Atomoxetine Hydrochloride: Clinical Drug-Drug Interaction Prediction and Outcome

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.

Cytochrome P450 3A Time-Dependent Inhibition Assays Are Too Sensitive for Identification of Drugs Causing Clinically Significant Drug-Drug Interactions: A Comparison of Human Liver Microsomes and Hepatocytes and Definition of Boundaries for Inactivation Rate Constants

Time-dependent inhibition (TDI) of CYP3A is an important mechanism underlying numerous drug-drug interactions (DDIs), and assays to measure this are done to support early drug research efforts. However, measuring TDI of CYP3A in human liver microsomes (HLMs) frequently yields overestimations of clinical DDIs and thus can lead to the erroneous elimination of many viable drug candidates from further development. In this investigation, 50 drugs were evaluated for TDI in HLMs and suspended human hepatocytes (HHEPs) to define appropriate boundary lines for the TDI parameter rate constant for inhibition (k_obs) at a concentration of 30 µM. In HLMs, a k_obs value of 0.002 minute^-1 was statistically distinguishable from control; however, many drugs show k_obs greater than this but do not cause DDI. A boundary line defined by the drug with the lowest k_obs that causes a DDI (diltiazem) was established at 0.01 minute^-1 Even with this boundary, of the 33 drugs above this value, only 61% cause a DDI (true positive rate). A corresponding analysis was done using HHEPs; k_obs of 0.0015 minute^-1 was statistically distinguishable from control, and the boundary was established at 0.006 minute^-1 Values of k_obs in HHEPs were almost always lower than those in HLMs. These findings offer a practical guide to the use of TDI data for CYP3A in early drug-discovery research. SIGNIFICANCE STATEMENT: Time-dependent inhibition of CYP3A is responsible for many drug interactions. In vitro assays are employed in early drug research to identify and remove CYP3A time-dependent inhibitors from further consideration. This analysis demonstrates suitable boundaries for inactivation rates to better delineate drug candidates for their potential to cause clinically significant drug interactions.

Non-Linear Pharmacokinetics of Atomoxetine in Adult Japanese Patients With ADHD

Objective: The objective was to reveal the relationship between dose and concentration of atomoxetine. Method: Fifty-five blood samples of 33 patients with ADHD were examined using high-performance liquid chromatography. Results: The plasma concentrations were 53.2 ± 67.0, 298.0 ± 390.5, and 639.3 ± 831.9 ng/mL at doses of 40 mg, 80 mg, and 120 mg, and the concentration/dose were 1.33 ± 1.67, 3.73 ± 4.88, and 5.33 ± 6.93 ng/mL/mg, respectively. Statistical analyses revealed a significant correlation between the concentration and the dose of atomoxetine (p = .004), and a trending toward significance in the difference in the concentration/dose in the three dosage groups (p = .064). The concentration/dose at 40 and 80 + 120 mg/day were 1.33 ± 1.67 and 4.22 ± 5.53 ng/mL/mg, the latter was significantly higher than the former (p = .006), which suggested non-linear pharmacokinetics. Conclusion: Clinicians should carefully titrate in high dose atomoxetine treatment.

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.

Clinically Significant Drug-Drug Interactions with Agents for Attention-Deficit/Hyperactivity Disorder

