Targeted Photodynamic Therapy—An Assimilation of Successes, Challenges and Future Directions
Targeted approaches in photodynamic therapy (PDT) promise to alleviate many of the limitations of the treatment, such as it being restricted to localized treatments and the most common adverse biological effects of non-specific phototoxicity. Tumor selectivity during conventional PDT is predominantly achieved through the enhanced permeability and retention effect and the regional control of the area of irradiation, restricting the application to pre-established and localized tumor foci. Continuing scientific and clinical advances in our understanding of tumor physiology and the biochemistry of tumor regulation, progression and treatment evasion have been exploited in order to target neoplastic tissue for selective photodynamic destruction. Such multifaceted approaches to targeting and sensitizing tumors through numerous strategies expand the application of PDT to locoregional or systemic neoplastic control and can enhance its therapeutic outcome by combating related treatment escape pathways. In this chapter, we discuss the developments in targeted PDT and the multiple approaches taken in order to maximize treatment outcomes. The chapter is not meant to be entirely comprehensive; rather, a broad but selective choice of approaches is discussed. Although the major portion of the chapter addresses targeted PDT of cancers, some specific examples of non-cancer applications are also discussed.