Concentrations of atomoxetine and its metabolites in plasma and oral fluid from paediatric patients with attention deficit/hyperactivity disorder

Inhibitory effect of atomoxetine on Nav1.2 voltage-gated sodium channel currents

BACKGROUND

Atomoxetine (ATX), a norepinephrine reuptake inhibitor (NRI), is used to attenuate the symptoms of Attention Deficit/Hyperactivity Disorder (AD/HD) by increasing neurotransmitter concentrations at the synaptic cleft. Although Nav1.2 voltage-gated sodium channels (VGSCs) are thought to play a role in monoamine transmitter release in the synaptic junction, it is unclear how atomoxetine affects Nav1.2 VGSCs.

METHODS

In this study, we investigated the effect of ATX on Nav1.2 VGSC-transfected HEK293 cells with the whole-patch clamp technique.

RESULTS

Nav1.2 VGSC current decreased by 51.15 ± 12.75% under treatment with 50 µM ATX in the resting state (holding membrane potential at - 80 mV). The IC₅₀ of ATX against Nav1.2 VGSC current was 45.57 µM. The activation/inactivation curve of Nav1.2 VGSC currents was shifted toward hyperpolarization by 50 µM ATX. In addition, the inhibitory effect of ATX increased with membrane depolarization (holding membrane potential at - 50 mV) and its IC₅₀ was 10.16 µM. Moreover, ATX showed the time-dependent interaction in the inactivation state.

CONCLUSION

These findings suggest that ATX interacts with Nav1.2 VGSCs producing the inhibition of current and the modification of kinetic properties in the state-dependent manner.

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.

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.

Determination of Guanfacine in Oral Fluid and Serum of Children and Adolescents with Attention-Deficit/Hyperactivity Disorder: A Short Communication

BACKGROUND

Guanfacine, a selective α2A-adrenoreceptor agonist, is a second-line medication for treating children and adolescents with attention-deficit/hyperkinetic disorder. The dosage administered as milligram per body weight to balance the potential benefits and risks of treatment. Therapeutic drug monitoring (TDM) is useful for identifying a patient's therapeutic window to optimize individual drug dosing and reduce the risk of adverse drug reactions. However, in children and adolescents, intravenous sample collection is especially stressful and thus remains a primary challenge, restricting the use of TDM. Therefore, evaluating alternative specimens to facilitate TDM is a worthwhile task. The aim of this study was to assess the feasibility of using oral fluid for TDM of guanfacine in children and adolescents.

METHODS

In this article, 9 patients (median age 8.1 years; 6 boys and 3 girls) undergoing treatment with guanfacine were included. Simultaneously collected oral fluid and serum samples were deproteinized using methanol containing a stable isotope-labeled internal standard before the determination of guanfacine by liquid chromatography-tandem mass spectrometry. Pearson correlation and paired t test were used for statistical analysis.

RESULTS

The mean serum guanfacine concentration was 3 times higher than that detected in oral fluid (7.47 ng/mL versus 2.36 ng/mL; t (8) = 5.94; P < 0.001). A strong positive linear correlation (r = 0.758, P = 0.018) was identified between oral fluid and serum concentrations. A strong but nonsignificant negative correlation (r = -0.574, P = 0.106) was detected between the oral fluid pH and oral fluid-to-serum concentration ratio.

CONCLUSIONS

The strong correlation between oral fluid and serum concentration and the probable small effect of oral fluid pH on oral fluid-to-serum concentration ratio supports guanfacine as a suitable candidate for TDM in oral fluid.

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.

Determination of atomoxetine or escitalopram in human plasma by HPLC: Applications in neuroscience research studies

BACKGROUND

Atomoxetine and escitalopram are potent and selective drugs approved for noradrenergic or serotonergic modulation of neuronal networks in attention-deficit hyperactivity disorder (ADHD) or depression, respectively. High-performance liquid chromatography (HPLC) methods still play an important role in the therapeutic drug monitoring (TDM) of psychopharmacological drugs, and coupled with tandem mass spectrometry are the gold standard for the quantification of drugs in biological matrices, but not available everywhere. The aim of this work was to develop and validate a HPLC method for neuroscientific studies using atomoxetine or escitalopram as a test drug.

