d/l threo-methylphenidate enantiomers influence on catecholaminergic enzyme activities

Introduction: Racemate of d/l-threo-methylphenidate (MPH; Ritalin) is an effective first-line treatment for the symptoms associated with attention-deficit/hyperactivity disorder (ADHD). Although MPH has long been administered as a racemic mixture of the two enantiomers, d-MPH and l-MPH, converging lines of evidence drawn from investigations using in vitro systems indicate that it is predominantly, d-threo MPH which mediates the pharmacological/therapeutic actions of MPH. Aim: In the present study, we investigated which of the two enantiomers has stronger effects on the tyrosine hydroxylase (TH) and monoamine-oxidase B (MAO-B) enzyme activity in vitro, as both enzymes are important for dopamine efficacy. Methods: Using homogenates of rat pheochromocytoma cells (PC-12) we investigated dose dependent effects of d-threo and l-threo MPH. Homogenates were treated with 0/1/10/100 nM and 1/10/100 µM of each MPH enantiomer. TH activity was detected via high-performance liquid chromatography (HPLC) methodology, while MAO-B activity was measured using fluorescent based enzyme-linked immunosorbent assay (ELISA). Results: We could observe dose dependent higher TH as well as MAO-B activity after treatment with d-threo MPH compared to l-threo MPH. Discussion: This exploratory investigation revealed in vitro pharmacological evidence for a potential difference between MPH enantiomers on dopaminergic enzyme activity. This finding might point to the therapeutic effects in the treatment of ADHD.