PET study examining pharmacokinetics, detection and likeability, and dopamine transporter receptor occupancy of short- and long-acting oral methylphenidate

In Memoriam: Professor Joseph Biederman's Contributions to Child and Adolescent Psychiatry

OBJECTIVE

To provide an overview of Joe Biederman's contributions to child and adolescent psychiatry.

METHOD

Nine colleagues described his contributions to: psychopharmacology, comorbidity and genetics, pediatric bipolar disorder, autism spectrum disorders, Tourette's and tic disorders, clinical and neuro biomarkers for pediatric mood disorders, executive functioning, and adult ADHD.

RESULTS

Joe Biederman left us with many concrete indicators of his contributions to child and adolescent psychiatry. He set up the world's first pediatric psychopharmacology clinic and clinical research program in child adolescent psychiatry. As a young faculty member he began a research program that led to many awards and eventual promotion to full professor at Harvard Medical School. He was for many years the most highly cited researcher in ADHD. He achieved this while maintaining a full clinical load and was widely respected for his clinical acumen.

CONCLUSION

The world is a better place because Joe Biederman was here.

Preclinical Models of Attention Deficit Hyperactivity Disorder: Neurobiology, Drug Discovery, and Beyond

OBJECTIVE

We offer an overview of ADHD research using mouse models of nicotine exposure.

METHOD

Nicotine exposure of C57BL/6 or Swiss Webster mice occurred during prenatal period only or during the prenatal and the pre-weaning periods. Behavioral, neuroanatomical and neurotransmitter assays were used to investigate neurobiological mechanisms of ADHD and discover candidate ADHD medications.

RESULTS

Our studies show that norbinaltorphimine, a selective kappa opioid receptor antagonist is a candidate novel non-stimulant ADHD treatment and that a combination of methylphenidate and naltrexone has abuse deterrent potential with therapeutic benefits for ADHD. Other studies showed transgenerational transmission of ADHD-associated behavioral traits and demonstrated that interactions between untreated ADHD and repeated mild traumatic brain injury produced behavioral traits not associated with either condition alone.

CONCLUSION

Preclinical models contribute to novel insights into ADHD neurobiology and are valuable tools for drug discovery and translation to benefit humans with ADHD.

A systematic review of the potential effects of medications and drugs of abuse on dopamine transporter imaging using [123I]I-FP-CIT SPECT in routine practice

PURPOSE

In routine practice, dopamine transporter (DAT) imaging is frequently used as a diagnostic tool to support the diagnosis of Parkinson's disease or dementia with Lewy bodies. In 2008, we published a review on which medications and drugs of abuse may influence striatal [123I]I-FP-CIT binding and consequently may influence the visual read of an [123I]I-FP-CIT SPECT scan. We made recommendations on which drugs should be withdrawn before performing DAT imaging in routine practice. Here, we provide an update of the original work based on published research since 2008.

METHODS

We performed a systematic review of literature without language restriction from January 2008 until November 2022 to evaluate the possible effects of medications and drugs of abuse, including the use of tobacco and alcohol, on striatal DAT binding in humans.

RESULTS

The systematic literature search identified 838 unique publications, of which 44 clinical studies were selected. Using this approach, we found additional evidence to support our original recommendations as well as some new findings on potential effect of other medications on striatal DAT binding. Consequently, we updated the list of medications and drugs of abuse that may influence the visual read of [123I]I-FP-CIT SPECT scans in routine clinical practice.

CONCLUSION

We expect that a timely withdrawal of these medications and drugs of abuse before DAT imaging may reduce the incidence of false-positive reporting. Nevertheless, the decision to withdraw any medication must be made by the specialist in charge of the patient's care and considering the pros and cons of doing so.

An exploratory analysis of the performance of methylphenidate regimens based on a PKPD model of dopamine and norepinephrine transporter occupancy

Methylphenidate (MPH) is a psychostimulant which inhibits the uptake of dopamine and norepinephrine transporters, DAT and NET, and is mostly used to treat Attention Deficit/Hyperactivity Disorder. The current dose optimization is done through titration, a cumbersome approach for patients. To assess the therapeutic performance of MPH regimens, we introduce an in silico framework composed of (i) a population pharmacokinetic model of MPH, (ii) a pharmacodynamic (PD) model of DAT and NET occupancy, (iii) a therapeutic box delimited by time and DAT occupancy, and (iv) a performance score computation. DAT occupancy data was digitized (n = 152) and described with Emax models. NET occupancy was described with a KPD model. We used this integrative framework to simulate the performance of extended-release (18-99 mg) and tid MPH regimens (25-40 mg). Early blood samples of MPH seem to lead to higher DAT occupancy, consistent with an acute tolerance observed in clinical rating scales. An Emax model with a time-dependent tolerance was fitted to available data to assess the observed clockwise hysteresis. Peak performance is observed at 63 mg. While our analysis does not deny the existence of an acute tolerance, data precision in terms of formulation and sampling times does not allow a definite confirmation of this phenomenon. This work justifies the need for a more systematic collection of DAT and NET occupancy data to further investigate the presence of acute tolerance and assess the impact of low MPH doses on its efficacy.

The effects of first-dose methylphenidate on the neural signatures of visual selective attention in children with attention-deficit/hyperactivity disorder

Although methylphenidate (MPH) has been shown to significantly improve selective attention in children with attention-deficit/hyperactivity disorder (ADHD), the neural mechanism of this effect remains unclear. We investigated the effects of first-dose MPH on the neural signatures of visual selective attention in children with ADHD. We measured the impact of first-dose MPH on electrophysiological indexes from eighteen children with ADHD (8.9-15.2 years; 15 boys) while they performed a visual search task. MPH was administered in a double-blind placebo-controlled crossover design. MPH led to decreases in behavioral error rates and reaction times. For the electrophysiological indexes, MPH significantly increased the target-elicited N2pc amplitude and posterior P3 amplitude during the selective attention process. The trial-based correlation analysis revealed that the enhanced N2pc (more negative) and P3 (more positive) promoted the behavioral response speed for children with ADHD. The lower individual P3 amplitude was associated with higher severity of inattention symptoms. The severer inattention symptoms were related to weaker MPH effect on N2pc amplitude. These findings suggest that N2pc and P3 are closely related to the mechanism of MPH in the ADHD treatment.

Oral, intranasal, and intravenous abuse potential of serdexmethylphenidate, a novel prodrug of d-methylphenidate

OBJECTIVES

Serdexmethylphenidate (SDX) chloride (Cl) is a novel prodrug of d-methylphenidate (d-MPH). These studies evaluated the abuse potential of SDX Cl when administered orally, intranasally (IN), and intravenously (IV).

