Negligible urinary cysteamine loss in cystinosis patients with Fanconi syndrome

Cystinosis is an inborn error of lysosomal cystine transporter, resulting in cystine accumulation in lysosomes of all cells. Renal Fanconi syndrome is an early sign of kidney involvement in cystinosis patients. Cysteamine, a small amino-thiol, depletes intralysosomal cystine content and reduces organ damage. However, it does not reverse renal Fanconi syndrome and only postpones the progression to renal failure. We examined whether cysteamine could be lost in the urine of cystinosis patients with Fanconi syndrome, which may explain the inefficiency of treatment. Urinary cysteamine loss was studied in 6 cystinosis patients with and without Fanconi syndrome and was less than I% of ingested dose in all patients.