Long-term treatment of erythropoietic protoporphyria with cysteine.

A Phase III 3 year placebo-controlled trial, in which patients were blinded as to when placebo was given, was conducted to determine if the administration of L-cysteine-HCl was effective in preventing or ameliorating the photosensitivity associated with erythropoietic protoporphyria. Forty-seven patients participated in the trial. Placebo was administered in the first month of the trial and then 500 mg of cysteine was given in two divided doses per day for the duration of the study. To measure efficacy, baseline and follow-up visit history forms and light-exposure diaries were filled in by the patients, and those seen in Boston were phototested. Analysis of the data from the questionnaires, the minutes of symptom-free light exposure recorded in diaries and history forms and the results of the phototests showed that cysteine increased the time of symptom-free light exposure to a statistically significant degree.