Enhancing Type I Photochemistry in Photodynamic Therapy Under Near Infrared Light by Using Antennae–Fullerene Complexes

Abstract

Photodynamic therapy (PDT) has been developed as salvage or palliative treatment for a wide range of tumors. The principle underlying this therapy is the generation of reactive oxygen species through two types of photochemical pathways. As compared with the type II pathway, the type I pathway offers a higher oxidizing ability for photosensitive residues such as tryptophan, lower oxygen dependency, and deeper tissue penetration ability. In this review, we focus on the enhancement of the type I pathway in the near infrared (NIR) optical therapeutic window of wavelength 700-1,100 nm by using antennae-fullerene complexes. When photo-induced electron transfer occurs from antennas to fullerene, the pathway switches from type II to type I. Enhancing the type I pathway leads to less oxygen dependency and stronger capacity for tryptophan oxidation. Then, antennae that transfer long-wavelength light (e.g., NIR light) energy to fullerene via photo-induced energy and/or electron transfer have the ability to affect deep-seated tumors. On the basis of theories on photo-induced energy/electron transfer in other fields such as solar cells, here, we summarize the mechanism by which the switch from the type II to type I pathway occurs in antenna-fullerene-based PDT.