Effect of the side chain composition of mixed-charge polymers on pH-selective cell–membrane interactions

Zwitterionic polymers are characterized by their antifouling ability due to their net-neutral charge and affinity for cancer cell surfaces. We developed six pH-responsive mixed-charge polymers and investigated the effect of their side chain hydrophobicity on their interaction with cancer cell membranes. The polymers were synthesized using [2-(methacryloyloxy)ethyl]trimethylammonium chloride, carboxylic acid monomers, and butyl methacrylate (BMA) as the neutral spacer moiety, and differences in hydrophobicity were created by altering the type of carboxylic acid monomer and ratio of the spacer. The surface charge of the polymers was designed to switch from net-neutral to positive in response to the weakly acidic tumor environment. This tuning occurred in a more cancer-like environment with the usage of a more hydrophobic anionic monomer and a higher ratio of BMA due to the weakening of electrostatic interactions. This trend was further confirmed by evaluation of the interaction between the polymers and heparin, an anionic polysaccharide that is used as a model for cancer cell surfaces. The affinity of the mixed-charge polymers for cells was then evaluated, and both the hydrophobicity and pH-responsive nature of these polymers led to pH-selective toxicity, which was assumed to be caused by the disruption of cell membrane integrity.