Dalbavancin exposure in vitro selects for dalbavancin-nonsusceptible and vancomycin-intermediate strains of methicillin-resistant Staphylococcus aureus.

Abstract Objectives Dalbavancin is a lipoglycopeptide active against methicillin-resistant Staphylococcus aureus (MRSA). Its long half-life (8.5-16 days) allows for once-weekly or single-dose treatments but could prolong the mutant selection window, promoting resistance and cross-resistance to related antimicrobials like vancomycin. The objective of this study was to evaluate the capacity of post-distributional pharmacokinetic exposures of dalbavancin to select for resistance and cross-resistance in MRSA. Methods We simulated average, post-distributional exposures of single-dose (1500mg) dalbavancin (fCmax 9.9μg/mL, β-elimination t1/2 204h) in an in vitro PK/PD model for 28 days (672h) against 5 MRSA and 1 methicillin-susceptible strain. Samples were collected at least daily, and surviving colonies were enumerated and screened for resistance on drug-free and dalbavancin-supplemented medium respectively. Isolates from resistance screening plates were subjected to whole genome sequencing (WGS) and susceptibly testing against dalbavancin, vancomycin, daptomycin, and 6 beta-lactams with varying penicillin-binding protein affinities. Results Dalbavancin was bactericidal against most strains for days 1-4, before regrowth of less-susceptible subpopulations occurred. Isolates with 8-fold increases in dalbavancin MIC were detected as early as day-4 but increased 64-128-fold in all models by day-28. Vancomycin and daptomycin MICs increased by 4-16-fold, exceeding susceptibly breakpoints for both antibiotics. Beta-lactam MICs generally decreased by 2-8-fold, suggesting a dalbavancin-beta-lactam seesaw effect, but increased by ≥8-fold in certain isolates. Resistant isolates carried mutations in a variety of genes, most commonly walKR, apt, stp1, and atl. Conclusions In our in vitro system, post-distributional dalbavancin exposures selected for stable mutants with reduced susceptibility to dalbavancin, vancomycin, and daptomycin, and generally increased susceptibility to beta-lactams in all strains of MRSA tested. The clinical significance of these findings remains unclear but created an opportunity to genotype a unique collection of dalbavancin-resistant strains for the first time. Mutations involved genes previously associated with vancomycin-intermediate and daptomycin non-susceptibility, most commonly walKR-associated genes.