Synthesis and evaluation of novel mitochondria-targeted, water-soluble phenoxazine-porphyrins for efficient photodynamic therapy

In this study, we use cationic imidazole as the targeting group and phenoxazine as the electron donor, attaching phenoxazine to porphyrin via two different linking methods to synthesize two phenoxazine-porphyrin photosensitizers (PSs), P1 and P2, intended for photodynamic therapy (PDT) applications. Additionally, we synthesized a molecule, P3, that does not contain the cationic imidazole group, to serve as a control PS. We comprehensively evaluate the performance of P1 and P2 through steady-state and transient spectroscopy analysis, theoretical calculations, photooxidation experiments, and in vitro cell experiments. The results showed that P1 and P2 exhibit excellent water solubility, longer triplet state lifetimes (P1 at 200 μs, P2 at 170 μs), outstanding photostability, higher photooxidation rates and quantum yields of singlet oxygen (1O2). Notably, in the photooxidation experiment, P1's photooxidation rate is 1.1 times that of P2, and 1.5 times that of tetraphenyl zinc porphyrin (ZnTPP) without phenothiazine. This may be due to the larger Gibbs free energy difference between the singlet and triplet excited states in P1 (ΔGS-T of P1 is 0.87 eV, compared to 0.83 eV for P2), which results in a greater driving force for intersystem crossing (ISC) in P1 than in P2. The introduction of the cationic imidazole group in P1 and P2 significantly enhances water solubility and tumor cell uptake compared to P3, particularly showing higher intracellular accumulation and 1O2 generation capabilities in HepG2 tumor cells. Flow cytometry test results further confirmed the significant photocytotoxicity of P1 and P2, with a marked decrease in survival rates after light exposure, where the survival rate of P1 nanoparticles (NPs) dropped to 41.7 %, P2 NPs to 35.1 %, and P3 NPs to 75.0 %. The findings of this study provide crucial theoretical support for the design of new PSs.