Cystinosis: Therapy adherence and metabolic monitoring in patients treated with immediate-release cysteamine

BACKGROUND: Cystinosis is a metabolic disease caused by intracellular accumulation of cystine within lysosomes. Development of symptoms can be delayed significantly by a life-long therapy with cysteamine, a drug that enters the lysosome and reacts with cystine thereby enabling its export from the organelle. METHODS: During a period of 16 years, blood samples of 330 cystinosis patients were analyzed to investigate therapeutic adherence and metabolic control in patients treated with immediate-release cysteamine. The accepted therapeutic goal is to measure intracellular cystine levels in white blood cells every 3 months and to keep them below 0.5 nmol cystine/mg protein (= 1 nmol hemicystine/mg protein). RESULTS: 42% of measurements were within the desired 3-month interval, 38% were done every 3–5 months, 11% every 6–8 months, 5% every 9–12 months and 4% after a 12-month interval only. 64.4% of the measurements were higher than the therapeutic target value. Median cystine levels increased with longer control intervals. CONCLUSIONS: The majority of the cystinosis patients showed insufficient metabolic adjustment. Intracellular cystine levels were not done as often as recommended and were not within therapeutic range. Poor therapy adherence is likely to be caused by gastrointestinal side effects of immediate-release cysteamine. Incorrect intervals between drug intake and blood sampling could contribute to the results.