Prodrugs in photodynamic anticancer therapy

Photodynamic therapy (PDT), the concept of cancer treatment through the selective uptake of a light-sensitive agent followed by exposure to a specific wavelength, is limited by the transport of a photosensitizer (PS) to the tumor tissue. Porphyrin, an important PS class, can be used in PDT in the form of its prodrug molecule 5-aminolevulinic acid (5-ALA). Unfortunately, its poor pharmacokinetic properties make this compound difficult to administer. Two different methods for eliminating this problem can be distinguished. The first approach is to play with its formulation in order to improve the drug's applicability. The second approach, which is to find possible 5- ALA prodrugs, is an example of the double-prodrug method, a strategy often used in modern drug design. In this approach, the biological mechanisms in a long biosynthetic pathway involving several steps must be completed before the active drug appears. Recently, an idea of enhancing PDT sensitization using the so-called iron chelators seemed to increase the accumulation of protoporphyrin in cells. At the same time, iron chelators can destroy tumor cells by producing active oxygen after the formation of an active drug by chelating iron in the cancer cells. Thus, in the latter case, the therapy resembles a prodrug strategy. The mechanism can be explained by the Fenton reaction. Vitamin C is another example of a potential anticancer agent of this type.