Disposition and metabolic fate of atomoxetine hydrochloride: the role of CYP2D6 in human disposition and metabolism

Population Pharmacokinetic Analysis of Atomoxetine and its Metabolites in Children and Adolescents with Attention-Deficit/Hyperactivity Disorder

Atomoxetine (ATX) is a non-stimulant used to treat attention-deficit/hyperactivity disorder (ADHD) and systemic exposure is highly variable due to polymorphic cytochrome P450 2D6 (CYP2D6) activity. The objective of this study was to characterize the time course of ATX and metabolites (4-hydroxyatomoxetine (4-OH); N-desmethylatomoxetine (NDA); and 2-carboxymethylatomoxetine (2-COOH)) exposure following oral ATX dosing in children with ADHD to support individualized dosing. A nonlinear mixed-effect modeling approach was used to analyze ATX, 4-OH, and NDA plasma and urine, and 2-COOH urine profiles obtained over 24-72 hours from children with ADHD (n = 23) following a single oral ATX dose. Demographics and CYP2D6 activity score (AS) were evaluated as covariates. Simulations were performed to explore the ATX dosing in subjects with various CYP2D6 AS. A simultaneous pharmacokinetic modeling approach was used in which a model for ATX, 4-OH, and NDA in plasma and urine, and 2-COOH in urine was developed. Plasma ATX, 4-OH, and NDA were modeled using two-compartment models with first-order elimination. CYP2D6 AS was a significant determinant of ATX apparent oral clearance (CL/F), fraction metabolized to 4-OH, and systemic exposure of NDA. CL/F of ATX varied almost 7-fold across the CYP2D6 AS groups: AS 2: 20.02 L/hour; AS 1: 19.00 L/hour; AS 0.5: 7.47 L/hour; and AS 0: 3.10 L/hour. The developed model closely captures observed ATX, 4-OH, and NDA plasma and urine, and 2-COOH urine profiles. Application of the model shows the potential for AS-based dosing recommendations for improved individualized dosing.

Personalizing atomoxetine dosing in children with ADHD: what can we learn from current supporting evidence

PURPOSE

There is marked heterogeneity in treatment response of atomoxetine in patients with attention deficit/hyperactivity disorder (ADHD), especially for the pediatric population. This review aims to evaluate current evidence to characterize the dose-exposure relationship, establish clinically relevant metrics for systemic exposure to atomoxetine, define a therapeutic exposure range, and to provide a dose-adaptation strategy before implementing personalized dosing for atomoxetine in children with ADHD.

METHODS

A comprehensive search was performed across electronic databases (PubMed and Embase) covering the period of January 1, 1985 to July 10, 2022, to summarize recent advances in the pharmacokinetics, pharmacogenomics/pharmacogenetics (PGx), therapeutic drug monitoring (TDM), physiologically based pharmacokinetics (PBPK), and population pharmacokinetics (PPK) of atomoxetine in children with ADHD.

RESULTS

Some factors affecting the pharmacokinetics of atomoxetine were summarized, including food, CYP2D6 and CYP2C19 phenotypes, and drug‒drug interactions (DDIs). The association between treatment response and genetic polymorphisms of genes encoding pharmacological targets, such as norepinephrine transporter (NET/SLC6A2) and dopamine β hydroxylase (DBH), was also discussed. Based on well-developed and validated assays for monitoring plasma concentrations of atomoxetine, the therapeutic reference range in pediatric patients with ADHD proposed by several studies was summarized. However, supporting evidence on the relationship between systemic atomoxetine exposure levels and clinical response was far from sufficient.

CONCLUSION

Personalizing atomoxetine dosage may be even more complex than anticipated thus far, but elucidating the best way to tailor the non-stimulant to a patient's individual need will be achieved by combining two strategies: detailed research in linking the pharmacokinetics and pharmacodynamics in pediatric patients, and better understanding in nature and causes of ADHD, as well as environmental stressors.

Physiologically Based Pharmacokinetic Modeling to Describe the CYP2D6 Activity Score-Dependent Metabolism of Paroxetine, Atomoxetine and Risperidone

The cytochrome P450 2D6 (CYP2D6) genotype is the single most important determinant of CYP2D6 activity as well as interindividual and interpopulation variability in CYP2D6 activity. Here, the CYP2D6 activity score provides an established tool to categorize the large number of CYP2D6 alleles by activity and facilitates the process of genotype-to-phenotype translation. Compared to the broad traditional phenotype categories, the CYP2D6 activity score additionally serves as a superior scale of CYP2D6 activity due to its finer graduation. Physiologically based pharmacokinetic (PBPK) models have been successfully used to describe and predict the activity score-dependent metabolism of CYP2D6 substrates. This study aimed to describe CYP2D6 drug–gene interactions (DGIs) of important CYP2D6 substrates paroxetine, atomoxetine and risperidone by developing a substrate-independent approach to model their activity score-dependent metabolism. The models were developed in PK-Sim^®, using a total of 57 plasma concentration–time profiles, and showed good performance, especially in DGI scenarios where 10/12, 5/5 and 7/7 of DGI AUC_last ratios and 9/12, 5/5 and 7/7 of DGI C_max ratios were within the prediction success limits. Finally, the models were used to predict their compound’s exposure for different CYP2D6 activity scores during steady state. Here, predicted DGI AUC_ss ratios were 3.4, 13.6 and 2.0 (poor metabolizers; activity score = 0) and 0.2, 0.5 and 0.95 (ultrarapid metabolizers; activity score = 3) for paroxetine, atomoxetine and risperidone active moiety (risperidone + 9-hydroxyrisperidone), respectively.

Characterisation of seven medications approved for attention-deficit/hyperactivity disorder using in vitro models of hepatic metabolism

The metabolism of most medications approved for the treatment of attention deficit/hyperactivity disorder (ADHD) is not fully understood.In vitro studies using cryopreserved, plated human hepatocytes (cPHHs) and pooled human liver microsomes (HLMs) were performed to more thoroughly characterise the metabolism of several ADHD medications.The use of enzyme-specific chemical inhibitors indicated a role for CYP2D6 in atomoxetine (ATX) metabolism, and roles for CYP3A4/5 in guanfacine (GUA) metabolism.The 4-hydroxy-atomoxetine and N-desmethyl-atomoxetine pathways represented 98.4% and 1.5% of ATX metabolism in cPHHs, respectively. The 3-OH-guanfacine pathway represented at least 2.6% of GUA metabolism in cPHHs, and 71% in HLMs.The major metabolising enzyme for methylphenidate (MPH) and dexmethylphenidate (dMPH) could not be identified using these methods because these compounds were too unstable. Hydrolysis of these medications was spontaneous and did not require the presence of protein to occur.Clonidine (CLD), amphetamine (AMPH), and dextroamphetamine (dAMPH) did not deplete substantially in cPHHs nor HLMs, suggesting that these compounds may not undergo considerable hepatic metabolism. The major circulating metabolites of AMPH and dAMPH (benzoic acid and hippuric acid) were not observed in either system, and therefore could not be characterised. Additionally, inhibition experiments suggested a very minimal role for CYP2D6 in CLD and AMPH metabolism.

The Development of a PBPK Model for Atomoxetine Using Levels in Plasma, Saliva and Brain Extracellular Fluid in Patients with Normal and Deteriorated Kidney Function

BACKGROUND

Atomoxetine is a treatment for attention-deficit hyperactivity disorder. It inhibits Norepinephrine Transporters (NET) in the brain. Renal impairment can reduce hepatic CYP2D6 activity and atomoxetine elimination which may increase its body exposure. Atomoxetine can be secreted in saliva.

OBJECTIVE

The objective of this work was to test the hypothesis that atomoxetine saliva levels (sATX) can be used to predict ATX brain Extracellular Fluid (bECF) levels and their pharmacological effects in healthy subjects and those with End-Stage Renal Disease (ESRD).

