In vitro activity of rifabutin against 293 contemporary carbapenem-resistant Acinetobacter baumannii clinical isolates and characterization of rifabutin mode of action and resistance mechanisms

Background: Rifabutin, an oral drug approved to treat Mycobacterium avium infections, demonstrated potent activity against Acinetobacter baumannii in nutrient-Limited medium enabled by rifabutin cellular uptake through the siderophore receptor FhuE. Objectives: To determine rifabutin in vitro activity and resistance mechanisms in a Large panel of A. baumannii isolates. Methods: Two hundred and ninety-three carbapenem-resistant A. baumannii clinical isolates collected from Europe, the USA and Asia during 2017-19 were used for MIC determination. Sequencing/genotyping of fhuE, rpoB and arr-2 genes in isolates with elevated rifabutin MIC combined with genetic engineering and gene expression quantification was used to characterize rifabutin's mode of action and resistance mechanisms. Results: Rifabutin showed excellent activity on the strain panel, with an MIC50/90 of 0.008/1 mg/L, and was superior to all other antibiotics tested, including colistin, tigecycline and cefiderocol (MIC90 of 8 mg/L). Rifabutin remained active on resistant subpopulations, including strains resistant to the siderophore-drug conjugate cefiderocol (MIC90 of 2 mg/L, n = 23). At Least two independent resistance mechanisms were required to abolish rifabutin activity, which is in Line with the dose-dependent mutational resistance frequency reaching 10(-9) at rifabutin concentrations at or above 2 mg/L. Conclusions: This study demonstrated the potent activity of rifabutin against carbapenem-resistant A. baumannii. We propose that FhuE-mediated active uptake of rifabutin enables activity against rifampicin-resistant isolates. To achieve clinically meaningful strain coverage and to avoid rapid resistance development, rifabutin concentrations >= 2 mg/L are required, something rifabutin oral formulations cannot deliver.