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.

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.

The lowest effective plasma concentration of atomoxetine in pediatric patients with attention deficit/hyperactivity disorder: A non-randomized prospective interventional study.

BACKGROUND: Atomoxetine (ATX) is used as a first-line, non-stimulant treatment for attention-deficit/hyperactivity disorder (ADHD), although no studies have systematically examined the relationship between plasma concentration and clinical efficacy. We conducted this non-randomized prospective interventional study to examine the relationship between plasma concentration of ATX and clinical efficacy. METHODS: Forty-three ADHD pediatric patients received ATX, and the steady-state through plasma concentration of the last daily dose that was maintained for at least 4 weeks were determined by high-performance liquid chromatography. RESULTS: The receiver operating characteristic curve suggested that when plasma concentration exceeded 64.60 ng/mL, scores on the ADHD-Rating Scale improved by 50% or more (P = .14). Although 6 of the 8 final responders were unresponsive at the initial dose (.72 ±â€Š.04 mg/kg [mean ±â€Šstandard deviation]), they responded after increasing the ATX dose to the final dose (1.52 ±â€Š.31 mg/kg). Excluding 7 outlier participants, the concentration was 83.3 ±â€Š32.3 ng/mL in 7 responders and was significantly higher than 29.5 ±â€Š23.9 ng/mL (P < .01) for the 29 non-responders. CONCLUSIONS: These results suggest that a minimum effective plasma concentration of ATX is required to achieve sufficient clinical efficacy. We hypothesized a mechanism that results in the realization of a clinical effect when the plasma concentration exceeds a certain threshold in the potential response group, whereas will not improve even if the plasma concentration is increased in the unqualified non-responder group.

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 Cmax and AUC0-24 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, AUC0-24 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, Cmax, AUC0-24 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.

Taste masking of water-soluble drug by solid lipid microspheres: a child-friendly system established by reversed lipid-based nanoparticle technique.

OBJECTIVES: A child-friendly taste-masking strategy using solid lipid microsphere (SLM) has been proposed to obscure the undesirable taste of some water-soluble drugs. In this study, the reversed lipid-based nanoparticle (RLBN) technique was used to encapsulate a water-soluble drug to facilitate the preparation of SLM. METHODS: The model drug used was atomoxetine hydrochloride (ATX), and a three-step method was used to prepare ATX-RLBN. Taste-masking microsphere (ATX-RLBN-SLM) was prepared by the spray chilling method. The drug release mechanism was studied by high-performance liquid chromatography and scanning electron microscopy. Moreover, in vitro taste evaluation method was established and ATX bioavailability was investigated employing pharmacokinetic studies. KEY FINDINGS: The obtained ATX-RLBN-SLM had smooth spherical particles with a size of about 80 µm. The drug encapsulation and loading efficiencies were 98.28% ± 0.59% and 0.89% ± 0.04%, respectively. In vitro drug release studies showed that nearly 96% drug was retained in the microspheres within 10 min at pH 6.8 and a complete release was triggered by lipase, accompanied by variation in the morphology. Taste assessment revealed that ATX-RLBN-SLM could efficiently mask the bitter taste and improved the bioavailability of ATX. CONCLUSIONS: Atomoxetine hydrochloride-reversed lipid-based nanoparticle-solid lipid microsphere exhibited excellent taste-masking effect with negligible leakage in the oral cavity environment and thorough collapse upon lipase stimulation, simultaneously enhancing the bioavailability of ATX. The study paves a new way to efficiently mask the undesirable taste of some water-soluble drugs.

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).

Synergistic Pressor Effect of Atomoxetine and Pyridostigmine in Patients With Neurogenic Orthostatic Hypotension.

