**2.3 Mechanism of PDT**

As discussed, photodynamic therapy (PDT) opens a promising treatment approach for the management of a variety of solid tumors. Here, cell death is caused by oxidative damage to the cellular organelles through the reactive oxygen species (ROS). It has been demonstrated that cell death in PDT works by apoptosis [programmed cell death] and necrosis [Unprogrammed cell death] [57]. For example, necrotic cell death was observed in the PDT of human lung adenocarcinoma cells using Palladium 2 – tetraphenyl porphycene [58]. Lutetium texaphyrins showed induction of cellular apoptosis through selective modulation of Bcl-2 family proteins. The choice of cell death mechanisms (i.e., whether apoptosis or necrosis) in PDT varies based on:


PDT works by the accumulation of photosensitizer (PS) in the tumor or targeted diseased tissue before being exposed to the laser or visible light of a specific wavelength. The applied light should be of a higher wavelength and have to match the absorption band of PS. As shown in **Figure 3**, the higher wavelength light possesses better penetration and less absorption will get lost inside the tissues [60]. By absorbing the light, PS dye gets electronically excited and comes to its singlet excited state.

*Porphyrinoid Photosensitizers for Targeted and Precise Photodynamic Therapy: Progress… DOI: http://dx.doi.org/10.5772/intechopen.109071*

The specially designed PS dyes show better intersystem crossing ability and move to a triplet excited state. Before intersystem crossing, the singlet state PS drops its energy either as light emission (fluorescence) or through heat production (nonradiative relaxation) [61]. From the triplet excited state, PS relaxes in different ways, among them two mechanisms are beneficial for the generation of ROS. They are generally classified as Type I and II.

In type I, the excited state PS can generate radicals which will then retaliate with cellular oxygen to form ROS, such as superoxide radical anions (O2 .<sup>−</sup>), hydrogen peroxide (H2O2), and hydroxyl radicals (OH).

The type II mechanism happens by directly transferring the energy to cellular oxygen (molecular oxygen in triplet state 3 O2) and converts to singlet oxygen (1 O2) [33]. Singlet oxygen, the putative cytotoxic agent is one of the major outcomes of such photochemical reactions (**Figure 1**).

Even though the singlet oxygen has only around a 10–55 nm radius of destruction inside cells and a very short lifetime of 10–320 ns [62], it can enable a considerable and complicated chain of events that modify the immune response. These events progress to local, regional, and systemic levels, making it feasible to reliably regulate tumor growth. Same time PDT can also trigger cell signaling and leads to killing cells directly. These all totally depend on the physiochemical properties, concentration, and intracellular location of the PS. Besides that, the availability and concentration of cellular oxygen also play a key role in PDT [30]. Various steps involved during photodynamic therapy include:

#### *2.3.1 Subcellular localization of the photosensitizer*

The photosensitizer is brought into the target cellularly or subcellular level or intracellularly using mechanisms like low-density lipoprotein binding, receptormediated phagocytosis or pinocytosis, uptake via epidermal growth factor/tyrosine kinase receptors, etc. PS molecules can also accumulate inside various cell organelles like mitochondria, golgi apparatus, ribosomes, cytoplasm, etc. [59]. For example, in the case of Photofrin, rather than accumulating inside the cell membrane, it can bind with the mitochondria [61]. It is observed that, usually, PS localize selectively into the tumors, because of i) its leaky microvasculature, ii) its low extracellular pH, iii) it contains a large number of macrophages, iv) its large interstitial volume, v) it contains more lipoproteins and receptors, and vi) its poor lymphatic drainage [63].

#### *2.3.2 Activation of the photosensitizer by light*

Once the PS is localized in the tumor cells, the appropriate light with the desired wavelength can electronically excite the dye. This can give rise to type I or type II reactions as **Figure 1**. The formed singlet oxygen and other ROS will carry out cellular destruction through the chromosomal and cytoplasmic breaking of the localized cell [61]. The outcome of PDT will limit to the localized cells only, it will not be progressing to the neighboring cells, due to the short lifetime of the produced singlet oxygen and as it is active around 10–55 nm in cells [64].

#### *2.3.3 Necrosis and apoptosis: modes of cell death*

As discussed, Necrosis and apoptosis were found to be the major mode of cytotoxicity after PDT [65]. Necrosis is the sudden cell death occurring through the influence of external factors, while apoptosis is natural cell death. It is believed that a

**Figure 6.**

*A series of events during PDT (a) a cluster of cancer cells (blue) and normal cells (light rose); (b) PS accumulates in both the cells; (c) after a period PS localize into tumor tissue; (d) porphyrinoid PS fluoresce inside cancer tissue; (e) laser/visible light can induce PDT action.*

low dose of light will cause apoptosis and meanwhile higher dose will cause necrosis. Found that around 70% of cytotoxicity was due to apoptosis and 99% was because of necrosis (**Figure 6**) [66].

In the case of tumor-specific PS with good fluorescence, the ability can be effectively employed for tumor detection and fluorescence-guided surgery. On the other hand, fluorescence-guided surgery can be achieved by effective utilization of different fluorophores and PS in a nanocarrier (e.g., liposomes, niosmes, cubosomes, and other nanoparticles), which readily accumulate in tumor tissue [17]. Lee et al. utilized such combined fluorescence-assisted surgery and PDT toward glioblastoma using indocyanine-green and chlorin-e6 in a nanocluster [67].
