Experimental evidence involving the photosensitizing properties of hematoporphyrin is presented. Hematoporphyrin is shown by fluorescent microscopy to be taken up into the epidermis after intradermal injection. Such a circumstance gives rise, on exposure to light, to a sunburn-like response. This response is demonstrated to be dependent on oxygen, just as has been recognized for the urticarial response with hematoporphyrin's dermal location. These studies support the contention that the cutaneous responses resulting from both so-called photodynamic and phototoxic agents are governed by the same principles. Whether the response is urticarial or sunburn-like depends on whether these agents are located in the dermis or epidermis. The penetrability of the wavelengths representing the photosensitizer's absorption spectrum determines the thickness of skin within which the photosensitizer must reside in order to be activated.
McGRAE JD, PERRY HO. Relationship of Photodynamic Action to Phototoxicity: Determinants of Morphologic Response. Arch Dermatol. 1963;87(2):252–257. doi:10.1001/archderm.1963.01590140114018
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