A Preliminary Comparison of the Photosensitizing Properties of Porphyrins in Aqueous Solution and Liposomal Systems

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Abstract
Most of the detailed mechanistic studies on porphyrins as photosensitizers have been carried out in more or less homogeneous solution. Sensitized photoreactions in cells, as exemplified by the porphyrin-sensitized photokilling of tumor cells, must be a much more complex process. Cells are non-homogeneous with abrupt phase changes and with major differences in chemical composition in different regions. Because of localized differences, sensitizers tend to selectively dissolve in or bind non-covalently to different micro-regions of the cell; on illumination this could result in selective, localized damage to cell structure and function. Because of these complexities, we know relatively little about the specific lesions in photosensitized cells leading to injury and death (1).