A Positive Charge in an Antimalarial Compound Unlocks Broad-spectrum Antibacterial Activity

In this study, we synthesised a library of eNTRy-rule-complying compounds by introducing ionisable nitrogens to an antimalarial compound. These positively-charged derivatives gained activity against both Gram-negative and -positive bacteria, Mycobacterium tuberculosis and boosted Plasmodium falciparum inhibition to the double-digit nanomolar range. Overcoming and remaining inside the cell envelope of Gram-negative bacteria is one of the major difficulties in antibacterial drug development. The eNTRy rules (N = ionisable nitrogen, T = low three-dimensionality, R = rigidity) can be a useful structural guideline to improve accumulation of small molecules in Gram-negative bacteria. With the aim of unlocking Gram-negative activity, we added amines and (cyclic) N-alkyl guanidines to an already flat and rigid pyrazole-amide class. To test their performance, we compared these eNTRy-rule-complying compounds to closely related non-complying ones through phenotypic assay screenings of various pathogens (P. falciparum, Escherichia coli, Acinetobacter baumannii, Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus pneumoniae, and M. tuberculosis) obtaining a handful of broad-spectrum hits. The results support the working hypothesis and even extend its applicability, the studied pyrazole-amide class adheres to the eNTRy rules; non-compliant compounds do not kill any of the bacteria tested, while compliant compounds largely showed inhibition of Gram-negative, -positive, and M. tuberculosis bacteria in the single-digit micromolar range.