This article provides an overview of the pharmacokinetic drug-drug interactions (DDIs) for agents prescribed for attention-deficit/hyperactivity disorder (ADHD). Polypharmacy in the treatment of patients with ADHD leads to high exposures to DDIs and possibly adverse safety outcomes. We performed a systematic search of DDI reports for ADHD agents in Embase and Medline. We also searched for agents in the pharmacological pipeline, which include (1) mazindol, molindone and viloxazine, which were previously prescribed for other indications; (2) centanafadine and AR-08, never before approved; and (3) two extracts (Polygala tenuifolia extract and the French maritime pine bark extracts). The identified literature included case reports, cross-sectional, cross-over and placebo-controlled studies of patient cohorts and healthy volunteers. The DDIs were classified as follows: ADHD agents acting as perpetrators, i.e., affecting the clearance of co-prescribed agents (victim drugs), or ADHD agents being the victim drugs, being affected by other agents. Ratios for changes in pharmacokinetic parameters before and after the DDI were used as a rough estimate of the extent of the DDI. Alcohol may increase plasma dextroamphetamine concentrations by presystemic effects. Until studies are done to orient clinicians regarding dosing changes, clinicians need to be aware of the potential for cytochrome P450 (CYP) 2D6 inhibitors to increase amphetamine levels, which is equivalent to increasing dosages. Atomoxetine is a wide therapeutic window drug. The CYP2D6 poor metabolizers who do not have CYP2D6 activity had better atomoxetine response, but also an increased risk of adverse effects. CYP2D6 inhibitors have been used to increase atomoxetine response in CYP2D6 extensive metabolizers. Guanfacine is mainly metabolized by CYP3A4, which can be induced and inhibited. The package insert recommends that in guanfacine-treated patients, after adding potent CYP3A4 inducers, the guanfacine dose should be doubled; after adding potent CYP3A4 inhibitors the guanfacine dose should be halved. Based on a phenobarbital case report and our experience with CYP3A4-metabolized antipsychotics, these correction factors may be too low. According to two case reports, carbamazepine is a clinically relevant inducer of methylphenidate (MPH). A case series study suggested that MPH may be associated with important elevations in imipramine concentrations. Due to the absence of or limitations in the data, no comments for clinicians can be provided on the pharmacokinetic DDIs for clonidine, centanafadine, mazindol, molindone, AR-08, P. tenuifolia extract and the French maritime pine bark extracts. According to currently available data, clinicians should not expect that ADHD drugs modify each other's serum concentrations. A summary table for clinicians provides our current recommendations on pharmacokinetic DDIs of ADHD agents based on our literature review and the package inserts; whenever it was possible, we provide information on serum concentrations and dose correction factors. There will be a need to periodically update these recommendations and these correction factors as new knowledge becomes available.

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.

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.

Acute Dystonia Following a Switch in Treatment from Atomoxetine to Low-dose Aripiprazole

The present report describes the cases of a 17-year-old male patient and a 13-year-old female patient who developed acute dystonia following the administration of low-dose aripiprazole (5 mg/day) after the cessation of atomoxetine treatment. Although aripiprazole-induced dystonia has been previously reported in the literature, it is rare, and most of these cases were associated with doses higher than 5 mg/day. Furthermore, both of the patients in the present study discontinued atomoxetine prior to the initiation of aripiprazole treatment; thus, this report also discussed the possible mechanisms underlying the manifestation of dystonia from the perspective of neurotransmitter 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.

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.

Pharmacokinetic and pharmacodynamic interactions between almorexant, a dual orexin receptor antagonist, and desipramine

Almorexant is a dual orexin receptor antagonist (DORA) with sleep-enabling effects in humans. Insomnia is often associated with mental health problems, including depression. Hence, potential interactions with antidepressants deserve attention. Desipramine was selected as a model drug because it is mainly metabolized by CYP2D6, which is inhibited by almorexant in vitro. A single-center, randomized, placebo-controlled, two-way crossover study in 20 healthy male subjects was conducted to evaluate the pharmacokinetic and pharmacodynamic interactions between almorexant and desipramine. Almorexant 200mg or matching placebo (double-blind) was administered orally once daily in the morning for 10 days, and a single oral dose of 50mg desipramine (open-label) was administered on Day 5. Almorexant increased the exposure to desipramine 3.7-fold, suggesting that almorexant is a moderate inhibitor of desipramine metabolism through inhibition of CYP2D6. Conversely, desipramine showed no relevant effects on the pharmacokinetics of almorexant. Pharmacodynamic evaluations indicated that almorexant alone reduced visuomotor coordination, postural stability, and alertness, and slightly increased calmness. Desipramine induced a reduction in subjective alertness and an increase in pupil/iris ratio. Despite the increase in exposure to desipramine, almorexant and desipramine in combination showed the same pharmacodynamic profile as almorexant alone, except for prolonging reduced alertness and preventing the miotic effect of almorexant. Co-administration also prolonged the mydriatic effect of desipramine. Overall, repeated administration of almorexant alone or with single-dose desipramine was well tolerated. The lack of a relevant interaction with antidepressants, if confirmed for other DORAs, would be a key feature for a safer class of hypnotics.

