Toward targeted photodynamic therapy

The sensitizers in common use for photodynamic therapy (PDT) are summarized, and approaches to the improvement of these outlined. Selectivity in the targeting of sensitizers to tumor cells and tissue is highly desirable, as is water solubility and prevention of aggregation. Some new free sensitizers are described, based upon the pyropheophorbide a (PPa) structure, and their photophysical properties, distribution in cells via confocal fluorescence microscopy, and cell kill properties described. A novel approach to targeting is to covalently attach such sensitizers to monoclonal antibody fragments, and recent work on the attachment of pyropheophorbide a to such monoclonal antibody fragments is reviewed, with a demonstration of the increased efficiency of cell kill, and the treatment of a solid human tumor in a mouse model described. Finally, an alternative method of achieving selectivity based upon two-photon excitation (TPE) using porphyrin dimer sensitizers is reviewed, and the use of these to kill tumor cells is compared with the use of a commercially available PDT sensitizer (Visudyne). TPE of a porphyrin dimer sensitizer is shown to be capable of sealing blood vessels in a mouse model.