Antibacterial Screening Reveals Multiple Potent Compounds Against Multidrug-Resistant Pathogens

We characterized the microbiological properties of 204 Gram-positive-specific compounds from a previously unexplored novel compound library of compounds with minimum inhibitory concentration (MIC) ≤ 32 mg/L against Staphylococcus aureus). We identified 23 compounds with MICs 0.5 - 4 mg/L against S. aureus. All 23 retained activity against multidrug-resistant Gram-positive clinical isolates including MRSA, VRSA, hVISA, and VRE strains and no resistant mutants could be selected (frequency of resistance < 5,9 x 10-9). Most compounds were non-haemolytic (22/23, haemolysis <1%), but only 2/23 had an acceptable level of cytotoxicity (HepG2 IC50 > 50 µM). All 23 compounds lost activity in the presence of human serum (MIC >64 mg/L), suggesting high protein binding. We also characterized one Gram-negative-specific compound, JBL-198, active against efflux-defective Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii (MICs of 2, 16, 0,06 and 0,5 mg/L respectively). JBL-198 is non-haemolytic but cytotoxic against HepG2 (IC50 8,5 µM) and loses activity in the presence of human serum. Resistant mutants selected at 4xMIC occur at frequencies of »10-8 and »10-6. Whole genome sequencing revealed mutations in betI (osmolyte regulation) and baeS (efflux regulation) in E. coli; in PA4989 (lipopolysaccharide synthesis) in P. aeruginosa; and in adeL (efflux pump) in A. baumannii. The mutants showed low cross-resistance against six different antibiotic classes. The preliminary conclusion is that these compounds, have significant liabilities that would need to be overcome before they could be developed as antibiotics for systemic therapy, although topical applications might be a possible development path for the Gram-positive-specific compounds.