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.

Clinical implications and management of drug-drug interactions between antiretroviral agents and psychotropic medications

Medications used in the treatment of human immunodeficiency virus (HIV) often have drug-drug interactions which complicate treatment of psychiatric illnesses in HIV-infected patients. Protease inhibitors (PIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) are the two classes of HIV medications most likely to be involved with interactions, with the majority occurring via the cytochrome P450 (CYP450) system. These interactions can result in either increased or decreased exposure to psychotropic and antiretroviral medications, often requiring dosage adjustments and increased monitoring. This article reviews some of the major drug interactions with antiretroviral agents.

The use of lisdexamfetamine dimesylate for the treatment of ADHD

ADHD is a common neurobehavioral disorder characterized by significant impairment in attention, hyperactivity and impulsivity. Symptoms begin in childhood and can persist into adulthood. Current data suggest that abnormal functioning of the prefrontal cortex, cortical and subcortical regions of the brain have roles in ADHD. All currently approved drugs used to treat ADHD enhance dopamine and norepinephrine signals in these regions. Lisdexamfetamine dimesylate (LDX) is a long-acting amphetamine prodrug indicated for the treatment of ADHD and has been shown to be effective in children, adolescents and adults. The prodrug properties of LDX make it a desirable treatment because of its long duration of effect, and low intrasubject and intersubject pharmacokinetic variability, and attenuated response on measures of abuse liability when compared with immediate-release amphetamine. However, LDX is still classified as a controlled substance. In this article, the pharmacokinetic parameters and efficacy and safety of LDX are reviewed.

In vitro-to-in vivo predictions of drug-drug interactions involving multiple reversible inhibitors

INTRODUCTION

Predictions of drug-drug interactions (DDIs) are commonly performed for single inhibitors, but interactions involving multiple inhibitors also frequently occur. Predictions of such interactions involving stereoisomer pairs, parent/metabolite combinations and simultaneously administered multiple inhibitors are increasing in importance. This review provides the framework for predicting inhibitory DDIs of multiple inhibitors with any combination of reversible inhibition mechanism.

AREAS COVERED

The review provides an overview of the reliability of the in vitro determined reversible inhibition mechanism. Furthermore, the article provides a method to predict DDIs for multiple reversible inhibitors that allows substituting the inhibition constant (K(i)) with an inhibitor affinity (IC(50)) value determined at S << K(M).

EXPERT OPINION

A better understanding and the prediction methods of DDIs, resulting from multiple inhibitors, are important. The inhibition mechanism of a reversible inhibitor is often equivocal across studies and unreliable. Determination of the K(i) requires the assignment of reversible inhibition mechanism but in vitro-to-in vivo prediction of DDI risk can be achieved for multiple inhibitors from estimates of the inhibitor affinity (IC(50)) only, regardless of the inhibition mechanism.

Synergistic effect of norepinephrine transporter blockade and α-2 antagonism on blood pressure in autonomic failure

