Enantioselective gas chromatographic assay with electron-capture detection for dl-ritalinic acid in plasma

Differential pharmacokinetics and pharmacodynamics of methylphenidate enantiomers: does chirality matter?

d,l-threo-methylphenidate (MPH) is an effective first-line treatment for the symptoms associated with attention-deficit/hyperactivity disorder. threo-methylphenidate inhibits the dopamine transporter and the norepinephrine transporter, resulting in elevations of these monoamines after impulse release. Although MPH has long been administered as a racemic mixture of the 2 enantiomers, d-MPH and l-MPH, converging lines of evidence drawn from investigations using in vitro systems, animal models, and humans indicate that it is predominantly, if not exclusively, d-MPH that mediates the pharmacological/therapeutic actions of MPH. In both rodent and primate animal models, the binding of radiolabeled d-MPH to dopamine transporter was found to be selective, saturable, and reversible, whereas binding of l-MPH was diffuse and nonspecific. The behavioral effects of the enantiomers of MPH have been tested in several animal models, and results indicate these observed behavioral changes are likewise mediated by d-MPH, whereas l-MPH has little or no effect.The contribution of the l-isomer to the overall pharmacological profile of the racemate remains unclear, owing to several studies suggesting that l-MPH may not be merely an inert isomeric ballast. For example, behavioral studies conducted in rats demonstrate an attenuation of the effect of d-MPH in animals pretreated with l-MPH, suggesting that l-MPH may interfere with the action of the active enantiomer. The importance of MPH chirality to central nervous system MPH receptor targeting has culminated in human imaging studies revealing that d-MPH binds specifically to striatal structures, whereas l-MPH binding is nonspecific. Taken together, data from in vitro, animal, and human studies support the premise that the d-enantiomer of MPH mediates the neurophysiological actions of MPH and therefore likely mediates its clinical efficacy.

Methylphenidate and its isomers: their role in the treatment of attention-deficit hyperactivity disorder using a transdermal delivery system

dl-threo-Methylphenidate is a highly efficacious drug for treating attention-deficit hyperactivity disorder (ADHD) that is currently administered as immediate- or controlled-release and osmotically controlled-released formulations. The drug exists as two enantiomers, d-threo-methylphenidate and l-threo-methylphenidate, with the former having been developed as a medication to treat ADHD in its own right. dl-threo-Methylphenidate undergoes enantioselective metabolism in the liver, which results in marked differences in the plasma concentrations of its isomers, depending on the route of administration and formulation. When dl-threo-methylphenidate is orally administered, the plasma concentrations of d-threo-methylphenidate are higher than those of l-threo-methylphenidate. However, with the recently developed methylphenidate transdermal system (MTS), 'first-pass' metabolism is circumvented and, as a consequence, plasma concentrations of d-threo-methylphenidate are consistent with those produced by oral formulations, but the relative concentrations of l-threo-methylphenidate are much higher, i.e. 50-60% of those of d-threo-methylphenidate. In this article, we review the pharmacokinetics and pharmacology of dl-threo-methylphenidate and its isomers to assess the extent to which their mechanism of action as noradrenaline (norepinephrine) and dopamine reuptake inhibitors is responsible for their efficacy and commonly occurring adverse effects. The major findings are that d-threo-methylphenidate and l-threo-methylphenidate share the same pharmacological profile as the parent racemate, i.e. catecholamine-selective reuptake inhibition with higher potency against dopamine versus noradrenaline reuptake in vivo. However, d-threo-methylphenidate is approximately 10-fold more potent than the l-isomer in this regard. For these drugs, their abilities not only to ameliorate the behavioural and cognitive dysfunctions in ADHD, but also to induce the common adverse effects of reduced appetite, nausea/vomiting and stomach ache, are almost certainly due to their ability to potentiate noradrenergic and/or dopaminergic function in the central and peripheral nervous systems. The sympathomimetic actions of ADHD drugs on cardiovascular function are currently an issue of concern. Since noradrenaline reuptake inhibition is the likely mediator for the effects of dl-threo-methylphenidate on blood pressure and heart rate, the more potent d-isomer will therefore be predominantly responsible. Motor and vocal tics are the other important adverse event to be considered in the treatment of ADHD. It is now accepted that tics are a frequently occurring behavioural manifestation of ADHD itself and the evidence for or against their exacerbation by treatment with dl-threo-methylphenidate or other stimulants remains highly contradictory. Focusing on the enantiomers of dl-threo-methylphenidate, it can be concluded that d-threo-methylphenidate, which is the more potent and abundant of the two isomers, is the major contributor of both efficacy and adverse effects, irrespective of the formulation or route of administration of the racemate. Moreover, for the oral, extended-release formulations of dl-threo-methylphenidate, the d-isomer represents the only pharmacologically active moiety when these medications are used in the clinic. With the MTS, plasma concentrations of l-threo-methylphenidate are higher than are achieved using oral formulations, but even in this case, it is likely that the contribution of this enantiomer to the efficacy and adverse effects of the racemate is no greater than 5-10% of the total.

