NONLINEAR KINETICS OF THREO-METHYLPHENIDATE ENANTIOMERS IN A PATIENT WITH NARCOLEPSY AND IN HEALTHY-VOLUNTEERS

We have studied the pharmacokinetics of methylphenidate enantiomers after the oral administration of different doses of racemic methylphenidate to one patient with narcolepsy and to four healthy volunteers. The plasma concentrations of (+)-methylphenidate were much higher than those of (-)-methylphenidate after each dose in all subjects. In the patient the oral clearance (CL/f) of (+)-methylphenidate fell 3-fold and the area under the concentration-time curve (AUC) rose 7-fold when the dose was increased from 20 to 40 mg (from 0.27 to 0.53 mg . kg-1), in spite of the relatively constant terminal half-life of 2.6-2.7 h. Similar dose-dependency was also observed in the healthy volunteers in the dose range of 10-60 mg (0.120. 77 mg . kg-1). The mean value of CL/f for the 40 mg dose was significantly lower than that for the 20 mg dose. The mean AUC of the (+)-isomer corrected to a dose of 10 mg increased significantly between the 20 mg and 40 mg doses. In the urine (+)- and (-)-ritalinic acid were excreted for 48 h after each dose as 32-31 % and 34-40 % of the dose respectively. The mean total recoveries (sum of enantiomers of methylphenidate and its metabolite, ritalinic acid) in the urine were relatively constant (63-78 % of the doses), suggesting that the changes in AUC with dose may not be due to a change in the intestinal absorption of racemic methylphenidate. We conclude that the nonlinear kinetics of (+)-methylphenidate may be due to saturation of its presystemic elimination.