Patients with autonomic failure have disabling orthostatic hypotension because of impaired sympathetic activity. Norepinephrine transporter blockade with atomoxetine raises blood pressure in autonomic failure by increasing synaptic norepinephrine concentrations in postganglionic sympathetic neurons. This effect requires tonic release of norepinephrine, which is decreased in patients with low sympathetic tone. We hypothesized that increasing residual sympathetic outflow with the α-2 antagonist yohimbine would potentiate the pressor effect of norepinephrine transporter blockade with atomoxetine and improve orthostatic tolerance in peripheral autonomic failure. Seventeen patients received a single oral dose of either placebo, yohimbine 5.4 mg or atomoxetine 18.0 mg, and the combination yohimbine and atomoxetine in a single blind, crossover study. Blood pressure was assessed while patients were seated and standing for ≤10 minutes before and 1 hour postdrug. Neither yohimbine nor atomoxetine significantly increased seated systolic blood pressure or orthostatic tolerance compared with placebo. The combination, however, significantly increased seated systolic blood pressure and orthostatic tolerance (P<0.001 and P=0.016, respectively) in a synergistic manner. The maximal increase in seated systolic blood pressure seen with the combination was 31±33 mm Hg at 60 minutes postdrug. Only the combination showed a significant improvement in orthostatic symptoms. In conclusion, the combination of yohimbine and atomoxetine had a synergistic effect on blood pressure and orthostatic tolerance in peripheral autonomic failure, which may be explained by an increased release of norepinephrine in peripheral sympathetic neurons by α-2 antagonism combined with a reduced norepinephrine clearance by norepinephrine transporter blockade. Safety studies are required to address the clinical usefulness of this pharmacological approach.

Age-related changes in antidepressant pharmacokinetics and potential drug-drug interactions: a comparison of evidence-based literature and package insert information

BACKGROUND

Antidepressants are among the most commonly prescribed psychotropic agents for older patients. Little is known about the best source of pharmacotherapy information to consult about key factors necessary to safely prescribe these medications to older patients.

OBJECTIVE

The objective of this study was to synthesize and contrast information in the package insert (PI) with information found in the scientific literature about age-related changes of antidepressants in systemic clearance and potential pharmacokinetic drug-drug interactions (DDIs).

METHODS

A comprehensive search of two databases (MEDLINE and EMBASE from January 1, 1975 to September 30, 2011) with the use of a combination of search terms (antidepressants, pharmacokinetics, and drug interactions) was conducted to identify relevant English language articles. This information was independently reviewed by two researchers and synthesized into tables. These same two researchers examined the most up-to-date PIs for the 26 agents available at the time of the study to abstract quantitative information about age-related decline in systemic clearance and potential DDIs. The agreement between the two information sources was tested with κ statistics.

RESULTS

The literature reported age-related clearance changes for 13 antidepressants, whereas the PIs only had evidence about 4 antidepressants (κ < 0.4). Similarly, the literature identified 45 medications that could potentially interact with a specific antidepressant, whereas the PIs only provided evidence about 12 potential medication-antidepressant DDIs (κ < 0.4).

CONCLUSION

The evidence-based literature compared with PIs is the most complete pharmacotherapy information source about both age-related clearance changes and pharmacokinetic DDIs with antidepressants. Future rigorously designed observational studies are needed to examine the combined risk of antidepressants with age-related decline in clearance and potential DDIs on important health outcomes such as falls and fractures in older patients.

Rationalization and prediction of in vivo metabolite exposures: the role of metabolite kinetics, clearance predictions and in vitro parameters

IMPORTANCE OF THE FIELD

Due to growing concerns over toxic or active metabolites, significant efforts have been focused on qualitative identification of potential in vivo metabolites from in vitro data. However, limited tools are available to quantitatively predict their human exposures.

AREAS COVERED IN THIS REVIEW

Theory of clearance predictions and metabolite kinetics is reviewed together with supporting experimental data. In vitro and in vivo data of known circulating metabolites and their parent drugs were collected and the predictions of in vivo exposures of the metabolites were evaluated.

WHAT THE READER WILL GAIN

The theory and data reviewed will be useful in early identification of human metabolites that will circulate at significant levels in vivo and help in designing in vivo studies that focus on characterization of metabolites. It will also assist in rationalization of metabolite-to-parent ratios used as markers of specific enzyme activity.

TAKE HOME MESSAGE

The relative importance of a metabolite in comparison to the parent compound as well as other metabolites in vivo can only be predicted using the metabolite's in vitro formation and elimination clearances, and the in vivo disposition of a metabolite can only be rationalized when the elimination pathways of that metabolite are known.