METHODS

Three randomized, double-blind, placebo- and active-controlled crossover studies were conducted in recreational drug users to evaluate the abuse-related effects of oral SDX (120 and 240 mg) vs. extended-release (ER) d-MPH (80 mg) and phentermine (60 mg); IN SDX (80 mg) vs. d-MPH (40 mg), and IV SDX (30 mg) vs. d-MPH (15 mg). Abuse-related subjective measures, pharmacokinetics, and safety were assessed.

RESULTS

The primary endpoint of maximum (E_max) Drug Liking (DL) (0-100-point scale) was significantly higher following d-MPH vs. placebo, validating the studies. In the oral study, DL E_max was significantly higher following 80 mg ER d-MPH (E_max = 81.5) than 120 mg SDX (E_max = 62.8, p < .001) and 240 mg SDX (E_max = 63.8, p = .006); and following 60 mg phentermine (E_max = 80.2) than 120 mg SDX (p = .0195), but not 240 mg SDX (p = .0665). DL E_max scores were significantly higher following IN d-MPH vs SDX (E_max = 93.2 vs. 71.0, p < .0001) and following IV d-MPH vs. SDX (E_max = 84.3 vs. 56.6, p = .001). Intravenous SDX was non-inferior to placebo (p = .001) for DL E_max. Secondary endpoints (e.g. Take Drug Again) were generally consistent with the primary endpoint. Maximal and overall d-MPH exposure was lower for SDX than d-MPH for all routes. Adverse events typical of stimulants were more frequent with d-MPH than SDX.

CONCLUSIONS

These findings indicate that the novel d-MPH prodrug, SDX, has lower abuse potential than d-MPH and support its classification as a C-IV controlled substance.

Prediction of Carboxylesterase 1-mediated In Vivo Drug Interaction between Methylphenidate and Cannabinoids using Static and Physiologically Based Pharmacokinetic Models

The use of cannabis products has increased substantially. Cannabis products have been perceived and investigated as potential treatments for attention-deficit/hyperactivity disorder (ADHD). Accordingly, co-administration of cannabis products and methylphenidate (MPH), a first-line medication for ADHD, is possible. Oral MPH undergoes extensive presystemic metabolism by carboxylesterase 1 (CES1), a hepatic enzyme which can be inhibited by two prominent cannabinoids, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). This prompts further investigation into the likelihood of clinical interactions between MPH and these two cannabinoids through CES1 inhibition. In the present study, inhibition parameters were obtained from a human liver S9 system and then incorporated into static and physiologically-based pharmacokinetic (PBPK) models for prediction of potential clinical significance. The inhibition of MPH hydrolysis by THC and CBD was reversible, with estimated unbound inhibition constants (Ki,u) of 0.031 and 0.091 µM, respectively. The static model predicted a mild increase in MPH exposure by concurrent THC (34%) and CBD (94%) from smoking a cannabis cigarette and ingestion of prescriptive CBD, respectively. PBPK models suggested no significant interactions between single doses of MPH and CBD (2.5 - 10 mg/kg) when administered simultaneously, while a mild interaction (area under drug concentration-time curve increased by up to 55% and maximum concentration by up to 45%) is likely if multiple doses of CBD (10 mg/kg twice daily) are administered. In conclusion, the pharmacokinetic disposition of MPH can be potentially influenced by THC and CBD under certain clinical scenarios. Whether the magnitude of predicted interactions translates into clinically relevant outcomes requires verification in an appropriately designed clinical study. SIGNIFICANCE STATEMENT: This work demonstrated a potential mechanism of drug-drug interactions between methylphenidate (MPH) and two major cannabinoids (Δ9-tetrahydrocannabinol [THC] and cannabidiol [CBD]) not previously reported. We predicted a mild interaction between MPH and THC when the cannabinoid exposure occurred via cannabis smoking. Mild interactions between MPH and CBD were predicted with multiple oral administrations of CBD.

ADHD stimulant medication misuse and considerations for current prescribing practice: a literature review

BACKGROUND

Stimulant medications have been prescribed to effectively treat childhood Attention Deficit Hyperactivity Disorder (ADHD) since the 1960's, with improved outcomes observed in the three core symptom domains. Over the course of these decades researchers and clinicians have debated the issue of negative outcomes with regard to later development of substance use disorders (SUD) for these children.

AIMS

To chronicle the development of medical and scientific opinion on the subject of SUD outcomes in ADHD and to appraise most recently published research in this sphere.

METHODS

A systematic search of the literature was conducted over 4 databases. Removal of duplicates, application of exclusion criteria and inclusion of publications identified through manual and citation-based search yielded 9 papers.

RESULTS

Prescriptions for stimulant medications are increasing worldwide and in tandem the prevalence of stimulant misuse. Much research focuses on non-medical stimulant misuse as a study aid; however, they are also used as recreational drugs with action on dopaminergic neurotransmitter pathways implicated in addiction disorders. Considering the risks and benefits of stimulant prescribing on later SUD development research in recent decades has produced inconsistent results. Current research supports the hypothesis of improved SUD outcomes for young people treated early and intensely, with poorer outcomes for those with less robust treatment histories.

CONCLUSIONS

Consideration of the impact that variable treatment trajectories may have on the risk of later SUD development is recommended, with further research potentially leading to the development of different management pathways based on an individual's multivariate treatment profile.

Doctor shopping for methylphenidate as a proxy for misuse and potential abuse in the 67 million inhabitants in France

Doctor shopping enables subjects to receive more than the prescribed dose out of any medical management, which suggests a search for high doses and makes doctor shopping a relevant proxy for misuse and potential abuse. Therefore, this study aimed to identify and characterize profiles of subjects with doctor-shopping behavior for methylphenidate in the entire French population. This study is a cross-sectional study of doctor-shopping behavior for methylphenidate in France, in 2016, using the Système national des données de santé, and accounting for overlapping prescriptions. Subjects who obtained >840 mg by doctor shopping were defined as subjects with heavy doctor-shopping behavior, and subjects who obtained >0 mg and ≤840 mg by doctor shopping were defined as subjects with light doctor-shopping behavior. A total of 63 739 subjects were included, and received 339.6 kg of methylphenidate. Among them, 216 (0.3%) subjects had heavy doctor-shopping behavior, and 313 (0.5%) subjects had light doctor-shopping behavior. Compared with subjects with light doctor-shopping behavior, subjects with heavy doctor-shopping behavior were older (64% of 30- to 49-year-old subjects vs. 77% of ≤17-year-old subjects; P < 0.001), received more concomitant dispensings of antipsychotics (37% vs. 26%; P = 0.008) and opioid maintenance treatments (50% vs. 6%; P < 0.001), and had more prescribers (4 [IQR = 2-5] vs. 2 [IQR = 2-3]; P < 0.001). In the French context where prescription and dispensing of methylphenidate are highly regulated and methylphenidate is much less used than in other countries, these results are a warning signal to avoid trivializing methylphenidate in adults.

