The majority of nanoparticles designed for cellular delivery of drugs and imaging agents enter the cell via endocytotic pathways leading to their entrapment in endosomes that present a robust barrier to further trafficking of the nanoparticles within the cells. A few materials, such as the cell penetrating peptides (CPPs), are known to enter cells not only via endocytosis, but also via translocation through the cell membrane into the cytoplasm, successfully bypassing the endosomes. We report here that random copolymers of 3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate and poly(ethylene glycol) methacrylate, p(DMAPS-ran-PEGMA), are internalized in cells primarily via translocation through the cell membrane rather than endocytosis. The properties of the polymers and their modes of uptake were investigated systematically by dynamic light scattering, confocal fluorescence microscopy, and flow cytometry. Using specific inhibitors of the cellular uptake machinery in a human cervical carcinoma cell line (HeLa), we show that these nontoxic synthetic polyzwitterions exist in cell media as self-assembled nanospheres that unravel as they adsorb on the plasma membrane and translocate through it. Conjugates of p(DMAPS-ran-PEGMA) with rhodamine B were delivered selectively to the mitochondria, whereas doxorubicin (Dox)-p(DMAPS-ran-PEGMA) conjugates were accumulated in both the nucleus and the mitochondria, effectively inducing apoptosis in HeLa cells. These findings suggest that the noncytotoxic and readily synthesized p(DMAPS-ran-PEGMA) can find applications as bioimaging tools and drug nanocarriers.

Mitochondrial targeting and entry, two crucial steps in fighting severe diseases resulting from mitochondria dysfunction, pose important challenges in current nanomedicine. Cell-penetrating peptides or targeting groups, such as Rhodamine-B (Rho), are known to localize in mitochondria, but little is known on how to enhance their effectiveness through structural properties of polymeric carriers. To address this issue, we prepared 8 copolymers of 3-dimethyl(methacryloyloxyethyl)ammonium propane sulfonate and poly(ethyleneglycol) methacrylate, p(DMAPS-ran-PEGMA) (molecular weight, 18.0 < Mn < 74.0 kg/mol) with two different endgroups. We labeled them with Rho groups attached along the chain or on one of the two endgroups (α or ω). From studies by flow cytometry and confocal fluorescence microscopy of the copolymers internalization in HeLa cells in the absence and presence of pharmacological inhibitors, we established that the polymers cross the cell membrane foremost by translocation and also by endocytosis, primarily clathrin-dependent endocytosis. The most effective mitochondrial entry was achieved by copolymers of Mn < 30.0 kg/mol, lightly grafted with PEG chains (< 5 mol %) labeled with Rho in the ω-position. Our findings may be generalized to the uptake and mitochondrial targeting of prodrugs and imaging agents with a similar polymeric scaffold.