Are circulating metabolites important in drug-drug interactions?: Quantitative analysis of risk prediction and inhibitory potency

The potential of metabolites to contribute to drug-drug interactions (DDIs) is not well defined. The aim of this study was to determine the quantitative role of circulating metabolites in inhibitory DDIs in vivo. The area under the plasma concentration-time curve (AUC) data related to at least one circulating metabolite was available for 71% of the 102 inhibitor drugs identified. Of the 80 metabolites characterized at steady state, 78% had AUCs >10% of that of the parent drug. A comparison of the inhibitor concentration/inhibition constant ([I]/K(i)) ratios of metabolites and the respective parent drugs showed that 17 of the 21 (80%) reversible inhibitors studied had metabolites that were likely to contribute to in vivo DDIs, with some metabolites predicted to have inhibitory effects greater than those of the parent drug. The in vivo drug interaction risks associated with amiodarone, bupropion, and sertraline could be identified from in vitro data only, when data pertaining to metabolites were included in the predictions. In conclusion, cytochrome P450 (CYP) inhibitors often have circulating metabolites that contribute to clinically observed CYP inhibition.

Accurate prediction of dose-dependent CYP3A4 inhibition by itraconazole and its metabolites from in vitro inhibition data

Inhibitory drug metabolites may contribute to drug-drug interactions (DDIs). The aim of this study was to determine the importance of inhibitory metabolites of itraconazole (ITZ) in in vivo cytochrome P450 (CYP) 3A4 inhibition. The pharmacokinetics of ITZ and midazolam (MDZ) were determined in six healthy volunteers in four sessions after administration of MDZ with and without oral ITZ. After doses of 50, 200, and 400 mg of ITZ, the clearance of orally administered MDZ decreased by 27, 74, and 83%, respectively. The in vivo half maximal inhibitory concentration (IC(50)) for ITZ ranged from 5 to 132 nmol/l in the six subjects. The metabolites of ITZ were estimated to account for ~50% of the total CYP3A4 inhibition, with the relative contribution increasing with time after ITZ dosing. Of the total of 18 interactions observed, 15 (84%) could be predicted within a twofold error margin, with improved accuracy observed when ITZ metabolites were included in the predictions. This study shows that the metabolites of ITZ contribute to CYP3A4 inhibition and need to be accounted for in quantitative rationalization of ITZ-mediated DDIs.

Anticoagulation and psychotropic medications

Anticoagulants and psychotropic medications are commonly prescribed together. Thus, the potential for interaction exists. Whereas thrombolytics and heparins are implicated in few pharmacokinetic interactions, warfarin and platelet inhibitors have been implicated in various interactions with psychotropic medications. In this article, I review the various psychotropic drug classes' relationships to anticoagulants. In a field devoid of randomized, placebo-controlled trials, clinical vigilance is advised when managing patients on concomitant therapy.

InforMatrix for attention deficit hyperactivity disorder

The purpose of this review is to facilitate discussion on drug selection for the treatment of ADHD by using only clinically relevant selection criteria and providing an up-to-date overview. The InforMatrix method was used to select drugs to treat attention deficit hyperactivity disorder (ADHD). The following selection criteria were applied: clinical efficacy, safety, tolerability, ease of use, applicability, and cost. The drugs approved for ADHD in the Netherlands were included in the analysis, namely: atomoxetine, immediate-release methylphenidate, and various formulations of slow-release methylphenidate (Concerta, Equasym and Medikinet). Most studies are of limited quality, duration, and size. In one study, Concerta was more effective than atomoxetine. Although no relevant differences were seen in other comparative studies, the clinical experience with atomoxetine is still limited and unexpected toxicity cannot be excluded; few studies have been published with Equasym and Medikinet. No major differences were seen in general tolerability between the drugs. The ease of use of immediate-release methylphenidate is less than for the other drugs. The acquisition cost of immediate-release methylphenidate is considerably lower than that of the slow-release formulations. Atomoxetine is the most expensive drug. The InforMatrix program is available in an interactive format. It enables the user to judge both the importance of the selection criteria and the properties of each therapeutic option per criterion on the basis of his or her own personal expertise and/or the present document.