Patients with autonomic failure are characterized by disabling orthostatic hypotension because of impaired sympathetic activity, but even severely affected patients have residual sympathetic tone which can be harnessed for their treatment. For example, norepinephrine transporter blockade with atomoxetine raises blood pressure (BP) in autonomic failure patients by increasing synaptic norepinephrine concentrations; acetylcholinesterase inhibition with pyridostigmine increases BP by facilitating ganglionic cholinergic neurotransmission to increase sympathetic outflow. We tested the hypothesis that pyridostigmine will potentiate the pressor effect of atomoxetine and improve orthostatic tolerance and symptoms in patients with severe autonomic failure. Twelve patients received a single oral dose of either placebo, pyridostigmine 60 mg, atomoxetine 18 mg or the combination on separate days in a single blind, crossover study. BP was assessed seated and standing before and 1-hour postdrug. In these severely affected patients, neither pyridostigmine nor atomoxetine improved BP or orthostatic tolerance compared with placebo. The combination, however, significantly increased seated BP in a synergistic manner (133±9/80±4 versus 107±6/66±4 mm Hg for placebo, 105±5/67±3 mm Hg for atomoxetine, and 99±6/64±4 mm Hg for pyridostigmine; P<0.001); the maximal increase in seated BP with the combination was 33±8/18±3 mm Hg at 60 minutes postdrug. Only the combination showed a significant improvement of orthostatic tolerance and symptoms. In conclusion, the combination pyridostigmine and atomoxetine had a synergistic effect on seated BP which was associated with improvement in orthostatic tolerance and symptoms. This pharmacological approach could be useful in patients with severe autonomic failure but further safety and long-term efficacy studies are needed.

Management of Attention-Deficit Disorder and Attention-Deficit/Hyperactivity Disorder Drug Intoxication in Dogs and Cats: An Update.

Amphetamines and the nonamphetamine atomoxetine are commonly used in the treatment of attention-deficit disorder/attention-deficit/hyperactivity disorder in humans. Because these medications are often found in homes, dog and cat exposure to these medications is a common intoxication. Amphetamine intoxication can cause life-threatening central nervous system and cardiovascular stimulation, even when small amounts are ingested.

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.

Evaluation of the Potential Pharmacokinetic Interaction between Atomoxetine and Fluvoxamine in Healthy Volunteers.

BACKGROUND/AIMS: Attention deficit hyperactivity disorder (ADHD) is frequently associated with other psychiatric pathologies. Therefore, the present study investigated a possible pharmacokinetic interaction between atomoxetine (ATX), a treatment option for ADHD, and an antidepressant, namely, fluvoxamine (FVX). METHODS: Designed as an open-label, non-randomized clinical trial, the study included 2 periods. In period 1 (reference), each subject received ATX 25 mg (single-dose), whereas in period 2 (test), all subjects were given a combination of ATX 25 mg + FVX 100 mg, following a 6-day pretreatment regimen with the enzymatic inhibitor. Non-compartmental methods were employed to determine the pharmacokinetic parameters of ATX and its main active metabolite (glucuronidated form), 4-hydroxyatomoxetine-O-glucuronide. RESULTS: The results revealed significant differences between the study periods for Cmax, AUC0-t and AUC0-∞ values corresponding to ATX and its metabolite. Small, but statistically significant increases in AUC values were reported for both parent drug (1,583.05 ± 1,040.29 vs. 2,111.55 ± 1,411.59 ng*h/ml) and 4-hydroxyatomoxetine-O-glucuronide (5,754.71 ± 1,235.5 vs. 6,293.17 ± 1,219.34 ng*h/ml) after combined treatment of ATX and the enzymatic inhibitor. CONCLUSION: FVX had a modest effect on the pharmacokinetics of ATX and 4-hydroxyatomoxetine-O-glucuronide. The presence or absence of any clinical consequences associated with this pharmacokinetic drug-drug interaction needs to be established in future studies.

The Effect of Myricetin on Pharmacokinetics of Atomoxetine and its Metabolite 4-Hydroxyatomoxetine In Vivo and In Vitro.

