Single-dose pharmacokinetics of methylphenidate in CYP2D6 extensive and poor metabolizers

Six adults phenotyped as either extensive (N = 4) or poor (N = 2) metaboliz ers for cytochrome P450 (CYP) 2D6 were given a 10-mg oral dose of methylphe nidate (MPH) on two separate occasions with and without quinidine, a potent CYP2D6 inhibitor. Quinidine had no significant effect on the pharmacokinet ics of either MPH or ritalinic acid, its major metabolite, in either group of CYP2D6 metabolizers. These data suggest a lack of involvement of CYP2D6 in the metabolism of MPH. Drugs that are inhibitors of CYP2D6 when taken co ncurrently with MPH should not affect its plasma concentration.