In vitro activity of omiganan pentahydrochloride tested against vancomycin-tolerant, -intermediate, and -resistant Staphylococcus aureus

Omiganan, a novel topical cationic peptide active against a broad spectrum of bacteria and yeast, is targeted for the prevention of catheter-associated infections. The spectrum of this agent was evaluated against contemporary methicillin-(oxacillin)-resistant Staphylococcus aureus (MRSA; 109 isolates), including subgroups displaying reduced susceptibility to vancomycin. Strain phenotypes included: vancomycin-tolerant (MBC/MIC ratio, ≥ 32-fold);vancomycin-intermediate (VISA; MIC values, 4–8 μg/ml); heterogeneous vancomycin-intermediate (hVISA); and vancomycin-resistant (VRSA; MIC values, ≥ 16 μg/ml) S. aureus. All S. aureus tested were inhibited by ≤64 μg/ml of omiganan, with MIC₅₀/MIC₉₀ values of 16/32 μg/ml, respectively. Compared to wild-type S. aureus, MIC₉₀ values were only 2-fold greater for vancomycin-tolerant, hVISA and VISA strains. The VRSA isolates, representing the most resistant strains tested, were inhibited by 16 μg/ml (mode for all groups). Omiganan demonstrated potent activity against S. aureus, regardless of harbored resistance mechanism. Given the worrisome emergence of S. aureus with reduced susceptibility to vancomycin, the demonstration that omiganan remains equally active against all isolates of this species at a level significantly below the clinical formulation concentration (1% gel; 10,000 μg/ml) is an important attribute.