Urinary Excretion of Free Cystine and the Tiopronin-Cysteine-Mixed Disulfide during Long-Term Tiopronin Treatment of Cystinuria

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We report the results of a biochemical evaluation of long-term treatment of cystinuria with the SH compound tiopronin (2-mercaptopropionylglycine). The effects of tiopronin were studied by monitoring the urinary excretion of free cysteine and the mixed disulfide between tiopronin and cysteine. Thirty-one patients with homozygous cystinuria were treated with tiopronin for 0.4-12 years (mean 7.8 years). The urinary concentration of free cysteine was used to adjust the tiopronin dose. In 28 of the 31 patients a mean urinary cystine concentration of less than 1,200 mumol/1(288 mg/l) was achieved with the final dose. The final daily doses of tiopronin ranged from 250 mg (1.5 mmol) to 3,000 mg (18.4 mmol; mean 1,540 mg; 9.4 mmol). In a majority of the patients the treatment reduced the 24-hour urinary free cystine excretion effectively, on average by 0.61 mumol (0.15 mg)/mg of tiopronin administered. No changes in the efficacy of tiopronin over time were observed, and the frequency of adverse effects was acceptable. To evaluate the effects of tiopronin on the metabolism of cystine we calculated the total urinary excretion of cystine as the sum of free cystine and the amount of cystine corresponding to the cysteine content of the tiopronin-cysteine disulfide. At low doses of tiopronin there was an increase in urinary excretion of the mixed disulfide as well as of total cystine. Monitoring urinary cystine concentration is necessary to achieve adequate individualized doses of tiopronin. Assessment of the mixed tiopronin-cysteine disulfide and the urinary excretion of total cystine shows that tiopronin may interfere with cystine metabolism in a more complex way than through a simple disulfide exchange reaction with urinary cystine.