Effect of cyclic topology versus linear terpolymers on antibacterial activity and biocompatibility: antimicrobial peptide avatars

Host-defense peptides (HDPs) and their analogs hold significant potential for combating multidrug-resistant (MDR) bacterial infections. However, their clinical use has been hindered by susceptibility to proteases, high production costs, and cytotoxicity towards mammalian cells. Synthetic polymers with diverse topologies and compositions, designed to mimic HDPs, show promise for treating bacterial infections. In this study, we explored the antibacterial activity and biocompatibility of synthetic amphiphilic linear (LPs) and cyclic terpolymers (CPs) containing hydrophobic groups 2-ethylhexyl (E) and 2-phenylethyl (P) at 20% and 30% content. LPs were synthesized via RAFT polymerization and then cyclized into CPs through a hetero-Diels-Alder click reaction. The bioactivity of these terpolymers was correlated with their topology (LPs vs. CPs) and hydrophobic composition. LPs demonstrated superior antibacterial efficacy compared to CPs against four Gram-negative bacterial strains, with terpolymers containing (P) outperforming those with (E). Increasing the hydrophobicity from 20% to 30% in the terpolymers increased toxicity to both bacterial and mammalian cells. Notably, our terpolymers inhibited the MDR Gram-negative bacterial strain PA37 more effectively than gentamicin and ciprofloxacin. Furthermore, our terpolymers were able to disrupt cell membranes and rapidly eliminate Gram-negative bacteria (99.99% within 15 minutes). Interestingly, CPs exhibited higher hemocompatibility and biocompatibility with mammalian macrophage cells compared to LPs, showcasing a better safety profile (CPs > LPs). These findings underscore the importance of tailoring polymer architectures and optimizing the hydrophilic/hydrophobic balance to address challenges related to toxicity and selectivity in developing antimicrobial polymers.