Clinical course and cystine stone formation during tiopronin treatment.

The formation of stones in patients with cystinuria can be counteracted by reducing the urinary concentration of cystine and by increasing its solubility. Thirty-one patients with homozygous cystinuria and treated with tiopronin (2-mercaptopropionylglycine) were followed for between 0.4 and 12 years (median 8.8). With the aim of avoiding cystine concentrations above 1200 mumol/l, the daily dose varied between 500 and 3000 mg (median 1500). The therapeutic effect was evaluated from the clinical symptoms and repeated radiographic examinations. The rate of stone formation during the treatment period was reduced by 60% in comparison with the pretreatment period (P < 0.001). The frequency of active stone removal was reduced by 72% (P < 0.05). The formation of new stones was associated with a higher cystine concentration than was the case during periods when stone formation and stone growth were excluded (P < 0.05). The probability of new stone formation increased with increasing concentrations of cystine up to 1100 mumol/l, but stone formation was not accentuated above 1200 mumol/l. There was no significant relationship between the 24 h excretion of cystine and stone formation. It is concluded that the formation of cystine stones can be efficiently counteracted during treatment with tiopronin, guided by analysis of the concentration of urinary cystine.