METHODS

The pharmacokinetics of atomoxetine after intravenous administration to rats with chemically induced acute and chronic renal impairments were investigated. A physiologically-based pharmacokinetic (PBPK) model was built and verified in rats using previously published measured atomoxetine levels in plasma and brain tissue. The rat PBPK model was then scaled to humans and verified using published measured atomoxetine levels in plasma, saliva, and bECF.

RESULTS

The rat PBPK model predicted the observed reduced atomoxetine clearance due to renal impairment in rats. The PBPK model predicted atomoxetine exposure in human plasma, sATX and bECF. Additionally, it predicted that ATX bECF levels needed to inhibit NET are achieved at 80 mg dose. In ESRD patients, the developed PBPK model predicted that the previously reported 65% increase in plasma exposure in these patients can be associated with a 63% increase in bECF. The PBPK simulations showed that there is a significant correlation between sATX and bECF in human.

CONCLUSION

Saliva levels can be used to predict atomoxetine pharmacological response.

Therapeutic drug monitoring of atomoxetine in children and adolescents with attention-deficit/ hyperactivity disorder: a naturalistic study

The selective norepinephrine reuptake inhibitor atomoxetine is potentially among the first-line pharmacotherapy options for ADHD. Therapeutic drug monitoring (TDM) with the quantification and interpretation of atomoxetine serum concentrations is used to determine an individual dose followed by an optimal effectiveness and minimal side effects. The aim of this retrospective pharmacokinetic–pharmacodynamic analysis was to derive age-appropriate recommendations for the implementation of TDM to improve the efficacy and tolerability of atomoxetine in children and adolescents. Using the analytical method of high-performance liquid chromatography with UV detection, 94 serum concentrations of 74 patients between 6 and 21 years of age were determined. Therapeutic effectiveness and side effects were evaluated according to the categories “low”, “moderate”, and “significant”. As part of TDM, a time interval with maximum concentrations of 1–3 h after the administration of atomoxetine was determined for blood sampling. In this time interval, a significant correlation between the weight-normalized dose and the serum concentrations was found. The efficacy as well as the tolerability proved to be mainly moderate or significant. A preliminary therapeutic reference range was between 100 and 400 ng/ml. Naturalistic studies have limitations. Therefore, and due to a limited study population, the results have to be regarded as preliminary observations that must be confirmed in further studies. The preliminary therapeutic reference range for children and adolescents proved to be narrower than the reference range for adult patients. However, due to good efficacy and tolerability an exact reference range remained difficult to determine.

The Pharmacogenetic Impact on the Pharmacokinetics of ADHD Medications

ADHD is a common condition in both children and adults. The most prescribed medications for the treatment of ADHD include methylphenidate, mixed amphetamine salts, atomoxetine, guanfacine, and clonidine. While each of these medications have their own distinct pharmacokinetic profile, the extent to which pharmacogenetics effects their pharmacokinetic parameters is best described in atomoxetine, followed by methylphenidate. Atomoxetine is predominantly metabolized by cytochrome p450 2D6 (CYP2D6), while methylphenidate is metabolized by carboxylesterase 1 (CES1). Both CYP2D6 and CES1 have multiple variants resulting in varying levels of enzyme activity; however, to date, the functional consequence of variants and alleles for CYP2D6 is better characterized as compared to CES1. Regarding CYP2D6, individuals who are poor metabolizers prescribed atomoxetine experience up to ten-fold higher exposure as compared to normal metabolizers at comparable dosing. Additionally, individuals prescribed methylphenidate with the rs71647871 variant may experience up to 2.5-fold higher exposure as compared to those without. Having this pharmacogenetic information available may aid clinicians and patients when choosing medications and doses to treat ADHD.

Metabolic Activation of Atomoxetine Mediated by Cytochrome P450 2D6

Atomoxetine (ATX) is a neurological drug widely used for the treatment of attention deficit-hyperactivity disorder. Liver injury has been documented in patients administered ATX. The mechanism of ATX's toxic action is less clear. This study is aimed to characterize reactive metabolites of ATX in vitro and in vivo to assist our understanding of the mechanisms of ATX hepatotoxicity. A hydroxylated metabolite, along with an O-dealkylation metabolite, was found in ATX-supplemented rat liver microsome incubations. Additionally, two glutathione (GSH) conjugates and two N-acetylcysteine (NAC) conjugates were observed in rat liver microsome incubations containing ATX, NADPH, and GSH or NAC. The corresponding GSH conjugates and NAC conjugates were found in bile and urine of ATX-treated rats, respectively. Recombinant P450 enzyme incubation study demonstrated that CYP2D6 dominated the metabolic activation of ATX. The insights gained from this study may be of assistance to illuminate the mechanisms of ATX-induced hepatotoxicity.

Determination of atomoxetine levels in human plasma using LC-MS/MS and clinical application to Chinese children with ADHD based on CPIC guidelines

The Clinical Pharmacogenetic Implementation Consortium (CPIC) guidelines for personalized atomoxetine therapy are based on the CYP2D6 genotype information and the peak plasma concentrations of atomoxetine. Therefore, a highly rapid, sensitive, and reproducible method is critical for the clinical implementation of the guidelines. In this study, an LC-MS/MS approach was developed and validated for the determination of atomoxetine levels in human plasma using atomoxetine-d3 as the internal standard. Samples were prepared by simple protein precipitation method with MeOH. The analyte was separated using a Kinetex C18 column (2.1 mm × 50 mm, 2.6 μm, Phenomenex) with a flow rate of 0.25 mL min^-1, using a gradient elution. A MeOH and water solution containing 5 mM ammonium acetate and 0.1 mM formic acid (pH 6.26) was used as the mobile phase and successfully solved the problem of inconsistent retention time between the plasma samples and the solution samples of atomoxetine. Detection was performed under positive-electrospray-ion multiple reaction-monitoring mode using the 256.4 → 43.8 and 259.3 → 47.0 transitions for atomoxetine and atomoxetine-d3, respectively. Linearity was achieved using an extremely wide range, from 0.500 to 2000 ng mL^-1 in plasma. The intra- and inter-batch precision and accuracy, dilution accuracy, recovery, and stability of the method were all within the acceptable limits and no matrix effect was observed. With a complex needle wash solution containing ACN : MeOH : isopropanol : H₂O (4 : 4:1 : 1, v/v/v/v), carryover contamination was eliminated successfully. This method was successfully implemented on pediatric patients with attention-deficit/hyperactivity disorder and provided valuable information to enable clinicians to do dose selection and titration.

Metabolic N-Dealkylation and N-Oxidation as Elucidators of the Role of Alkylamino Moieties in Drugs Acting at Various Receptors

Metabolic reactions that occur at alkylamino moieties may provide insight into the roles of these moieties when they are parts of drug molecules that act at different receptors. N-dealkylation of N,N-dialkylamino moieties has been associated with retaining, attenuation or loss of pharmacologic activities of metabolites compared to their parent drugs. Further, N-dealkylation has resulted in clinically used drugs, activation of prodrugs, change of receptor selectivity, and providing potential for developing fully-fledged drugs. While both secondary and tertiary alkylamino moieties (open chain aliphatic or heterocyclic) are metabolized by CYP450 isozymes oxidative N-dealkylation, only tertiary alkylamino moieties are subject to metabolic N-oxidation by Flavin-containing monooxygenase (FMO) to give N-oxide products. In this review, two aspects will be examined after surveying the metabolism of representative alkylamino-moieties-containing drugs that act at various receptors (i) the pharmacologic activities and relevant physicochemical properties (basicity and polarity) of the metabolites with respect to their parent drugs and (ii) the role of alkylamino moieties on the molecular docking of drugs in receptors. Such information is illuminative in structure-based drug design considering that fully-fledged metabolite drugs and metabolite prodrugs have been, respectively, developed from N-desalkyl and N-oxide metabolites.