Methylphenidate boosts choices of mental labor over leisure depending on striatal dopamine synthesis capacity

The cognitive enhancing effects of methylphenidate are well established, but the mechanisms remain unclear. We recently demonstrated that methylphenidate boosts cognitive motivation by enhancing the weight on the benefits of a cognitive task in a manner that depended on striatal dopamine. Here, we considered the complementary hypothesis that methylphenidate might also act by changing the weight on the opportunity cost of a cognitive task, that is, the cost of foregoing alternative opportunity. To this end, 50 healthy participants (25 women) completed a novel cognitive effort-discounting task that required choices between task and leisure. They were tested on methylphenidate, placebo, as well as the selective D2-receptor agent sulpiride, the latter to strengthen inference about dopamine receptor selectivity of methylphenidate's effects. Furthermore, they also underwent an [18F]DOPA PET scan to quantify striatal dopamine synthesis capacity. Methylphenidate boosted choices of cognitive effort over leisure across the group, and this effect was greatest in participants with more striatal dopamine synthesis capacity. The effects of sulpiride did not reach significance. This study strengthens the motivational account of methylphenidate's effects on cognition, and suggests that methylphenidate reduces the cost of mental labor by increasing striatal dopamine.

Medication utilization among adult patients with attention-deficit/hyperactivity disorder after reimbursement criteria change

This study analyzed the pattern of attention-deficit/hyperactivity disorder (ADHD) medication initiation in adult patients with ADHD after the reimbursement criteria change and identified the influencing factors associated with it using the claim data. We identified 243 adult patients with ADHD who had not been prescribed ADHD drugs before 1 September 2016. We conducted Kaplan-Meier survival analysis to calculate the time to initial prescription of ADHD medications, and Cox proportional hazard regression analysis to estimate the influencing factors. Approximately one-third of the patients (n = 76, 31.3%) were first prescribed ADHD medications after reimbursement approval, and 40 of them (16.5%) started treatment with osmotic release oral system methylphenidate. The patient's age group (30-39 years) and the status of diagnosis before the index date were associated with early initiation of pharmacotherapy. The odds of starting ADHD medications increased approximately 2.7-fold in the 30-39 age group and 0.2-fold in the case of patients who were diagnosed before the approval. Our findings show that both diagnosis and treatment of adult ADHD remains inadequate despite the change in reimbursement criteria. Improving awareness of adult ADHD among both the public and the professionals is essential to increase its chances of diagnosis and treatment.

An update on the clinical pharmacology of methylphenidate: therapeutic efficacy, abuse potential and future considerations

INTRODUCTION

Methylphenidate remains a first-line medication for treating ADHD in children and adults. However, its behavioral pharmacological similarities to methamphetamine and cocaine have historically created concern for its potential as a drug of abuse. In September 2019, the FDA published a docket requesting comments for the development of abuse deterrent formulations for CNS stimulants, emphasizing the abuse of methylphenidate as a public health concern.

AREAS COVERED

We conducted a narrative review of research on the clinical pharmacology, therapeutic efficacy, and abuse potential of methylphenidate.

EXPERT OPINION

Several studies indicate that methylphenidate has at least some abuse potential. Methylphenidate, amphetamine, methamphetamine, and cocaine overlap in their subjective, reinforcing, and discriminative stimulus effects. Regardless, methylphenidate remains an efficacious treatment for ADHD in children and adults when properly adhered to, especially when paired with non-pharmacological treatments. The development of abuse deterrent formulations of methylphenidate is warranted.

Investigation of Efficacy of Short-Acting Methylphenidate (Ritalin) and Long-Acting (Matoride) on Symptoms of Attention Deficit Hyperactivity Disorder in Children Aged 6-18 Years: A Single-Blind, Randomized Clinical Trial

Background

This study aimed to evaluate the efficacy and safety of long-acting versus short-acting methylphenidate on the symptoms of attention deficit hyperactivity disorder (ADHD) in children and adolescents aged 6-18 years.

Materials and Methods

This single-blind, randomized clinical trial was conducted on 150 children and adolescents aged 6-18 years with ADHD based on the Diagnostic and Statistical Manual of Mental Disorder-5 criteria. The patients were randomly assigned to two groups (Matoride or Ritalin). In the first group, Ritalin was prescribed 2/3 times a day, and in the second group, Matoride was prescribed once a day for 3 weeks. The Conner's questionnaire was completed by the parents of the participants for evaluation of the performance and symptoms of ADHD in both groups at the beginning and 3 weeks after treatment. In addition, the incidence of any drug complications at the end of 3-week treatment period was evaluated.

Results

There were no statistically significant differences between the two groups before the intervention (P > 0.05) in the dimensions of attention deficit, emotional reaction, behavioral disorder, learning disorder, and impulsivity. At the postinterventional periods, behavioral disorder of the Ritalin group was statistically significantly lower than that of the Matoride group (P = 0.001). This treatment did not have a statistically significant effect on the total score of Conners (P = 0.255). Complications were seen in 58 cases (77.3%) of Matoride group and 49 ones (67.1%) of the Ritalin group. Weight loss in the Ritalin group was higher than that of the Matoride group (P = 0.019). Compared to the Ritalin group, anxiety was higher in the Matoride group (P = 0.022).

Conclusion

Given the similar effect of Matoride and Ritalin and no significant difference in drug complications, it seems that Matoride (slow release) can be used as an alternative to Ritalin (short acting).

Human Abuse Potential of Oral NKTR-181 in Recreational Opioid Users: A Randomized, Double-Blind, Crossover Study

OBJECTIVE

To evaluate the human abuse potential, pharmacokinetics, pharmacodynamics, and safety of oral NKTR-181 (oxycodegol), a novel full mu-opioid receptor agonist, relative to oral oxycodone.

DESIGN

This double-blind, randomized, single-dose, crossover human abuse potential study was conducted in healthy, adult, non-physically dependent recreational opioid users.

SETTING

Inpatient clinical research site.

SUBJECTS

Seventy-one subjects randomized (95.7% male, 65.2% African American, mean age = 31.7 years).

