Chiral drugs: comparison of the pharmacokinetics of [11C]d-threo and L-threo-methylphenidate in the human and baboon brain

Toxicokinetics of homosalate in humans after dermal application: applicability of oral-route data for exposure assessment by human biomonitoring

Homosalate (HMS) is a UV filter used in sunscreens and personal care products as a mixture of cis- and trans-isomers. Systemic absorption after sunscreen use has been demonstrated in humans, and concerns have been raised about possible endocrine activity of HMS, making a general population exposure assessment desirable. In a previous study, it was shown that the oral bioavailability of cis-HMS (cHMS) is lower than that of trans-HMS (tHMS) by a factor of 10, calling for a separate evaluation of both isomers in exposure and risk assessment. The aim of the current study is the investigation of HMS toxicokinetics after dermal exposure. Four volunteers applied a commercial sunscreen containing 10% HMS to their whole body under regular-use conditions (18-40 mg HMS (kg bw)-1). Parent HMS isomers and hydroxylated and carboxylic acid metabolites were quantified using authentic standards and isotope dilution analysis. Further metabolites were investigated semi-quantitatively. Elimination was delayed and slower compared to the oral route, and terminal elimination half-times were around 24 h. After dermal exposure, the bioavailability of cHMS was a factor of 2 lower than that of tHMS. However, metabolite ratios in relation to the respective parent isomer were very similar to the oral route, supporting the applicability of the oral-route urinary excretion fractions for dermal-route exposure assessments. Exemplary calculations of intake doses showed margins of safety between 11 and 92 (depending on the approach) after single whole-body sunscreen application. Human biomonitoring can reliably quantify oral and dermal HMS exposures and support the monitoring of exposure reduction measures.

Methylphenidate induces a different response in the dorsal raphe as compared to ventral tegmental area and locus coeruleus: behavioral and concomitant neuronal recordings in adult rats

Methylphenidate (MPD) is a psychostimulant used to treat attention deficit hyperactivity disorder. MPD exerts its neurocognitive effects through increasing concentrations of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the neuronal synapse. This study recorded from adult freely behaving rats a total of 1170 neurons, 403 from the ventral tegmental area (VTA), 409 from locus coeruleus (LC), and 356 from dorsal raphe (DR) nucleus, which are the main sources of DA, NE, and 5-HT to the mesocorticolimbic circuitry, respectively. Electrophysiological and behavioral activities were recorded simultaneously following acute and repetitive (chronic) saline or 0.6, 2.5, or 10.0 mg/kg MPD. The uniqueness of this study is the evaluation of neuronal activity based on the behavioral response to chronic MPD. Animals received daily saline or MPD administration on experimental days 1-6 (ED1-6), followed by a 3-day wash-out period, and then MPD rechallenge on ED10. Each chronic MPD dose elicits behavioral sensitization in some animals, while in others, behavioral tolerance. Neuronal excitation following chronic MPD was observed in brains areas of animals exhibiting behavioral sensitization, while neuronal attenuation following chronic MPD was observed in those animals expressing behavioral tolerance. DR neuronal activity was most affected in response to acute and chronic MPD administration and responded differently compared to the neurons recorded from VTA and LC neurons at all doses. This suggests that although not directly related, DR and 5-HT are involved in the acute and chronic effects of MPD in adult rats, but exhibit a different role in response to MPD.

Methylphenidate undermines or enhances divergent creativity depending on baseline dopamine synthesis capacity

Catecholamine-enhancing psychostimulants, such as methylphenidate have long been argued to undermine creative thinking. However, prior evidence for this is weak or contradictory, stemming from studies with small sample sizes that do not consider the well-established large variability in psychostimulant effects across different individuals and task demands. We aimed to definitively establish the link between psychostimulants and creative thinking by measuring effects of methylphenidate in 90 healthy participants on distinct creative tasks that measure convergent and divergent thinking, as a function of individuals' baseline dopamine synthesis capacity, indexed with 18F-FDOPA PET imaging. In a double-blind, within-subject design, participants were administered methylphenidate, placebo or selective D2 receptor antagonist sulpiride. The results showed that striatal dopamine synthesis capacity and/or methylphenidate administration did not affect divergent and convergent thinking. However, exploratory analysis demonstrated a baseline dopamine-dependent effect of methylphenidate on a measure of response divergence, a creativity measure that measures response variability. Response divergence was reduced by methylphenidate in participants with low dopamine synthesis capacity but enhanced in those with high dopamine synthesis capacity. No evidence of any effect of sulpiride was found. These results show that methylphenidate can undermine certain forms of divergent creativity but only in individuals with low baseline dopamine levels.

Dopamine, Norepinephrine and Serotonin Participate Differently in Methylphenidate Action in Concomitant Behavioral and Ventral Tegmental Area, Locus Coeruleus and Dorsal Raphe Neuronal Study in Young Rats

Methylphenidate (MPD), known as Ritalin, is a psychostimulant used to treat children, adults, and the elderly. MPD exerts its effects through increasing concentrations of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the synaptic cleft. Concomitant behavioral and neuronal recording from the ventral tegmental area (VTA), locus coeruleus (LC), and from the dorsal raphe (DR) nucleus, which are the sources of DA, NE, and 5-HT to the mesocorticolimbic circuit, were investigated following acute and repetitive (chronic) saline, 0.6, 2.5, or 10.0 mg/kg MPD. Animals received daily saline or MPD administration on experimental days 1 to 6 (ED1-6), followed by a 3-day washout period and MPD rechallenge on ED10. Each chronic MPD dose elicits behavioral sensitization in some animals while inducing behavioral tolerance in others. The uniqueness of this study is in the evaluation of neuronal activity based on the behavioral response to chronic MPD. Neuronal excitation was observed mainly in brain areas of animals exhibiting behavioral sensitization, while neuronal attenuation following chronic MPD was observed in animals expressing behavioral tolerance. Different ratios of excitatory/inhibitory neuronal responses were obtained from the VTA, LC, or DR following chronic MPD. Thus, each brain area responds differently to each MPD dose used, suggesting that DA, NE, and 5-HT in the VTA, LC, and DR exert different effects.

Time-varying SUVr reflects the dynamics of dopamine increases during methylphenidate challenges in humans

Dopamine facilitates cognition and is implicated in reward processing. Methylphenidate, a dopamine transporter blocker widely used to treat attention-deficit/hyperactivity disorder, can have rewarding and addictive effects if injected. Since methylphenidate's brain uptake is much faster after intravenous than oral intake, we hypothesize that the speed of dopamine increases in the striatum in addition to its amplitude underly drug reward. To test this we use simulations and PET data of [¹¹C]raclopride's binding displacement with oral and intravenous methylphenidate challenges in 20 healthy controls. Simulations suggest that the time-varying difference in standardized uptake value ratios for [¹¹C]raclopride between placebo and methylphenidate conditions is a proxy for the time-varying dopamine increases induced by methylphenidate. Here we show that the dopamine increase induced by intravenous methylphenidate (0.25 mg/kg) in the striatum is significantly faster than that by oral methylphenidate (60 mg), and its time-to-peak is strongly associated with the intensity of the self-report of "high". We show for the first time that the "high" is associated with the fast dopamine increases induced by methylphenidate.

