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Notes: It is well recognized that patients who have undergone renal transplantation are at an increased risk of developing cutaneous malignancy. The incidence of this problem rises as survival time increases, and multiple dysplastic lesions may develop, especially on sun-exposed sites. Conventional treatment by excision, curettage, cryotherapy or topical cytotoxic agents is often unsatisfactory because of the number and extent of lesions present. In non-immunosuppressed patients, treatment with etretinate has been shown to be helpful in treating both basal cell carcinomas and solar keratoses. There are, however, no previous studies of its safety or efficacy in renal transplant patients who have developed dysplastic lesions, and the immuno-adjuvant effect of retinoids is a potential limitation to this type of treatment. We have therefore undertaken a study in which patients with functioning renal allografts who had developed multiple dysplastic lesions were treated with etretinate, 1 mg/kg/day for a period of 6 months. Six patients (five males, one female), who had been immunosuppressed for between 166 and 240 months (mean 210 months), were entered into the study. All patients had histological evidence of severe epidermal dysplasia, and four patients had previously developed invasive squamous cell carcinomas. In addition, all patients had numerous viral warts, some of which showed evidence of dysplastic change. No deterioration of renal function was detected in any of the patients during treatment, as measured by serial plasma creatinine estimations. Circulating lymphocyte numbers were unaffected by treatment and there was no consistent change in the relative numbers of T helper or suppressor cells. Intradermal challenge with multiple antigens was undertaken at o and 6 months, and all patients remained completely anergic. Serum cholesterol and triglyceride levels were monitored throughout the study, and only one patient had a rise in lipid values which did not respond to dietary manipulation. Four patients had almost complete clearing of hyperkeratotic lesions, one patient had a partial response, and one patient developed two further squamous cell carcinomas during treatment. Side-effects were no more severe than those normally encountered with retinoid therapy. Etretinate appears to be a safe and useful treatment in this group of patients. Fears concerning its immuno-modulating effects proved groundless, and its effects on a heavily suppressed immune system appear to be negligible.