Chiral drugs: comparison of the pharmacokinetics of [11C]d-threo and l-threo-methylphenidate in the human and baboon brain |
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Authors: | Y-S Ding J S Fowler N D Volkow S L Dewey G-J Wang J Logan S J Gatley N Pappas |
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Institution: | (1) Chemistry and Medical Departments, Brookhaven National Laboratory, Upton, NY 11973-5000, USA Fax (+1) 516/344-7902, e-mail: ding@simbrain.chm.bnl.gov, US;(2) Department of Psychiatry, SUNY-Stony Brook, Stony Brook, NY 11794, USA, US |
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Abstract: | A
bstract
Methylphenidate (Ritalin) is the most commonly prescribed psychoactive medication for children in the US where it is used for
the treatment of attention deficit hyperactivity disorder. Methylphenidate is marketed as a racemic mixture of the d-threo and l-threo enantiomers. It is believed that the d enantiomer is responsible for the therapeutic effect of methylphenidate. In this study we labeled the individual enantiomers
with carbon-11 and compared their binding and pharmacokinetics in the human and baboon brain. Microdialysis studies in the
rat were performed to compare their potency in elevating striatal dopamine concentration. Positron emission tomographic (PET)
studies with 11C]d-threo-methylphenidate (11C]d-threo-MP) demonstrated highest regional uptake in basal ganglia. In contrast, 11C]l-threo-methylphenidate (11C]l-threo-MP) displayed similar uptakes in all brain regions. The ratios of distribution volumes at the steady-state for the basal
ganglia to cerebellum (DVBG/DVCB) ranged from 2.2 to 3.3 for 11C]d-threo-MP in baboon and human, and only 1.1 for 11C]l-threo-MP. Pretreatment with unlabeled methylphenidate (0.5 mg/kg) or GBR12909 (1.5 mg/kg) markedly reduced the striatal but not
the cerebellar uptake of 11C]d-threo-MP, whereas there was no effect on DVBG/DVCB for 11C]l-threo-MP. In the rat, d-threo-MP increased extracellular dopamine concentration by 650% whereas l-threo-MP did not affect dopamine levels. These results indicate that pharmacological specificity of MP resides entirely in the
d-threo isomer and directly show that binding of the l-isomer in human brain is mostly non-specific.
Received: 18 September 1996 / Final version: 18 November 1996 |
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Keywords: | Methylphenidate Ritalin Dopamine transporter Positron emission tomography Chiral drugs Stereoselectivity |
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