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.