Polymyxin B adjuvants against polymyxin B- and carbapenem-resistant Gram-negative bacteria

Aim: Polymyxin B (PMB) is one of the few therapeutic options for treating infections caused by carbapenem-resistant Gram-negative bacteria (CR-GNB). However, the emergence of PMB-resistant CR-GNB strains has prompted the exploration of antibiotic adjuvants as potential therapeutic avenues. Thus, this study evaluates the potential of 3,5-dinitrobenzoic acid derivatives (DNH01, DNH11, DNH13 and DNH20) and isoniazid-N-acylhydrazones (INZ1-7, INZ9 and INZ11) as adjuvants to enhance PMB efficacy against CR-GNB. Materials & methods: MIC, MBC and drug combination assays were conducted using multidrug-resistant clinical isolates of Enterobacterales and Acinetobacter baumannii. In addition, the effects of PMB and PMB + DNH derivatives were assessed through flow cytometry and scanning electron microscopy (SEM). Results: DNH01, DNH11 and DNH20, unlike the INH-acylhydrazones, significantly restored PMB activity (MIC <= 2 mu g/ml) in 80% of the tested isolates. Flow cytometry and SEM assays confirmed that DNH derivatives rescued the activity of PMB, yielding results comparable to those expected for PMB alone but at 256-fold lower concentrations. Conclusion: These findings suggest DNH derivatives hold substantial promise as PMB adjuvants to combat PMB-resistant CR-GNB infections.