Simulated exposures of oritavancin in in vitro pharmacodynamic models select for methicillin-resistant Staphylococcus aureus with reduced susceptibility to oritavancin but minimal cross-resistance or seesaw effect with other antimicrobials.

Background: Dalbavancin exposures select for VAN and daptomycin cross-resistance in Staphylococcus aureus often by walK-related mutations. Oritavancin is another long-acting lipoglycopeptide, but its proclivity to select for cross-resistance is unknown. The objective of this study was to determine if post-distributional pharmacokinetic oritavancin exposures select for meaningful susceptibility changes in S. aureus.

Methods: We simulated average post-distributional, free-drug exposures of oritavancin 1200 mg IV once (fCmax 11.2 µg/mL; β-elimination t1/2 13.4 h; γ-elimination t1/2 245 h) in an in vitro pharmacodynamic model for 28 days against five S. aureus including four MRSA. Samples were taken daily for colony enumeration and resistance screening. Susceptibility testing was repeated on isolates from resistance screening plates against oritavancin, vancomycin, daptomycin, dalbavancin and 6 beta-lactams with varying penicillin-binding protein affinities.

Results: Tested oritavancin exposures were bactericidal against 5/5 strains for 2-17 days before regrowth of less-susceptible subpopulations occurred. Isolates with reduced susceptibility to oritavancin were detected as early as 5 days, but the MIC increased above the susceptibility breakpoint (>0.125 mg/L) in 4/5 strains eventually. Vancomycin and daptomycin MICs increased by 2- to 8-fold but did not exceed the susceptibility breakpoints in most isolates. β-lactam MICs were largely unchanged among the recovered isolates with reduced oritavancin susceptibility. Mutations were diverse but often involved purR with 13 unique variants identified among 4/5 strains.

Conclusions: Oritavancin-selected resistance was primarily associated with purR mutation and less frequently associated with cross-resistance and walK mutation than dalbavancin-selected resistance in similar strains and conditions. The reason for this is unclear but may stem from differences in the mechanism(s) and divergent mutational pathways.