Steady-state pharmacokinetics and pharmacodynamics of cysteamine bitartrate in paediatric nephropathic cystinosis patients

Aims: Cysteamine is used to reduce tissue cystine content in patients suffering from nephropathic cystinosis. The objectives of the current study were to investigate pharmacokinetics and pharmacodynamics of cysteamine bitartrate in children and young adults with nephropathic cystinosis. Methods: Cysteamine bitartrate was administered to 11 cystinosis patients at their regular dose level in a single-dose, open-label, steady-state study. Blood samples were collected and analysed for plasma cysteamine and white blood cell cystine content and pharmacokinetic and pharmacodynamic parameters estimated by NONMEM analysis using a linked pharmacokinetic-pharmacodynamic model. Results: Cysteamine was rapidly cleared from the plasma (mean CL/F = 32.3 ml min ^-1 kg^-1, range = 17.3-52.2), appeared to be extensively distributed (mean Vss/F =15.11, range 2.7-32.3) and exhibited a mean T _max of 1.4 h. White blood cell cystine content post-dosing was significantly decreased compared with pre- and post-dose values (average decrement approximately 47%). A counter-clockwise hysteresis was noted in all patients, suggestive of a lag time (mean T_lag = 0.44 h, range 0.22-0.92) between drug concentration and effect. Conclusions: The results of this study establish that cysteamine is rapidly cleared from the plasma but that an every 6 h dosing interval adequately maintains white blood cell cystine content below the target of 1 nmol cystine per mg protein.