As drug-resistant bacteria become more prevalent, stressing our economy and healthcare systems, developing novel antimicrobial agents is critical. While antimicrobial peptides (AMPs) are promising alternatives to conventional antibiotics, they are limited by toxicity to mammalian cells and instability in biological environments. Attaching AMPs to neutral, hydrophilic polymers can overcome the above challenges. We recently found that comb-like conjugates of the chemokine-derived, hydrophobic, and cationic AMP (stapled P9) with poly(ethylene glycol methacrylate) (PEGMA) are non-toxic and display resoundingly superior antimicrobial activity and stability compared to the AMP alone and to analogous star-shaped conjugates. Yet, several reports note prolonged exposure to PEG prompts the immune system to develop antibodies that specifically target PEG, thereby limiting its performance as a drug carrier. Alternatively, zwitterionic polymers featuring both a cationic and an anionic group on each repeating unit are still charge-neutral and are not yet known to incite an immune response. These polymers are also more hydrophilic than PEG, providing opportunities to improve solubility and perhaps to enhance the stability of conjugates. Moreover, the differing orientations of the charged groups with respect to the polymer backbone on sulfobetaine (SB) and phosphorylcholine (PC) zwitterions offer the opportunity to tailor the surface charge of the conjugates and potentially enhance antimicrobial activity.
In the preparation of these conjugates by reversible addition-fragmentation chain transfer (RAFT) polymerization, we found zwitterionic SB methacrylate to polymerize markedly slower than PEGMA, reaching only 60% conversion in 1 h, whereas PEGMA reached full conversion within 30 min, as quantified by proton nuclear magnetic resonance (1H NMR). These differing kinetics may alter the distribution of AMP-containing monomers along the chain and, thus, the resulting properties and performance of the comb-like conjugates. This presentation will discuss the impact of different reaction conditions and monomer composition (PEG vs. SB vs. PC) on the kinetics of these copolymerizations and the properties of the resulting conjugates. We use dynamic light scattering (DLS), transmission electron microscopy (TEM), and high-performance liquid chromatography (HPLC) to study the surface charge, morphology, and proteolytic stability of the conjugates, respectively. We anticipate that these findings will guide the selection of neutral, hydrophilic monomers as better alternatives for PEGMA to ultimately generate more effective antimicrobial agents.