BACKGROUND AND OBJECTIVES: Atomoxetine is the first non-stimulant drug to be approved for the treatment of ADHD, while the effect of myricetin on the pharmacokinetic of atomoxetine in rats or human is still unknown. The present work was to study the impact of myricetin on the metabolism of atomoxetine both in vivo and in vitro. METHODS: Twenty healthy male Sprague-Dawley rats were randomly divided into four groups: A (control group), B (100 mg/kg myricetin), C (50 mg/kg myricetin), and D (25 mg/kg myricetin). A single dose of atomoxetine (10 mg/kg) was administrated half an hour later. In addition, human and rat liver microsomes were performed to determine the effect of myricetin on the metabolism of atomoxetine in vitro. RESULTS: Group B, C, D all increased the C max and AUC of atomoxetine, but decreased the C max and AUC of 4-hydroxyatomoxetine. Moreover, myricetin showed inhibitory effect on human and rat microsomes, the IC50 of myricetin was 8.651 and 35.45 µmol/L, respectively. CONCLUSIONS: Our study showed that myricetin could significantly inhibit the formation of atomoxetine metabolite both in vivo and in vitro. It is recommended that the effect of myricetin on the metabolism of atomoxetine should be noted and atomoxetine plasma concentration should be monitored.

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.

Evaluation of a Potential Metabolism-Mediated Drug-Drug Interaction Between Atomoxetine and Bupropion in Healthy Volunteers.

PURPOSE: To evaluate the impact of bupropion on the pharmacokinetic profile of atomoxetine and its main active metabolite (glucuronidated form), 4-hydroxyatomoxetine-O-glucuronide, in healthy volunteers. METHODS: An open-label, non-randomized, two-period, sequential clinical trial was conducted as follows: during Period I (Reference), each volunteer received a single oral dose of 25 mg atomoxetine, whilst during Period II (Test), a combination of 25 mg atomoxetine and 300 mg bupropion was administered to all volunteers, after a pretreatment regimen with bupropion for 7 days. Next, after determining atomoxetine and 4-hydroxyatomoxetine-O-glucuronide plasma concentrations, their pharmacokinetic parameters were calculated using a noncompartmental method and subsequently compared to determine any statistically significant differences between the two periods. RESULTS: Bupropion intake influenced all the pharmacokinetic parameters of both atomoxetine and its metabolite. For atomoxetine, Cmax increased from 226±96.1 to 386±137 ng/mL and more importantly, AUC0-∞ was significantly increasedfrom 1580±1040 to 8060±4160 ng*h/mL, while the mean t1/2 was prolonged after bupropion pretreatment. For 4-hydroxyatomoxetine-O-glucuronide, Cmax and AUC0-∞  were decreased from 707±269 to 212±145 ng/mL and from 5750±1240 to 3860±1220 ng*h/mL, respectively. CONCLUSIONS: These results demonstrated that the effect of bupropion on CYP2D6 activity was responsible for an increased systemic exposure to atomoxetine (5.1-fold) and also for a decreased exposure to its main metabolite (1.5-fold). Additional studies are required in order to evaluate the clinical relevance of this pharmacokinetic drug interaction.This article is open to POST-PUBLICATION REVIEW. Registered readers (see "For Readers") may comment by clicking on ABSTRACT on the issue's contents page.

Pharmacokinetic Profile and Palatability of Atomoxetine Oral Solution in Healthy Japanese Male Adults.

