Action spectra of topical psoralens: a re-evaluation

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Abstract
The action spectra of producing minimal phototoxic erythema [in man] with topical 0.1% trimethyl psoralen (TMP) and 8-methoxypsoralen (8-MOP) were determined with a double monochromator in the range of 295-380 nm. Both psoralens induced photosensitivity in the range of 313-365 nm; TMP was 54% more effective than 8-MOP. There was no difference in the dose needed to produce minimal UV erythema or phototoxic erythema with 8-MOP and TMP at 295 and 305 nm, but at 313 nm with 8-MOP photosensitivity was enhanced 3.5 times and with TMP sensitivity was enhanced 5.5 times. The peak sensitivity with 8-MOP was at 330 nm and for TMP at 335 nm. No photosensitivity occurred above 380 nm. Apparently TMP and 8-MOP are significantly photoreactive at 320-335 nm. Commonly used UV-A light sources show peak emission at .apprx. 360 nm. If there is a relationship between development of erythema and therapeutic effectiveness, this raises the possibility of alternative UV light sources for phototherapy with psoralens.

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