Beyond the “2 + 2 Pharmacophore” Electronic and Hydrophobic Effects Control the Activity of Cationic Amphiphilic Antimicrobial 2,5-Diketopiperazines

Mimics of natural cationic antimicrobial peptides have demonstrated high antimicrobial activity, and their essential pharmacophore can be effectively incorporated into the cyclic 2,5-diketopiperazine scaffold. Control over bioactivity and selectivity can be achieved by altering the balance between charge and hydrophobicity and by employing a range of small electron-withdrawing and electron-donating substituents; compounds with good antibacterial potency (low μM) could be prepared. The lead compounds prepared in this study had comparable antimicrobial potency to the antibiotic positive controls and the antimicrobial peptide LTX-109, against both Gram-positive and Gram-negative bacteria and multidrug-resistant strains. The off-target toxicity against human red blood cells and VERO cells revealed a good cellular selectivity. Nontoxic and highly potent small compounds were thus prepared, which were comparable or superior to several short antimicrobial peptide mimics under clinical development. A clear link to N-substituent hydrophobicity was revealed, and hydrophobicity thresholds for both cellular selectivity and antimicrobial activity were identified.