5 Conclusions and perspectives
Research on porphyrin photosensitizers has made great progress in overcoming the problems of insufficient water solubility, limited tissue penetration and low light energy capture efficiency in traditional porphyrin photosensitizers. Porphyrin photosensitizers with red-shifted spectral absorption bands, metal-modified porphyrins, nanoparticle-modified porphyrins or self-assembled nanoparticles have solved the problem that photosensitizers can easily lead to electronic excited state is quenched through aggregation in living organisms while enhancing the light energy capture efficiency of porphyrin photosensitizers. In PDT of tumors, a fluorescent donor is used to indirectly activate the photosensitizer by FRET to porphyrins under light irradiation conditions. The introduction of the donor fluorescent group is able to red-shift the absorption spectral region of the porphyrin photosensitizer, which makes PDT of tumors in the near-infrared spectral region possible. Near-infrared light is more penetrating, thus enabling PDT of deep tumor tissues. In addition, the larger photon absorption cross-section of the two-photon excited donor enables efficient light energy capture, which ultimately leads to the production of large amounts of 1O2 and enhances the PDT effect. However, it should be noted that the study of FRET with porphyrin photosensitizers in tumor PDT is still in its infancy, and many problems remain to be explored and solved.
Avoiding or minimizing toxicity to normal tissues is extremely important in tumor PDT. It has been shown that some nanoscale photosensitizers, such as Au NRs and silica nanoparticles, have high redox reactivity on their surfaces, which may generate unnecessary1O2 to be toxic to normal cells(Guo et al. 2015). Some nanoparticles containing heavy metals have been shown to cause damage to normal cells(Khalil et al. 2011; Tsoi et al. 2013; Yu et al. 2017). In addition, photosensitizing drugs can be quickly cleared in the human body, resulting in low drug concentration in tumor tissue and unable to exert a good PDT effect(Wang et al. 2015; Zhu et al. 2019b). Therefore, the development of biocompatible materials in the porphyrin photosensitizer PDT to reduce the toxic and side effects of photosensitizers on normal cells is more important.
In conclusion, there has been a growing interest in developing novel porphyrin photosensitizers for PDT in recent years. Although the application of PDT technology to the clinic still needs further efforts. However, with the rapid development of optical technology and nanotechnology, it is believed that safer and more effective photosensitizers will be developed in the future.