An intraindividual comparative study of psoralen-UVA erythema induced by bath 8-methoxypsoralen and 4, 5', 8-trimethylpsoralen.

BACKGROUND

Limited work has been conducted on the characteristics of topical trimethylpsoralen (TMP) psoralen-UVA (PUVA) erythema.

OBJECTIVE

We sought to determine the time-course and dose-response characteristics of erythema induced by topical TMP, and to compare these parameters with those for topical 8-methoxypsoralen (MOP) within patients.

METHODS

After photosensitization of one forearm with topical TMP, test sites were exposed to a UVA dose series. The procedure was repeated on the other forearm using 8-MOP solution. Erythema was assessed visually and with a reflectance instrument every 24 hours for 7 days.

RESULTS

TMP PUVA erythema followed a similar time course to 8-MOP PUVA erythema. The majority of patients were at maximal erythema at or beyond 96 hours. TMP PUVA had a significantly steeper dose-response curve at 48, 72, and 96 hours compared with 8-MOP PUVA.

CONCLUSION

On the basis of these data, the optimal time to read the TMP minimal phototoxic dose is 96 hours. In view of the steeper dose-response curve for TMP PUVA, a lower UVA incremental regimen should be considered compared with that for 8-MOP PUVA.