Charge Transfer Reactions (PDT Reaction I) Induced at 1064 nm (NIR-II) between Rhodamines and Folic Acid as the Basis for Biphotonic Photodynamic Therapy.

Rhodamine-6G, rhodamine-123, and rhodamine-B undergo type I charge transfer reactions with folic acid under irradiation at 532 nm. Therefore, these compounds are potential therapeutic agents for photodynamic therapy in hypoxic media. As 532 nm light penetrates poorly into tissues, an alternative is to use biphotonic absorption at 1064 nm as an excitation source. In this study, the ability of rhodamine-6G, rhodamine-123, and rhodamine-B to induce charge transfer reactions with folic acid by biphotonic absorption at 1064 nm was evaluated via fluorescence. If the reaction takes place, folic acid breaks up and fluorescence is increased. The cytotoxicity induced by these rhodamines in breast cancer cells upon irradiation at 1064 nm after passive internalization and internalization via reconstituted high-density lipoprotein nanoparticles was also studied. Rhodamine-123 did not undergo a type I reaction at 1064 nm in solution; rhodamine-6G and rhodamine-B did so to a similar extent. Irradiation of cancer cells after 1 h of incubation with rhodamine/(folic acid) solutions and 24 h of incubation post-treatment revealed rhodamine-6G- and rhodamine-B-induced toxicity via the production of reactive oxygen species. Such species damage mitochondria and induce apoptosis and necrosis. The cell internalization of rhodamines via rHDL nanoparticles did not compromise the cytotoxic effect previously exhibited. Rhodamine-6G and rhodamine-B transported in nanoparticles are potential candidates for biphotonic photodynamic therapy at 1064 nm.