Synergistic efficacy and diminished adverse effect profile of composite treatment of several ADHD medications

Although attention-deficit/hyperactivity disorder (ADHD) is widely studied, problems regarding the adverse effect risks and non-responder problems still need to be addressed. Combination pharmacotherapy using standard dose regimens of existing medication is currently being practiced mainly to augment the therapeutic efficacy of each drug. The idea of combining different pharmacotherapies with different molecular targets to alleviate the symptoms of ADHD and its comorbidities requires scientific evidence, necessitating the investigation of their therapeutic efficacy and the mechanisms underlying the professed synergistic effects. Here, we injected male ICR mice with MK-801 to induce ADHD behavioral condition. We then modeled a "combined drug" using sub-optimal doses of methylphenidate, atomoxetine, and fluoxetine and investigated the combined treatment effects in MK-801-treated mice. No sub-optimal dose monotherapy alleviated ADHD behavioral condition in MK-801-treated mice. However, treatment with the combined drug attenuated the impaired behavior of MK-801-treated animals. Growth impediment, sleep disturbances, or risk of substance abuse were not observed in mice treated subchronically with the combined drugs. Finally, we observed that the combined ADHD drug rescued alterations in p-AKT and p-ERK1/2 levels in the prefrontal cortex and hippocampus, respectively, of MK-801-treated mice. Our results provide experimental evidence of a possible new pharmacotherapy option in ameliorating the ADHD behavioral condition without the expected adverse effects. The detailed mechanism of action underlying the synergistic therapeutic efficacy and reduced adverse reaction by combinatorial drug treatment should be investigated further in future studies.

Sources of Interindividual Variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in others. A significant source of this variability in drug response is drug metabolism, where differences in presystemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C_max, and/or C_min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is recognized that both intrinsic factors (e.g., genetics, age, sex, and disease states) and extrinsic factors (e.g., diet , chemical exposures from the environment, and the microbiome) play a significant role. For drug-metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, upregulation and downregulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less predictable and time-dependent manner. Understanding the mechanistic basis for variability in drug disposition and response is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that will improve outcomes in maintaining health and treating disease.

Effects of paroxetine on the pharmacokinetics of atomoxetine and its metabolites in different CYP2D6 genotypes

The aim of this study was to investigate the  effects of paroxetine, a potent inhibitor of CYP2D6, on the pharmacokinetics of atomoxetine and its two metabolites, 4-hydroxyatomoxetine and N-desmethylatomoxetine, in different CYP2D6 genotypes. Twenty-six healthy subjects were recruited and divided into CYP2D6*wt/*wt (*wt=*1 or *2, n = 10), CYP2D6*wt/*10 (n = 9), and CYP2D6*10/*10 groups (n = 7). In atomoxetine phase, all subjects received a single oral dose of atomoxetine (20 mg). In paroxetine phase, after administration of a single oral dose of paroxetine (20 mg) for six consecutive days, all subjects received a single oral dose of atomoxetine with paroxetine. Plasma concentrations of atomoxetine and its metabolites were determined up to 24 h after dosing. During atomoxetine phase, there were significant differences in C_max and AUC_0-24 of atomoxetine and N-desmethylatomoxetine among three genotype groups, whereas significant differences were not found in relation to CYP2D6*10 allele after administration of paroxetine. AUC ratios of 4-hydroxyatomoxetine and N-desmethylatomoxetine to atomoxetine were significantly different among three genotype groups during atomoxetine phase (all, P < 0.001), but after paroxetine treatment significant differences were not found. After paroxetine treatment, AUC_0-24 of atomoxetine was increased by 2.3-, 1.7-, and 1.3-fold, in CYP2D6*wt/*wt, CYP2D6*wt/*10, and CYP2D6*10/*10 groups in comparison to atomoxetine phase, respectively. AUC ratio of 4-hydroxyatomoxetine to atomoxetine in each group was significantly decreased, whereas AUC ratio of N-desmethylatomoxetine to atomoxetine significantly increased after administration of paroxetine. In conclusion, paroxetine coadministration significantly affected pharmacokinetic parameters of atomoxetine and its two metabolites, 4-hydroxyatomoxetine and N-desmethylatomoxetine. When atomoxetine was administered alone, C_max, AUC_0-24 and CL/F of atomoxetine were significantly different among the three CYP2D6 genotype groups. However, after paroxetine coadministration, no significant differences in these pharmacokinetic parameters were observed among the CYP2D6 genotype groups.

Metabolic profiling of norepinephrine reuptake inhibitor atomoxetine

Atomoxetine (ATX), a selective and potent inhibitor of the presynaptic norepinephrine transporter, is used mainly to treat attention-deficit hyperactivity disorder. Although multiple adverse effects associated with ATX have been reported including severe liver injuries, the mechanisms of ATX-related toxicity remain largely unknown. Metabolism frequently contributes to adverse effects of a drug through reactive metabolites, and the bioactivation status of ATX is still not investigated yet. Here, we systematically investigated ATX metabolism, bioactivation, species difference in human, mouse, and rat liver microsomes (HLM, MLM, and RLM) and in mice using metabolomic approaches as mice and rats are commonly used animal models for the studies of drug toxicity. We identified thirty one ATX metabolites and adducts in LMs and mice, 16 of which are novel. In LMs, we uncovered two methoxyamine-trapped aldehydes, two cyclization metabolites, detoluene-ATX, and ATX-N-hydroxylation for the first time. Detoluene-ATX and one cyclization metabolite were also observed in mice. Using chemical inhibitors and recombinant CYP enzymes, we demonstrated that CYP2C8 and CYP2B6 mainly contribute to the formation of aldehyde; CYP2D6 is the dominant enzyme for the formation of ATX cyclization and detoluene-ATX; CYP3A4 is major enzyme responsible for the hydroxylamine formation. The findings concerning aldehydes should be very useful to further elucidate the mechanistic aspects of adverse effects associated with ATX from metabolic angles. Additionally, the species differences for each metabolite should be helpful to investigate the contribution of specific metabolites to ATX toxicity and possible drug-drug interactions in suitable models.

Estimation of an Appropriate Dose of Trazodone for Pediatric Insomnia and the Potential for a Trazodone-Atomoxetine Interaction

There is a paucity of clinical trials for the treatment of pediatric insomnia. This study was designed to predict the doses of trazodone to guide dosing in a clinical trial for pediatric insomnia using physiologically‐based pharmacokinetic (PBPK) modeling. Data on the pharmacokinetics of trazodone in children are currently lacking. The interaction potential between trazodone and atomoxetine was also predicted. Doses predicted in the following age groups, with exposures corresponding to adult dosages of 30, 75, and 150 mg once a day (q.d.), respectively, were: (i) 2‐ to 6‐year‐old group, doses of 0.35, 0.8, and 1.6 mg/kg q.d.; (ii) >6‐ to 12‐year‐old group, doses of 0.4, 1.0, and 1.9 mg/kg q.d.; (iii) >12‐ to 17‐year‐old group, doses of 0.4, 1.1, and 2.1 mg/kg q.d. An interaction between trazodone and atomoxetine was predicted to be unlikely. Clinical trials based on the aforementioned predicted dosing are currently in progress, and pharmacokinetic data obtained will enable further refinement of the PBPK models.

