Add time:08/02/2019         Source:sciencedirect.com

Publisher SummaryThis chapter discusses those drugs where one or more photoderivatives have been positively identified and photosensitizers which, although unchanged themselves, photocatalyse the degradations of biomolecules. Precautions against the photodegradation of drugs in storage are easily taken, but light-induced changes after the patient has taken the medicine may lead to serious side-effects––typically erythema, oedema, and vesicle formation of exaggerated sunburn. Sunlight penetrates the skin to a depth sufficient to reach blood circulating in surface capillaries. Two types of photosensitivity as a result of foreign chemicals circulating in the blood stream has been described––namely, phototoxicity and photoallergy. In phototoxicity, the excited state of the drug molecule can induce cellular damage in at least one of three ways: by inducing radical formation in an endogenous molecule, by converting oxygen from its ground (triplet) to the highly toxic singlet state, or by electron transfer, it can generate the very reactive superoxide radical. Phototoxicity occurs only in areas of skin exposed to the light and is characterized by the relief of symptoms when the drug is withdrawn. It can be effective with only one dose in combination with exposure to the sun (or an ultraviolet lamp). With a high enough dose, the phototoxic response can be elicited from all exposed individuals. Examples of drugs that can generate singlet oxygen are chloroquine, frusemide, nalidixic acid, and naproxen. Superoxide generators are chlorpromazine, mercaptopurine, some tetracyclines, and psoralens.

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