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Lookup NU author(s): Professor Peter Farr
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Methoxsalen (microcrystalline preparation) is given at a standard dose of 0.6 mg per kg or 25 mg per m2 in oral PUVA therapy but there is great variation in serum psoralen concentration between patients, and therefore in degree of photosensitivity achieved. In patients who have a high UVA minimal phototoxic dose (MPD), an increase in psoralen dose may be considered, likewise a dose reduction may be appropriate in highly photosensitive patients. Limited information is available on the erythemal effect of changing psoralen dose (Ljunggren et al, J Invest Dermatol 1981; 76:73). We studied in 15 subjects the effect of varying the methoxsalen dose over a clinically relevant range. Two hours after methoxsalen ingestion, serum concentration was measured (HPLC, P Barnfield, St Thomas’ Hospital, London) and the subjects phototested at 350 6 30 nm (0.45–14 J per cm2). The MPD at 72 h was recorded and intensity of erythema measured with a reflectance instrument. Each subject was tested on three occasions using doses of 25 mg per m2 (conventional dose; CD) or CD 6 10 mg. Median serum psoralen concentration increased from 85 to 143–239 ng per mL with dose increases from CD-10 mg to CD and CD 1 10 mg, respectively (P , 0.05). Median MPD and D0.025 (the objective equivalent of the MPD, derived from the dose– response curve), were significantly reduced with increasing methoxsalen dose from CD-10 mg (MPD 1.7 J per cm2; D0.025 2.8 J per cm2) to CD (1.2 and 1.4 J per cm2) and CD 1 10 mg (0.9 and 1.0 J per cm2) (P , 0.01). The psoralen dose had no significant effect on the slope of the dose–response curve. Thus, changing the dose of methoxsalen over a narrow therapeutic range appears to significantly altered threshold erythema but not the rate of increase of erythema.
Author(s): Farr P; Ibbotson S
Publication type: Conference Proceedings (inc. Abstract)
Publication status: Published
Conference Name: 60th Annual Meeting of the Society for Investigative Dermatology
Year of Conference: 1999
Publisher: Nature Publishing Group
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