Small-Molecule Inhibitors of the NusB–NusE Protein–Protein Interaction with Antibiotic Activity

The NusB–NusE protein–protein interaction (PPI) is critical to the formation of stable antitermination complexes required for stable RNA transcription in all bacteria. This PPI is an emerging antibacterial drug target. Pharmacophore-based screening of the mini-Maybridge compound library (56 000 molecules) identified N,N′-[1,4-butanediylbis­(oxy-4,1-phenylene)]­bis­(N-ethyl)­urea 1 as a lead of interest. Competitive enzyme-linked immunosorbent assay screening validated 1 as a 20 μM potent inhibitor of NusB–NusE. Four focused compound libraries based on 1, comprising 34 compounds in total were designed, synthesized, and evaluated as NusB–NusE PPI inhibitors. Ten analogues displayed NusB–NusE PPI inhibition ≥50% at 25 μM concentration in vitro. In contrast to representative Gram-negative Escherichia coli and Gram-positive Bacillus subtilis species, these analogues showed up to 100% growth inhibition at 200 μM. 2-((Z)-4-(((Z)-4-(4-((E)-(Carbamimidoylimino)­methyl)­phenoxy)­but-2-en-1-yl)­oxy)­benzylidene)­hydrazine-1-carboximidamide 22 showed excellent activity against important pathogens. With minimum inhibitory concentration values of ≤3 μg/mL for Gram-positive Streptococcus pneumoniae and methicillin-resistant Staphylococcus aureus and ≤51 μg/mL for Gram-negative Pseudomonas aeruginosa and Acinetobacter baumannii, 22 is a potent lead for a novel antibacterial target. Epifluorescence studies in live bacteria were consistent with 22, inhibiting the NusB–NusE PPI as proposed.