Antimicrobial Potency of Nor-Pyochelin Analogs and Their Cation Complexes Against Multidrug-Resistant Pathogens

The Gram-negative, opportunistic pathogen Pseudomonas aeruginosa has become a serious threat to global health, with increasing resistance towards even the most potent antibiotics. Like other bacteria, the pathogen produces a number of virulence factors that promote its pathogenicity. Of these, metallophores constitute an important group, able to scavenge metal ions from the surrounding environment to secure the metabolic functions of the bacterium. Pseudomonads produce the iron-chelating metallophore (siderophore) pyochelin (PCH), which, in addition to its iron scavenging ability, is an effector for the transcriptional regulator PchR in its Feᴵᴵᴵ-bound form (ferripyochelin). In the present study, docking studies predicted a major ferripyochelin binding site in PchR, which prompted the exploration of nor-pyochelin analogs to produce tight binding to PchR, and thereby establish a lasting effector response associated with upregulation of the genes involved in the PCH metabolism. In addition, we investigated the effects of using the analogs to bind the antimicrobial cations Gaᴵᴵᴵ and Inᴵᴵᴵ, leading to enhanced uptake and further disruption of the cell machinery. Selected analogs of nor-pyochelin (nor-PCH) were synthesized and their Gaᴵᴵᴵ and Inᴵᴵᴵ-based complexes were assessed for antimicrobial activity. The results indicate that the Gaᴵᴵᴵ-complexes inhibit the pathogens under iron-limited conditions, while the Inᴵᴵᴵ-based systems are more effective in iron-rich media. Several of the Gaᴵᴵᴵ-complexes were shown to be highly effective against an MDR P. aeruginosa clinical isolate, with minimum inhibitory concentrations (MICs) of ≤1 µg/mL. Similarly, two of the Inᴵᴵᴵ-based systems were particularly effective against the isolate, with an MIC of 8 µg/mL. These results show high promise in comparison with other, traditionally potent antibiotics. Preliminary mechanistic investigations of the antimicrobial activity using pseudomonal transposon mutants suggested that the inhibitory effects of the Inᴵᴵᴵ-based systems could be due to acute iron deficiency as a result of Inᴵᴵᴵ-bound bacterioferritin (BfrB). Cytotoxiticy analysis indicated that the compounds generally show low toxicity toward mammalian cells under the experimental conditions.