Simple pharmacokinetic models accounting for drug monitoring results of atomoxetine and its 4-hydroxylated metabolites in Japanese pediatric patients genotyped for cytochrome P450 2D6

Atomoxetine is an approved medicine for attention-deficit/hyperactivity disorder and a cytochrome P450 2D6 (CYP2D6) probe substrate. Simple physiologically based pharmacokinetic (PBPK) models and compartment models were set up to account for drug monitoring results of 33 Japanese patients (6-15 years of age) to help establish the correct dosage for the evaluation of clinical outcomes. The steady-state one-point drug monitoring data for the most participants indicated the extensive biotransformation of atomoxetine to 4-hydroxyatomoxetine under individually prescribed doses of atomoxetine. However, 5 participants (with impaired CYP2D6 activity scores based on the CYP2D6 genotypes) showed high plasma concentrations of atomoxetine (0.53-1.5 μM) compared with those of total 4-hydroxyatomoxetine (0.49-1.4 μM). Results from full PBPK models using the in-built Japanese pediatric system of software Simcyp, one-compartment models, and new simple PBPK models (using parameters that reflected the subjects' small body size and normal/reduced CYP2D6-dependent clearance) could overlay one-point measured drug/metabolite plasma concentrations from almost common 28 participants within threefold ranges. Validated one-compartment or simple PBPK models can be used to predict steady-state plasma concentrations of atomoxetine and/or its primary metabolites in Japanese pediatric patients (>6 years) who took a variety of individualized doses in a clinical setting.

Clinical Pharmacogenetics Implementation Consortium Guideline for Cytochrome P450 (CYP)2D6 Genotype and Atomoxetine Therapy

Atomoxetine is a nonstimulant medication used to treat attention-deficit/hyperactivity disorder (ADHD). Cytochrome P450 (CYP)2D6 polymorphisms influence the metabolism of atomoxetine thereby affecting drug efficacy and safety. We summarize evidence from the published literature supporting these associations and provide therapeutic recommendations for atomoxetine based on CYP2D6 genotype (updates at www.cpicpgx.org).

Meeting the Challenge of Predicting Hepatic Clearance of Compounds Slowly Metabolized by Cytochrome P450 Using a Novel Hepatocyte Model, HepatoPac

Generating accurate in vitro intrinsic clearance data is an important aspect of predicting in vivo human clearance. Primary hepatocytes in suspension are routinely used to predict in vivo clearance; however, incubation times have typically been limited to 4-6 hours, which is not long enough to accurately evaluate the metabolic stability of slowly metabolized compounds. HepatoPac is a micropatterened hepatocyte-fibroblast coculture system that can be used for continuous incubations of up to 7 days. This study evaluated the ability of human HepatoPac to predict the in vivo clearance (CL) of 17 commercially available compounds with low to intermediate clearance (<12 ml/min/kg). In vitro half-life for disappearance of each compound was converted to hepatic clearance using the well stirred model, with and without correction for plasma protein binding. Hepatic CL, using three individual donors, was accurately predicted for 11 of 17 compounds (59%; predicted clearance within 2-fold of observed human in vivo clearance values). The accuracy of prediction increased to 82% (14 of 17 compounds) with an acceptance criterion defined as within 3-fold. When considering only low clearance compounds (<5 ml/min per kg), which represented 10 of the 17 compounds, the accuracy of prediction was 70% within 2-fold and 100% within 3-fold. In addition, the turnover of three slowly metabolized compounds (alprazolam, meloxicam, and tolbutamide) in HepatoPac was directly compared with turnover in suspended hepatocytes. The turnover of alprazolam and tolbutamide was approximately 2-fold greater using HepatoPac compared with suspended hepatocytes, which was roughly in line with the extrapolated values (correcting for the longer incubation time and lower cell number with HepatoPac). HepatoPac, but not suspended hepatocytes, demonstrated significant turnover of meloxicam. These results demonstrate the utility of HepatoPac for prediction of in vivo hepatic clearance, particularly with low clearance compounds.

Le TDAH chez les enfants et les adolescents, partie 2 : le traitement

Le trouble de déficit de l’attention/hyperactivité (TDAH) est un trouble neurodéveloppemental chronique. La Société canadienne de pédiatrie a préparé trois documents de principes après avoir effectué des analyses bibliographiques systématiques. Leurs objectifs s’établissent comme suit : 1) Résumer les données probantes cliniques à jour sur le TDAH. 2) Établir une norme pour les soins du TDAH. 3) Aider les cliniciens canadiens à prendre des décisions éclairées et fondées sur des données probantes pour rehausser la qualité des soins aux enfants et aux adolescents qui présentent cette affection.

La partie 2, axée sur le traitement, porte sur les données probantes et le contexte entourant diverses approches cliniques, la combinaison des interventions comportementales et pharmacologiques pour assurer un traitement plus efficace, le rôle de la formation des parents et des enseignants (ou des autres personnes qui s’occupent de l’enfant), le recours aux stimulants et aux non-stimulants, leurs effets et leurs risques, leur posologie et les protocoles de surveillance. Les recommandations thérapeutiques reposent sur les lignes directrices à jour, les données probantes tirées de publications scientifiques et le consensus d’experts.

ADHD in children and youth: Part 2-Treatment

Attention-deficit hyperactivity disorder (ADHD) is a chronic neurodevelopmental disorder. Three position statements have been developed by the Canadian Paediatric Society, following systematic literature reviews. Statement objectives are to: 1) Summarize the current clinical evidence regarding ADHD,2) Establish a standard for ADHD care, and3) Assist Canadian clinicians in making well-informed, evidence-based decisions to enhance care of children and youth with this condition. Specific topics reviewed in Part 2, which focuses on treatment, include: evidence and context for a range of clinical approaches, combining behavioural and pharmacological interventions to address impairment more effectively, the role of parent and teacher (or other caregiver) training, the use of stimulant and nonstimulant medications, with effects and risks, and dosing and monitoring protocols. Treatment recommendations are based on current guidelines, evidence from the literature, and expert consensus.

Physiologically based pharmacokinetic modelling of atomoxetine with regard to CYP2D6 genotypes

Atomoxetine is a norepinephrine reuptake inhibitor indicated in the treatment of attention-deficit/hyperactivity disorder. It is primarily metabolized by CYP2D6 to its equipotent metabolite, 4-hydroxyatomoxetine, which promptly undergoes further glucuronidation to an inactive 4-HAT-O-glucuronide. Clinical trials have shown that decreased CYP2D6 activity leads to substantially elevated atomoxetine exposure and increase in adverse reactions. The aim of this study was to to develop a pharmacologically based pharmacokinetic (PBPK) model of atomoxetine in different CYP2D6 genotypes. A single 20 mg dose of atomoxetine was given to 19 healthy Korean individuals with CYP2D6*wt/*wt (*wt = *1 or *2) or CYP2D6*10/*10 genotype. Based on the results of this pharmacokinetic study, a PBPK model for CYP2D6*wt/*wt individuals was developed. This model was scaled to those with CYP2D6*10/*10 genotype, as well as CYP2D6 poor metabolisers. We validated this model by comparing the predicted pharmacokinetic parameters with diverse results from the literature. The presented PBPK model describes the pharmacokinetics after single and repeated oral atomoxetine doses with regard to CYP2D6 genotype and phenotype. This model could be utilized for identification of appropriate dosages of atomoxetine in patients with reduced CYP2D6 activity to minimize the adverse events, and to enable personalised medicine.

Pharmacokinetics of CYP2C9, CYP2C19, and CYP2D6 substrates in healthy Chinese and European subjects

PURPOSE

The aim of this analysis is to compare the pharmacokinetics of drug substrates in healthy Chinese and European subjects of aligned CYP2C9, CYP2C19, or CYP2D6 enzyme activity, providing further insight into drivers of interethnic differences in pharmacokinetics.

METHODS

Following identification of appropriate drug substrates, a comprehensive and structured literature search was conducted to identify single-dose pharmacokinetic data in healthy Chinese or European subjects with reported CYP2C9, CYP2C19, or CYP2D6 activity (genotype or phenotype). The ratio of drug AUC in the Chinese and European subjects classified with aligned enzyme activity was calculated (ethnicity ratio (ER)).

