Anthracimycin activity against contemporary methicillin-resistant Staphylococcus aureus.

Anthracimycin is a recently discovered novel marine-derived compound with activity against Bacillus anthracis. We tested anthracimycin against an expanded panel of Staphylococcus aureus strains in vitro and in vivo. All strains of S. aureus tested, including methicillin-susceptible, methicillin-resistant (MRSA) and vancomycin-resistant strains of S. aureus, were susceptible to anthracimycin at MIC values of ⩽0.25 mg l(-1). Although its postantibiotic effects were minimal, anthracimycin exhibited potent and rapid bactericidal activity, with a >4-log kill of USA300 MRSA within 3 h at five times its MIC. At concentrations significantly below the MIC, anthracimycin slowed MRSA growth and potentiated the bactericidal activity of the human cathelicidin, LL-37. The bactericidal activity of anthracimycin was somewhat mitigated in the presence of 20% human serum, and the compound was minimally toxic to human cells, with an IC50 (inhibitory concentration 50)=70 mg l(-1) against human carcinoma cells. At concentrations near the MIC, anthracimycin inhibited S. aureus nucleic acid synthesis as determined by optimized macromolecular synthesis methodology, with inhibition of DNA and RNA synthesis occurring in the absence of DNA intercalation. Anthracimycin at a single dose of 1 or 10 mg kg(-1) was able to protect mice from MRSA-induced mortality in a murine peritonitis model of infection. Anthracimycin provides an interesting new scaffold for future development of a novel MRSA antibiotic.

Structure-based design of new dihydrofolate reductase antibacterial agents: 7-(benzimidazol-1-yl)-2,4-diaminoquinazolines.

A new series of dihydrofolate reductase (DHFR) inhibitors, the 7-(benzimidazol-1-yl)-2,4-diaminoquinazolines, were designed and optimized for antibacterial potency and enzyme selectivity. The most potent inhibitors in this series contained a five-membered heterocycle at the 2-position of the benzimidazole, leading to highly potent and selective compounds that exploit the differences in the size of a binding pocket adjacent to the NADPH cofactor between the bacterial and human DHFR enzymes. Typical of these compounds is 7-((2-thiazol-2-yl)benzimidazol-1-yl)-2,4 diaminoquinazoline, which is a potent inhibitor of S. aureus DHFR (Ki = 0.002 nM) with 46700-fold selectivity over human DHFR. This compound also has high antibacterial potency on Gram-positive bacteria with an MIC versus wild type S. aureus of 0.0125 µg/mL and a MIC versus trimethoprim-resistant S. aureus of 0.25 µg/mL. In vivo efficacy versus a S. aureus septicemia was demonstrated, highlighting the potential of this new series.

Distinguishing on-target versus off-target activity in early antibacterial drug discovery using a macromolecular synthesis assay.

The macromolecular synthesis assay was optimized in both S. aureus and E. coli imp and used to define patterns of inhibition of DNA, RNA, protein, and cell wall biosynthesis of several drug classes. The concentration of drug required to elicit pathway inhibition differed among the antimicrobial agents tested, with inhibition detected at concentrations significantly below the minimum inhibitory concentration (MIC) for tedizolid; within 4-fold of the MIC for ciprofloxacin, cefepime, vancomycin, tetracycline, and chloramphenicol; and significantly above the MIC for rifampicin and kanamycin. In a DNA gyrase/topoisomerase IV structure-based drug design optimization program, the assay rapidly identified undesirable off-target activity within certain chemotypes, altering the course of the program to focus on the series that maintained on-target activity.

Identification of bacteria-selective threonyl-tRNA synthetase substrate inhibitors by structure-based design.

A series of potent and bacteria-selective threonyl-tRNA synthetase (ThrRS) inhibitors have been identified using structure-based drug design. These compounds occupied the substrate binding site of ThrRS and showed excellent binding affinities for all of the bacterial orthologues tested. Some of the compounds displayed greatly improved bacterial selectivity. Key residues responsible for potency and bacteria/human ThrRS selectivity have been identified. Antimicrobial activity has been achieved against wild-type Haemophilus influenzae and efflux-deficient mutants of Escherichia coli and Burkholderia thailandensis.

Tricyclic GyrB/ParE (TriBE) inhibitors: a new class of broad-spectrum dual-targeting antibacterial agents.

Increasing resistance to every major class of antibiotics and a dearth of novel classes of antibacterial agents in development pipelines has created a dwindling reservoir of treatment options for serious bacterial infections. The bacterial type IIA topoisomerases, DNA gyrase and topoisomerase IV, are validated antibacterial drug targets with multiple prospective drug binding sites, including the catalytic site targeted by the fluoroquinolone antibiotics. However, growing resistance to fluoroquinolones, frequently mediated by mutations in the drug-binding site, is increasingly limiting the utility of this antibiotic class, prompting the search for other inhibitor classes that target different sites on the topoisomerase complexes. The highly conserved ATP-binding subunits of DNA gyrase (GyrB) and topoisomerase IV (ParE) have long been recognized as excellent candidates for the development of dual-targeting antibacterial agents with broad-spectrum potential. However, to date, no natural product or small molecule inhibitors targeting these sites have succeeded in the clinic, and no inhibitors of these enzymes have yet been reported with broad-spectrum antibacterial activity encompassing the majority of Gram-negative pathogens. Using structure-based drug design (SBDD), we have created a novel dual-targeting pyrimidoindole inhibitor series with exquisite potency against GyrB and ParE enzymes from a broad range of clinically important pathogens. Inhibitors from this series demonstrate potent, broad-spectrum antibacterial activity against Gram-positive and Gram-negative pathogens of clinical importance, including fluoroquinolone resistant and multidrug resistant strains. Lead compounds have been discovered with clinical potential; they are well tolerated in animals, and efficacious in Gram-negative infection models.