Positron emission tomography studies in adult patients with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by inattention, motor hyperactivity, impulsivity, and psychosocial as well as cognitive dysfunction. Although characteristic clinical manifestations have been described, no definitive biomarkers to diagnose ADHD have been established. In this review article, we summarize positron emission tomography (PET) studies conducted in adult patients with ADHD. We found that, although, disturbances of dopamine, serotonin, and norepinephrine functions have been implicated in ADHD, no characteristic findings have been identified from PET studies in patients with ADHD. Several previous PET studies on the central dopaminergic transmission-related ligands in patients with ADHD have shown altered binding of dopamine markers in the basal ganglia. However, no consistent results were observed in the binding characteristics for dopamine transporters and receptors. Findings from PET studies with ligands related to serotonin and norepinephrine pathways showed either unclear clinical significance or low replicability. Therefore, whether alterations of monoamine function may be involved in the pathophysiological mechanism remains to be clarified. The limitations of previous PET studies include their small sample sizes, focus on several kinds of existing ligands, and a questionable validity of the diagnosis (lack of biological diagnostic criteria). To determine the characteristic findings for diagnosing ADHD, further research is needed, and particularly, studies that evaluate new active ligands with specific binding to monoamine pathways should be undertaken.

Methylphenidate for attention-deficit/hyperactivity disorder in adults: a narrative review

RATIONALE

Psychostimulants, including methylphenidate (MPH), are the mainstay of pharmacotherapy for attention-deficit/hyperactivity disorder (ADHD) in adults. Even though MPH is the most commonly used medication for ADHD these days, there are relatively few resources available that provide comprehensive insight into the pharmacological and clinical features of the compound.

OBJECTIVE

The aim of this paper is to provide an up-to-date outline of the pharmacology and clinical utility of MPH for ADHD in adult patients.

METHODS

While conducting the narrative review, we applied structured search strategies covering the two major online databases (MEDLINE and Cochrane Central Register of Controlled Trials). In addition, we performed handsearching of reference lists of relevant papers.

RESULTS

Methylphenidate exhibits multimodal mechanism of action, working primarily as a dopamine and noradrenaline reuptake inhibitor. It also protects the dopaminergic system against the ongoing 'wearing off' (by securing a substantial reserve pool of the neurotransmitter, stored in the presynaptic vesicles). In placebo-controlled trials, MPH was shown to be moderately effective both against the core ADHD symptoms (standardized mean difference [SMD], 0.49; 95% confidence interval [CI], 0.35-0.64), and the accompanying emotion regulation deficits (SMD, 0.34; 95% CI, 0.23-0.45). The most common adverse events related to long-term treatment with MPH are decreased appetite (~ 20%), dry mouth (15%), heart palpitations (13%), gastrointestinal infections (~ 10%), and agitation/feeling restless (~ 10%).

CONCLUSIONS

There is substantial body of evidence to suggest that MPH is an effective and safe treatment option for adults with ADHD.

The role of neuroimaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder that affects millions of people worldwide. Two hallmarks of PD are the accumulation of alpha-synuclein and the loss of dopaminergic neurons in the brain. There is no cure for PD, and all existing treatments focus on alleviating the symptoms. PD diagnosis is also based on the symptoms, such as abnormalities of movement, mood, and cognition observed in the patients. Molecular imaging methods such as magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET) can detect objective alterations in the neurochemical machinery of the brain and help diagnose and study neurodegenerative diseases. This review addresses the application of functional MRI, PET, and SPECT in PD patients. We provide an overview of the imaging targets, discuss the rationale behind target selection, the agents (tracers) with which the imaging can be performed, and the main findings regarding each target's state in PD. Molecular imaging has proven itself effective in supporting clinical diagnosis of PD and has helped reveal that PD is a heterogeneous disorder, which has important implications for the development of future therapies. However, the application of molecular imaging for early diagnosis of PD or for differentiation between PD and atypical parkinsonisms has remained challenging. The final section of the review is dedicated to new imaging targets with which one can detect the PD-related pathological changes upstream from dopaminergic degeneration. The foremost of those targets is alpha-synuclein. We discuss the progress of tracer development achieved so far and challenges on the path toward alpha-synuclein imaging in humans.

Short- and Long-Term Stability of Methylphenidate and its Metabolites in Blood

Methylphenidate is a medication used to combat attention-deficit/hyperactivity disorder by speeding up brain activity. Methylphenidate has two chiral centers; however, d-threo-methylphenidate is responsible for its effects. Few studies have analyzed methylphenidate and its metabolites, ritalinic acid and ethylphenidate, in blood. Stability studies are crucial in a forensic setting to provide insight on ideal storage conditions and analysis time. In this study, d,l-methylphenidate, d,l-ethylphenidate and ritalinic acid were analyzed at two concentrations (15 and 150 ng/mL) over 5 months at room temperature (~25°C), refrigerated (4°C), frozen (-20°C), and elevated (35°C) temperatures. Analytes were analyzed using a validated liquid-chromatography mass spectrometry method. Ritalinic acid concentrations increased 53% at 25°C after 24 h while d- and l-methylphenidate concentrations dropped 18.1% and 20.6%, respectively. Additionally, d- and l-ethylphenidate concentrations decreased 22.3% and 28.8%, respectively. All analytes were stable at 4°C for one week (±17% change). At -20°C, all analytes were stable for 5 months. At 35°C, l-ethylphenidate remained stable for 24 h (14.4% loss) at the high concentration while ritalinic acid increased 244%. Losses of 64.1%, 68.7% and 27.2% were observed for d-methylphenidate, l-methylphenidate and d-ethylphenidate, respectively. Due to this, a follow up study was designed to assess the breakdown of methylphenidate. The short term experiment assessed d,l-methylphenidate at two concentrations for one month in the same conditions. As methylphenidate decreased, ritalinic acid concentrations rose. At 25°C, it took two weeks for methylphenidate to metabolize completely into ritalinic acid. In refrigerated and frozen temperatures, methylphenidate did not completely metabolize to ritalinic acid. In elevated temperatures, methylphenidate broke down to ritalinic acid within two weeks. Due to this, it was concluded that d,l-methylphenidate breaks down in the blood to its metabolite ritalinic acid and may make data interpretation difficult if samples are not properly stored. The optimal storage for these analytes is recommended at -20°C.

Application of chiral chromatography in radiopharmaceutical fields: A review

Chiral column chromatography (CCC) is a revolutionary analytical methodology for the enantioseparation of novel positron emission tomography (PET) tracers in the primary stages of drug development. Due to the different behaviors of tracer enantiomers (e.g. toxicity, metabolism and side effects) in administrated subjects, their separation and purification is a challenging endeavor. Over the last three decades, different commercial chiral columns have been applied for the enantioseparation of PET-radioligand (PET-RL) or radiotracers (PET-RT), using high-performance liquid chromatography (HPLC). The categorization and reviewing of them is a vital topic. This review presents a brief overview of advances, applications, and future prospectives of CCC in radiopharmaceutical approaches. In addition, the effective chromatographic parameters and degravitation trends to enhance enantioseparation resolution are addressed. Moreover, the application and potential of chiral super fluidical chromatography (CSFC) as an alternative for enantioseparation in the field of radiopharmaceutical is discussed. Finally, the crucial application challenges of CCC are explained and imminent tasks are suggested.

Intrinsic connections between thalamic sub-regions and the lateral prefrontal cortex are differentially impacted by acute methylphenidate

BACKGROUND

The thalamus is a major target of dopaminergic projections and is densely connected with the prefrontal cortex. A better understanding of how dopamine changes thalamo-cortical communication may shed light on how dopamine supports cognitive function. Methylphenidate has been shown to facilitate cognitive processing and reduce connectivity between the thalamus and lateral prefrontal cortex.

AIMS

The thalamus is a heterogeneous structure, and the present study sought to clarify how the intrinsic connections of thalamic sub-regions are differentially impacted by acute dopamine transporter blockade.