METHODS

The primary objective was to compare two therapeutic doses of NKTR-181 (400 and 600 mg) with 40 and 60 mg of oxycodone and a supratherapeutic dose (1200 mg) of NKTR-181 with 60 mg of oxycodone using visual analog scale (VAS) ratings for Drug Liking "at this moment" (Drug Liking). Secondary objectives included VAS ratings for other subjective measures, and central nervous system (CNS) mu-opioid effects were assessed using pupillometry. Each subject received single oral doses of five treatments and matching placebo.

RESULTS

Compared with 40 and 60 mg of oxycodone, the maximum mean Drug Liking score at 400 and 600 mg NKTR-181 was significantly lower, and the rate of onset and extent of Drug Liking for all NKTR-181 doses in the first two hours postdose were also significantly lower. Delayed attenuated Drug Liking and pupillary miosis response following administration of NKTR-181 vs oxycodone were consistent with slower NKTR-181 CNS entry kinetics and mu-opioid receptor binding. No adverse events were rated as severe, and somnolence and dizziness occurred more frequently when subjects received oxycodone.

CONCLUSIONS

NKTR-181 at oral doses of 400 and 600 mg showed significantly fewer and less severe subjective effects accepted as representative of opioid abuse potential, such as lower peak Drug Liking in recreational opioid users, than 40 and 60 mg of oxycodone.

Guanfacine treatment improves ADHD phenotypes of impulsivity and hyperactivity in a neurofibromatosis type 1 mouse model

Background

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder with a mutation in one copy of the neurofibromin gene (NF1^+/−). Even though approximately 40–60% of children with NF1 meet the criteria for attention deficit hyperactivity disorder (ADHD), very few preclinical studies, if any, have investigated alterations in impulsivity and risk-taking behavior. Mice with deletion of a single NF1 gene (Nf1^+/−) recapitulate many of the phenotypes of NF1 patients.

Methods

We compared wild-type (WT) and Nf1^+/− mouse strains to investigate differences in impulsivity and hyperactivity using the delay discounting task (DDT), cliff avoidance reaction (CAR) test, and open field. We also investigated whether treatment with the clinically effective alpha-2A adrenergic receptor agonist, guanfacine (0.3 mg/kg, i.p.), would reverse deficits observed in behavioral inhibition.

Results

Nf1^+/− mice chose a higher percentage of smaller rewards when both 10- and 20-s delays were administered compared to WT mice, suggesting Nf1^+/− mice are more impulsive. When treated with guanfacine (0.3 mg/kg, i.p.), Nf1^+/− mice exhibited decreased impulsive choice by waiting for the larger, delayed reward. Nf1^+/− mice also exhibited deficits in behavioral inhibition compared to WT mice in the CAR test by repetitively entering the outer edge of the platform where they risk falling. Treatment with guanfacine ameliorated these deficits. In addition, Nf1^+/− mice exhibited hyperactivity as increased distance was traveled compared to WT controls in the open field. This hyperactivity in Nf1^+/− mice was reduced with guanfacine pre-treatment.

Conclusions

Overall, our study confirms that Nf1^+/− mice exhibit deficits in behavioral inhibition in multiple contexts, a key feature of ADHD, and can be used as a model system to identify alterations in neural circuitry associated with symptoms of ADHD in children with NF1.

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.

Prescription Stimulants: From Cognitive Enhancement to Misuse

The nonmedical use of prescription stimulants has become increasingly pervasive among transitional age youth (TAY), aged 16 years to 26 years. Although therapeutically administered stimulants are regarded as safe and effective in TAY with attention-deficit/hyperactivity syndrome (ADHD), stimulant misuse is of concern due to prevalence, behavioral health and substance use correlates, and negative short-term and long-term outcomes. Although academic motivations primarily drive misuse, it is unclear whether prescription ADHD stimulants enhance cognition. Providers are advised to exercise precautions when prescribing ADHD medications, enhance surveillance for misuse, and screen those with misuse for ADHD and other psychopathology, executive dysfunction, and substance use disorders.

Baicalin regulates the dopamine system to control the core symptoms of ADHD

We aimed to test the therapeutic effects of baicalin on attention deficit hyperactivity disorder (ADHD) in an animal model and to explain the potential mechanism. We investigated the therapeutic effects and mechanisms of baicalin in a spontaneously hypertensive rat (SHR) model of ADHD depending on the dopamine (DA) deficit theory. In this study, fifty SHRs were randomly divided into five groups: methylphenidate (MPH), baicalin (50 mg/kg, 100 mg/kg, or 150 mg/kg), and saline-treated. Ten Wistar Kyoto (WKY) rats were used as controls. All rats were orally administered the treatment for four weeks. Motor activity, spatial learning and memory ability were assessed with the open-field and Morris water-maze tests. The mRNA and protein levels of tyrosine hydroxylase (TH), vesicular monoamine transporter 2 (VMAT2), synaptosomal-associated protein of molecular mass 25kD (SNAP25) and synataxin 1a in synaptosomes were detected with real-time polymerase chain reaction (PCR) and Western blot. In addition, DA levels were measured in the prefrontal cortex and striatum. The results indicated that both MPH and baicalin at doses of 150 mg/kg and 100 mg/kg significantly decreased the hyperactivity and improved the spatial learning memory deficit in the SHRs and increased the synaptosomal mRNA and protein levels of TH, SNAP25, VMAT2 and synataxin 1a compared with saline treatment. MPH significantly increased DA levels in both the prefrontal cortex (PFC) and striatum, while baicalin significantly increased DA levels only in the striatum. The results of the present study showed that baicalin treatment was effective for controlling the core symptoms of ADHD. Baicalin increased DA levels only in the striatum, which suggested that baicalin may target the striatum. The increased DA levels may partially be attributed to the increased mRNA and protein expression of TH, SNAP25, VMAT2, and syntaxin 1a. Therefore, these results suggested that the pharmacological effects of baicalin were associated with the synthesis, vesicular localization, and release of DA and might be effective in treating ADHD. However, further studies are required to better understand the molecular mechanisms underlying these findings.

Effects of methylphenidate on cognition and behaviour in children with neurofibromatosis type 1: a study protocol for a randomised placebo-controlled crossover trial

INTRODUCTION

Dopamine dysregulation has been identified as a key modulator of behavioural impairment in neurofibromatosis type 1 (NF1) and a potential therapeutic target. Preclinical research demonstrates reduced dopamine in the brains of genetically engineered NF1 mouse strains is associated with reduced spatial-learning and attentional dysfunction. Methylphenidate, a stimulant medication that increases dopaminergic and noradrenergic neurotransmission, rescued the behavioural and dopamine abnormalities. Although preliminary clinical trials have demonstrated that methylphenidate is effective in treating attention deficit hyperactivity disorder (ADHD) symptoms in children with NF1, its therapeutic effect on cognitive performance is unclear. The primary aim of this clinical trial is to assess the efficacy of methylphenidate for reducing attention deficits, spatial working memory impairments and ADHD symptoms in children with NF1.