New methylphenidate formulations for the treatment of attention-deficit/hyperactivity disorder

dl-Methylphenidate (MPH) remains the most widely used pharmacological agent in the treatment of attention-deficit/hyperactivity disorder (ADHD). The predominantly dopaminergic mechanism of the psychostimulant actions has become more clearly defined. Neuroimaging and genetic studies are revealing the underlying neuropathology in ADHD. Novel extended-release (ER) MPH formulations now offer drug delivery options to overcome both the short-term actions of immediate-release (IR) MPH and the acute tolerance associated with the first-generation ER-MPH products. These novel MPH products apply proprietary technologies such as OROS (Alza), Diffucaps (Eurand) and SODAS (Elan) to offer both the convenience of once-a-day administration and absorption profiles resembling, to varying degrees, the standard multiple dose schedules of IR-MPH. The pharmacodynamics of the separate MPH enantiomers is in the process of further neuropharmacological characterisation. It is well established that dl-MPH undergoes marked stereoselective metabolism. Although l-MPH exhibits only minimal oral absorption, it may preferentially penetrate the brain, and interacts with ethanol to form the metabolite ethylphenidate. The newly approved resolved enantiomer product d-MPH is now available in an IR formulation, and when administered at one-half the dose to that of the racemate, is purported to produce a longer duration of clinical effect, despite essentially identical pharmacokinetics. A long-acting formulation of d-MPH, which employs the SODAS technology, is in the advanced stages of clinical development.

Review: derivatization in mass spectrometry--5. Specific derivatization of monofunctional compounds

The present paper is complementary to the foregoing reviews and describes some additional methods of the derivatization of particular functional groups mainly to enhance the structural information content of electron ionization and chemical ionization mass spectra. Derivatization approaches for the modification of unsaturated compounds, alcoholic, carboxylic, carbonyl, amine and other functional groups, are discussed. Derivatization for separation and quantitative determination of chiral enantiomeric compounds is also considered. Preliminary chemical and physicalchemical degradation for structure elucidation of high molecular weight compounds (biopolymers, synthetic polymers) is mentioned. Chemical aspects of derivatizations and characteristic mass spectral features of derivatives are described briefly. Some particular applications of chemical modification, in conjunction with mass spectral measurements for the analysis of various important bioorganic compounds and compounds in biological fluids, air, environmental etc., are considered.

Pharmacokinetic considerations in the treatment of attention-deficit hyperactivity disorder with methylphenidate

