Mitochondria-targeting nanostructures from enzymatically degradable fluorescent amphiphilic polyesters

Water-soluble π-conjugated luminescent bioprobes have been broadly used in biomedical research but are limited by the nonbiodegradability associated with their rigid C-C backbones. In the present work, we introduced three naphthalene monoimide (NMI)-functionalized amphiphilic fluorescent polyesters (P1, P2, and P3) prepared by transesterification of functional diols with an activated diester monomer of adipic acid. These polyesters featured a side-chain NMI fluorophore, imparting the required hydrophobicity for self-assembly in water and endowing the polymeric nanoassemblies with green fluorescence. Two polymers (P1 and P2) were intrinsically cationic at physiological pH (7.4), while neutral P3 exhibited pH-triggered (pH ∼6.2) cationic features due to the protonation of the tertiary amine groups present in its backbone. These biocompatible polymers revealed around 85% cellular uptake after 1 hour of incubation. However, the initial uptake for the cationic polymers (P1 and P2) within 15 minutes was significantly greater than that of the neutral P3 because of their stronger electrostatic interactions with the negatively charged cell membranes. Notably, cationic P1 and P2 could specifically target mitochondria in cancerous HeLa cells by escaping the initial endosome/lysosome trap. In contrast, neutral P3 exhibited cell-selective mitochondria targeting in cancerous (HeLa) cells over non-cancerous (NKE) cells. This is attributed to P3's protonation-induced positive charge accumulation in the acidic environment of cancer cells, unlike in the non-acidic environment of non-cancerous cells. This possibly causes P3 nanoassemblies to behave similarly to P1 and P2 in HeLa cells despite P3 being intrinsically neutral. The insights gained from this work may be relevant for future development of cell-specific, mitochondria-targeted drug delivery systems from enzymatically degradable polyester backbones.