Positive Charge in an Antimalarial Compound Unlocks Broad-Spectrum Antibacterial Activity

In this study, we synthesized a library of eNTRy-rule-compliant compounds by introducing ionizable nitrogen atoms to an antimalarial compound. These positively charged derivatives gained activity against both Gram-negative and -positive bacteria, <i>Mycobacterium tuberculosis</i>, and boosted <i>Plasmodium falciparum</i> inhibition to the double-digit nanomolar range. Overcoming and remaining inside the cell envelope of Gram-negative bacteria (GNB) is one of the major difficulties in antibacterial drug discovery and development. The eNTRy rules (N = ionizable nitrogen, T = low three-dimensionality, R = rigidity) can be a useful structural guideline to improve accumulation of small molecules in GNB. With the aim of unlocking Gram-negative activity, we added amines and (cyclic) <i>N</i>-alkyl guanidines to an already flat and rigid pyrazole-amide class as a representative example for our investigation. To test their performance, we compared these eNTRy-rule-compliant compounds to closely related noncompliant ones through phenotypic screening of various pathogens (<i>P. falciparum</i>, <i>Escherichia coli</i>, <i>Acinetobacter baumannii</i>, <i>Pseudomonas aeruginosa</i>, <i>Staphylococcus aureus</i>, <i>Streptococcus pneumoniae</i>, and <i>M. tuberculosis</i>), 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; noncompliant compounds do not kill any of the bacteria tested, while compliant compounds largely showed growth inhibition of Gram-negative, -positive, and <i>M. tuberculosis</i> bacteria in the single-digit micromolar range.