METHODS AND ANALYSIS

A randomised, double-blind, placebo-controlled trial of methylphenidate with a two period crossover design. Thirty-six participants with NF1 aged 7-16 years will be randomised to one of two treatment sequences: 6 weeks of methylphenidate followed by 6 weeks of placebo or; 6 weeks of placebo followed by 6 weeks of methylphenidate. Neurocognitive and behavioural outcomes as well as neuroimaging measures will be completed at baseline and repeated at the end of each treatment condition (week 6, week 12). Primary outcome measures are omission errors on the Conners Continuous Performance Test-II (attention), between-search errors on the Spatial Working Memory task from the Cambridge Neuropsychological Test Automated Battery (spatial working memory) and the Inattentive and Hyperactivity/Impulsivity Symptom Scales on the Conners 3-Parent. Secondary outcomes will examine the effect of methylphenidate on executive functions, attention, visuospatial skills, behaviour, fine-motor skills, language, social skills and quality of life.

ETHICS AND DISSEMINATION

This trial has hospital ethics approval and the results will be disseminated through peer-reviewed publications and international conferences.

TRIAL REGISTRATION NUMBER

ACTRN12611000765921.

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.

The Mixed Opioid Receptor Antagonist Naltrexone Mitigates Stimulant-Induced Euphoria: A Double-Blind, Placebo-Controlled Trial of Naltrexone

OBJECTIVE

Supratherapeutic doses of methylphenidate activate μ-opioid receptors, which are linked to euphoria. This study assessed whether naltrexone, a mixed μ-opioid antagonist, may attenuate the euphoric effects of stimulants, thereby minimizing their abuse potential in subjects with attention-deficit/hyperactivity disorder (ADHD).

METHODS

We conducted a 6-week, double-blind, placebo-controlled, randomized clinical trial of naltrexone in adults with DSM-IV ADHD receiving open treatment with a long-acting formulation of methylphenidate (January 2013 to June 2015). Spheroidal Oral Drug Absorption System methylphenidate (SODAS-MPH) was administered twice daily, was titrated to ~1 mg/kg/d over 3 weeks, and was continued for 3 additional weeks depending on response and adverse effects. Subjects were adults with ADHD preselected for having experienced euphoria with an oral test dose of 60 mg of immediate-release methylphenidate (IR-MPH). The primary outcome measure was Question 2 (Liking a Drug Effect) on the Drug Rating Questionnaire, Subject version, which was assessed after oral test doses of 60 mg of IR-MPH were administered after the third and sixth weeks of treatment with SODAS-MPH.

RESULTS

Thirty-seven subjects who experienced stimulant-induced (mild) euphoria at a baseline visit were started in the open trial of SODAS-MPH and randomized to naltrexone 50 mg/d or placebo. Thirty-one subjects completed through week 3, and 25 completed through week 6. Naltrexone significantly diminished the euphoric effect of IR-MPH during the heightened-risk titration phase (primary outcome; first 3 weeks) (χ² = 5.07, P = .02) but not the maintenance phase (weeks 4-6) (χ² = 0.22, P = .64) of SODAS-MPH treatment.

CONCLUSIONS

Preclinical findings are extended to humans showing that naltrexone may mitigate stimulant-associated euphoria. Our findings provide support for further studies combining opioid receptor antagonists with stimulants to reduce abuse potential.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT01673594.

Disrupted Glutamatergic Transmission in Prefrontal Cortex Contributes to Behavioral Abnormality in an Animal Model of ADHD

Spontaneously hypertensive rats (SHR) are the most widely used animal model for the study of attention deficit hyperactivity disorder (ADHD). Here we sought to reveal the neuronal circuits and molecular basis of ADHD and its potential treatment using SHR. Combined electrophysiological, biochemical, pharmacological, chemicogenetic, and behavioral approaches were utilized. We found that AMPAR-mediated synaptic transmission in pyramidal neurons of prefrontal cortex (PFC) was diminished in SHR, which was correlated with the decreased surface expression of AMPAR subunits. Administration of methylphenidate (a psychostimulant drug used to treat ADHD), which blocks dopamine transporters and norepinephrine transporters, ameliorated the behavioral deficits of adolescent SHR and restored AMPAR-mediated synaptic function. Activation of PFC pyramidal neurons with a CaMKII-driven Gq-coupled designer receptor exclusively activated by designer drug also led to the elevation of AMPAR function and the normalization of ADHD-like behaviors in SHR. These results suggest that the disrupted function of AMPARs in PFC may underlie the behavioral deficits in adolescent SHR and enhancing PFC activity could be a treatment strategy for ADHD.

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.

Methylphenidate for Treating ADHD: A Naturalistic Clinical Study of Methylphenidate Blood Concentrations in Children and Adults With Optimized Dosage

Background

Methylphenidate (MPH), along with behavioral and psychosocial interventions, is the first-line medication to treat attention-deficit hyperactivity disorder (ADHD) in Sweden. The dose of MPH for good symptom control differs between patients. However, studies of MPH concentration measurement in ADHD treatment are limited.

Objective

To describe blood and oral fluid (OF) concentrations of MPH after administration of medication in patients with well-adjusted MPH treatment for ADHD, and to identify the most suitable matrix for accurate MPH concentration during treatment.

Methods

Patients were recruited from Child and Adolescent Psychiatry (CAP), General Psychiatry (GP), and the Department of Dependency (DD). Blood and OF samples were collected in the morning before MPH administration as well as 1 and 6 h after administration of the prescribed morning dose of MPH.

Results

Fifty-nine patients aged between 9 and 69 years, 76 % males. The daily dose of MPH varied from 18 to 180 mg, but the median daily dose per body weight was similar, approximately 1.0 mg/kg body weight. The median MPH concentration in blood 1 and 6 h after the morning dose was 5.4 and 9.3 ng/mL, respectively. Highly variable OF-to-blood ratios for MPH were found at all time points for all three groups.

Conclusions

Weight is a reliable clinical parameter for optimal dose titration. Otherwise, MPH blood concentration might be used for individual dose optimization and for monitoring of the prescribed dose. Relying only on the outcome in OF cannot be recommended for evaluation of accurate MPH concentrations for treatment monitoring.

