Cumbre Pharmaceuticals, Inc.

Rifamycins have proven efficacy in the treatment of persistent bacterial infections. However, the frequency with which bacteria develop resistance to rifamycin agents restricts their clinical use to antibiotic combination regimens. In a program directed toward the synthesis of rifamycins with a lower propensity to elicit resistance development, a series of compounds were prepared that covalently combine rifamycin and quinolone pharmacophores to form stable hybrid antibacterial agents. We describe mode-of-action studies with Staphylococcus aureus of CBR-2092, a novel hybrid that combines the rifamycin SV and 4 H -4-oxo-quinolizine pharmacophores. In biochemical studies, CBR-2092 exhibited rifampin-like potency as an inhibitor of RNA polymerase, was an equipotent (balanced) inhibitor of DNA gyrase and DNA topoisomerase IV, and retained activity against a prevalent quinolone-resistant variant. Macromolecular biosynthesis studies confirmed that CBR-2092 has rifampin-like effects on RNA synthesis in rifampin-susceptible strains and quinolone-like effects on DNA synthesis in rifampin-resistant strains. Studies of mutant strains that exhibited reduced susceptibility to CBR-2092 further substantiated RNA polymerase as the primary cellular target of CBR-2092, with DNA gyrase and DNA topoisomerase IV being secondary and tertiary targets, respectively, in strains exhibiting preexisting rifampin resistance. In contrast to quinolone comparator agents, no strains with altered susceptibility to CBR-2092 were found to exhibit changes consistent with altered efflux properties. The combined data indicate that CBR-2092 may have potential utility in monotherapy for the treatment of persistent S. aureus infections.

We present data from antimicrobial assays performed in vitro that pertain to the potential clinical utility of a novel rifamycin-quinolone hybrid antibiotic, CBR-2092, for the treatment of infections mediated by gram-positive cocci. The MIC 90 s for CBR-2092 against 300 clinical isolates of staphylococci and streptococci ranged from 0.008 to 0.5 μg/ml. Against Staphylococcus aureus , CBR-2092 exhibited prolonged postantibiotic effects (PAEs) and sub-MIC effects (SMEs), with values of 3.2, 6.5, and >8.5 h determined for the PAE (3× MIC), SME (0.12× MIC), and PAE-SME (3× MIC/0.12× MIC) periods, respectively. Studies of genetically defined mutants of S. aureus indicate that CBR-2092 is not a substrate for the NorA or MepA efflux pumps. In minimal bactericidal concentration and time-kill studies, CBR-2092 exhibited bactericidal activity against staphylococci that was retained against rifampin- or intermediate quinolone-resistant strains, with apparent paradoxical cidal characteristics against rifampin-resistant strains. In spontaneous resistance studies, CBR-2092 exhibited activity consistent with balanced contributions from its composite pharmacophores, with a mutant prevention concentration of 0.12 μg/ml and a resistance frequency of <10 −12 determined at 1 μg/ml in agar for S. aureus . Similarly, CBR-2092 suppressed the emergence of preexisting rifamycin resistance in time-kill studies undertaken at a high cell density. In studies of the intracellular killing of S. aureus , CBR-2092 exhibited prolonged bactericidal activity that was superior to the activities of moxifloxacin, rifampin, and a cocktail of moxifloxacin and rifampin. Overall, CBR-2092 exhibited promising activity in a range of antimicrobial assays performed in vitro that pertain to properties relevant to the effective treatment of serious infections mediated by gram-positive cocci.

Biofilms are communal structures of microorganisms encased in an exopolymeric coat that form on both natural and abiotic surfaces and have been associated with a variety of persistent infections that respond poorly to conventional antibiotic chemotherapy. Biofilm infections of certain indwelling medical devices by common pathogens such as staphylococci are not only associated with increased morbidity and mortality but are also significant contributors to the emergence and dissemination of antibiotic resistance traits in the nosocomial setting. Current treatment paradigms for biofilm-associated infections of semipermanent indwelling devices typically involve surgical replacement of the device combined with long-term antibiotic therapy and incur high health care costs. This review summarizes the existing data relating to the nature, prevalence, and treatment of biofilm-associated infections and highlights experimental approaches and therapies that are being pursued toward more effective treatments.