RESULTS

For 22/25 drugs identified, the ERs calculated indicated no or only limited interethnic differences in exposure (<twofold) in Chinese and European subjects with aligned polymorphic enzyme activity. The interethnic differences observed can reflect differences across populations in additional determinants of pharmacokinetics, although the notable between study variation and change over time in methods used to assign enzyme activity may also be contributing factors. There was no association between drug substrate fraction metabolized (fm) for CYP2C9, CYP2C19, or CYP2D6 and the ERs calculated.

CONCLUSION

The spectrum of pharmacokinetic determinants for each drug substrate and their differences across ethnic groups must be considered on a case-by-case basis in addition to metabolism by CYP2C9, CYP2C19, or CYP2D6. This analysis has also highlighted the challenges which arise when comparing published datasets if consistent methods to assign polymorphic enzyme activity have not been used.

Physiologically Based Pharmacokinetic Model of the CYP2D6 Probe Atomoxetine: Extrapolation to Special Populations and Drug-Drug Interactions

Physiologically based pharmacokinetic (PBPK) modeling of drug disposition and drug-drug interactions (DDIs) has become a key component of drug development. PBPK modeling has also been considered as an approach to predict drug disposition in special populations. However, whether models developed and validated in healthy populations can be extrapolated to special populations is not well established. The goal of this study was to determine whether a drug-specific PBPK model validated using healthy populations could be used to predict drug disposition in specific populations and in organ impairment patients. A full PBPK model of atomoxetine was developed using a training set of pharmacokinetic (PK) data from CYP2D6 genotyped individuals. The model was validated using drug-specific acceptance criteria and a test set of 14 healthy subject PK studies. Population PBPK models were then challenged by simulating the effects of ethnicity, DDIs, pediatrics, and renal and hepatic impairment on atomoxetine PK. Atomoxetine disposition was successfully predicted in 100% of healthy subject studies, 88% of studies in Asians, 79% of DDI studies, and 100% of pediatric studies. However, the atomoxetine area under the plasma concentration versus time curve (AUC) was overpredicted by 3- to 4-fold in end stage renal disease and hepatic impairment. The results show that validated PBPK models can be extrapolated to different ethnicities, DDIs, and pediatrics but not to renal and hepatic impairment patients, likely due to incomplete understanding of the physiologic changes in these conditions. These results show that systematic modeling efforts can be used to further refine population models to improve the predictive value in this area.

The Influence of CYP2D6 Phenotype on the Pharmacokinetic Profile of Atomoxetine in Caucasian Healthy Subjects

Objective: To analyze a potential phenotypic variation within the studied group based on the pharmacokinetic profile of atomoxetine and its active metabolite, and to further investigate the impact of CYP2D6 phenotype on atomoxetine pharmacokinetics. Methods: The study was conducted as an open-label, non-randomized clinical trial which included 43 Caucasian healthy volunteers. Each subject received a single oral dose of atomoxetine 25 mg. Subsequently, atomoxetine and 4-hydroxyatomoxetine-O-glucuronide (glucuronidated active metabolite) plasma concentrations were determined and a noncompartmental method was used to calculate the pharmacokinetic parameters of both compounds. Further on, the CYP2D6 metabolic phenotype was assessed using the area under the curve (AUC) metabolic ratio (atomoxetine/ 4-hydroxyatomoxetine-O-glucuronide) and specific statistical tests (Lilliefors (Kolgomorov-Smirnov) and Anderson-Darling test). The phenotypic differences in atomoxetine disposition were identified based on the pharmacokinetic profile of the parent drug and its metabolite. Results: The statistical analysis revealed that the AUC metabolic ratio data set did not follow a normal distribution. As a result, two different phenotypes were identified, respectively the poor metabolizer (PM) group which included 3 individuals and the extensive metabolizer (EM) group which comprised the remaining 40 subjects. Also, it was demonstrated that the metabolic phenotype significantly influenced atomoxetine pharmacokinetics, as PMs presented a 4.5-fold higher exposure to the parent drug and a 3.2-fold lower exposure to its metabolite in comparison to EMs. Conclusions: The pharmacokinetic and statistical analysis emphasized the existence of 2 metabolic phenotypes: EMs and PMs. Furthermore, it was proved that the interphenotype variability had a marked influence on atomoxetine pharmacokinetic profile.

Dihydroxyphenylglycol as a Biomarker of Norepinephrine Transporter Inhibition by Atomoxetine: Human Model to Assess Central and Peripheral Effects of Dosing

To assess the primary metabolite of norepinephrine, 3,4-dihydroxyphenylglycol (DHPG), as a sensitive biomarker for norepinephrine transporter (NET) function and the relationship of DHPG measured peripherally and centrally, NET was antagonized with 80 mg/d atomoxetine for 18 days. Twelve healthy subjects were treated with atomoxetine in an open-label, multiple-dose exploratory study. Plasma atomoxetine reached steady state by day 6, and the pharmacokinetic results demonstrated availability of atomoxetine to the central nervous system. The cerebrospinal fluid (CSF)/plasma ratios of atomoxetine based on area under concentration-time curve from 0 to 12 hours postdose (AUC0-12), maximum concentration (Cmax), and predose were 0.3%, 0.2%, and 11%, respectively. Plasma from atomoxetine-treated subjects (ex vivo) significantly inhibited radioligand binding to human NET (P < 0.001) only 1 hour after dosing. Plasma DHPG and DHPG/norepinephrine (ratio) during repeated posture tests were reduced significantly (P < 0.001) on day 5 and stayed significantly reduced up to 1 day after treatment. In CSF, both DHPG and the ratio were significantly reduced (P < 0.001) on day 18. Urine results showed significant decreases for both DHPG and the ratio (P = 0.010 to P < 0.001). Brain-derived neurotrophic factor in CSF was lesser than the limits of detection. The findings suggest that NET blockade can be assessed with DHPG concentration or with the ratio in plasma, CSF, and urine. The data suggest that DHPG is a useful biomarker to proactively assess the pharmacological activity of compounds intended to inhibit NET activity within the brain. The study shows that CSF is a medium for early identification and quantification of biomarkers useful in assessing novel neuroscience targets.

A neurological perspective on the enhancement debate: Lessons learned from Parkinson's disease

Cognitive enhancement is signified by adaptive behavioural change following an intervention that targets the brain. Although much of the discussion and research into cognitive enhancement focuses on the effects of neural interventions in healthy individuals, it is useful to consider evidence from clinical populations. Diseases of the central nervous system represent the primary and richest source of evidence on the effects of brain manipulations, which are in the first instance therapeutic. Parkinson's disease (PD) is used as a model for understanding the effects of pharmacological agents that target systems with a central role in cognition. The mixed outcomes of deep brain stimulation on cognition will also be discussed. By illustrating the psychopharmacological principle of diverse and malleable neurochemical optima for different cognitive functions, and the role of individual differences, it will be argued that the entire spectrum of cognitive effects in any one individual following any given manipulation, such as the administration of a drug, often includes enhancement as well as impairment. Predicting these effects represents a complex multivariate problem, and the accuracy of this predictive effort, as well as the harm prevention it connotes, is determined by our evolving understanding of the brain and cognition. A manipulation can be said to confer cognitive enhancement; however, it is argued that using the global term cognitive enhancer to refer to such a manipulation without qualification is of limited utility.