Tyrosinase-Based Biosensors for Selective Dopamine Detection

A novel tyrosinase-based biosensor was developed for the detection of dopamine (DA). For increased selectivity, gold electrodes were previously modified with cobalt (II)-porphyrin (CoP) film with electrocatalytic activity, to act both as an electrochemical mediator and an enzyme support, upon which the enzyme tyrosinase (Tyr) was cross-linked. Differential pulse voltammetry was used for electrochemical detection and the reduction current of dopamine-quinone was measured as a function of dopamine concentration. Our experiments demonstrated that the presence of CoP improves the selectivity of the electrode towards dopamine in the presence of ascorbic acid (AA), with a linear trend of concentration dependence in the range of 2–30 µM. By optimizing the conditioning parameters, a separation of 130 mV between the peak potentials for ascorbic acid AA and DA was obtained, allowing the selective detection of DA. The biosensor had a sensitivity of 1.22 ± 0.02 µA·cm⁻²·µM⁻¹ and a detection limit of 0.43 µM. Biosensor performances were tested in the presence of dopamine medication, with satisfactory results in terms of recovery (96%), and relative standard deviation values below 5%. These results confirmed the applicability of the biosensors in real samples such as human urine and blood serum.

An Evaluation Approach for the Performance of Dosing Regimens in Attention-Deficit/Hyperactivity Disorder Treatment

OBJECTIVE

Stimulant medications, with methylphenidate as the main agent, are the most prescribed for the treatment of attention-deficit/hyperactivity disorder. Nevertheless, real challenges still remain for clinicians concerned with adaptation of the therapeutic regimens, in terms of doses and timing, to children's daily activities. The aim of this study was to optimize short-acting methylphenidate regimens according to specific children's needs by evaluating the performance of a particular regimen through a web-based application.

METHODS

In this article, accounting for day-to-day children's activities and using up-to-date pharmacokinetic knowledge of methylphenidate, we propose a computational approach for the identification of the most suitable dosing regimens of immediate-release formulations of methylphenidate based on constraints on drug concentration and time frame of activities, defined through therapeutic boxes. To assess the performance of these regimens, time- and concentration-based therapeutic indicators, as well as a roller coaster effect, are proposed.

RESULTS

A web-based interface that can serve as an educational tool for clinicians and patients has been developed based on the proposed approach for the evaluation of dosing regimens. Comparison of those optimal regimens identified by our method with the well-accepted regimens defined in the NIMH Collaborative Multisite Multimodal Treatment study of Children with attention-deficit/hyperactivity disorder indicates that there is still room for improvement in the current practice especially for the last dose administration to avoid side effects such as sleep disturbance.

CONCLUSION

The developed approach and its associated web-based interface provide an efficient way to evaluate and adapt the methylphenidate regimens to children's daily activities. In addition, this approach could be used as proof of concept to further implement combination of short- and long-acting methylphenidate.

Contribution of Clinical Neuroimaging to the Understanding of the Pharmacology of Methylphenidate

Methylphenidate (MPH) is currently the most widely used molecule in the pharmacologic treatment of attention-deficit hyperactivity disorder (ADHD). Although experience of its application now extends over several decades, its psychotropic nature, prolonged use in children, and chemical relation to amphetamines still raise doubts in the minds of prescribers and the families of the patients. Brain imaging has shed considerable light on the neuropharmacology of MPH. The two main in vivo neuroimaging techniques are positron-emission tomography (PET) and magnetic resonance imaging (MRI), and these can be applied in both animal models and humans. The present review seeks to show how human molecular and functional imaging has contributed to determining not only the molecular targets of MPH, and the action kinetics of the various pharmaceutical forms available, but also the connectivity and brain networks activated by treatment. We also discuss the perspectives opened up by new hybrid PET-MRI techniques that enable multimodal tracking of the impact of methylphenidate on neurotransmission.

The role of impulsivity in psychostimulant- and stress-induced dopamine release: Review of human imaging studies

Drug addiction is a debilitating disorder and its pivotal problem is the high relapse rate. To solve this problem, the aim is to prevent people from becoming addicted in the first place. One of the key questions that is still unanswered is why some people become addicted to drugs and others, who take drugs regularly, do not. In recent years extensive research has been done to untangle the many factors involved in this disorder. Here, we review some of the factors that are related to dopamine, i.e., impulsivity and stress (hormones), and aim to integrate this into a neurobiological model. Based on this, we draw two conclusions: (1) in order to understand the transition from recreational drug use to addiction, we need to focus more on these recreational users; and (2) research should be aimed at finding therapies that can restore inhibitory control/frontal functioning and improve stress resiliency in addicts.

Nonmedical Stimulant Use in College Students: Association With Attention-Deficit/Hyperactivity Disorder and Other Disorders

OBJECTIVE

The nonmedical use of stimulants (misuse) in the college setting remains of utmost public health and clinical concern. The objective of this study was to evaluate comprehensively the characteristics of college students who misused stimulants, attending to rates of attention-deficit/hyperactivity disorder (ADHD), other psychopathology, and substance use disorders.

METHODS

The data presented are from a cross-sectional study of college students who misused prescription stimulant medications (not including cocaine or methamphetamine) and controls (college students without stimulant misuse). Between May 2010 and May 2013, college students were assessed blindly for psychopathology and substance use disorder by way of Structured Clinical Interview for DSM-IV-TR Axis I Disorders, Research Version, Patient Edition (SCID-I/P) and completion of self-report questionnaires.

RESULTS

The analysis included 198 controls (mean ± SD age = 20.7 ± 2.6 years) and 100 stimulant misusers (20.7 ± 1.7 years). Misusers, when compared to controls, were more likely to endorse alcohol, drug, alcohol + drug, and any substance use disorder (all P values < .01). When a subset of stimulant misusers (n = 58) was examined, 67% had a full or subthreshold prescription stimulant use disorder. Misusers also had higher rates of conduct disorder (10% vs 3%; P = .02) and ADHD (including subthreshold cases; 27% vs 16%; P = .02) in addition to lower Global Assessment of Functioning score (P < .01). Higher rates of misuse of immediate-release—relative to extended-release—stimulants were reported.

CONCLUSIONS

Our data suggest that, compared to controls, college students who misuse stimulant medications are more likely to have ADHD, conduct disorder, stimulant and other substance use disorder, and overall dysfunction.

