3 Mechanism of PDT of porphyrin
The anti-tumor mechanism of PDT consists of two main phases (Fig. 5). Photosensitizer (PS) accumulates at the tumor site after intravenous injection and then irradiates the tumor tissue at a specific wavelength. In the first stage, PS changes from the ground state (single-linear state (S0)) to the excited single-linear state (S1) after being irradiated (nanosecond range). The excited state of the photosensitizer is very unstable and loses excess energy through non-radiative (thermal emission) or radiative (fluorescence emission) pathways(Bouramtane et al. 2019; Castano et al. 2005; Robertson et al. 2009). The excited single-linear state can produce a more stable excited trilinear state (T1) with parallel spins (microsecond to millisecond range) by inter-system crossover. In the T1, PS can undergo two types of reactions (Type I reactions and Type II reactions). In the first type of pathway, electron or hydrogen atom transfer occurs between the T1 photosensitizer and the cell membrane of the biomolecule(Z. 2003). This process forms free radicals and radical ions, leading to the production of cytotoxic hydroxyl radicals (•OH), hydrogen peroxide (H2O2), and other ROS. The second type of reaction involves the interaction between electronically excited trilinear state photosensitizers and the ground state trilinear state molecular oxygen (3O2). The excited PS transfers energy to 3O2 to form the singlet oxygen. The product1O2 can react with a variety of biomolecules and is a key factor in the induction of apoptosis and tissue destruction in cancer cells(Buytaert et al. 2007; Kessel and Oleinick 2010; Mehraban and Freeman 2015). In addition, it has been demonstrated that type I and type II reactions can occur simultaneously and independently, and that type II reactions play a more important role in PDT(Castano et al. 2004; Ethirajan et al. 2011; Gomes et al. 2018; Lin et al. 2020).