PUVA ERYTHEMAL SENSITIVITY DEPENDS ON PLASMA PSORALEN CONCENTRATION AND UVA SENSITIVITY

The variation in erythemal sensitivity of the skin during PUVA therapy with oral 8-methoxypsoralen (8-MOP) was examined by measuring both UV A and PUVA erythemal responses, together with plasma 8-MOP concentrati on, in 2 7 patients about to start PUVA therapy for psoriasis. The ery thema responses were judged visually, and also measured using a reflec tance instrument in order to construct dose-response curves. No signif icant association was found between the UVA and PUVA minimal erythema responses. The plasma psoralen concentration showed significant associ ation with the slope of the PUVA erythema dose-response curve. The slo pes of the UVA and PUVA erythema dose response curves were significant ly associated, and this association became much stronger when allowanc e was made for plasma psoralen concentration. These results show that erythemal sensitivity during PUVA therapy is related to both plasma ps oralen concentration and inherent UVA sensitivity, but that this relat ionship is not apparent when sensitivity is judged visually as the min imal erythema response. The association between PUVA and UVA erythemal sensitivity suggests a common pathway in the vascular response induce d by UVA radiation, with or without psoralen.