A new proof of evidence of cysteamine quantification for therapeutic drug monitoring in patients with cystinosis.

Background: To date, measurement of intracellular cystine is used for the therapeutic monitoring of patients affected by cystinosis in treatment with cysteamine. Since this method is time and sample consuming, development of a faster method to quantify cysteamine would be extremely useful in order to help clinicians to adjust dosages of cysteamine and to define better the pharmacokinetic profile of this drug. The aim of the study was to develop a liquid chromatography tandem mass spectrometry method for the quantification of cysteamine in plasma samples and to test its applicability on plasma samples derived from patients with nephropathic infantile cystinosis in treatment with cysteamine.

Results: The percentage of accuracy of the developed method varied between 97.80 and 106.00% and CV% between 0.90 and 6.93%. There was no carry over. The calibration curves were built from 2.5 to 50 µM. The limit of detection and the lower limit of quantification occurred at 0.25 and 1.25 µM respectively. Cysteamine was stable up to 2 months at -20 °C. Concentrations of cysteamine and intracellular cystine of 4 patients were in line with data previously reported.

Conclusion: The proposed method showed an appropriate selectivity, specificity, linearity, sensibility, accuracy, precision and good applicability to samples.