In Vitro Evaluation of CBR-2092, a Novel Rifamycin-Quinolone Hybrid Antibiotic: Studies of the Mode of Action in Staphylococcus aureus

Abstract

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 modeof-action studies with Staphylococcus aureus of CBR-2092, a novel hybrid that combines the rifamycin SV and 4H-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.

Challenges of Antibacterial Discovery

“…No clinical candidate emerged from these studies, presumably indicating PK, absorption, distribution, metabolism, and excretion (ADME), or toxicity difficulties. One hybrid that reached the clinic was Cumbre's CBR-2092, a rifamycin-FQ (strictly speaking, 2-pyridone instead of an FQ) hybrid that displayed both mechanisms of intracellular action and had the desired characteristics of much lowered resistance selection relative to those of its components and activity on strains with preexisting resistance to a component (313). Two phase I trials were completed by 2008, but no public reports were issued, and the company soon went out of business.…”

In Vitro Evaluation of CBR-2092, a Novel Rifamycin-Quinolone Hybrid Antibiotic: Microbiology Profiling Studies with Staphylococci and Streptococci

“…Finally, data from studies on the isolation and characterization of mutant strains that exhibit reduced susceptibility to CBR-2092 substantiated that RNA polymerase is 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, strains with altered susceptibility to CBR-2092 were not found to exhibit changes consistent with altered efflux properties (18). Overall, the combined data suggest that CBR-2092 may have potential utility as monotherapy for the treatment of S. aureus infections and those mediated by other susceptible cocci.…”

“…The emergence and spread of invasive MRSA isolates, including those exhibiting multidrug-resistant traits (15), have spurred renewed interest in the potential use of older classes of antistaphylococcal agents for the treatment of serious infections. In an accompanying article (18), we describe detailed mode-of-action studies for CBR-2092, a novel rifampinquinolone hybrid antibiotic that combines the rifamycin SV and a 4H-4-oxo-quinolizine pharmacophores via a chiral linking group.…”

“…Table 1. The isolation and characterization of derivatives of S. aureus ATCC 29213 (CB190) that exhibit stable resistance to agents of the rifamycin or quinolone classes are described in the accompanying article (18). Derivatives of Streptococcus pyogenes CB1208 that exhibit intermediate resistance (i.e., MIC, 1 g/ml; strain CB1456) or high-level resistance (i.e., MIC, Ͼ32 g/ml; strain CB1504) to rifampin were isolated sequentially by spontaneous selection on Trypticase soy agar medium supplemented with 5% sheep's blood (TSBA) and 1 or 16 g/ml rifampin, respectively.…”

“…Derivatives of Streptococcus pyogenes CB1208 that exhibit intermediate resistance (i.e., MIC, 1 g/ml; strain CB1456) or high-level resistance (i.e., MIC, Ͼ32 g/ml; strain CB1504) to rifampin were isolated sequentially by spontaneous selection on Trypticase soy agar medium supplemented with 5% sheep's blood (TSBA) and 1 or 16 g/ml rifampin, respectively. Genotypic analysis of these strains was undertaken by standard methods (18).…”