Synthesis and antimicrobial testing of 5-fluorouracil derivatives

Antibiotic resistance has increased the demand for novel treatments against multidrug-resistant microorganisms. In the research literature, 5-fluorouracil (5-FU) was proposed as an alternative due to its intrinsic antibacterial property. However, given its toxicity profile at high doses, its use in antibacterial therapy is dubious. In the quest for improving the efficacy of 5-FU, the present study intends to synthesise 5-FU derivatives and assess their susceptibility and mechanism against pathogenic bacteria. It was found that the compounds having tri-hexylphosphonium substitution on both nitrogen groups of 5-FU (6a, 6b and 6c) had considerable activity against both Gram-positive and Gram-negative bacteria. Among the active compounds, those with an asymmetric linker group 6c were found to have higher antibacterial efficacy. However, no conclusive efflux inhibition activity was found. As elucidated by electron microscopy studies, these self-assembling active phosphonium-based 5-FU derivatives caused considerable septal damage and cytosolic alterations in Staphylococcus aureus cells. In Escherichia coli, these compounds triggered plasmolysis. Interestingly, the minimal inhibitory concentration (MIC) of the most potent 5-FU derivative 6c remained constant, regardless of the bacteria's resistance profile. Further analysis revealed that compound 6c generated significant alterations in membrane permeabilization and depolarization in S. aureus and E. coli cells at the MIC. Compound 6c was found to substantially impede bacterial motility, suggesting its importance in regulating bacterial pathogenicity. Additionally, the nonhaemolytic activity of 6c suggested that it could be a potential therapeutic option for treating multidrug-resistant bacterial infections.