Characterization of Atomoxetine Biotransformation and Implications for Development of PBPK Models for Dose Individualization in Children

Atomoxetine (ATX) is a second-line nonstimulant medication used to control symptoms of attention deficit hyperactivity disorder (ADHD). Inconsistent therapeutic efficacy has been reported with ATX, which may be related to variable CYP2D6-mediated drug clearance. We characterized ATX metabolism in a panel of human liver samples as a basis for a bottom-up PBPK model to aid in ATX exposure prediction and control. Km, Vmax, and Clint values in pooled human liver microsomes (HLMs) were 2.4 µM, 479 pmol/min/mg protein, and 202 µl/min/mg protein, respectively. Mean population values of kinetic parameters are not adequate to describe variability in a population, given that Km, Vmax, and Clint values from single-donor HLMs ranged from 0.93 to 79.2 µM, 20.0 to 1600 pmol/min/mg protein, and 0.3 to 936 µl/min/mg protein. All kinetic parameters were calculated from 4-hydroxyatomoxetine (4-OH-ATX) formation. CYP2E1 and CYP3A contributed to 4-OH-ATX formation in livers with CYP2D6 intermediate and poor metabolizer status. In HLMs with lower CYP2D6 activity levels, 2-hydroxymethylatomoxetine (2-CH2OH-ATX) formation became a more predominant pathway of metabolism, which appeared to be catalyzed by CYP2B6. ATX biotransformation at clinically relevant plasma concentrations was characterized in a panel of pediatric HLM (n = 116) samples by evaluating primary metabolites. Competing pathways of ATX metabolism [N-desmethylatomoxetine (NDM-ATX) and 2-CH2OH-ATX formation] had increasing importance in livers with lesser CYP2D6 activity, but, overall ATX clearance was still compromised. Modeling ATX exposure to individualize therapy would require comprehensive knowledge of factors that affect CYP2D6 activity as well as an understanding of competing pathways, particularly for individuals with lower CYP2D6 activity.

Atomoxetine: A Review of Its Pharmacokinetics and Pharmacogenomics Relative to Drug Disposition

Atomoxetine is a selective norepinephrine (NE) reuptake inhibitor approved for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children (≥6 years of age), adolescents, and adults. Its metabolism and disposition are fairly complex, and primarily governed by cytochrome P450 (CYP) 2D6 (CYP2D6), whose protein expression varies substantially from person to person, and by race and ethnicity because of genetic polymorphism. These differences can be substantial, resulting in 8-10-fold differences in atomoxetine exposure between CYP2D6 poor metabolizers and extensive metabolizers. In this review, we have attempted to revisit and analyze all published clinical pharmacokinetic data on atomoxetine inclusive of public access documents from the new drug application submitted to the United States Food and Drug Administration (FDA). The present review focuses on atomoxetine metabolism, disposition, and genetic polymorphisms of CYP2D6 as they specifically relate to atomoxetine, and provides an in-depth discussion of the fundamental pharmacokinetics of the drug including its absorption, distribution, metabolism, and excretion in pediatric and adult populations. Further, a summary of relationships between genetic variants of CYP2D6 and to some degree, CYP2C19, are provided with respect to atomoxetine plasma concentrations, central nervous system (CNS) pharmacokinetics, and associated clinical implications for pharmacotherapy. Lastly, dosage adjustments based on pharmacokinetic principles are discussed.

Single dose, CYP2D6 genotype-stratified pharmacokinetic study of atomoxetine in children with ADHD

The effect of CYP2D6 genotype on the dose-exposure relationship for atomoxetine has not been well characterized in children. Children 6-17 years of age diagnosed with attention-deficit hyperactivity disorder (ADHD) were stratified by CYP2D6 genotype into groups with 0 (poor metabolizers [PMs], n = 4), 0.5 (intermediate metabolizers [IMs], n = 3), one (extensive metabolizer [EM]1, n = 8) or two (EM2, n = 8) functional alleles and administered a single 0.5 mg/kg oral dose of atomoxetine (ATX). Plasma and urine samples were collected for 24 (IM, EM1, and EM2) or 72 hours (PMs). Dose-corrected ATX systemic exposure (area under the curve [AUC]0-∞ ) varied 29.6-fold across the study cohort, ranging from 4.4 ± 2.7 μM*h in EM2s to 5.8 ± 1.7 μM*h, 16.3 ± 2.9 μM*h, and 50.2 ± 7.3 μM*h in EM1s, IMs, and PMs, respectively (P < 0.0001). Simulated steady state profiles at the maximum US Food and Drug Administration (FDA)-recommended dose suggest that most patients are unlikely to attain adequate ATX exposures. These data support the need for individualized dosing strategies for more effective use of the medication.

The influence of paroxetine on the pharmacokinetics of atomoxetine and its main metabolite

Background and aims

To evaluate the effects of paroxetine on the pharmacokinetics of atomoxetine and its main metabolite, 4-hydroxyatomoxetine-O-glucuronide, after coadministration of atomoxetine and paroxetine in healthy volunteers.

Methods

22 healthy volunteers, extensive metabolizers, took part in this open-label, non-randomized, clinical trial. The study consisted of two periods: Reference, when a single oral dose of 25 mg atomoxetine was administrated to each subject and Test, when 25 mg atomoxetine and 20 mg paroxetine were coadministered. Between the two periods, the volunteers received an oral daily dose of 20–40 mg paroxetine, for 6 days. Atomoxetine and 4-hydroxyatomoxetine-O-glucuronide plasma concentrations were determined within the first 48 hours following drug administration. The pharmacokinetic parameters of both compounds were assessed using a non-compartmental method and the analysis of variance aimed at identifying any statistical significant differences between the pharmacokinetic parameters of atomoxetine and its main metabolite, corresponding to each study period.

Results

Paroxetine modified the pharmacokinetic parameters of atomoxetine. C_max increased from 221.26±94.93 to 372.53±128.28 ng/mL, while AUC_0-t and AUC_0-∞ also increased from 1151.19±686.52 to 6452.37±3388.76 ng*h/mL, and from 1229.15±751.04 to 7111.74±4195.17 ng*h/mL respectively. The main metabolite pharmacokinetics was also influenced by paroxetine intake, namely C_max, AUC_0-t and AUC_0-∞ decreased from 688.76±270.27 to 131.01±100.43 ng*h/mL, and from 4810.93±845.06 to 2606.04±923.88 and from 4928.55±853.25 to 3029.82 ±941.84 respectively.

Conclusions

Multiple-dose paroxetine intake significantly influenced atomoxetine and its active metabolite pharmacokinetics, causing a 5.8-fold increased exposure to atomoxetine and 1.6-fold reduced exposure to 4-hydroxyatomoxetine-O-glucuronide.

Translation in Data Mining to Advance Personalized Medicine for Health Equity

Personalized medicine is the development of 'tailored' therapies that reflect traditional medical approaches, with the incorporation of the patient's unique genetic profile and the environmental basis of the disease. These individualized strategies encompass disease prevention, diagnosis, as well as treatment strategies. Today's healthcare workforce is faced with the availability of massive amounts of patient- and disease-related data. When mined effectively, these data will help produce more efficient and effective diagnoses and treatment, leading to better prognoses for patients at both the individual and population level. Designing preventive and therapeutic interventions for those patients who will benefit most while minimizing side effects and controlling healthcare costs, requires bringing diverse data sources together in an analytic paradigm. A resource to clinicians in the development and application of personalized medicine is largely facilitated, perhaps even driven, by the analysis of "big data". For example, the availability of clinical data warehouses is a significant resource for clinicians in practicing personalized medicine. These "big data" repositories can be queried by clinicians, using specific questions, with data used to gain an understanding of challenges in patient care and treatment. Health informaticians are critical partners to data analytics including the use of technological infrastructures and predictive data mining strategies to access data from multiple sources, assisting clinicians' interpretation of data and development of personalized, targeted therapy recommendations. In this paper, we look at the concept of personalized medicine, offering perspectives in four important, influencing topics: 1) the availability of 'big data' and the role of biomedical informatics in personalized medicine, 2) the need for interdisciplinary teams in the development and evaluation of personalized therapeutic approaches, and 3) the impact of electronic medical record systems and clinical data warehouses on the field of personalized medicine. In closing, we present our fourth perspective, an overview to some of the ethical concerns related to personalized medicine and health equity.

