Photodynamic Therapy: Use of Nanocarrier Systems to Improve Its Effectiveness

Photodynamic Therapy (PDT) is a medical modality that has been applied against several types of cancer, macular degeneration, pointed condyloma, actinic keratosis as well as infections caused by fungi, viruses, and bacteria. When PDT is applied against microorganisms, the technique is called as antimicrobial photodynamic therapy (aPDT). PDT/aPDT principle involves the association of a light source (performed by a LASER, LED, and optical fiber), a non-toxic photosensitizer (PS), and molecular oxygen dissolved in the tissue of interest. The photosensitizer is excited by a light source of a specific wavelength which, in the presence of oxygen, generate high-cytotoxic reactive oxygen species (ROS) as well as superoxide anion (O2·−), hydroxyl radical (HO·), hydrogen peroxide (H2O2) and singlet oxygen (1O2). These species cause damage to tumor cells and vasculatures by apoptosis, necrosis, and activating the immune responses. Among the main advantages of PDT is the specificity. This is guaranteed by the preferential accumulation of the photosensitizer in the cells of interest and the targeting of the lighting system without compromising healthy tissues. Additionally, it is a cheaper and less invasive than most known treatment, such as surgery, chemotherapy, and radiotherapy. Several classes of photosensitizers have been proposed for application in PDT treatment. It is necessary to mention phthalocyanines, porphyrins, bacteriochlorins, chlorines, chlorophyll-based compounds, phenothiazinium salts, and xanthene dyes. Generally, better PS compounds are hydrophobic once they accumulate in the interest tumors most effectively. However, the direct application of these in the body is harmful once PS can precipitate in the body, forming aggregates. Additionally, the pre-solubilization in an organic solvent before the application is not recommended once these are high toxicity in the cell. In this way, strategies have been proposed to solubilize hydrophobic PS in aqueous solutions and increase their biocompatibility. One of the most successful is the incorporation of PS in nanocarrier systems such as liposomes, copolymeric micelles, cyclodextrins, gold nanoparticles, microemulsions, self-assembled peptide-based nanomaterials, and others. Each nanocarrier has this specificity in order of its vantages and advantages. The main objective of this chapter is the description of PDT principles and understands more about formulations that have been used for PDT treatment.