PUVA therapy prevents sensitization to mechlorethamine in patients with psoriasis.

Two groups of 20 patients with psoriasis were treated with mechlorethamine applied topically (group A) or with PUVA combined with mechlorethamine (group B). In group B mechlorethamine was started after six PUVA treatments. Results showed a significant decrease of the incidence of contact dermatitis in group B (30%) compared with group A (75%). Allergic dermatitis, demonstrated by a positive patch test to mechlorethamine with an histology of eczema, was observed in 55% of patients in group A and 20% in group B. The incidence of irritant dermatitis was not significantly different in the two groups. Allergic dermatitis was observed later in group B: after an average of 32.2 applications of mechlorethamine compared with 25 applications in group A. Possible mechanisms responsible for these results are reduction of epidermal Langerhans cells by PUVA therapy and induction of antigen-specific suppressor T cells. Patients living far from a specialized centre might be treated initially with PUVA therapy then with mechlorethamine alone, at home. This schedule may reduce the incidence of contact dermatitis to mechlorethamine.