Identification and bioevaluation of SRI-12742 as an antimicrobial agent against multidrug-resistant Acinetobacter baumannii.

Multidrug-resistant Acinetobacter baumannii (MDR-Ab) is one of the most significant nosocomial pathogens that is being increasingly isolated in healthcare settings worldwide. Owing to its inherent drug-resistant nature, coupled with its ability to readily acquire resistance to other antibiotic classes, there is a real dearth of antibiotics available to treat infections with MDR-Ab. A commercially available library was screened against MDR-Ab BAA-1605 to identify novel inhibitory molecules. The selectivity index of a hit was tested against Vero cells and in vitro efficacy was profiled against a panel of clinical MDR-Ab. The bacteriostatic or bactericidal nature was determined by time-kill experiments, and synergy with clinically approved drugs was determined by the chequerboard method. Additionally, in vivo efficacy was measured in a murine neutropenic A. baumannii thigh infection model. SRI-12742 was identified as a potent active hit, with a minimum inhibitory concentration (MIC) of 4 mg/L against BAA-1605. Its activity was then profiled against a MDR-Ab clinical strain panel (MICs 4 mg/L to >64 mg/L). SRI-12742 exhibited concentration-dependent bactericidal activity and caused an ca. 16 log10 CFU/mL reduction at 10 × MIC in 24 h, which is comparable with minocycline. In a murine neutropenic thigh infection model of A. baumannii infection, SRI-12742 reduced CFU counts by ca. 0.9 log10 CFU, which is comparable with polymyxin B. In addition, SRI-12742 synergised with all classes of antibiotics tested. SRI-12742 exhibits all of the criteria necessary to be positioned as a novel lead with potential to be deployed for the treatment of infections caused by MDR-Ab.

Discovery and optimization of benzotriazine di-N-oxides targeting replicating and nonreplicating Mycobacterium tuberculosis.

Compounds bactericidal against both replicating and nonreplicating Mtb may shorten the length of TB treatment regimens by eliminating infections more rapidly. Screening of a panel of antimicrobial and anticancer drug classes that are bioreduced into cytotoxic species revealed that 1,2,4-benzotriazine di-N-oxides (BTOs) are potently bactericidal against replicating and nonreplicating Mtb. Medicinal chemistry optimization, guided by semiempirical molecular orbital calculations, identified a new lead compound (20q) from this series with an MIC of 0.31 µg/mL against H37Rv and a cytotoxicity (CC(50)) against Vero cells of 25 µg/mL. 20q also had equivalent potency against a panel of single-drug resistant strains of Mtb and remarkably selective activity for Mtb over a panel of other pathogenic bacterial strains. 20q was also negative in a L5178Y MOLY assay, indicating low potential for genetic toxicity. These data along with measurements of the physiochemical properties and pharmacokinetic profile demonstrate that BTOs have the potential to be developed into a new class of antitubercular drugs.

Evaluation of gyrase B as a drug target in Mycobacterium tuberculosis.

OBJECTIVES: New classes of drugs are needed to treat tuberculosis (TB) in order to combat the emergence of resistance to existing agents and shorten the duration of therapy. Targeting DNA gyrase is a clinically validated therapeutic approach using fluoroquinolone antibiotics to target the gyrase subunit A (GyrA) of the heterotetramer. Increasing resistance to fluoroquinolones has driven interest in targeting the gyrase subunit B (GyrB), which has not been targeted for TB. The biological activities of two potent small-molecule inhibitors of GyrB have been characterized to validate its targeting as a therapeutic strategy for treating TB. MATERIALS AND METHODS: Novobiocin and aminobenzimidazole 1 (AB-1) were tested for their activity against Mycobacterium tuberculosis (Mtb) H37Rv and other mycobacteria. AB-1 and novobiocin were also evaluated for their interaction with rifampicin and isoniazid as well as their potential for cytotoxicity. Finally, AB-1 was tested for in vivo efficacy in a murine model of TB. RESULTS: Novobiocin and AB-1 have both been shown to be active against Mtb with MIC values of 4 and 1 mg/L, respectively. Only AB-1 exhibited time-dependent bactericidal activity against drug-susceptible and drug-resistant mycobacteria, including a fluoroquinolone-resistant strain. AB-1 had potent activity in the low oxygen recovery assay model for non-replicating persistent Mtb. Additionally, AB-1 has no interaction with isoniazid and rifampicin, and has no cross-resistance with fluoroquinolones. In a murine model of TB, AB-1 significantly reduced lung cfu counts in a dose-dependent manner. CONCLUSIONS: Aminobenzimidazole inhibitors of GyrB exhibit many of the characteristics required for their consideration as a potential front-line antimycobacterial therapeutic.

Mannich reaction derivatives of novobiocin with modulated physiochemical properties and their antibacterial activities.

Synthetic derivatives of the natural product antibiotic novobiocin were synthesized in order to improve their physiochemical properties. A Mannich reaction was used to introduce new side chains at a solvent-exposed position of the molecule, and a diverse panel of functional groups was evaluated at this position. Novobiocin and the new derivatives were tested for their binding to gyrase B and their antibacterial activities against Staphylococcus aureus, Mycobacterium tuberculosis, Francisella tularensis and Escherichia coli. While the new derivatives still bound the gyrase B protein potently (0.07-1.8 µM, IC(50)), they had significantly less antibacterial activity. Two compounds were identified with increased antibacterial activity against M. tuberculosis, with a minimum inhibitory concentration of 2.5 µg/ml.

Identification of antimicrobial activity among FDA-approved drugs for combating Mycobacterium abscessus and Mycobacterium chelonae.

OBJECTIVES: Rapidly growing mycobacteria have long been neglected in drug discovery efforts and this neglect is reflected in the paucity of therapeutic options available for diseases resulting from these infections. The purpose of this work is to identify new candidate drugs for treating non-tuberculous mycobacteria (NTM) by testing FDA-approved drugs for antimicrobial activity against Mycobacterium abscessus and Mycobacterium chelonae, two emerging NTM drug-resistant pathogens. METHODS: In this study, we screened 1040 FDA-approved drugs against M. abscessus and M. chelonae. RESULTS: Of the drugs screened, 32 compounds exhibited significant antimicrobial activity, with an MIC ≤ 8 mg/L, against M. chelonae, while only 7 compounds showed such activity against M. abscessus. Notably, neostigmine bromide and cinnarizine exhibited highly significant antimicrobial activity against M. chelonae, but had little potency against M. abscessus. Metronidazole and puromycin were the only drugs that acted equipotently against both strains, in decreasing order of effectiveness. CONCLUSIONS: The dearth of identified compounds active against M. abscessus exemplifies its ability to resist drugs as well as the resilience of rapidly growing NTM. Repurposing of approved drugs is a viable alternative to de novo drug discovery and development.