BACKGROUND AND OBJECTIVES: There is a clinical need for a liquid formulation of atomoxetine. We assessed the safety and bioequivalence of an atomoxetine oral solution. METHODS: This was an open-label, randomized, crossover study. Healthy adult male Japanese subjects (n = 42) with a cytochrome P450 2D6 extensive (including intermediate and ultrarapid) metabolizer genotype were administered atomoxetine 50 mg as oral solution and capsules once each, with a washout period >5 days between doses. Blood samples were used to analyze pharmacokinetic parameters, particularly maximum observed drug concentration (C max) and area under the concentration vs. time curve from time zero to the last time point with a measurable concentration (AUC0-last). Bioequivalence was concluded if the 90 % confidence interval of the ratio of geometric means between formulations for both C max and AUC0-last were within the interval of 0.8-1.25. Safety assessments included determination of adverse events. Taste was evaluated via a five-item questionnaire immediately and 10 min after taking atomoxetine oral solution. RESULTS: Forty subjects completed the study. Plasma concentration-time profiles of atomoxetine oral solution and capsules were similar, and the statistical analysis of systemic exposure showed that the two formulations were bioequivalent. Adverse events were mild and similar in type and frequency between the formulations. For taste acceptability, only 7.1 % of subjects responded that the oral solution would be difficult to take every day. CONCLUSION: Atomoxetine oral solution is bioequivalent to atomoxetine capsules and potentially fulfills the need for an oral solution atomoxetine formulation that will facilitate treatment of children with attention-deficit hyperactivity disorder.

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.

Effect of 24 Cytochrome P450 2D6 Variants Found in the Chinese Population on Atomoxetine Metabolism in vitro.

OBJECTIVE: The aim of this article was to assess the catalytic activities of 24 cytochrome P450 2D6 (CYP2D6) variants found in the Chinese population toward atomoxetine in vitro as well as CYP2D6.1. METHODS: In this study, the co-expression enzyme of human recombinant CYPOR, CYPb5, and CYP2D6.1 or other CYP2D6 variants with the baculovirus-mediated insect cells (Sf21) was used to study the catalytic activities of 24 CYP2D6 variants toward atomoxetine metabolism. The metabolite of atomoxetine (4-hydroxyatomoxetine) was detected by ultra-high performance liquid chromatography-mass spectrometry method. RESULTS: The intrinsic clearance (Vmax/Km) values of most variants were significantly altered when compared with CYP2D6.1. CYP2D6.94, CYP2D6.D336N, CYP2D6.R440C exhibited marked increased values 172, 126, 121% respectively. CYP2D6.89 and CYP2D6.98 exhibited similar catalytic activity as the wild type, whereas 17 variants exhibited significantly decreased values (from 5 to 87%) due to increase Km and/or decrease Vmax values. However, CYP2D6.92 and CYP2D6.96 showed no or few activity because of producing nothing. CONCLUSIONS: Our results suggest that most of these newly found variants exhibit significantly changed catalytic activities compared with the wild type. And these findings provide valuable information for the growth and development of personalized medicine in China.

Quantification of Methylphenidate, Dexamphetamine, and Atomoxetine in Human Serum and Oral Fluid by HPLC With Fluorescence Detection.

BACKGROUND: For psychostimulants, a marked individual variability in the dose-response relationship and large differences in plasma concentrations after similar doses are known. Therefore, optimizing the efficacy of these drugs is at present the most promising way to exploit their full pharmacological potential. Moreover, it seems important to examine oral fluid as less invasive biological matrix for its benefit in therapeutic drug monitoring for patients with hyperkinetic disorder. METHODS: A high-performance liquid chromatography method for quantification of methylphenidate (MPH), dexamphetamine (DXA), and atomoxetine in serum and oral fluid has been developed and validated. The analytical procedure involves liquid-liquid extraction, derivatization with 4-(4,5-diphenyl-1H-imidazol-2-yl)benzoyl chloride as a label and chromatographic separation on a Phenomenex Gemini-NX C18 analytical column using gradient elution with water-acetonitrile. The derivatized analytes were detected at 330 nm (excitation wavelength) and 440 nm (emission wavelength). To examine the oral fluid/serum ratios, oral fluid samples were collected simultaneously to blood samples from patients with hyperkinetic disorder. RESULTS: The method allows quantification of all analytes in serum and oral fluid within 16 minutes under the same or similar conditions. Oral fluid/serum ratios for MPH and DXA were highly variable and showed an accumulation of these drugs in oral fluid. CONCLUSIONS: The developed method covers the determination of MPH, DXA, and atomoxetine concentrations in serum and oral fluid after the intake of therapeutic doses. Oral fluid samples are useful for the qualitative detection of MPH and DXA.