METHODS

Sixty healthy volunteers were orally administered either 20 mg of methylphenidate (N = 29) or placebo (N = 31) in a double-blind, randomized, between-subject design. Multi-echo fMRI was used to assess intrinsic functional connectivity of sub-regions of the thalamus during a resting state scan. An N-back working-memory paradigm provided a measure of cognitive performance.

RESULTS

Acute methylphenidate significantly reduced connectivity of the lateral prefrontal cortex with the motor and somatosensory sub-regions of the thalamus and reduced connectivity with the parietal and visual sub-regions at a trend level. Connectivity with the premotor, prefrontal, and temporal sub-regions was not impacted. The intrinsic connectivity between the thalamus and the lateral prefrontal cortex was not associated with working-memory performance.

CONCLUSIONS

Methylphenidate decreases functional connections between the lateral prefrontal cortex and thalamus broadly, while sparing intrinsic connectivity with thalamic sub-regions involved with working-memory and language related processes. Collectively, our results suggest that the dopamine transporter regulates functional connections between the prefrontal cortex and non-cognitive areas of the thalamus.

The rate of dasotraline brain entry is slow following intravenous administration

RATIONALE

Drugs that rapidly increase dopamine levels have an increased risk of abuse. Dasotraline (DAS) is a dopamine and norepinephrine reuptake inhibitor characterized by slow oral absorption with low potential for abuse. However, it remains unclear whether intravenous (i.v.) administration would facilitate the rapid elevation of dopamine levels associated with stimulant drugs.

OBJECTIVE

To assess the kinetics of DAS across the blood-brain barrier and time to onset of dopamine transporters (DAT) inhibition.

METHODS

We compared the onset of DAT occupancy and the associated elevation of synaptic dopamine levels in rhesus monkey following i.v. administration of DAS or methylphenidate (MPH) using positron emission tomography (PET). Brain entry times were estimated by reductions in [¹⁸F]-FE-PE2I binding to DAT in rhesus monkeys. Elevations of synaptic dopamine were estimated by reductions in [¹¹C]-Raclopride binding to D₂ receptors.

RESULTS

Intravenous administration of DAS (0.1 and 0.2 mg/kg) resulted in striatal DAT occupancies of 54% and 68%, respectively; i.v. administered MPH (0.1 and 0.5 mg/kg) achieved occupancies of 69% and 88% respectively. Brain entry times of DAS (22 and 15 min, respectively) were longer than for MPH (3 and 2 min). Elevations in synaptic dopamine were similar for both DAS and MPH however the time for half-maximal displacement by MPH (t = 23 min) was 4-fold more rapid than for DAS (t = 88 min).

CONCLUSIONS

These results demonstrate that the pharmacodynamics effects of DAS on DAT occupancy and synaptic dopamine levels are more gradual in onset than those of MPH even with i.v. administration that is favored by recreational drug abusers.

Population Pharmacokinetic-Pharmacodynamic Modeling of a Novel Methylphenidate Extended-Release Orally Disintegrating Tablet in Pediatric Patients With Attention-Deficit/Hyperactivity Disorder

PURPOSE/BACKGROUND

A methylphenidate (MPH) extended-release orally disintegrating tablet (MPH XR-ODT) formulation was recently approved for attention-deficit/hyperactivity disorder treatment in children 6 to 17 years of age. This analysis sought to develop a population pharmacokinetic (PK)/pharmacodynamic (PD) model to describe MPH XR-ODT PD-response data in a classroom study and use the model to simulate PD responses for a range of body weights and doses.

METHODS/PROCEDURES

The MPH XR-ODT PK/PD model was developed with pediatric and adult PK data from prior studies and efficacy data from a laboratory classroom study in children with attention-deficit/hyperactivity disorder. In these studies, the safety profile of MPH XR-ODT was consistent with other extended-release MPH formulations. The PK/PD model efficacy end point was the Swanson, Kotkin, Agler, M-Flynn, and Pelham Scale Combined score. Body weight effects on MPH clearance and volume of distribution were included in the resulting model. Simulations using the PK/PD model were performed for patients with body weights between 7 and 100 kg and MPH XR-ODT doses of 10 to 60 mg MPH hydrochloride equivalents.

FINDINGS/RESULTS

In the PK/PD model, the maximal reduction in the Swanson, Kotkin, Agler, M-Flynn, and Pelham Scale Combined score was approximately 38 units, and the MPH concentration required to achieve 50% of the maximal reduction was 14.24 ng/mL, suggesting favorable efficacy for MPH XR-ODT. Simulations showed a direct correlation between the effective MPH XR-ODT dose and body weight, with heavier participants requiring higher doses for symptom control.

IMPLICATIONS/CONCLUSION

This model may help facilitate the dose-titration process by identifying an effective MPH XR-ODT target dose.

The pharmacology of amphetamine and methylphenidate: Relevance to the neurobiology of attention-deficit/hyperactivity disorder and other psychiatric comorbidities

Psychostimulants, including amphetamines and methylphenidate, are first-line pharmacotherapies for individuals with attention-deficit/hyperactivity disorder (ADHD). This review aims to educate physicians regarding differences in pharmacology and mechanisms of action between amphetamine and methylphenidate, thus enhancing physician understanding of psychostimulants and their use in managing individuals with ADHD who may have comorbid psychiatric conditions. A systematic literature review of PubMed was conducted in April 2017, focusing on cellular- and brain system-level effects of amphetamine and methylphenidate. The primary pharmacologic effect of both amphetamine and methylphenidate is to increase central dopamine and norepinephrine activity, which impacts executive and attentional function. Amphetamine actions include dopamine and norepinephrine transporter inhibition, vesicular monoamine transporter 2 (VMAT-2) inhibition, and monoamine oxidase activity inhibition. Methylphenidate actions include dopamine and norepinephrine transporter inhibition, agonist activity at the serotonin type 1A receptor, and redistribution of the VMAT-2. There is also evidence for interactions with glutamate and opioid systems. Clinical implications of these actions in individuals with ADHD with comorbid depression, anxiety, substance use disorder, and sleep disturbances are discussed.

Ethanol Interactions With Dexmethylphenidate and dl-Methylphenidate Spheroidal Oral Drug Absorption Systems in Healthy Volunteers

BACKGROUND/PURPOSE

Ethanol coadministered with immediate-release dl-methylphenidate (dl-MPH) or dexmethylphenidate (d-MPH) significantly increases the geomean maximum plasma concentration (Cmax) of d-MPH 22% and 15%, respectively, and elevates overall drug exposure and psychostimulant effects. We asked the question: Are these ethanol-MPH interactions based more fundamentally on (1) inhibition of postabsorption d-MPH metabolism or (2) acceleration of MPH formulation gastric dissolution by ethanol in the stomach? This was investigated using the pulsatile, distinctly biphasic, spheroidal oral drug absorption systems of dl-MPH and d-MPH.

METHODS

In a randomized, 4-way crossover study, 14 healthy subjects received pulsatile dl-MPH (40 mg) or d-MPH (20 mg), with or without ethanol (0.6 g/kg), dosed 4 hours later. These 4 hours allowed the delayed-release second MPH pulse to reach a more distal region of the gut to preclude gastric biopharmaceutical influences. Plasma was analyzed using a highly sensitive chiral method. Subjective/physiological effects were recorded.