Methylphenidate Causes Behavioral Impairments and Neuron and Astrocyte Loss in the Hippocampus of Juvenile Rats

Although the use, and misuse, of methylphenidate is increasing in childhood and adolescence, there is little information about the consequences of this psychostimulant chronic use on brain and behavior during development. The aim of the present study was to investigate hippocampus biochemical, histochemical, and behavioral effects of chronic methylphenidate treatment to juvenile rats. Wistar rats received intraperitoneal injections of methylphenidate (2.0 mg/kg) or an equivalent volume of 0.9 % saline solution (controls), once a day, from the 15th to the 45th day of age. Results showed that chronic methylphenidate administration caused loss of astrocytes and neurons in the hippocampus of juvenile rats. BDNF and pTrkB immunocontents and NGF levels were decreased, while TNF-α and IL-6 levels, Iba-1 and caspase 3 cleaved immunocontents (microglia marker and active apoptosis marker, respectively) were increased. ERK and PKCaMII signaling pathways, but not Akt and GSK-3β, were decreased. SNAP-25 was decreased after methylphenidate treatment, while GAP-43 and synaptophysin were not altered. Both exploratory activity and object recognition memory were impaired by methylphenidate. These findings provide additional evidence that early-life exposure to methylphenidate can have complex effects, as well as provide new basis for understanding of the biochemical and behavioral consequences associated with chronic use of methylphenidate during central nervous system development.

Individual differences in timing of peak positive subjective responses to d-amphetamine: Relationship to pharmacokinetics and physiology

Rate of delivery of psychostimulants has been associated with their positive euphoric effects and potential addiction liability. However, information on individual differences in onset of d-amphetamine's effects remains scarce. We examined individual differences in the time to peak subjective and physiological effects and the pharmacokinetics/pharmacodynamics of oral d-amphetamine. We considered two independent studies that used different dosing regimens where subjects completed the drug effects questionnaire at multiple time points post d-amphetamine. Based on the observation of distinct individual differences in time course of drug effects questionnaire "feel", "high", and "like" ratings (DEQH+L+F) in Study 1, subjects in both studies were categorized as early peak responders (peak within 60 minutes), late peak responders (peak > 60 minutes) or nonresponders; 20-25% of participants were categorized as early peak responders, 50-55% as late peak responders and 20-30% as nonresponders. Physiological (both studies) and plasma d-amphetamine (Study 1) were compared among these groups. Early peak responders exhibited an earlier rise in plasma d-amphetamine levels and more sustained elevation in heart rate compared to late peak responders. The present data illustrate the presence of significant individual differences in the temporal pattern of responses to oral d-amphetamine, which may contribute to heightened abuse potential.

Absorption Differences between Immediate-Release Dexmethylphenidate and dl-Methylphenidate

The postulate that twice the milligram/kilogram dose of dl-methylphenidate (dl-MPH) would result in equal exposure to d-MPH compared with half that milligram/kilogram dose of the chiral switch product dexmethylphenidate (d-MPH) was tested. Using a randomized, crossover study design, 12 men and 12 women received either immediate-release (IR) dl-MPH (0.3 mg/kg) or IR d-MPH (0.15 mg/kg). Relative bioavailability comparisons included partial area under the plasma concentration-time curves (pAUC0-3 h) for d-MPH. The pAUC0-3 h is a new regulatory metric presently only required for bioequivalence testing of a specific dl-MPH modified-release product. The geometric mean ratios for both the Cmax and area under the plasma concentration-time curve (AUC0-∞) were within the 90% confidence interval (CI) regulatory range of 0.8-1.25, indicating that these two drugs were bioequivalent in terms of d-MPH. However, the pAUC0-3 h geometric mean ratio for d-MPH after IR dl-MPH versus IR d-MPH was 0.76 (P < 0.001; 90% CI, 0.67-0.87), showing significantly less early exposure to the d-isomer than IR d-MPH. The 1-hour d-MPH concentration after dl-MPH was 56% of that after the enantiopure drug. The maximum d-MPH plasma concentration (Cmax) for dl-MPH was also significantly lower for dl-MPH (P < 0.05; CI, 1.02-1.19), whereas the AUC0-∞ ratio of 0.89 was not significantly different (P = 0.21; CI, 0.98-1.13). The AUC0-3 h difference reported here points to the potential limitations of using bioequivalence for sound predictions of dose-response relationships. Knowledge of the greater early exposure to d-MPH after the pure d-isomer drug compared with the racemate may contribute to drug individualization/optimization in the treatment of attention deficit hyperactivity disorder.

Comparative Ethanol-Induced Potentiation of Stimulatory Responses to Dexmethylphenidate Versus Methylphenidate

The potentiation of positive subjective responses to immediate-release dexmethylphenidate (d-MPH) or dl-methylphenidate (dl-MPH) by ethanol was investigated over the time course of maximal drug exposure after a single dose. In a 4-way, randomized, crossover study design, 12 men and 12 women normal volunteers received d-MPH (0.15 mg/kg) or dl-MPH (0.3 mg/kg) with or without ethanol (0.6 g/kg). Serial visual analog scales were used as surrogates for drug abuse liability ("high," "good," "like," "stimulated," and "any drug effect"). Combining pure d-MPH with ethanol significantly (P < 0.005) increased the area under the effect curves (AUC(0-5.25h)) of all 5 subscales. The dl-MPH-ethanol combination significantly (P < 0.05) increased these AUCs with the exception of like (P = 0.08). Effects of the pure d-MPH-ethanol combination exhibited delayed potentiation relative to dl-MPH-ethanol. A pharmacokinetic interaction between the l-isomer of dl-MPH and ethanol has previously been shown to increase early exposure to d-MPH. Administration of the pure isomer d-MPH precludes this absorption phase pharmacokinetic interaction with ethanol. This notwithstanding, the pure d-MPH-ethanol combination resulted in comparable, if not greater, cumulative stimulant potentiation than the dl-MPH-ethanol combination. These findings provide evidence of a pharmacodynamic component to d-MPH-ethanol synergistic interactions and carry implications for the rational drug individualization in the treatment of attention-deficit/hyperactivity disorder.

Antidepressant drug development: Focus on triple monoamine reuptake inhibition

Many patients with major depressive disorder (MDD) only partially respond, and some have no clinically meaningful response, to current widely used antidepressant drugs. Due to the purported role of dopamine in the pathophysiology of depression, triple-reuptake inhibitors (TRIs) that simultaneously inhibit serotonin (5-HT), norepinephrine (NE) and dopamine reuptake could be a useful addition to the armamentarium of treatments for MDD. A TRI should more effectively activate mesolimbic dopamine-related reward-networks, restore positive mood and reduce potent 5-HT reuptake blockade associated "hypodopaminergic" adverse effects of decreased libido, weight gain and "blunting" of emotions. On the other hand, dopaminergic effects raise concern over abuse liability and TRIs may have many of the cardiovascular effects associated with NET inhibition. Several clinical development programs for potential TRI antidepressants have failed to demonstrate significantly greater efficacy than placebo or standard of care. Successful late-stage clinical development of a TRI is more likely if experimental research studies in the target population of depressed patients have demonstrated target engagement that differentially and dose-dependently improves assessments of reward-network dysfunction relative to existing antidepressants. TRI treatment could be individualized on the basis of predictive markers such as the burden of decreased positive mood symptoms and/or neuroimaging evidence of reward network dysfunction. This review focuses on how the next generation of monoamine-based treatments could be efficiently developed to address unmet medical need in MDD.

