Rapid in vitro activity of telavancin against Bacillus anthracis and in vivo protection against inhalation anthrax infection in the rabbit model

Anthrax, caused by the bacterium Bacillus anthracis, is a zoonotic disease that manifests in various forms in human infection, depending on the route of infection. Inhalation anthrax, the most detrimental form of the disease, comes about from the inhalation of anthrax spores and progresses to severe life-threatening conditions late in infection. Notably, there are FDA-approved antibiotics that are effective at treating the disease when administered promptly; however, these antibiotics would be rendered useless against strains of B. anthracis that were genetically modified to be resistant to these antibiotics. Consequently, the search for new and effective therapeutics to combat anthrax infection continues. In this study, telavancin (Vibativ®), a semisynthetic lipoglycopeptide antibiotic, was assessed for in vitro activity against 17 B. anthracis strains and tested for the protective efficacy against inhalation anthrax infection in the rabbit model. Telavancin demonstrated potent in vitro activity against B. anthracis which led us to test its efficacy in the rabbit inhalation anthrax model. Rabbits were infected with a lethal dose of anthrax spores via the inhalation route and treated intravenously with telavancin at 30 mg/kg every 12 hours, a dose that mimics the levels measured in the serum of humans, for 5 days upon detection of antigenemia. Blood samples were collected at various times post-infection to assess the level of bacteremia and antibody production, and tissues were collected to determine bacterial load. The animals’ body temperatures were also recorded. Telavancin conveyed 100% survival in this model. Moreover, the dosage of telavancin used for the study effectively cleared B. anthracis from the bloodstream and organ tissues, even more effectively than a humanized dose of levofloxacin. Collectively, the low MICs against all strains tested and rapid bactericidal in vivo activity demonstrate that telavancin has the potential to be an effective alternative for the treatment or prophylaxis of anthrax infection. Author Summary Bacillus anthracis, the causative agent of anthrax, continues to interest the research community due to its past and future potential use as bioweapon. Importantly, as a bacterial pathogen, B. anthracis is capable of developing resistance to the antibiotics currently used to treat the infection, either naturally or by deliberate, nefarious means. Consequently, there remains a need to discover, develop, and/or repurpose new antibiotics that would be effective at treating anthrax infection. In this study, we evaluated the antibacterial activity of telavancin, a semisynthetic glycopeptide antibiotic clinically approved to treat complicated skin and skin structure infections, against various strains of B. anthracis in vitro, and we assessed the protective efficacy of telavancin against inhalation anthrax infection in the rabbit model. We show that telavancin is very potent against numerous B. anthracis strains in vitro, and its level of potency surpassed that of another antibiotic currently approved and used to prevent anthrax infection. Moreover, we show that telavancin protects against inhalation anthrax infection in vivo. Overall, our findings support the use of telavancin as an effective therapeutic for anthrax infection.


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Bacillus anthracis is a Gram-positive, spore-forming bacillus bacterium that causes 68 anthrax infection. It is a category A bioterrorism agent because it is "…infective in low doses…suitable for mass production, storage, and weaponization; [and] stable during  Telavancin (TD-6424, trade name Vibativ) is the only clinically approved 99 semisynthetic glycopeptide antibiotic derived from vancomycin. It differs most significantly 100 from its parent structure by the decylaminoethyl modification on the vancosamine unit, a 101 modification that is responsible for telavancin's enhanced potency against Gram-positive 102 strains [11,12]. This modification introduced unfavorable excretion and distribution 103 properties, so an additional (phosphonomethyl) aminomethyl moiety was appended to 104 ring 7, leading to an improved ADME profile [11,12]. 105 Telavancin has a dual mode of action. First, it retains the mechanism of action of 106 vancomycin by binding lipid II, thereby inhibiting bacterial cell wall biosynthesis [13,14]. 107 This interaction is promoted by the decylaminoethyl lipid, which anchors into the 108 cytoplasmic membrane and brings telavancin into proximity with peptidoglycan 109 precursors. For this reason, telavancin displays a higher binding affinity for the bacterial 110 cell surface and increased inhibition of transglycosylation [14]. These results led us to test the protective efficacy of telavancin against inhalation 149 anthrax infection using the rabbit model. The in vivo study was performed using the rabbit 150 model, which is a superior model for inhalation anthrax infection relative to rodent models