FINDINGS/RESULTS

Ethanol increased the second pulse of d-MPH Cmax for dl-MPH by 35% (P < 0.01) and the partial area under the plasma concentration curve from 4 to 8 hours by 25% (P < 0.05). The respective values for enantiopure d-MPH were 27% (P = 0.001) and 20% (P < 0.01). The carboxylesterase 1-mediated transesterification metabolite ethylphenidate served as a biomarker for coexposure. Ethanol significantly potentiated stimulant responses to either formulation.

IMPLICATIONS/CONCLUSIONS

These findings support drug dispositional interactions between ethanol and MPH as dominant over potential biopharmaceutical considerations. Understanding the pharmacology underlying the frequent coabuse of MPH-ethanol provides rational guidance in the selection of first-line pharmacotherapy for comorbid attention-deficit/hyperactivity disorder-alcohol use disorder.

Pharmacokinetic Characterization of [18F]UCB-H PET Radiopharmaceutical in the Rat Brain

The synaptic vesicle glycoprotein 2A (SV2A), a protein essential to the proper nervous system function, is found in presynaptic vesicles. Thus, SV2A targeting, using dedicated radiotracers combined with positron emission tomography (PET), allows the assessment of synaptic density in the living brain. The first-in-class fluorinated SV2A specific radioligand, [¹⁸F]UCB-H, is now available at high activity through an efficient radiosynthesis compliant with current good manufacturing practices (cGMP). We report here a noninvasive method to quantify [¹⁸F]UCB-H binding in rat brain with microPET. Validation study in rats confirmed the need of high enantiomeric purity to target SV2A in vivo. We demonstrated the reliability of a population-based input function to quantify SV2A in preclinical microPET setting. Finally, we investigated the in vivo metabolism of [¹⁸F]UCB-H and confirmed the negligible amount of radiometabolites in the rat brain. Hence, the in vivo quantification of SV2A using [¹⁸F]UCB-H microPET seems a promising tool for the assessment of the synaptic density in the rat brain, and opens the way for longitudinal follow-up in neurodegenerative disease rodent models.

The impact of CES1 genotypes on the pharmacokinetics of methylphenidate in healthy Danish subjects

AIMS

This study investigated the influence of CES1 variations, including the single nucleotide polymorphism (SNP) rs71647871 (G143E) and variation in copy number, on the pharmacokinetics of a single oral dose of 10 mg methylphenidate.

METHODS

CES1 genotype was obtained from 200 healthy Danish Caucasian volunteers. Based on the genotype, 44 (19 males and 25 females) were invited to participate in an open, prospective trial involving six predefined genotypes: three groups with two, three and four CES1 copies, respectively; a group of carriers of the CES1 143E allele; a group of individuals homozygous for CES1A1c (CES1VAR); and a group having three CES1 copies, in which the duplication, CES1A2, had increased transcriptional activity. Plasma concentrations of methylphenidate and its primary metabolites were determined at scheduled time points.

RESULTS

Median AUC of d-methylphenidate was significantly larger in the group carrying the 143E allele (53.3 ng ml^-1  h^-1 , range 38.6-93.9) than in the control group (21.4 ng ml^-1  h^-1 , range 15.7-34.9) (P < 0.0001). Median AUC of d-methylphenidate was significantly larger in the group with four CES1 copies (34.5 ng ml^-1  h^-1 , range 21.3-62.8) than in the control group (P = 0.01) and the group with three CES1 copies (23.8 ng ml^-1  h^-1 , range 15.3-32.0, P = 0.03). There was no difference between the groups with two and three copies of CES1.

CONCLUSIONS

The 143E allele resulted in an increased AUC, suggesting a significantly decreased CES1 enzyme activity. Surprisingly, this was also the case in subjects with homozygous duplication of CES1, perhaps reflecting an undiscovered mutation affecting the activity of the enzyme.

Advantages of nonhuman primates as preclinical models for evaluating stem cell-based therapies for Parkinson's disease

The derivation of dopaminergic neurons from induced pluripotent stem cells brings new hope for a patient-specific, stem cell-based replacement therapy to treat Parkinson's disease (PD) and related neurodegenerative diseases; and this novel cell-based approach has already proven effective in animal models. However, there are several aspects of this procedure that have yet to be optimized to the extent required for translation to an optimal cell-based transplantation protocol in humans. These challenges include pinpointing the optimal graft location, appropriately scaling up the graft volume, and minimizing the risk of chronic immune rejection, among others. To advance this procedure to the clinic, it is imperative that a model that accurately and fully recapitulates characteristics most pertinent to a cell-based transplantation to the human brain is used to optimize key technical aspects of the procedure. Nonhuman primates mimic humans in multiple ways including similarities in genomics, neuroanatomy, neurophysiology, immunogenetics, and age-related changes in immune function. These characteristics are critical to the establishment of a relevant model in which to conduct preclinical studies to optimize the efficacy and safety of cell-based therapeutic approaches to the treatment of PD. Here we review previous studies in rodent models, and emphasize additional advantages afforded by nonhuman primate models in general, and the baboon model in particular, for preclinical optimization of cell-based therapeutic approaches to the treatment of PD and other neurodegenerative diseases. We outline current unresolved challenges to the successful application of stem cell therapies in humans and propose that the baboon model in particular affords a number of traits that render it most useful for preclinical studies designed to overcome these challenges.

Prenatal exposure to methylphenidate affects the dopamine system and the reactivity to natural reward in adulthood in rats

BACKGROUND

Methylphenidate (MPH) is a commonly-used medication for the treatment of children with Attention-Deficit/Hyperactivity Disorders (ADHD). However, its prescription to adults with ADHD and narcolepsy raises the question of how the brain is impacted by MPH exposure during pregnancy. The goal of this study was to elucidate the long-term neurobiological consequences of prenatal exposure to MPH using a rat model.

METHODS

We focused on the effects of such treatment on the adult dopamine (DA) system and on the reactivity of animals to natural rewards.

RESULTS

This study shows that adult male rats prenatally exposed to MPH display elevated expression of presynaptic DA markers in the DA cell bodies and the striatum. Our results also suggest that MPH-treated animals could exhibit increased tonic DA activity in the mesolimbic pathway, altered signal-to-noise ratio after a pharmacological stimulation, and decreased reactivity to the locomotor effects of cocaine. Finally, we demonstrated that MPH rats display a decreased preference and motivation for sucrose.

CONCLUSIONS

This is the first preclinical study reporting long-lasting neurobiological alterations of DA networks as well as alterations in motivational behaviors for natural rewards after a prenatal exposure to MPH. These results raise concerns about the possible neurobiological consequences of MPH treatment during pregnancy.

Brain dopaminergic system changes in drug addiction: a review of positron emission tomography findings

Dopamine (DA) is considered crucial for the rewarding effects of drugs of abuse, but its role in addiction remains unclear. Positron emission tomography (PET) is the first technology used for in vivo measurement of components of the dopaminergic system in the human brain. In this article, we review the major findings from PET imaging studies on the involvement of DA in drug addiction, including presynaptic DA synthesis, vesicular monoamine transporter 2, the DA transporter, and postsynaptic DA receptors. These results have corroborated the role of DA in addiction and increased the understanding of its underlying mechanisms.

