Quantitative tests for psoralens in the blood: methods and uses in monitoring psoralen and longwave radiation therapy.

Variability in 2 factors determines the therapeutic success of photochemotherapy with psoralens and UV radiation at 320-400 nm (PUVA): biochemical properties of the photosensitizing drug used and characteristics of the source of irradiation. Fine-tuning of PUVA therapy is achieved by delivery to the patient of the phototoxic dose of UV radiation at the precise time of peak bioavailability of the photosensitizing psoralen; thus it depends on accurate means for measuring the 2 variables. The UV emission is easily measured, but bioavailability of photosensitizing drugs is not. Blood levels provide information about the photosensitivity of the patient and thus relate to therapeutic efficacy. The oral dose of 8-methoxypsoralen (8-MOP) cannot be used to predict what plasma levels of the drug will be achieved. Dissolution and absorption of 8-MOP are affected by the form of the preparation. Absorption rates vary far more than do elimination rates in the same population of patients, but individual differences in metabolism of the drug have not been documented. Maximum therapeutic benefit varies from individual to individual. It may be influenced by factors affecting absorption, distribution, and metabolism of the drug and by skin type and degree of skin pigmentation.