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Molecules 2020, 25, 4102 3 of 22 Molecules 2020, 25, x FOR PEER REVIEW 3 of 22 source is commonly based on PS nature, absorption spectra of PS, location, and size and characteristics of tbhaesiendfeoctnedPStisnsuaetu[r7e,3, 2a]b. sorption spectra of PS, location, and size and characteristics of the infected tissuMe [o7r,3e2t]h.an 300 chemical compounds have already been identified as potential candidates to be used MasoPreSst.hAanm3o0n0gcsht etmheiscea,l acofemwpowuenredsauhtahvoeriasleredafdoyr cbleiennicaidl eanptpifliiecdataiosnpiontePnDtiTa,l acnanddoitdhaetres wtoebre used as PSs. Amongst these, a few were authorised for clinical application in PDT, and others were medically evaluated, whereas some are still under examination [33,34]. We have tabulated some PSs medically evaluated, whereas some are still under examination [33,34]. We have tabulated some PSs which are used in various cancer treatments in Table 1. Photosensitisers are naturally or chemically which are used in various cancer treatments in Table 1. Photosensitisers are naturally or chemically produced compound conjugated with a visible light-absorbing chromophore group with a strong produced compound conjugated with a visible light-absorbing chromophore group with a strong chemical absorbance. Choosing the correct PS is the most important phase in PDT for a successful chemical absorbance. Choosing the correct PS is the most important phase in PDT for a successful outcome [33,34]. The purity and the presence of a tetrapyrrole structure with good storage stability are outcome [33,34]. The purity and the presence of a tetrapyrrole structure with good storage stability the preferable properties of most PSs used in PDT. The potent and effective PS should have the ability are the preferable properties of most PSs used in PDT. The potent and effective PS should have the to initiate a photodynamic reaction after irradiation with 600–800 nm lights and should not cause any ability to initiate a photodynamic reaction after irradiation with 600–800 nm lights and should not toxicity under dark conditions. It should be easily distinguishable from the body with no or minimum cause any toxicity under dark conditions. It should be easily distinguishable from the body with no phototoxic side effects [35]. The better diffusion of PS through the cells after long administration or minimum phototoxic side effects [35]. The better diffusion of PS through the cells after long might contribute to the effectiveness of PDT [36]. The production of a significant amount of ROS after administration might contribute to the effectiveness of PDT [36]. The production of a significant irradiation that induces apoptosis with less inflammation is most likely to be a suitable PS for PDT amount of ROS after irradiation that induces apoptosis with less inflammation is most likely to be a application [37,38]. suitable PS for PDT application [37,38]. When a PS is subjected to a particular wavelength light, the electron of the outermost orbital will When a PS is subjected to a particular wavelength light, the electron of the outermost orbital will be shifted from the ground state (S0) to the first excited state (S1). Subsequently, the electromagnetic be shifted from the ground state (S0) to the first excited state (S1). Subsequently, the electromagnetic propulsion switches the molecule to an excited triplet state (T1) with a longer life span (Figure 1). propulsion switches the molecule to an excited triplet state (T1) with a longer life span (Figure 1). In In each of these excited states, PSs are quite unstable and lose their energy in the form of fluorescence, each of these excited states, PSs are quite unstable and lose their energy in the form of fluorescence, phosphorescence, and internal heat conversion. PSs in the T1 state may react photochemically in any phosphorescence, and internal heat conversion. PSs in the T1 state may react photochemically in any of the two pathways. In the type 1 pathway, the excited PS reacts through an electron transfer process of the two pathways. In the type 1 pathway, the excited PS reacts through an electron transfer process with the surrounding oxygen, which ultimately leads to generation of reactive oxygen species (ROS). with the surrounding oxygen, which ultimately leads to generation of reactive oxygen species (ROS). Such free radicals communicate readily with the biomolecules (lipids, peptides, proteins, and nucleic Such free radicals communicate readily with the biomolecules (lipids, peptides, proteins, and nucleic acids) and destroy them [39,40]. In contrast, in the type 2 pathway, the energy is directly transferred acids) and destroy them [39,40]. In contrast, in the type 2 pathway, the energy is directly transferred from the T1 state of the PS to the S0-state oxygen. This results in the ground-state PS transformation from the T1 state of the PS to the S0-state oxygen. This results in the ground-state PS transformation and excited-state singlet reactive oxygen. The disruption caused by PDT is local because both singlet and excited-state singlet reactive oxygen. The disruption caused by PDT is local because both singlet oxygen as well as free radicals have a short half-life between 10–300 nanoseconds and a small diffusion oxygen as well as free radicals have a short half-life between 10–300 nanoseconds and a small distance of 10–55 nm [41]. diffusion distance of 10–55 nm [41]. Figure 1. The general mechanism of photodynamic therapy..PDF Image | Role of Photoactive Phytocompounds in Photodynamic Therapy of Cancer
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