Development of a physiologically based model to describe the pharmacokinetics of methylphenidate in juvenile and adult humans and nonhuman primates

The widespread usage of methylphenidate (MPH) in the pediatric population has received considerable attention due to its potential effect on child development. For the first time a physiologically based pharmacokinetic (PBPK) model has been developed in juvenile and adult humans and nonhuman primates to quantitatively evaluate species- and age-dependent enantiomer specific pharmacokinetics of MPH and its primary metabolite ritalinic acid. The PBPK model was first calibrated in adult humans using in vitro enzyme kinetic data of MPH enantiomers, together with plasma and urine pharmacokinetic data with MPH in adult humans. Metabolism of MPH in the small intestine was assumed to account for the low oral bioavailability of MPH. Due to lack of information, model development for children and juvenile and adult nonhuman primates primarily relied on intra- and interspecies extrapolation using allometric scaling. The juvenile monkeys appear to metabolize MPH more rapidly than adult monkeys and humans, both adults and children. Model prediction performance is comparable between juvenile monkeys and children, with average root mean squared error values of 4.1 and 2.1, providing scientific basis for interspecies extrapolation of toxicity findings. Model estimated human equivalent doses in children that achieve similar internal dose metrics to those associated with pubertal delays in juvenile monkeys were found to be close to the therapeutic doses of MPH used in pediatric patients. This computational analysis suggests that continued pharmacovigilance assessment is prudent for the safe use of MPH.

Dopamine reuptake transporter (DAT) "inverse agonism"--a novel hypothesis to explain the enigmatic pharmacology of cocaine

The long held view is cocaine's pharmacological effects are mediated by monoamine reuptake inhibition. However, drugs with rapid brain penetration like sibutramine, bupropion, mazindol and tesofensine, which are equal to or more potent than cocaine as dopamine reuptake inhibitors, produce no discernable subjective effects such as drug "highs" or euphoria in drug-experienced human volunteers. Moreover they are dysphoric and aversive when given at high doses. In vivo experiments in animals demonstrate that cocaine's monoaminergic pharmacology is profoundly different from that of other prescribed monoamine reuptake inhibitors, with the exception of methylphenidate. These findings led us to conclude that the highly unusual stimulant profile of cocaine and related compounds, eg methylphenidate, is not mediated by monoamine reuptake inhibition alone. We describe the experimental findings which suggest cocaine serves as a negative allosteric modulator to alter the function of the dopamine reuptake transporter (DAT) and reverse its direction of transport. This results in a firing-dependent, retro-transport of dopamine into the synaptic cleft. The proposed mechanism of cocaine is, therefore, different from other small molecule negative allostereric modulators of the monoamine reuptake transporters, eg SoRI-6238, which merely reduce the rate of inward transport. Because the physiological role of DAT is to remove dopamine from the synapse and the action of cocaine is the opposite of this, we have postulated that cocaine's effect is analogous to an inverse agonist. If this hypothesis is validated then cocaine is the prototypical compound that exemplifies a new class of monoaminergic drugs; DAT "inverse agonists". This article is part of the Special Issue entitled 'CNS Stimulants'.

An evaluation of the pharmacokinetics of methylphenidate for the treatment of attention-deficit/ hyperactivity disorder

INTRODUCTION

Methylphenidate (MPH) plays a principal role in the multimodal treatment of attention-deficit/hyperactivity disorder (ADHD). Controlled studies have demonstrated an effective reduction in the core symptoms of the disorder following MPH therapy, although long-term studies also demonstrate that the therapeutic benefits dissipate in the absence of combined psychosocial interventions.

AREAS COVERED

This review article focuses on the pharmacological characteristics of MPH, examining its effects on brain metabolism and the neurotransmitter system. Neuropsychological and clinical effects of different immediate and extended release MPH formulations are discussed to aid clinicians in choosing the appropriate formulation. The drug's addictive potency and abuse potential is also discussed. Data came from a literature search of relevant studies performed using the PubMed database up to June 2013.

EXPERT OPINION

MPH is effective in the treatment of the core symptoms of ADHD. Considerable clinical expertise is required to identify an individually well-adapted dosage which will produce the optimal clinical effects with potential side effects minimized. Due to low adherence to medication, especially in adolescents, motivation to treatment and attentive clinical monitoring is mandatory, as is the consideration of risks of abuse or the presence of a comorbid addictive disorder.

Dexmethylphenidate extended-release capsules for attention deficit hyperactivity disorder

Medications for attention deficit hyperactivity disorder (ADHD) currently represent the ninth largest segment of the CNS market by sales, with 2.4 billion USD spent annually on this condition and 40% annual growth. Stimulant medications remain the most effective ADHD therapies and provide robust improvement in ADHD symptoms in both youth and adults. Current prescribing practices favor extended release preparations due to increased convenience, compliance and tolerability with once-daily dosing. Dexmethylphenidate extended release is a long-acting preparation of the ADHD medication Focalin (dexmethylphenidate immediate release) and was approved for marketing by the US Food and Drug administration in June 2005. Dexmethylphenidate consists of the single dextro-isomer form of d,l-methylphenidate commonly marketed as Ritalin. Dexmethylphenidate extended release utilizes spheroidal oral drug absorption system technology to achieve a 50% immediate medication delivery and 50% delayed release of dexmethylphenidate approximately 4 h after ingestion. Placebo-controlled, clinical trials in children and adults with ADHD have demonstrated efficacy for behavioral and academic ratings, with an analog classroom study showing medication effects up to 12 h after dosing. Dexmethylphenidate extended release was generally well tolerated with a side-effect profile similar to other stimulants. The most common reported side effects include diminished appetite and insomnia. Given its duration of effect, favorable tolerability and flexibility in dosing, dexmethylphenidate extended release is likely to gain considerable use as an ADHD treatment.

Preparation and structural analysis of (±)-threo-ritalinic acid

Hydrolysis of the methyl ester (±)-threo-methyl phenidate afforded the free acid in 40% yield, viz. (±)-threo-ritalinic acid, C13H17NO2. Hydrolysis and subsequent crystallization were accomplished at pH values between 5 and 7 to yield colourless prisms which were analysed by X-ray crystallography. Crystals of (±)-threo-ritalinic acid belong to the P21/n space group and form intermolecular hydrogen bonds. An antiperiplanar disposition of the H atoms of the (HOOC-)CH-CHpy group (py is pyridine) was found in both the solid (diffraction analysis) and solution state (NMR analysis). It was also determined that (±)-threo-ritalinic acid conforms to the minimization of negative gauche(+)-gauche(-) interactions.

Quantitative determination of d- and l-threo enantiomers of methylphenidate in brain tissue by liquid chromatography-mass spectrometry

Methylphenidate, a psychostimulant used for the treatment of attention deficit hyperactivity disorder and narcolepsy, is administered as a 50:50 racemic mixture, despite the fact that d-methylphenidate has been shown to have greater pharmacologic activity. This paper presents a validated LC-MS/MS approach to separation and quantification of methylphenidate enantiomers using a vancomycin column and triethylammonium acetate to enhance the chiral separation. The method is applicable to the monitoring of these enantiomers in mouse brain, with a limit of detection of 0.5 ng/mL and a lower limit of quantification of 7.5 ng/mL.

Dose-Proportional Pharmacokinetics ofd-threo-Methylphenidate After a Repeated-Action Release Dosage Form

A bimodal extended-release formulation of d-methylphenidate (d-MPH) has been developed to enable fast onset of action and once-daily administration in patients with attention deficit hyperactivity disorder. The authors studied the dose proportionality of extended-release d-MPH pharmacokinetics. Twenty-five healthy adult volunteers received 5, 10, 20, 30, and 40 mg d-MPH in a crossover study with 7 days between doses. All doses were well tolerated. Dose proportionality was shown for all dose-dependent pharmacokinetic parameters. Geometric means (%gCV) for the first Cmax peak, Cmax0-4, were 3.25 (29.0%), 6.05 (27.1%), 12.6 (31.9%), 18.5 (31.9%), and 25.2 ng/mL (29.3%) for d-MPH 5, 10, 20, 30, and 40 mg, respectively. Geometric means (%gCV) for Cmax4-10 were 3.18 (27.5%), 5.84 (27.7%), 12.5 (31.7%), 17.7 (31.6%), and 23.6 ng/mL (29.0%), respectively. Geometric means for AUC(0-infinity) were 24.3 (30.7%), 45.9 (30.2%), 96.4 (35.5%), 144 (33.3%), and 195 ng x h/mL (30.9%), respectively. The pharmacokinetics of once-daily extended-release d-MPH are proportional to the dose.