Methylphenidate is commonly used for the treatment of attention-deficit hyperactivity disorder (ADHD). Its efficacy in improving the core symptoms of ADHD, as well as some of the aggressive and oppositional behaviours, is well documented, based on a large volume of research. Methylphenidate has a wide margin of safety and relatively mild adverse effects, most commonly appetite suppression and insomnia. Methylphenidate is a rapidly absorbed medication that, in its d-isomer form, readily penetrates the CNS, particularly the striatum. It appears to function by blocking the reuptake of dopamine. Both the plasma concentrations and behavioural effects of methylphenidate demonstrate a time to maximum of between 1 and 3 hours, with the maximum behavioural effects occurring when the plasma concentrations are increasing. Because of the rapid onset of action, the effects of methylphenidate can be dramatic but usually last only about 4 hours with the immediate-release formulation. The behavioural responses of individuals are also highly variable, so that it is necessary to start treatment at a low dosage and increase up to a maximally effective dosage (usually starting at 10-15 mg/day with increases of 10-15mg at weekly intervals to a maximum dosage of 60 mg/day, irrespective of formulation). Because of the variability in behavioural responses, assessment of plasma concentrations is not clinically useful nor does weight help in deciding an appropriate dosage. The difficulties in administering methylphenidate multiple times a day, particularly during the school day, have been alleviated in the past few years by the development of extended-release preparations with varying behavioural effects lasting 8-12 hours. The 8-hour preparations (Metadate) CD and Ritalin) LA) utilise a microbead technology, while the 12-hour preparation (Concerta) utilises an osmotic pump system. All extended-release formulations effectively control the symptoms of ADHD. While pharmacokinetic differences appear to exist between some of these new formulations, there are currently no clinical data available to demonstrate clinical efficacy differences between them.

Methylphenidate is stereoselectively hydrolyzed by human carboxylesterase CES1A1

Methylphenidate is an important stimulant prescribed to treat attention-deficit hyperactivity disorder. It has two chiral centers, but most current commercial formulations consist of the racemic mixture of the threo pair of methylphenidate isomers (d-, l-threo-methylphenidate). The d-isomer is the pharmacologically active component. Numerous studies reported that oral administration of the methylphenidate racemate undergoes first-pass, stereoselective clearance in humans with l-methylphenidate being eliminated faster than d-methylphenidate. Accordingly, the kinetics of hydrolysis of individual enantiomers by purified native and recombinant human liver carboxylesterases CES1A1 and CES2 and a colon isoenzyme CES3 were examined with a liquid chromatography/mass spectrometry assay. The expression of CES1A1, CES2, and CES3 in Sf9 cells and the methods for purification of the three isoenzymes are reported. CES1A1 has a high catalytic efficiency for both d- and l-enantiomers of methylphenidate. No catalytic activity was detected with CES2 and CES3 for either enantiomer. The catalytic efficiency of CES1A1 for l-methylphenidate (k(cat)/K(m) = 7.7 mM(-1) min(-1)) is greater than that of d-methylphenidate (k(cat)/K(m) = 1.3-2.1 mM(-1) min(-1)). Hence, the catalytic efficiency of CES1A1 for methylphenidate enantiomers agrees with stereoselective clearance of methylphenidate reported in human subjects. Both enantiomers of methylphenidate can be fit into the three-dimensional model of CES1A1 to form productive complexes in the active site. We conclude that CES1A1 is the major enzyme responsible for the first-pass, stereoselective metabolism of methylphenidate.

Advances in the Pharmacotherapy of Attention-Deficit–Hyperactivity Disorder: Focus on Methylphenidate Formulations

The psychostimulant dl-methylphenidate (MPH) remains the most common drug therapy in child and adolescent psychiatry for the treatment of attention-deficit-hyperactivity disorder (ADHD). Evidence of a dopaminergic basis both for the actions of MPH and for the underlying neuropathology in ADHD continues to mount. Advances in the biopharmaceutics of MPH have been conspicuous. Novel approaches to formulation design have resulted in new MPH delivery options to overcome the short-term actions of both immediate-and sustained-release MPH. New modified-release MPH products offer the convenience of once-daily administration while providing extended absorption profiles that better mimic those of standard schedules of immediate-release MPH (i.e., the absorption phase of MPH better correlates with improved behavioral response than does the elimination phase). The oral bioavailability of MPH in females may be lower than in males. The l-MPH isomer exhibits only negligible oral bioavailability and, further, possesses little intrinsic activity at the dopamine transporter. This notwithstanding, a single-isomer d-MPH immediate-release product is now available for dosing recommended at one-half that of dl-MPH.

