Dimeric 2-aminoimidazoles are highly active adjuvants for gram-positive selective antibiotics against Acinetobacter baumannii

The Centers for Disease Control and Prevention (CDC) reports that hospital acquired infections have increased by 65% since 2019. One of the main contributors is the gram-negative bacterium Acinetobacter baumannii. Previously, we reported aryl 2-aminoimidazole (2-AI) adjuvants that potentiate macrolide antibiotics against A. baumannii. Macrolide antibiotics are typically used to treat infections caused by gram-positive bacteria, but are ineffective against most gram-negative bacteria. We describe a new class of dimeric 2-AIs that are highly active macrolide adjuvants, with lead compounds lowering minimum inhibitory concentrations (MICs) to or below the gram-positive breakpoint level against A. baumannii. The parent dimer lowers the clarithromycin (CLR) MIC against A. baumannii 5075 from 32 μg/mL to 1 μg/mL at 7.5 μM (3.4 μg/mL), and a subsequent structure activity relationship (SAR) study identified several compounds with increased activity. The lead compound lowers the CLR MIC to 2 μg/mL at 1.5 μM (0.72 μg/mL), far exceeding the activity of both the parent dimer and the previous lead aryl 2-AI. Furthermore, these dimeric 2-AIs exhibit considerably reduced mammalian cell toxicity compared to aryl-2AI adjuvants, with IC50s of the two lead compounds against HepG2 cells of >200 μg/mL, giving therapeutic indices of >250. 2-aminoimidazole dimers in combination with the gram-positive selective antibiotic clarithromycin provide a new approach towards combating antibiotic resistant Gram-negative infections. Macrolides typically have limited efficacy against Gram-negative bacteria due to poor permeation across the Gram-negative outer membrane. However, in the presence of 2-aminoimidazole dimers, clarithromycin becomes efficacious against several Gram-negative species including Acinetobacter baumannii. [Display omitted] •Small molecule adjuvants overcome intrinsic antibiotic resistance in gram-negative bacteria.•2-aminoimidazole dimers are highly active macrolide adjuvants against A. baumannii.•Structure activity relationship study leads to higher potency.•High therapeutic indices will allow for exploration of in vivo potential.