Sucrose as chiral selector for determining enantiomeric composition of metalaxyl by UV-vis spectroscopy and PLS regression

This study revealed that it was possible to determine the enantiomeric composition of with multivariate regression models of spectral data obtained by ordinary UV-vis spectrophotometry of enantiomeric guest-host complexes. The total 60samples involving three concentration levels of metalaxyl as low, medium and high were prepared for spectral collecting. Four methods of modeling were subsequently proposed and compared including two common ways and two compensating ways for variations in total analyte concentration. Firstly, without normalization robust modeling was failed to achieve while employing the medium concentration levels as calibration and the other two levels as a validation. The same case occurred when full-cross validation was conducted. Besides, two enhanced methods were developed to account for the systematic variation. One of which normalized the spectra with respect to the total concentration of enantiomeric, along with spectral data, as a variable in the statistical analysis. The other one ignored variations in total concentration, relying on the specific band normalization to sort out any variations due to total concentration differences. The results clearly demonstrated that the spectra according to concentration provided the acceptable predictive ability in determining enantiomeric composition.

Conserved dorsal-ventral gradient of dopamine release and uptake rate in mice, rats and rhesus macaques

Although the vast majority of research on the dopamine system has been performed in rodents, and it is assumed that this work will inform us about the human condition, there have been very few direct comparisons of presynaptic dopamine terminal function across multiple species. Because it is difficult to query rapid sub-second dopamine signaling in humans using voltammetric methods, we chose to compare dopamine signals across multiple striatal subregions in slices from C57BL/6J mice, Sprague-Dawley rats and rhesus macaques. We found a dorsal to ventral gradient of dopamine uptake rates with highest levels in the dorsal striatum and lowest levels in the nucleus accumbens shell, which is conserved across species. In addition to uptake rates, there was also a dorsal to ventral, high to low, gradient in the magnitude of stimulated DA release observed in monkeys, mice, and rats. These data demonstrate that there is considerable functional homology across striatal regions in non-human primates and rodents, lending support to the use of rodents as model systems to study dopamine-related circuitry and disorders that are clinically relevant to the human population.

Effects of methylphenidate on basic and higher-order oculomotor functions

Eye movements are sensitive indicators of pharmacological effects on sensorimotor and cognitive processing. Methylphenidate (MPH) is one of the most prescribed medications in psychiatry. It is increasingly used as a cognitive enhancer by healthy individuals. However, little is known of its effect on healthy cognition. Here we used oculomotor tests to evaluate the effects of MPH on basic oculomotor and executive functions. Twenty-nine males were given 20mg of MPH orally in a double-blind placebo-controlled crossover design. Participants performed visually-guided saccades, sinusoidal smooth pursuit, predictive saccades and antisaccades one hour post-capsule administration. Heart rate and blood pressure were assessed prior to capsule administration, and again before and after task performance. Visually-guided saccade latency decreased with MPH (p<0.004). Smooth pursuit gain increased on MPH (p<0.001) and number of saccades during pursuit decreased (p<0.001). Proportion of predictive saccades increased on MPH (p<0.004), specifically in conditions with predictable timing. Peak velocity of predictive saccades increased with MPH (p<0.01). Antisaccade errors and latency were unaffected. Physiological variables were also unaffected. The effects on visually-guided saccade latency and peak velocity are consistent with MPH effects on dopamine in basal ganglia. The improvements in predictive saccade conditions and smooth pursuit suggest effects on timing functions.

Drug therapy of attention deficit hyperactivity disorder: current trends

Attention deficit hyperactivity disorder is a developmental disorder with an age onset prior to 7 years. Children with ADHD have significantly lower ability to focus and sustain attention and also score higher on impulsivity and hyperactivity. Stimulants, such as methylphenidate, have remained the mainstay of ADHD treatment for decades with evidence supporting their use. However, recent years have seen emergence of newer drugs and drug delivery systems, like osmotic release oral systems and transdermal patches, to mention a few. The use of nonstimulant drugs like atomoxetine and various other drugs, such as α-agonists, and a few antidepressants, being used in an off-label manner, have added to the pharmacotherapy of ADHD. This review discusses current trends in drug therapy of ADHD and highlights the promise pharmacogenomics may hold in the future.

Importance of pharmacokinetic profile and timing of coadministration of short- and long-acting formulations of methylphenidate on patterns of subjective responses and abuse potential

OBJECTIVE

Subjective responses (ie, liking, disliking) to stimulants are thought to be proxies for abuse potential. Greater subjective responses have been documented in formulations that are more rapidly absorbed. However, repeat dosing has not been examined.

METHODS

Subjective responses on the Drug Rating Questionnaire were compared in 26 healthy adults after administration of short- (immediate-release [IR] methylphenidate [MPH]) and long- (osmotically controlled-release oral delivery system [OROS] MPH) acting stimulant formulations. The second dose was administered 4 hours after initial dosing. All subjects received all 5 conditions (ie, placebo to placebo; IR-MPH to IR-MPH; IR-MPH to OROS-MPH; OROS-MPH to IR-MPH; or OROS-MPH to OROS-MPH) in a double-blind, counter-balanced design on 5 separate days.

RESULTS

Plasma levels and subjective patterns of detection were higher when an IR formulation was administered during the ascending phase of a first-administered long-acting dose (OROS).

CONCLUSION

These results emphasize the critical role that formulation type (IR vs OROS) and timing of administration (ascending vs descending phase) play when short- and long-acting formulations are coadministered. Such knowledge provides important information for clinicians about the safety and tolerability of the timing of repeat dosing of various permutations of coadministration of MPH formulations.

Positron emission tomography molecular imaging of dopaminergic system in drug addiction

Dopamine (DA) is involved in drug reinforcement, but its role in drug addiction remains unclear. Positron emission tomography (PET) is the first technology used for the direct measurement of components of the dopaminergic system in the living human brain. In this article, we reviewed the major findings of PET imaging studies on the involvement of DA in drug addiction, especially in heroin addiction. Furthermore, we summarized PET radiotracers that have been used to study the role of DA in drug addiction. To investigate presynaptic function in drug addiction, PET tracers have been developed to measure DA synthesis and transport. For the investigation of postsynaptic function, several radioligands targeting dopamine one (D1) receptor and dopamine two (D2) receptor are extensively used in PET imaging studies. Moreover, we also summarized the PET imaging findings of heroin addiction studies, including heroin-induced DA increases and the reinforcement, role of DA in the long-term effects of heroin abuse, DA and vulnerability to heroin abuse and the treatment implications. PET imaging studies have corroborated the role of DA in drug addiction and increase our understanding the mechanism of drug addiction.