Optimization of preparative electrophoretic chiral separation of ritalin enantiomers

Continuous free flow electrophoresis (CFFE) was applied to the preparative chiral separation of ritalin enantiomers. Sulfated beta-cyclodextrin (sbeta-CD) was used as the chiral additive. Liquid chromatography-mass spectrometry (LC-MS) experiments were applied to study the time averaged concentration of sbeta-CD in the separation chamber. The distribution of sbeta-CD in the separation chamber greatly influenced resolution and the angle of deflection. To optimize the separation, several parameters (methanol, concentration of sbeta-CD in the cathodic wash and in the separation buffer, and the introduction of a low conductivity zone) were investigated. The dependence of the resolution and deflection angles of ritalin enantiomers on the concentration of sbeta-CD in both the separation buffer and in the cathode wash solution appeared to be non-linear. Under close to optimal conditions, resolution of ritalin enantiomers was about 0.8 with an average processing rate of 0.5 mg/h. Overall, the enantiomeric purity of the individual isomers was approximately 83%; however, of the 20 vials containing ritalin, the presence of both enantiomers was only detected in three vials.

Negative ion chemical ionization for the determination of methylphenidate in human plasma by stable isotope dilution gas chromatography/mass spectrometry

A sensitive and specific method for the determination of methylphenidate in human plasma is presented. Methylphenidate was extracted from plasma by solvent extraction with hexane at pH 9.3 and derivatized to its heptafluorobutyrate derivative. The derivative was measured by gas chromatography/negative ion chemical ionization mass spectrometry without any further purification. Using this detection mode, a diagnostically useful fragment ion at m/z 369 was obtained at high relative abundance. (18)O(2)-labelled methylphenidate was used as an internal standard and its rapid and facile preparation from the unlabeled compound is described. Calibration graphs were linear within the range 0.14-18.25 ng ml(-1). The inter-assay precision was 8.7% (0.14 ng ml(-1)) and 3.1% (4.56 ng ml(-1)) and the intra-assay variability was 1.3% (0.14 ng ml(-1)) and 0.4% (4.56 ng ml(-1)). Accuracy determinations showed deviations of +0.7% (0.14 ng ml(-1)) and -2.5% (4.56 ng ml(-1)). The method is rugged, rapid and robust and has been applied to the batch analysis of methylphenidate during pharmacokinetic profiling of the drug.

Analysis of methylphenidate and its metabolite ritalinic acid in monkey plasma by liquid chromatography/electrospray ionization mass spectrometry

Methylphenidate (MP, Ritalin) is a psychotropic drug widely prescribed to children for treating the symptoms of attention deficit disorder with and without hyperactivity. Because little information exists about the effects of chronic MP administration on cognitive function in children, measures of behavior changes in non-human primates are important surrogates. An essential component of such studies is the determination of MP plasma levels under chronic and acute dosing conditions. An analytical method was developed that provided sufficient sensitivity to measure low levels of the active parent drug (lower limit of quantitation = 0.25 ng/mL) and the inactive metabolite, ritalinic acid (RA), in monkey plasma as well as the ability to conveniently analyze large numbers of samples. The method uses a polymeric reversed-phase sorbent for solid phase extraction, an efficient reverse-phase high performance liquid chromatography (HPLC) separation, deuterated internal standards for isotope dilution quantification of MP and RA, and detection by sensitive electrospray ionization mass spectrometry (ES-MS) with a single quadrupole instrument. The method responses are linear over the range of plasma concentrations of MP and RA observed in monkeys, gives respective analyte recoveries of 75 and 60% with reasonable precision and accuracy, and demonstrates robust MS performance for rapid determination of MP/RA plasma levels. The average peak MP concentration (ca. 16 ng/mL) and half-lives for MP and RA elimination in monkeys (1.79 and 2.31 h, respectively) were not significantly different under acute vs. chronic dosing conditions and were comparable to values previously reported from human studies.