Misuse of Methylphenidate

This chapter reviews methylphenidate misuse, abuse, dependence, diversion, and malingering associated with its use as a prescription medication for attention-deficit/hyperactivity disorder and the nonmedical use linked to its stimulant effects. Methylphenidate-induced regional elevations in brain dopamine appear to be integral to both efficacy in attention-deficit/hyperactivity disorder and potential for abuse, raising potential concerns for drug safety and prescription drug diversion costs associated with nonmedical use. Regardless, methylphenidate is an important treatment option, and detecting malingering for the purpose of illicit access to methylphenidate for subsequent misuse or diversion is a difficult challenge. Also discussed are the effects of methylphenidate in patients with comorbid substance use disorder and the potential linkage of methylphenidate use with subsequent substance abuse. The current data suggest that methylphenidate misuse and diversion are common health-care problems with a stimulant prescription drug diversion prevalence of approximately 5-10 % of high school students and 5-35 % of college students. The effectiveness and speed of action of methylphenidate are deemed desirable to enhance attention and focus performance for activities such as studying for exams, but methylphenidate is also misused recreationally. These data suggest a need for close screening and therapeutic monitoring of methylphenidate use in the treatment of attention-deficit/hyperactivity disorder.

Methylphenidate exerts dose-dependent effects on glutamate receptors and behaviors

BACKGROUND

Methylphenidate (MPH), a psychostimulant drug used to treat attention-deficit/hyperactivity disorder, produces the effects of increasing alertness and improving attention. However, misuse of MPH has been associated with an increased risk of aggression and psychosis. We sought to determine the molecular mechanism underlying the complex actions of MPH.

METHODS

Adolescent (4-week-old) rats were given one injection of MPH at different doses. The impact of MPH on glutamatergic signaling in pyramidal neurons of prefrontal cortex was measured. Behavioral changes induced by MPH were also examined in parallel.

RESULTS

Administration of low-dose (.5 mg/kg) MPH selectively potentiated N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory postsynaptic currents (EPSCs) via adrenergic receptor activation, whereas high-dose (10 mg/kg) MPH suppressed both NMDAR-mediated and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor-mediated EPSCs. The dual effects of MPH on EPSCs were associated with bidirectional changes in the surface level of glutamate receptor subunits. Behavioral tests also indicated that low-dose MPH facilitated prefrontal cortex-mediated temporal order recognition memory and attention. Animals injected with high-dose MPH exhibited significantly elevated locomotive activity. Inhibiting the function of synaptosomal-associated protein 25, a key SNARE protein involved in NMDAR exocytosis, blocked the increase of NMDAR-mediated EPSCs by low-dose MPH. In animals exposed to repeated stress, administration of low-dose MPH effectively restored NMDAR function and temporal order recognition memory via a mechanism dependent on synaptosomal-associated protein 25.

CONCLUSIONS

These results provide a potential mechanism underlying the cognitive-enhancing effects of low-dose MPH as well as the psychosis-inducing effects of high-dose MPH.

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.

Ethylphenidate as a selective dopaminergic agonist and methylphenidate-ethanol transesterification biomarker

We review the pharmaceutical science of ethylphenidate (EPH) in the contexts of drug discovery, drug interactions, biomarker for dl-methylphenidate (MPH)-ethanol exposure, potentiation of dl-MPH abuse liability, contemporary "designer drug," pertinence to the newer transdermal and chiral switch MPH formulations, as well as problematic internal standard. d-EPH selectively targets the dopamine transporter, whereas d-MPH exhibits equipotent actions at dopamine and norepinephrine transporters. This selectivity carries implications for the advancement of tailored attention-deficit/hyperactivity disorder (ADHD) pharmacotherapy in the era of genome-based diagnostics. Abuse of dl-MPH often involves ethanol coabuse. Carboxylesterase 1 enantioselectively transesterifies l-MPH with ethanol to yield l-EPH accompanied by significantly increased early exposure to d-MPH and rapid potentiation of euphoria. The pharmacokinetic component of this drug interaction can largely be avoided using dexmethylphenidate (dexMPH). This notwithstanding, maximal potentiated euphoria occurs following dexMPH-ethanol. C57BL/6 mice model dl-MPH-ethanol interactions: an otherwise depressive dose of ethanol synergistically increases dl-MPH stimulation; a substimulatory dose of dl-MPH potentiates a low, stimulatory dose of ethanol; ethanol elevates blood, brain, and urinary d-MPH concentrations while forming l-EPH. Integration of EPH preclinical neuropharmacology with clinical studies of MPH-ethanol interactions provides a translational approach toward advancement of ADHD personalized medicine and management of comorbid alcohol use disorder.

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.

Functional effects of dopamine transporter gene genotypes on in vivo dopamine transporter functioning: a meta-analysis

Much psychiatric genetic research has focused on a 40-base pair variable number of tandem repeats (VNTR) polymorphism located in the 3'-untranslated region (3'UTR) of the dopamine active transporter (DAT) gene (SLC6A3). This variant produces two common alleles with 9- and 10-repeats (9R and 10R). Studies associating this variant with in vivo DAT activity in humans have had mixed results. We searched for studies using positron emission tomography (PET) or single-photon emission computed tomography (SPECT) to evaluate this association. Random effects meta-analyses assessed the association of the 3'UTR variant with DAT activity. We also evaluated heterogeneity among studies and evidence for publication bias. We found twelve studies comprising 511 subjects, 125 from PET studies and 386 from SPECT studies. The PET studies provided highly significant evidence that the 9R allele was associated with increased DAT activity in human adults. The SPECT studies were highly heterogeneous. As a group, they suggested no association between the 3'UTR polymorphism and DAT activity. When the analysis was limited to the most commonly used ligand, [123I]β-CIT, stratification by affection status dramatically reduced heterogeneity and revealed a significant association of the 9R allele with increased DAT activity for healthy subjects. In humans, the 9R allele of the 3'UTR polymorphism of SLC6A3 regulates dopamine activity in the striatal brain regions independent of the presence of neuropsychiatric illness. Differences in study methodology account for the heterogeneous results across individual studies.