Outcomes of Tiopronin and D-Penicillamine Therapy in Pediatric Cystinuria: A Clinical Comparison of Two Cases

<b>Background and Clinical Significance:</b> Cystinuria is the most common genetic cause of pediatric nephrolithiasis, characterized by impaired renal cystine reabsorption and resulting in increased urinary cystine excretion. Due to the poor solubility of cystine at normal urine pH, increased urinary cystine excretion leads to urine supersaturation and precipitation of cystine, resulting in nephrolithiasis. <b>Case Presentation:</b> Here, we report two cases of female patients diagnosed with cystinuria caused by <i>SLC7A9</i> mutations. Both patients were initially managed with conservative treatments to minimize stone recurrence including increased oral fluid intake, a low-salt/low-protein diet, and potassium citrate supplementation with the goal of reducing urinary cystine levels and minimizing stone recurrences. Due to persistent stone formation, the patients were started on two distinct cystine-binding thiol medications. One patient was initiated on tiopronin, and the other on D-penicillamine. Tiopronin and D-penicillamine are both used in the treatment of pediatric cystinuria, although tiopronin is often preferred due to its more favorable side effect profile. However, due to insurance constraints, D-penicillamine was initiated for one patient in place of tiopronin. Since the initiation of these two distinct cystine-binding thiol medications, both patients have demonstrated reduced urinary cystine excretion and minimal to no recurrence of kidney stones. <b>Conclusions:</b> Cystine-binding thiols, including tiopronin and D-penicillamine, can both be used in the management of cystinuria in pediatric patients.