Derivatization procedures for gas chromatographic-mass spectrometric determination of xenobiotics in biological samples, with special attention to drugs of abuse and doping agents

The development of low cost MS detectors in recent years has promoted an important increase in the applicability of GC-MS system to analyze for the presence of foreign substances in the human body. Drugs and toxic agents are in vivo metabolized in such a way that more polar compounds are usually formed. Derivatization of these metabolites is often an unavoidable requirement for gas chromatographic analysis. Application of derivatization methods in recent years has been relevant, especially for silylation, acylation, alkylation and the formation of cyclic or diastereomeric derivatives. Given the relevance of drug of abuse testing in modern toxicology, main derivatization procedures for opiates, cocaine, cannabis, amphetamines, benzodiazepines and LSD have been reviewed. Papers describing the analyses of drugs of abuse in matrixes other than blood, such as hair or sweat, have received special attention. Advances in derivatization for sports drug testing have been particularly relevant for anabolic steroids, diuretics and corticosteroids. Among the several methodologies applied, the formation of trimethylsilyl, perfluoroacyl or methylated derivatives have proved to be both versatile and extensively used. Further advances in derivatization for GC-MS applications in clinical and forensic toxicology will depend on the one hand on the degree of further use of GC-MS for routine applications and, on the other hand, on the alternative progress made for developments in LC-MS or CE-MS. Last but not least, the appearance of comprehensive libraries in which reference spectra for different derivatives of many drugs and their metabolites are collected will have an important impact on the expansion of derivatization in GC-MS for toxicological applications.

Gas chromatographic determination of enantiomers as diastereomers following pre-column derivatization and applications to pharmacokinetic studies: a review

The introduction of chiral chromatographic methods has revolutionized the art of separation and quantitation of chiral drugs in biological fluids. A large number of chiral derivatization reagents for various functional groups are available commercially. Therefore, pre-column derivatization methods have become attractive and simple for the gas chromatographic assays in biological fluids. The intent of this article is to review the pre-column chiral derivatization reagents employed in gas chromatographic separations and analyses of enantiomers. A discussion of numerous procedures necessary to develop a quantitative gas chromatographic assay method for drug enantiomers is presented. In this regard, quantitative gas chromatographic assays based on this approach have been applied to investigate the stereoselective pharmacokinetics of several chiral drugs such as fenfluramine, norfenfluramine, methylphenidate, etodolac, propranolol, suprofen and methoxyphenamine.

Enantioselective pharmacokinetics of dl-threo-methylphenidate in humans

A definitive enantioselective pharmacokinetic evaluation of dl-threo-methylphenidate (MPH) was carried out in 11 healthy volunteers, all of whom received, in a randomized crossover design, three oral administrations of MPH: immediate release (IR), slow release (SR), and SR chewed before swallowing (CH). In addition, all subjects received MPH intravenously (IV) on a separate occasion. Both plasma and urine samples were collected for up to 16 hr after each drug administration. Significant enantioselective differences were found in pharmacokinetic parameters such as CL, MRT, Vdss, AUC infinity 0, and t1/2. A profound distortion of the enantiomeric ratio for MPH (d >> 1) was evident in all plasma samples harvested after oral administration. After IV MPH, however, there was no significant distortion in the plasma d/l ratio until 1.5 hr after dosing, whereafter there was a divergence of the plasma levels of the enantiomers. After oral administration of dl-MPH, the absolute bioavailability (F) of d-MPH was 0.23 and that of l-MPH was 0.05. There were no significant differences in renal clearance for d- or l-MPH after oral or IV administration, although the fraction of the dose excreted unchanged in the urine was significantly greater after IV MPH. These data suggest that enantioselective differences in the pharmacokinetics of oral MPH are the result of enantioselectivity in presystemic metabolism rather than in renal excretion, such that l-MPH is preferentially converted into l-ritalinic acid. Finally, it was found that chewing the slow release formulation led to a pharmacokinetic profile very similar to that of MPH-IR, suggesting that MPH-SR should not be prescribed for children who chew tablets.