Applications of positron emission tomography in animal models of neurological and neuropsychiatric disorders

Positron emission tomography (PET) provides dynamic images of the biodistribution of radioactive tracers in the brain. Through application of the principles of compartmental analysis, tracer uptake can be quantified in terms of specific physiological processes such as cerebral blood flow, cerebral metabolic rate, and the availability of receptors in brain. Whereas early PET studies in animal models of brain diseases were hampered by the limited spatial resolution of PET instruments, dedicated small-animal instruments now provide molecular images of rodent brain with resolution approaching 1mm, the theoretic limit of the method. Major applications of PET for brain research have consisted of studies of animal models of neurological disorders, notably Parkinson's disease (PD), Alzheimer's disease (AD), and Huntington's disease (HD), stroke, epilepsy and traumatic brain injury; these studies have particularly benefited from selective neurochemical lesion models (PD), and also transgenic rodent models (AD, HD). Due to their complex and uncertain pathophysiologies, corresponding models of neuropsychiatric disorders have proven more difficult to establish. Historically, there has been an emphasis on PET studies of dopamine transmission, as assessed with a range of tracers targeting dopamine synthesis, plasma membrane transporters, and receptor binding sites. However, notable recent breakthroughs in molecular imaging include the development of greatly improved tracers for subtypes of serotonin, cannabinoid, and metabotropic glutamate receptors, as well as noradrenaline transporters, amyloid-β and neuroinflammatory changes. This article reviews the considerable recent progress in preclinical PET and discusses applications relevant to a number of neurological and neuropsychiatric disorders in humans.

Methylphenidate: established and expanding roles in symptom management

Methylphenidate is a psychostimulant originally used for the treatment of attention-deficit disorder. Methylphenidate inhibits neuronal neurotransmitter transporters involved in the uptake of dopamine and norepinephrine at the level of the synapse. Inhibition of these transmitter transporters leads to increased concentrations of dopamine and norepinephrine in the synapse, which results in increasing alertness. The stimulant effect of methylphenidate has been used for the treatment of major depression, poststroke depression, cognitive enhancement in patients with brain tumors, neurodegenerative disorders, HIV disease, fatigue, and as a treatment for delirium and sedation associated with opioid use. Other areas where methylphenidate has been evaluated include gait disorders in the elderly individuals and the treatment of apathy in dementia. Analgesic effects have been demonstrated in preclinical models but true analgesic effects remain to be proven in humans. This article reviews the current use of methylphenidate for symptom management with a critical look at the evidence base for its efficacy in the conditions described.

Cingulate, frontal, and parietal cortical dysfunction in attention-deficit/hyperactivity disorder

Functional and structural neuroimaging have identified abnormalities of the brain that are likely to contribute to the neuropathophysiology of attention-deficit/hyperactivity disorder (ADHD). In particular, hypofunction of the brain regions comprising the cingulo-frontal-parietal cognitive-attention network have been consistently observed across studies. These are major components of neural systems that are relevant to ADHD, including cognitive/attention networks, motor systems, and reward/feedback-based processing systems. Moreover, these areas interact with other brain circuits that have been implicated in ADHD, such as the "default mode" resting state network. The ADHD imaging data related to cingulo-frontal-parietal network dysfunction will be selectively highlighted here to help facilitate its integration with the other information presented in this special issue. Together, these reviews will help shed light on the neurobiology of ADHD.

Efficacy of methylphenidate for adults with attention-deficit hyperactivity disorder: a meta-regression analysis

BACKGROUND

The efficacy of methylphenidate for adults with attention-deficit hyperactivity disorder (ADHD) shows wide between-study variability, which yields heterogeneous results in meta-analysis. The reasons for this variability have not been comprehensively investigated.

OBJECTIVES

To determine the influence of treatment-related covariates of methylphenidate for adults with ADHD by means of meta-analysis. Clinical and methodological moderators and clinical trial reporting quality were also collected to control for their potential confounding effect.

METHODS

We searched for randomized, placebo-controlled clinical trials investigating the efficacy of methylphenidate for adults with ADHD. The study outcome was the efficacy of methylphenidate for reducing ADHD symptom severity. Treatment-related covariates included dose, type of drug-release formulation (formulations with a continuous drug release vs those with a non-continuous drug release), dose regimen (fixed vs flexible) and treatment length. Clinical (presence of co-morbid substance use disorders [SUD]) and methodological (design and rater) covariates were also collected, in addition to clinical trial reporting quality. The standardized mean difference (SMD) was calculated for each study. The analysis of the influence of methylphenidate effect modifiers was performed by means of random-effects meta-regression.

RESULTS

Eighteen studies were included. Dose, type of formulation and SUD appeared to modify the efficacy of methylphenidate in the bivariate analysis. These variables were included in a multivariate meta-regression, which showed that methylphenidate, at an average dose of 57.4 mg/day, delivered by means of non-continuous-release formulations, had a moderate effect on ADHD symptoms compared with placebo (SMD 0.57-0.58). A dose-response relationship was found, indicating that efficacy could be increased by SMD 0.11-0.12 for every 10 mg increment of methylphenidate. Continuous-release formulations and co-morbid SUD appeared to reduce the efficacy of methylphenidate. Nevertheless, the effect of treatment formulation may have been confounded by co-morbid SUD, since all studies using this continuous-release formulation were conducted in dual ADHD-SUD patients. No residual heterogeneity was found.

CONCLUSIONS

This study shows that methylphenidate improves ADHD symptoms in adults in a dose-dependent fashion. The efficacy of methylphenidate appears to be reduced in patients with co-morbid SUD. It is unclear whether methylphenidate efficacy is influenced by the type of formulation, because the effect of this covariate is confounded by that of co-morbid SUD.

Clinically relevant doses of methylphenidate significantly occupy norepinephrine transporters in humans in vivo

BACKGROUND

Attention-deficit/hyperactivity disorder is a psychiatric disorder that starts in childhood. The mechanism of action of methylphenidate, the most common treatment for attention deficit hyperactivity disorder, is unclear. In vitro, the affinity of methylphenidate for the norepinephrine transporter (NET) is higher than that for the dopamine transporter (DAT). The goal of this study was to use positron emission tomography to measure the occupancy of brain norepinephrine transporter by methylphenidate in vivo in humans.

METHODS

We used (S,S)-[¹¹C] methylreboxetine ([¹¹C]MRB) to determine the effective dose 50 (ED₅₀) of methylphenidate for NET. In a within-subject design, healthy subjects (n = 11) received oral, single-blind placebo and 2.5, 10, and 40 mg of methylphenidate 75 min before [¹¹C]MRB injection. Dynamic positron emission tomography imaging was performed for 2 hours with the High Resolution Research Tomograph. The multilinear reference tissue model with occipital cortex as the reference region was used to estimate binding potential non-displaceable (BP(ND)) in the thalamus and other NET-rich regions.

RESULTS

BP(ND) was reduced by methylphenidate in a dose-dependent manner in thalamus and other NET-rich regions. The global ED₅₀ was estimated to be .14 mg/kg; therefore, the average clinical maintenance dose of methylphenidate (.35-.55 mg/kg) produces 70% to 80% occupancy of NET.

CONCLUSIONS

For the first time in humans, we demonstrate that oral methylphenidate significantly occupies NET at clinically relevant doses. The ED₅₀ is lower than that for DAT (.25 mg/kg), suggesting the potential relevance of NET inhibition in the therapeutic effects of methylphenidate in attention-deficit/hyperactivity disorder.