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[30] and was conducted as a trigger-to-treat study; therefore, treatment began after B.     Table 2). 185 The concentration of PA in the sera ranged from approximately 50 to 2,000 picograms 186 per milliliter (pg/ml).  Following infection, the animals were monitored at least twice daily for 14 days.

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The most common clinical signs included anorexia, lethargy, and respiratory distress.   Levofloxacin, at 12.5 mg/kg, administered once daily for 5 days, and saline, administered 207 once daily, were used as controls. Survival was monitored for 14 days post-infection.

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The percent survival rates for the two antibiotic-treated groups were significantly 209 (p<0.001) higher than that of the saline-treated group. The antibiotic-treated groups (telavancin and levofloxacin) exhibited comparable 213 temperature responses after the challenge (Fig 2). Specifically, animals in both groups 214 had febrile responses at approximately Day 1 post-infection that peaked at nearly 41⁰C 215 before Day 2. By Day 2, the mean temperatures for the antibiotic-treated groups returned 216 to baseline, most likely due to treatment which began at 18 to 30 hrs post-infection. 217 Interestingly, both antibiotic-treated groups appeared to have minor secondary febrile 218 responses from 7 to 9 days post-infection, which was after treatment was ended (Day 6), 219 but these temperature elevations subsided by Day 10 to 11. The mean temperature of 220 the animals treated with saline also began to rise by Day 1, and it remained elevated until   groups were significantly (p˂0.05) lower than that of the saline-treated group (Fig 3). 240 Approximately half the animals in both the telavancin-and levofloxacin-treated groups lower among the animals treated with telavancin (Fig 3). This would suggest that the  Overall, these results demonstrate that rabbits treated with telavancin more rapidly clear 251 B. anthracis from circulation than rabbits treated with levofloxacin.  (Fig 4). When comparing the two antibiotic-treated groups, the 276 average bacterial load in the lung tissue of the animals treated with telavancin (1.93 x 10 3 277 cfu/g) was significantly (p˂0.05) lower than that of the animals treated with levofloxacin 278 (6.93 x 10 3 cfu/g) (Fig 4), suggesting once again that the humanized telavancin dosage

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Histopathological analysis showed notable lesions in the tissues of the control 337 animals, with the lesions in the lungs, mediastinal lymph nodes, and spleen being the 338 most significant (Fig 6). Edema was consistently present in the lungs/alveoli of the control hemorrhage. Similarly, the spleen showed diffuse necrosis (Fig 6). The remaining 342 collected tissues in the control animals also exhibited pathology, albeit to a lesser extent.

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Specifically, there was evidence of degeneration and necrosis in the heart and liver; 344 bacteria were present in the liver, kidneys, and brain, and hemorrhage was present in the 345 brain (S1 Fig). Conversely, tissues from animals treated with telavancin and levofloxacin 346 showed only evidence of disease mitigation, and there were no substantial differences 347 among the tissues from these two groups (Fig 6 and S1 Fig). of both antibiotic-treated animals at termination. However, the bacterial load was 400 significantly less than that of the saline-treated animals (Fig. 4). Interestingly, the  [43], and the CSF was sterilized in six out of ten rabbits by telavancin 427 and only four out of ten rabbits in the comparator arm. Of note, it was reported that the 428 bacteria load at the initiation of therapy was significantly higher in the telavancin arm. The CDC's retrospective outcome study also underscores efforts to evaluate new 434 combination therapies for synergy will be critical for successful anthrax preparedness.