Targeted Photodynamic Activity of a Hyaluronic Acid-Protoporphyrin IX Complex via Receptor-Mediated Endocytosis

Hyaluronic acid (HA) is not only biocompatible, biodegradable, stable in vivo, nontoxic, and non-immunogenic, but also specifically binds to the cluster of differentiation 44 (CD44) receptor, which is overexpressed in many solid tumors. Therefore, we investigated unmodified HA as a drug carrier. In this study, protoporphyrin IX (1) was water-solubilized at high concentrations through complexation with unmodified HA using a high-speed vibration milling apparatus without the use of organic solvents. The resulting HA-1 complex formed a stable aqueous solution that remained intact for at least one week. Cellular uptake studies revealed approximately five times greater internalization of 1 from the HA-1 complex in human lung cancer cells (A549) than in murine fibroblast-like cells (L929). Its selectivity index ( = half maximal inhibitory concentration [IC₅₀] in normal cells (L929)/IC₅₀ in cancer cells [A549]) was approximately 10.3, demonstrating strong selectivity toward cancer cells. Further, the uptake of HA-1 in A549 cells was suppressed following the addition of free HA, indicating that its internalization occurred through CD44-receptor-mediated endocytosis. Moreover, the HA-1 complex exhibited better photodynamic activity against CD44-overexpressing cancer cells than both poly-L-lysine-1 complex and Photofrin, underscoring its potential as a targeted photosensitizer for photodynamic therapy.