Attention-deficit/hyperactivity disorder and attention networks

Research attempting to elucidate the neuropathophysiology of attention-deficit/hyperactivity disorder (ADHD) has not only shed light on the disorder itself, it has simultaneously provided new insights into the mechanisms of normal cognition and attention. This review will highlight and integrate this bidirectional flow of information. Following a brief overview of ADHD clinical phenomenology, ADHD studies will be placed into a wider historical perspective by providing illustrative examples of how major models of attention have influenced the development of neurocircuitry models of ADHD. The review will then identify major components of neural systems potentially relevant to ADHD, including attention networks, reward/feedback-based processing systems, as well as a 'default mode' resting state network. Further, it will suggest ways in which these systems may interact and be influenced by neuromodulatory factors. Recent ADHD imaging data will be selectively provided to both illustrate the field's current level of knowledge and to show how such data can inform our understanding of normal brain functions. The review will conclude by suggesting possible avenues for future research.

Dexmethylphenidate extended release: a review of its use in the treatment of attention-deficit hyperactivity disorder

Dexmethylphenidate extended release (XR) [Focalin XR] is a CNS stimulant indicated for the treatment of attention-deficit hyperactivity disorder (ADHD) in patients aged > or = 6 years. Dexmethylphenidate contains the d-threo-enantiomer of methylphenidate. Dexmethylphenidate XR capsules have a bimodal release profile, which mimics two doses of dexmethylphenidate immediate release (IR) given 4 hours apart, and allows once-daily administration. Once-daily dexmethylphenidate XR was effective and generally well tolerated in the treatment of ADHD in children, adolescents and adults in placebo-controlled trials. Improvements in ADHD symptoms were significantly greater for dexmethylphenidate XR versus placebo throughout the day, from as early as 0.5 hours after drug administration up to 11-12 hours after administration. Furthermore, dexmethylphenidate XR showed greater efficacy than osmotic release oral system (OROS) methylphenidate over the first half of the laboratory classroom day in crossover trials; however, assessments late in the day (10-12 hours post-dose) favoured OROS methylphenidate. The once-daily administration regimen with dexmethylphenidate XR avoids the need for a midday dose to be administered at school; administration options are extended in that the contents of the dexmethylphenidate XR capsule can be sprinkled on apple sauce for patients unable to swallow the capsule whole. Although dexmethylphenidate XR is a controlled substance in the US, this formulation appears to have a low risk of abuse or misuse. Thus, dexmethylphenidate XR extends the range of first-line pharmacological treatment options for children, adolescents or adults with ADHD, particularly for patients who require a rapid onset and prolonged duration of action, including children who require a reduction in ADHD symptoms throughout the school day.

Efficacy of dexmethylphenidate for the treatment of fatigue after cancer chemotherapy: a randomized clinical trial

Cancer and its treatment can induce subjective and objective evidence of diminished functional capacity encompassing physical fatigue and cognitive impairment. Dexmethylphenidate (D-MPH; the D-isomer of methylphenidate) was evaluated for treatment of chemotherapy-related fatigue and cognitive impairment. A randomized, double-blind, placebo-controlled, parallel-group study evaluated the potential therapeutic effect and safety of D-MPH in the treatment of patients with chemotherapy-related fatigue. Change from baseline in the Functional Assessment of Chronic Illness Therapy-Fatigue Subscale (FACIT-F) total score at Week 8 was the primary outcome measure. One hundred fifty-four patients (predominantly with breast and ovarian cancers) were randomized and treated. Compared with placebo, D-MPH-treated subjects demonstrated a significant improvement in fatigue symptoms at Week 8 in the FACIT-F (P=0.02) and the Clinical Global Impression-Severity scores (P=0.02), without clinically relevant changes in hemoglobin levels. Cognitive function was not significantly improved. There was a higher rate of study drug-related adverse events (AEs) (48 of 76 [63%] vs. 22 of 78 [28%]) and a higher discontinuation rate because of AEs (8 of 76 [11%] vs. 1 of 78 [1.3%]) in D-MPH-treated subjects compared with placebo-treated subjects. The most commonly reported AEs independent of study drug relationship in D-MPH-treated subjects were headache, nausea, and dry mouth, and in placebo-treated subjects were headache, diarrhea, and insomnia. D-MPH produced significant improvement in fatigue in subjects previously treated with cytotoxic chemotherapy. Further studies with D-MPH or other agents to explore treatment response in chemotherapy-associated fatigue should be considered.

Positron emission tomography imaging of D(2/3) agonist binding in healthy human subjects with the radiotracer [(11)C]-N-propyl-norapomorphine: preliminary evaluation and reproducibility studies

OBJECTIVE

(-)-N-[(11)C]-propyl-norapomorphine (NPA) is a full dopamine D(2/3) receptor agonist radiotracer suitable for imaging D(2/3) receptors configured in a state of high affinity for agonists using positron emission tomography. The aim of the present study was to define the optimal analytic method to derive accurate and reliable D(2/3) receptor parameters with [(11)C]NPA.

METHODS

Six healthy subjects (four females/two males) underwent two [(11)C]NPA scans in the same day. D(2/3) receptor-binding parameters were estimated using kinetic analysis (using one- and two-tissue compartment models) as well as simplified reference tissue method in the three functional subdivisions of the striatum (associative striatum, limbic striatum, and sensorimotor striatum). The test-retest variability and intraclass correlation coefficient were assessed for distribution volume (V(T)), binding potential relative to plasma concentration (BP(P)), and binding potential relative to nondisplaceable uptake (BP(ND)).

RESULTS

A two-tissue compartment kinetic model adequately described the functional subdivisions of the striatum as well as cerebellum time-activity data. The reproducibility of V(T) was excellent (<or=10%) in all regions, for this approach. The reproducibility of both BP(P) (<or=12%) and BP(ND) (<or=10%) was also excellent. The intraclass correlation coefficients of BP(P) and BP(ND) were acceptable as well (>0.75) in the three functional subdivisions of the striatum. Although SRTM led to an underestimation of BP(ND) values relative to that estimated by kinetic analysis by 8-13%, the values derived using both the methods were reasonably well correlated (r(2) = 0.89, n = 84). Both methods were similarly effective in detecting the differences in [(11)C]NPA BP(ND) between subjects.

CONCLUSION

The results of this study indicate that [(11)C]NPA can be used to measure D(2/3) receptors configured in a state of high affinity for the agonists with high reliability and reproducibility in the functional subdivisions of the human striatum.

Plasma and brain concentrations of oral therapeutic doses of methylphenidate and their impact on brain monoamine content in mice

Methylphenidate is a frequently prescribed stimulant for the treatment of attention deficit hyperactivity disorder (ADHD). An important assumption in the animal models that have been employed to study methylphenidate's effects on the brain and behavior is that bioavailability of methylphenidate in the animal models reflects that in human subjects. From this perspective, the dose and route of administration of methylphenidate assume critical importance because both these factors likely influence rate of uptake, plasma and brain concentrations of the drug. In the present study, plasma and brain concentrations of d- and l-methylphenidate and d- and l-ritalinic acid were measured in 2-month old mice (equivalent to young adulthood in humans) following a single oral administration of a racemic mixture. Our data show that oral administration of 0.75 mg/kg dose produced within 15 min, plasma levels of d-methylphenidate that correspond to the clinically effective plasma levels in human subjects (estimated to be 6-10 ng/ml). Brain concentrations of d- and l-methylphenidate tended to exceed their plasma concentrations, while the plasma concentrations of d- and l-ritalinic acid exceeded their brain concentrations. A single oral administration at 0.75 mg/kg dose increased dopamine content of the frontal cortex within 1 h, without producing statistically significant changes in serotonin or noradrenaline contents. Striatal monoamine levels remained unaltered. These data highlight disparities between plasma and brain concentrations of methylphenidate and its metabolites following oral administration and illustrate brain region- and monoamine-specific changes produced by the low oral dose of methylphenidate.