Kennerly S. Patrick

Clinical Studies of Methylphenidate Serum Levels in Children and Adults

Abstract Although methylphenidate (MPH) is used widely in the treatment of hyperactive children, and although it seems to be a safe and effective drug, very little is known about the pharmacology, pharmacokinetics, and metabolism of the drug, and the relationship between serum levels and subtle side effects. We report on the progress in applying a reliable and sensitive gas chromatography-mass spectrometric assay for MPH in serum to clinical studies of hyperactive children and adults. Although drug half-life in all subjects is remarkably constant (2 to 4 hours), there is a wide range of inter-individual variation in serum levels and, more disturbing, a wide range of intra-individual variations in serum levels. The clinical importance of this work is discussed.

Decreased serotonin content of embryonic raphe neurons following maternal administration of p-chlorophenylalanine: a quantitative immunocytochemical study.

Previous studies from this laboratory have suggested that serotonergic (5-HT) neurons may influence the differentiation of their embryonic target cells in the developing rat brain. The present study was designed to determine whether or not maternal p-chlorophenylalanine (pCPA) administration could deplete serotonin (5-HT) in developing 5-HT neurons during embryonic days 13-15, when the effects of pCPA on neuronal genesis have been observed previously. For this study, pCPA was administered to timed-pregnant rats and embryos were sacrificed at two different gestational ages, embryonic days 13-14 (E13-14) and 14-15 (E14-15). Immunotitration experiments were carried out on tissue sections, using an antiserum to 5-HT-hemocyanin conjugates to obtain a relative estimate of the amount of 5-HT contained within individual 5-HT neurons of embryos from pCPA-treated and control mothers. Diminished immunoreactivity as a consequence of addition of increasing amounts of antigen was then quantitated on a relative scale by comparison with the amount of immunoreactivity present when no antigen was added to the primary antiserum. Two major findings resulted from this study: maternal pCPA treatment depleted 5-HT by approximately 50% in developing 5-HT neurons at embryonic ages E13-14 and E14-15, but depletion appeared to be greatest in the youngest embryos; developing 5-HT neurons increased their content of neurotransmitter by approximately 10-fold during this one day of embryonic development, an effect which could be observed in both pCPA-treated and control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of stress-induced gastric ulcers by dopamine agonists in the rat

Dopamine (DA) and DA agonists have been shown to exert a protective role against the formation of duodenal ulcers. The effect of stimulation of DA receptors on the development of stress-induced gastric ulcers is currently unknown. Accordingly, we evaluated the effect of several DA agonists on the development of gastric ulcers induced by 3 h of cold + restraint stress (CRS) in rats. Apomorphine, d-amphetamine, methylphenidate, and threo-dl-p-hydroxymethylphenidate (an hydroxylated analog of methylphenidate), significantly reduced both the incidence and severity of CRS-induced gastric ulcers. The gastric cytoprotection afforded by these agents was dose-related, and completely antagonized by pretreatment with the peripherally acting DA antagonist domperidone. Because domperidone blocks peripheral, but not central, DA receptors, and since the entry of threo-dl-p-hydroxymethylphenidate across the blood-brain barrier into the brain is restricted to a great extent, we conclude that stimulation of peripheral DA receptors is primarily involved in the gastric cytoprotection induced by dopamimetics.

Clinical correlates of methylphenidate blood levels.

The clinical correlates of methylphenidate blood levels in hyperactive children and normal adults were examined in five studies. Although occasional correlations between blood levels and neuroendocrine response were noted within subjects along the pharmacokinetic time profile of the drug, no significant associations were found between blood levels and clinical response in behavioral measures or laboratory tests of attention or activity. It is unlikely that routine methylphenidate blood level determinations will become a part of the routine clinical management of hyperactive children.

Enantioselective behavioral effects of threo-methylphenidate in rats

The relative potency of d- and l-threo-methylphenidate (d-MPH and l-MPH) was evaluated using three behavioral paradigms for rats: Responding maintained by a fixed-interval schedule of reinforcement (FI), responding maintained by a concurrent variable-interval schedule of reinforcement (Conc VI VI), and consumption of sweetened condensed milk during a 15-min free-access period. In each case the potency of the d-MPH enantiomer greatly exceeded that of the l-MPH enantiomer. Temporal control of responding was reduced (FI) choice responding was equalized for most rats (Conc VI VI), and milk consumption was suppressed by d-MPH and dl-MPH.

Simultaneous determination of thioridazine and its S-oxidized and N-demethylated metabolites using high-performance liquid chromatography on radially compressed silica

A method for simultaneously quantifying thioridazine, northioridazine, thioridazine-2-sulfoxide, thioridazine-2-sulfone and thioridazine-5-oxide in serum and plasma is described. Following solvent extraction these compounds were separated by high-performance liquid chromatography on radially compressed silica gel and detected by UV absorbance at 254 nm. Chromatography time is less than 7 min. The relative retention of these compounds as a function of the methanol and methylamine content of the mobile phase is discussed. Practical limits of detection, based upon on assayed plasma or serum volume of 1 ml, were 20 ng/ml for thioridazine-5-oxide and 10 ng/ml for the other compounds. The coefficient of variation for all compounds was less than 13%. The method is compared with more conventional high-performance liqiud chromatographic and gas chromatographic methodology.

The discriminative stimulus properties of methylphenidate in C57BL/6J mice.

Methylphenidate (MPH) therapy for attention-deficit/hyperactivity disorder is common in children and adults. Concerns regarding abuse of MPH prompted studies to better understand its pharmacology. We used an established drug discrimination task to determine whether MPH could be discriminated by C57BL/6J (B6) mice. B6 mice learned to discriminate cues produced by racemic MPH (dl-MPH 5.0 mg/kg) or half the dose of pure d-isomer (2.5 mg/kg), and dose–response tests established appropriate reductions in discrimination with declining dose. Importantly, the two drug forms generalized to each other completely in substitution tests; consistent with reports that the l-isomer is pharmacodynamically inactive. An additional experiment indicated that lower doses (1 and 2 mg/kg) of dl-MPH did not support acquisition of MPH discrimination despite extensive training. Mice acquired discrimination of dl-MPH only when the dose was increased to 4 mg/kg. Thus, although these lower doses increased drug lever responding in mice trained on the higher dose, their stimuli were not sufficient to support acquisition of the discrimination task. These findings correspond to earlier studies conducted in our laboratory on threshold doses needed to produce stimulatory effects of motor activity inB6 mice. These preclinical findings provide insight intothe relative potency, and by extension, efficacy of dl-MPH versus d-MPH doses.