**1. Introduction**

Photodynamic therapy, also known as PDT, is a versatile treatment strategy, commonly used for cancer mitigation that combines three non-toxic components to cause the death of the target cell: light, a photosensitizer (PS), and tissue oxygen [1]. As we know conventional cancer therapy has various drawbacks due to serious adverse effects [2]. However, in the case of PDT such side effects are comparatively minimal and relatively less documented. PDT always ensures that cells are effectively targeted and damaged through necrosis or apoptosis [3].

Photosensitizers (PS) are organic or inorganic molecules that can be triggered by visible light and can produce reactive oxygen species (ROS) [4]. The PS can absorb the light energy and excite from the ground to its higher energy excited state. As shown in **Figure 1**, during the relaxation process, ROS production occurs by utilizing tissue/cellular oxygen to

**Figure 1.** *Modified Jablonski diagram showing the mechanism of PDT.*

cause cellular damage using apoptosis or necrosis [5]. The key benefits of this mechanism are that only localized cells are destroyed and limiting the toxicity in the tumor environment. The selectivity of PS to the target tissue is a key factor while designing a PDT system, otherwise irrespective of tissues, PS can damage the normal vegetative tissues also [6].

There are a few porphyrinoid-based PS with interesting ROS-producing abilities and better selectivity to the target tissue. A few US and Indian-approved porphyrinoid PS includes Photofrin II, ALA (prodrug), and Foscan. PDT now has a wide range of applications, ranging from illness prevention to diagnosis [3]. Every day, a new class of porphyrinoid photosensitizers is being discovered, allowing for the treatment of an expanding range of illnesses. PDT offers the advantage of treating solid tumors at all phases of the disease, from early to advanced, and that too non-invasively. Rather it has been shown to treat a variety of diseases ranging from skin disorders to cardiovascular and ocular diseases. Targeting takes place in both passive and active methods, with nanoparticles, peptides, proteins, and polyclonal as well as monoclonal antibodies (mAb), or any other systems that direct PS toward targeted cancer cells or tissue and avoid the localization of PS inside healthy normal cells. Passive targeting uses the EPR effect to deliver the medicine to the target location, whereas active targeting uses proteins, ligands, and pores to cross the cells and deliver the drug to the target site [7].

The ability of photosensitizers to act as diagnostic and theranostic agents is one of their growing applications. The concept has only lately gained popularity, although PDT has already established it through fluorescence imaging. Diagnostics refers to the use of fluorescence, MRI, NIR, photoacoustic PET, and SPECT imaging to aid in the diagnosis of a certain disease or metabolic pathway. Theranostic in the sense of possessing both diagnostic and disease-mitigating capabilities. Real-time imaging can assess dosimetry, medication efficacy, kinetics, singlet oxygen production, light delivery, oxygen pitch, blood gush, neighboring cells, and the status of the targeted cells or tissues [8].

For example, nano texaphyrin, a novel theranostic compound linked with a prostate-specific membrane antigen (PSMA) used in targeted radionucleotide imaging and focal photodynamic therapy. The creation of an activatable theranostic is currently the primary emphasis. As an inert chemical, some stimuli, such as pH and temperature, will activate inside the target. Another classic example is Photofrin having both photosensitizing and fluorescence capabilities.

Furthermore, the cost of this therapeutic procedure is not too expensive. With personalized PDT, PS dose, frequency, and side effects can be minimized. It will aid in the provision of individualized treatments, which is now more vital than ever. PDT is becoming more popular in the field of biotechnology, which will result in less pollution and greater human health compliance [9].