Atomoxetine pharmacogenetics: associations with pharmacokinetics, treatment response and tolerability

Atomoxetine is indicated for the treatment of attention deficit hyperactivity disorder and is predominantly metabolized by the CYP2D6 enzyme. Differences in pharmacokinetic parameters as well as clinical treatment outcomes across CYP2D6 genotype groups have resulted in dosing recommendations within the product label, but clinical studies supporting the use of genotype guided dosing are currently lacking. Furthermore, pharmacokinetic and clinical studies have primarily focused on extensive as compared with poor metabolizers, with little information known about other metabolizer categories as well as genes involved in the pharmacodynamics of atomoxetine. This review describes the pharmacogenetic associations with atomoxetine pharmacokinetics, treatment response and tolerability with considerations for the clinical utility of this information.

CYP2D6 predicted metabolizer status and safety in adult patients with attention-deficit hyperactivity disorder participating in a large placebo-controlled atomoxetine maintenance of response clinical trial

Atomoxetine, which is indicated for treatment of attention-deficit hyperactivity disorder (ADHD), is predominantly metabolized by genetically polymorphic cytochrome P450 2D6 (CYP2D6). Based on identified CYP2D6 genotypes, individuals can be categorized into 4 phenotypic metabolizer groups as ultrarapid, extensive, intermediate, and poor. Previous studies have focused on observed differences between poor and extensive metabolizers, but it is not well understood whether the safety profile of intermediate metabolizers differs from that of ultrarapid and extensive metabolizers. This study compared safety and tolerability among the different CYP2D6 metabolizer groups in the 12-week open-label phase of an atomoxetine study in adult patients with ADHD. Genotyping identified 1039 patients as extensive/ultrarapid metabolizers, 780 patients as intermediate metabolizers, and 117 patients as poor metabolizers. Common (≥5% frequency) treatment-emergent adverse events did not significantly differ between extensive/ultrarapid and intermediate metabolizers (odds ratios were <2.0 or >0.5). Poor metabolizers had higher frequencies of dry mouth, erectile dysfunction, hyperhidrosis, insomnia, and urinary retention compared with the other metabolizer groups. There were no significant differences between extensive/ultrarapid and intermediate metabolizers in changes from baseline in vital signs. These results suggest that data from CYP2D6 intermediate and extensive/ultrarapid metabolizers can be combined when considering safety analyses related to atomoxetine.

Differential response profiles in children and adolescents with attention-deficit/hyperactivity disorder: treatment with atomoxetine

Atomoxetine has been shown to be safe and effective in the treatment of attention-deficit/hyperactivity disorder (ADHD). The purpose of this post hoc analysis was to examine response trajectories of pediatric patients treated with atomoxetine. Data were pooled from 7 atomoxetine double-blind, placebo-controlled clinical trials conducted in pediatric patients between November 1998 and June 2004. Growth mixture modeling was applied to the investigator-rated ADHD rating scale (ADHDRS-Inv) and Clinical Global Impressions-ADHD-Severity (CGI-ADHD-S) scores in the randomized acute phase (6-9 weeks) to explore whether there were groups of patients who differed in their response to atomoxetine. Classification and regression tree analyses were performed to identify predictors that can help categorize subjects to different response profiles. Patients (N = 925) were mostly male (73%) and of the combined subtype (74%). Most patients had a response pattern characterized by gradual, modest improvement, while a smaller group experienced early, robust improvement.

Targeting impulsivity in Parkinson's disease using atomoxetine

Noradrenergic dysfunction may play a significant role in cognition in Parkinson's disease due to the early degeneration of the locus coeruleus. Converging evidence from patient and animal studies points to the role of noradrenaline in dopaminergically insensitive aspects of the parkinsonian dysexecutive syndrome, yet the direct effects of noradrenergic enhancement have not to date been addressed. Our aim was to directly investigate these, focusing on impulsivity during response inhibition and decision making. To this end, we administered 40 mg atomoxetine, a selective noradrenaline re-uptake inhibitor to 25 patients with Parkinson's disease (12 female /13 male; 64.4 ± 6.9 years old) in a double blind, randomized, placebo controlled design. Patients completed an extensive battery of neuropsychological tests addressing response inhibition, decision-making, attention, planning and verbal short term memory. Atomoxetine improved stopping accuracy on the Stop Signal Task [F(1,19) = 4.51, P = 0.047] and reduced reflection impulsivity [F(1,9) = 7.86, P = 0.02] and risk taking [F(1,9) = 9.2, P = 0.01] in the context of gambling. The drug also conferred effects on performance as a function of its measured blood plasma concentration: it reduced reflection impulsivity during information sampling [adjusted R(2) = 0.23, F(1,16) = 5.83, P = 0.03] and improved problem solving on the One Touch Stockings of Cambridge [adjusted R(2) = 0.29, F(1,17) = 8.34, P = 0.01]. It also enhanced target sensitivity during sustained attention [F(1,9) = 5.33, P = 0.046]. The results of this exploratory study represent the basis of specific predictions in future investigations on the effects of atomoxetine in Parkinson's disease and support the hypothesis that targeting noradrenergic dysfunction may represent a new parallel avenue of therapy in some of the cognitive and behavioural deficits seen in the disorder.

A review of atomoxetine effects in young people with developmental disabilities

This review summarizes the pharmacokinetic characteristics, pharmacodynamic properties, common side effects, and clinical advantages and disadvantages associated with atomoxetine (ATX) treatment in typically developing children and adults with ADHD. Then the clinical research to date in developmental disabilities (DD), including autism spectrum disorders (ASD), is summarized and reviewed. Of the 11 relevant reports available, only two were placebo-controlled randomized clinical trials, and both focused on a single DD population (ASD). All trials but one indicated clinical improvement in ADHD symptoms with ATX, although it was difficult to judge the magnitude and validity of reported improvement in the absence of placebo controls. Effects of ATX on co-occurring behavioral and cognitive symptoms were much less consistent. Appetite decrease, nausea, and irritability were the most common adverse events reported among children with DD; clinicians should be aware that, as with stimulants, irritability appears to occur much more commonly in persons with DD than in typically developing individuals. Splitting the dose initially, starting below the recommended starting dose, and titrating slowly may prevent or ameliorate side effects. Patience is needed for the slow build-up of benefit.

CONCLUSIONS

ATX holds promise for managing ADHD symptoms in DD, but properly controlled, randomized clinical trials of atomoxetine in intellectual disability and ASD are sorely needed. Clinicians and researchers should be vigilant for the emergence of irritability with ATX treatment. Effects of ATX on cognition in DD are virtually unstudied.

ADHD pharmacogenetics across the life cycle: New findings and perspectives

Attention-deficit/hyperactivity disorder (ADHD) is a complex and heterogeneous disorder, affecting individuals across the life cycle. Although its etiology is not yet completely understood, genetics plays a substantial role. Pharmacological treatment is considered effective and safe for children and adults, but there is considerable inter-individual variability among patients regarding response to medication, required doses, and adverse events. We present here a systematic review of the literature on ADHD pharmacogenetics to provide a critical discussion of the existent findings, new approaches, limitations, and recommendations for future research. Our main findings are: first, the number of studies continues to grow, making ADHD one of the mental health areas with more pharmacogenetic studies. Second, there has been a focus shift on ADHD pharmacogenetic studies in the last years. There is an increasing number of studies assessing gene-gene and gene-environment interactions, using genome-wide association approaches, neuroimaging, and assessing pharmacokinetic properties. Third and most importantly, the heterogeneity in methodological strategies employed by different studies remains impressive. The question whether pharmacogenetics studies of ADHD will improve clinical management by shifting from trial-and-error approach to a pharmacological regimen that takes into account the individual variability remains unanswered. © 2014 Wiley Periodicals, Inc.

