LAUSR

Colistin, typically viewed as the antibiotic of last resort to treat infections caused by multidrug-resistant (MDR) Gram-negative bacteria, had fallen out of favor due to toxicity issues. The recent increase in clinical usage of colistin has resulted in colistin-resistant isolates becoming more common. To counter this threat, we have investigated previously reported compounds, HSD07 and HSD17, and developed 13 compounds with more desirable drug-like properties for colistin sensitization against 16 colistin-resistant bacterial strains, three of which harbor the plasmid-borne mobile colistin resistance (mcr-1). Lead compound HSD1624, which has a lower LogDpH7.4 (2.46) compared to HSD07 (>5.58), reduces the minimum inhibitory concentration (MIC) of colistin against Pseudomonas aeruginosa strain TRPA161 to 0.03 μg/mL from 1024 μg/mL (34,000-fold reduction). Checkerboard assays revealed that HSD1624 and analogues are also synergistic with colistin against colistin-resistant strains of Escherichia coli, Acinetobacter baumannii, and Klebsiella pneumoniae. Preliminary mechanism of action studies indicate that HSD1624 exerts its action differently depending on the bacterial species. Time-kill studies suggested that HSD1624 in combination with 0.5 μg/mL colistin was bactericidal to extended-spectrum beta-lactamase (ESBL)-producing E. coli, as well as to E. coli harboring mcr-1, while against P. aeruginosa TRPA161, the combination was bacteriostatic. Mechanistically, HSD1624 increased membrane permeability in K. pneumoniae harboring a plasmid containing the mcr-1 gene but did not increase radical oxygen species (ROS), while a combination of 15 μM HSD1624 and 0.5 μg/mL colistin significantly increased ROS in P. aeruginosa TRPA161. HSD1624 was not toxic to mammalian red blood cells (up to 226 μM).