5-Methoxypsoralen, a naturally occurring linear furocoumarin, has been successfully used in combination with ultraviolet (UV) A irradiation [psoralen plus UV (PUVA)] to manage psoriasis and vitiligo. In patients and volunteers, PUVA 5-methoxypsoralen causes a dose-related increase in cutaneous photosensitivity. However, mean minimum phototoxic doses (MPD) were 30 to 50% greater with 5-methoxypsoralen than with 8-methoxypsoralen within individuals; this suggests lower photoactivity with 5-methoxypsoralen. In comparative clinical trials of parallel design, psoriasis clearance rates of > 90% or > 97% were observed in similar numbers of patients (60 to 77%) receiving oral PUVA 5-methoxypsoralen (typically 1.2 mg/kg) or oral PUVA 8-methoxypsoralen (0.6 mg/kg) treatment. Generally, 5-methoxypsoralen recipients required a greater total UVA exposure than 8-methoxypsoralen recipients to achieve end-point. However, study end-point was achieved sooner with oral or topical PUVA 5-methoxypsoralen in a small number of patients with psoriasis who received both treatments simultaneously and contralaterally. Up to 56% of patients with vitiligo achieved > 75% repigmentation with 5-methoxypsoralen (oral or topical) combined with UV irradiation (lamp or sun); the face and trunk were the most responsive areas. Lack of response to PUVA 5-methoxypsoralen treatment was observed in up to 16% of patients with psoriasis and, in 1 trial, in 22% of those with vitiligo. Lesion spreading during treatment of vitiligo was also observed in 7 (19%) patients in 1 study. The incidence and severity of adverse events was generally lower in PUVA 5-methoxypsoralen 1.2 mg/kg than in PUVA 8-methoxypsoralen 0.6 mg/kg recipients. Nausea and/or vomiting, pruritus and erythema were the most commonly reported adverse events in the short term; they occurred about 2 to 11 times more frequently in 8-methoxypsoralen than 5-methoxypsoralen recipients within clinical trials. Adverse hepatic events after oral administration of the drug were uncommon. Long term tolerability data for PUVA 5-methoxypsoralen are scarce; however, carcinogenicity was not reported during a 14-year observation period of 413 patients with psoriasis.
Conclusion: Similar lesion clearance rates were observed with oral 5- or 8-methoxypsoralen plus UVA exposure in patients with vitiligo or psoriasis, although patients given 5-methoxypsoralen often required a greater total UV exposure than 8-methoxypsoralen recipients. The incidence of short term cutaneous and gastrointestinal adverse effects is markedly less with 5-methoxypsoralen than with 8-methoxypsoralen, which is an advantage, although the long term tolerability of 5-methoxypsoralen has yet to be fully established. Nevertheless, in appropriately selected patients, PUVA 5-methoxypsoralen therapy may be recommended as an alternative first-line systemic treatment option for the management of vitiligo or psoriasis.