Drug-specific laterality effects on frontal lobe activation of atomoxetine and methylphenidate in attention deficit hyperactivity disorder boys during working memory

BACKGROUND

The catecholamine reuptake inhibitors methylphenidate (MPH) and atomoxetine (ATX) are the most common treatments for attention deficit hyperactivity disorder (ADHD). This study compares the neurofunctional modulation and normalization effects of acute doses of MPH and ATX within medication-naive ADHD boys during working memory (WM).

METHOD

A total of 20 medication-naive ADHD boys underwent functional magnetic resonance imaging during a parametric WM n-back task three times, under a single clinical dose of either MPH, ATX or placebo in a randomized, double-blind, placebo-controlled, cross-over design. To test for normalization effects, brain activations in ADHD under each drug condition were compared with that of 20 age-matched healthy control boys.

RESULTS

Relative to healthy boys, ADHD boys under placebo showed impaired performance only under high WM load together with significant underactivation in the bilateral dorsolateral prefrontal cortex (DLPFC). Both drugs normalized the performance deficits relative to controls. ATX significantly enhanced right DLPFC activation relative to MPH within patients, and significantly normalized its underactivation relative to controls. MPH, by contrast, both relative to placebo and ATX, as well as relative to controls, upregulated the left inferior frontal cortex (IFC), but only during 2-back. Both drugs enhanced fronto-temporo-striatal activation in ADHD relative to control boys and deactivated the default-mode network, which were negatively associated with the reduced DLPFC activation and performance deficits, suggesting compensation effects.

CONCLUSIONS

The study shows both shared and drug-specific effects. ATX upregulated and normalized right DLPFC underactivation, while MPH upregulated left IFC activation, suggesting drug-specific laterality effects on prefrontal regions mediating WM.

Effects of CYP2C19 genetic polymorphisms on atomoxetine pharmacokinetics

Atomoxetine is a selective norepinephrine reuptake inhibitor indicated for the treatment of attention-deficit/hyperactivity disorder. Atomoxetine metabolism is mediated by CYP2D6 and CYP2C19. This study aimed to investigate the effect of the CYP2C19 genetic polymorphism on the pharmacokinetics of atomoxetine and its metabolites, 4-hydroxyatomoxetine and N-desmethylatomoxetine. A single 40-mg oral dose of atomoxetine was administered to 40 subjects with different CYP2C19 genotypes (all participants carried the CYP2D6*1/*10 genotype). Concentrations of atomoxetine and its metabolites were analyzed using high-performance liquid chromatography with tandem mass spectrometry in plasma samples that were collected up to 24 hours after drug intake. For atomoxetine, the CYP2C19 poor metabolizer (PM) group showed significantly increased maximum plasma concentration and AUC0-∞ (area under the plasma concentration-time curve from 0 to infinity) and decreased apparent oral clearance compared with samples of the CYP2C19 extensive metabolizer (EM) and intermediate metabolizer (IM) groups (P < 0.001 for all). The half-life of atomoxetine in the CYP2C19PM group was also significantly longer than in the other genotype groups (P < 0.01 for CYP2C19EM and P < 0.05 for CYP2C19IM groups). The maximum plasma concentration and AUC 0-∞ of 4-hydroxyatomoxetine were significantly higher in the CYP2C19PM group compared with those in the CYP2C19EM and IM groups (P < 0.001 for CYP2C19EM and P < 0.05 for CYP2C19IM, respectively), whereas the corresponding values for N-desmethylatomoxetine in the CYP2C19PM group were significantly lower than those in the 2 genotype groups (P < 0.001 for both genotype groups). These results suggest that the genetic polymorphisms of CYP2C19 significantly affect the pharmacokinetics of atomoxetine.

Sources of interindividual variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in other individuals. A major source of this variability in drug response is drug metabolism, where differences in pre-systemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C max, and/or C min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is well recognized that both intrinsic (such as genetics, age, sex, and disease states) and extrinsic (such as diet, chemical exposures from the environment, and even sunlight) factors play a significant role. For the family of cytochrome P450 enzymes, the most critical of the drug metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, up- and down-regulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less reliably predictable and time-dependent manner. Understanding the mechanistic basis for drug disposition and response variability is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that brings with it true improvements in health outcomes in the therapeutic treatment of disease.

Active metabolites as antidepressant drugs: the role of norquetiapine in the mechanism of action of quetiapine in the treatment of mood disorders

Active metabolites of some antipsychotic drugs exhibit pharmacodynamic and pharmacokinetic properties that may be similar to or differ from the original compound and that can be translated by a different profile of responses and interactions to clinical level. Some of these antipsychotics' active metabolites might participate in mechanisms of antidepressant activity, as m-chlorophenylpiperazine (aripiprazole), 9-OH-risperidone and norquetiapine. Norquetiapine exhibits distinct pharmacological activity from quetiapine and plays a fundamental role in its antidepressant efficacy. In this review, we analyze the differential pharmacological aspects between quetiapine and norquetiapine, both from the pharmacokinetic and pharmacodynamic perspectives (affinity for dopaminergic, noradrenegic, and/or serotonergic receptors, etc.), as well as differential neuroprotective role. The pharmacological differences between the two drugs could explain the differential clinical effect, as well as some differences in tolerability profile and drug interactions. The available data are sufficient to arrive at the conclusion that antidepressant activity of quetiapine is mediated, at least in part, by the active metabolite norquetiapine, which selectively inhibits noradrenaline reuptake, is a partial 5-HT1A receptor agonist, and acts as an antagonist at presynaptic α2, 5-HT2C, and 5-HT7 receptors.

Meeting the challenge of predicting hepatic clearance of compounds slowly metabolized by cytochrome P450 using a novel hepatocyte model, HepatoPac

Generating accurate in vitro intrinsic clearance data is an important aspect of predicting in vivo human clearance. Primary hepatocytes in suspension are routinely used to predict in vivo clearance; however, incubation times have typically been limited to 4-6 hours, which is not long enough to accurately evaluate the metabolic stability of slowly metabolized compounds. HepatoPac is a micropatterened hepatocyte-fibroblast coculture system that can be used for continuous incubations of up to 7 days. This study evaluated the ability of human HepatoPac to predict the in vivo clearance (CL) of 17 commercially available compounds with low to intermediate clearance (<12 ml/min per kg). In vitro half-life for disappearance of each compound was converted to hepatic clearance using the well stirred model, with and without correction for plasma protein binding. Hepatic CL, using three individual donors, was accurately predicted for 10 of 17 compounds (59%; predicted clearance within 2-fold of observed human in vivo clearance values). The accuracy of prediction increased to 76% (13 of 17 compounds) with an acceptance criterion defined as within 3-fold. When considering only low clearance compounds (<5 ml/min per kg), which represented 10 of the 17 compounds, the accuracy of prediction was 60% within 2-fold and 90% within 3-fold. In addition, the turnover of three slowly metabolized compounds (alprazolam, meloxicam, and tolbutamide) in HepatoPac was directly compared with turnover in suspended hepatocytes. The turnover of alprazolam and tolbutamide was approximately 2-fold greater using HepatoPac compared with suspended hepatocytes, which was roughly in line with the extrapolated values (correcting for the longer incubation time and lower cell number with HepatoPac). HepatoPac, but not suspended hepatocytes, demonstrated significant turnover of meloxicam. These results demonstrate the utility of HepatoPac for prediction of in vivo hepatic clearance, particularly with low clearance compounds.