MATERIALS AND METHODS

A HPLC method from routine TDM determination of atomoxetine or citalopram in plasma was adapted and validated for use in neuroscientific research. Using photo diode array detection with UV absorption at 205 nm, the variation of internal standard within one chromatographic method enables separate drug monitoring for concentration-controlled explorative studies in healthy humans and patients with Parkinson's disease.

RESULTS

The method described here was found to be linear in the range of 0.002 - 1.4 mg/L for atomoxetine and 0.0012 - 0.197 mg/L for escitalopram, with overall mean intra-day and inter-day imprecision and accuracy bias < 10% for both drugs. The method was successfully applied in concentration-controlled neuroimaging studies in populations of healthy humans and patients with Parkinson's disease.

CONCLUSION

A simple, sensitive, robust HPLC method capable of monitoring escitalopram and atomoxetine is presented and validated, as a useful tool for drug monitoring and the study of pharmacokinetics in neuroscientific study applications.

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.

Using the five-choice serial reaction time task to examine the effects of atomoxetine and methylphenidate in the male spontaneously hypertensive rat

Attention deficit hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder and is normally treated with either stimulant or non-stimulant medication such as methylphenidate and atomoxetine respectively. The impact of these drugs on attention and impulsivity has been explored extensively in healthy animals but there is little research into their effects in an animal model of ADHD. In the present study we investigated the effects of both drugs on the spontaneously hypertensive rat (SHR) model of ADHD using the five-choice serial reaction time task (5CSRTT). We found a number of difficulties associated with training this vulnerable strain on such a complex task. However, where rats were able to learn the task we found very small effects of methylphenidate; increased incorrect responding and therefore decreased accuracy, a marker of attention at a single dose. There were no significant effects of atomoxetine on accuracy once multiple comparisons were taken into consideration. We found no effects of either drug on premature responding, a marker of impulsivity. These results indicate that the 5CSRTT may not be most sensitive to the impulsivity displayed in the SHR. Furthermore, they suggest that the SHR may lack predictive validity and further investigation is needed to optimise use of this model.

Sweat testing for the detection of atomoxetine from paediatric patients with attention deficit/ hyperactivity disorder: application to clinical practice

Atomoxetine (ATX) is a selective norepinephrine reuptake inhibitor approved since 2002 for the treatment of attention deficit hyperactivity disorder (ADHD) in children, adolescents, and adults as an alternative treatment to methylphenidate. Within the framework of a project evaluating the use of alternative biological matrices for therapeutic monitoring of psychoactive drugs in paediatric and non-paediatric individuals, the excretion of ATX and its principal metabolites has been recently studied in oral fluid and hair. The aim of this study was to describe the excretion profile of ATX and its metabolites 4-hydroxyatomoxetine (4-OH-ATX) and N-desmethylatomoxetine (N-des-ATX) in sweat following the administration of different dosage regimens (60, 40, 35, and 18 mg/day) of ATX to six paediatric patients. Sweat patches were applied to the back of each participant and removed at timed intervals. ATX and its metabolites were measured in patches using a previously validated liquid chromatography-tandem mass spectrometric (LC-MS/MS) method. Independently from the administered dose, ATX appeared in the sweat patches 1 h post administration and reached its maximum concentration generally at 24 h. Peak ATX concentrations ranged between 2.31 and 40.4 ng/patch and did not correlate with the administered drug dose, or with body surface area. Total ATX excreted in sweat ranged between 0.008 and 0.121 mg, corresponding to 0.02 and 0.3% of the administered drug. Neither 4-OH-ATX, nor N-des-ATX was detected in either of the collected sweat patches. Measuring ATX in sweat patches can provide information on cumulative drug use from patch application until removal.