | – | 4/11 (36) | 0.0451 |
ANTHRASIL 15 U/kg | 13/50 (26) | 0.0009 | 6/14 (43) | 0.0339 |
ANTHRASIL 30 U/kgd
| – | – | 7/10 (70) | 0.0031 |
ANTHRASIL Efficacy in Combination with Antibiotics
The efficacy of ANTHRASIL administered with levofloxacin was determined in New Zealand white rabbits with systemic disease. No significant difference between the control (normal immune globulin [IGIV] plus levofloxacin) and treatment groups (ANTHRASIL plus levofloxacin) was seen when combination treatment was delayed up to 60 hours post-challenge. There was no observed antagonism between levofloxacin and ANTHRASIL in this study. This study also supported that ANTHRASIL effectively cleared toxemia when administered with antibiotics. In ANTHRASIL treated groups, all animals cleared PA toxemia post-ANTHRASIL administration and only 4/31 (13%) of ANTHRASIL treated animals exhibited a single transient positive PA result for toxemia at the 12 or 18 hour time point post-dosing. Placebo control animals exhibited more persistent toxemia, with 26/32 (81%) having positive PA results for 18 to 90 hours post-treatment.
In a second study, treatment was delayed beyond 60 hours to simulate a clinical scenario. When combination treatment was initiated at 60, 72, 84 or 96 hours post anthrax exposure, differences in survival were seen, but no statistically significant added survival benefit was observed between groups that received placebo (IGIV plus levofloxacin) or ANTHRASIL (15 units per kg plus levofloxacin). An increase in survival was observed with ANTHRASIL when treatment was delayed to 96 hours post exposure, but was not statistically significant. When treatment was delayed to 96 hours, survival was 25% (2/8) in the antibiotic plus IGIV control group and 71% (5/7) in the ANTHRASIL plus levofloxacin group. A marginal improvement of 10 to 15% was observed at other time points, suggesting a trend in added benefit with ANTHRASIL. This study also demonstrated a significant effect of ANTHRASIL on toxemia. The majority of ANTHRASIL treated animals became negative for PA (toxemia) within one hour post-infusion of ANTHRASIL and remained negative, even with the delayed treatment from 60 to 96 hours post-anthrax challenge and high levels of toxemia pretreatment. In contrast, placebo treated animals remained toxemic up to three days after initiating antibiotic treatment.
The efficacy of ANTHRASIL co-administered with levofloxacin was evaluated in New Zealand white rabbits when treatment was delayed to 96 hours after anthrax spore inhalation. The dose of levofloxacin was chosen to yield a comparable exposure to that achieved by the recommended dose in humans. Of the animals that survived to be treated (19% of those challenged), antibacterial drug plus ANTHRASIL (15 units per kg) resulted in 58% (18/31) survival compared to 39% (13/33) survival in rabbits treated with antibacterial drug and IGIV placebo (p=0.14, Z-test).
When animals were stratified by pre-treatment toxemia (PA) in a post hoc analysis, added benefit was observed in animals treated with ANTHRASIL and levofloxacin when they had pre-treatment PA levels between 200 and 800 ng/mL (p=0.02, Fisher’s exact test). When pre-treatment toxemia was low (PA <200 ng/mL), survival was greater than 90% in all animals, regardless of treatment (Table 5). Animals with very high levels of toxemia (>800 ng/mL) did not survive irrespective of the treatment administered.
Table 5 Survival Rates in New Zealand White Rabbits Stratified by Pre-treatment PA LevelsPre-treatment PA (ng/mL) | IGIV Placebo + Levofloxacin (%) | ANTHRASIL + Levofloxacin (%) |
<200 | 11/12 (91.7) | 8/9 (88.9) |
200–800 | 2/11 (18.2) | 10/14 (71.4) |
>800 | 0/10 (0) | 0/8 (0) |
All pre-treatment PA levels | 13/33 (39.4) | 18/31 (58.1) |
ANTHRASIL and antibiotic combination treatment was also studied in the cynomolgus macaque model of inhalational anthrax. In this study, delay of initiation of treatment to 64 hours post anthrax exposure resulted in 75% (9/12) survival in the placebo plus ciprofloxacin treatment group versus 83% (10/12) survival in the ANTHRASIL (15 units per kg) plus ciprofloxacin group (p=1).
No antagonism of ANTHRASIL when administered with antibiotic as a concomitant therapy was observed.
ANTHRASIL in Post-exposure Prophylaxis
A post exposure prophylactic study assessed the survival following aerosol exposure to a lethal dose of anthrax spores (200 x LD50) in New Zealand white rabbits administered ANTHRASIL (7.5, 15 or 30 units TNA per kg) at 30 hours post-anthrax challenge compared to placebo controls. All three doses of ANTHRASIL improved survival when given 30 hours post-anthrax challenge. When animals that were both bacteremic and toxemic were treated at 30 hours following challenge, there was a 22% (2/9) survival with a dose of 15 units TNA per kg and a 33% (4/12) survival with a dose of 30 units TNA per kg. All rabbits in the placebo arm died.