Adult ADHD and comorbid depressive disorders: diagnostic challenges and treatment options

Attention-deficit/hyperactivity disorder (ADHD) and major depressive disorder (MDD) are separately common mental health conditions that can have an adverse effect on a patient's quality of life if left untreated. These disorders frequently co-occur with one another, which can lead to increased patient suffering and diagnostic challenges for the treating clinician. In the United States adult population, epidemiological data show that the prevalence rate for MDD is 6.7%, while the ADHD prevalence rate in US adults is 4.4%. This epidemiological data represents the general population and likely underestimates the prevalence rates in clinic or practice patients. Examining the concurrent comorbid rate, if a patient has MDD, the likelihood of that patient also having ADHD is 18.6%; if the patient has ADHD, the likelihood of that patient having comorbid MDD is 9.4%. If the patient has dysthymia, the comorbid rate of ADHD is 12.8%, while those adults with ADHD have a comorbid rate of dysthymia of 22.6%.

Qualitative analysis of the role of metabolites in inhibitory drug-drug interactions: literature evaluation based on the metabolism and transport drug interaction database

Guidance from the Food and Drug Administration on drug interaction studies does not include a specific section on contributions of metabolites to observed inhibitory drug-drug interactions, and the quantitative role of drug metabolites in inhibitory drug-drug interactions is not presently known. The current work was undertaken to evaluate what fraction of inhibitors of common drug-metabolizing enzymes [cytochrome P450 (P450) 1A2, 2E1, 2D6, 2C9, 2C19, 2C8, 2B6, and 3A4] have circulating metabolites that may contribute to observed in vivo interactions. A literature analysis was conducted using the Metabolism and Transport Drug Interaction Database to identify all precipitants (i.e., inhibitors) that cause more than a 20% increase in the area under the plasma concentration-time curve (AUC) of marker substrates. The database, PubMed, and product labels were then used to determine whether circulating metabolites were present after administration of these inhibitors. Of the total of 129 inhibitors identified, 106 were confirmed to have metabolites that circulate in plasma. An additional 14 inhibitors were identified that are extensively metabolized but whose metabolites either have not been identified or have not been investigated. Hence, only 7% of the inhibitors did not have circulating metabolites. Of the 21 potent inhibitors (>or=5-fold increase in AUC) currently known, 17 had circulating metabolites, and the remaining four were all extensively metabolized. On the basis of available in vitro data, 24 of all of the inhibitors were mechanism-based inactivators of P450 enzymes, while 105 were characterized as reversible inhibitors. In vitro evaluation of inhibition potential was conducted for only 32% of the circulating metabolites of the inhibitors. In conclusion, circulating metabolites are often present with inhibitors of P450 enzymes, suggesting a need for increased efforts to characterize the inhibitory potency of metabolites of candidate drugs and for newer models for in vitro to in vivo extrapolations.

Dyskinesias associated with atomoxetine in combination with other psychoactive drugs

Toxicity experience with atomoxetine, a selective norepinephrine reuptake inhibitor approved for Attention Deficit Hyperactivity Disorder (ADHD), is limited. We report two cases of neurologic complications requiring hospitalization in patients when atomoxetine was added to other psychoactive drugs. A 9-year-old taking clonidine and dextroamphetamine developed psychosis, abnormal involuntary movements, and insomnia. An 18-year-old also initiating venlafaxine developed facial tics, tremors, and speech disturbance. Acute symptoms did not respond to diphenhydramine in either case, but resolved after atomoxetine and other medications were discontinued. Possible explanations include atypical atomoxetine effect, excess atomoxetine or metabolites due to poor metabolizer status (CYP 2D6 polymorphism/deficiency), a drug-drug interaction leading to elevated drug levels or to excess synaptic norepinephrine or dopamine. Serotonin syndrome is a possibility in the second case, but not the first. Clinicians should be aware of emergent dyskinesias when combining atomoxetine with dopaminergic, noradrenergic, or serotonergic medications.

Switching from neurostimulant therapy to atomoxetine in children and adolescents with attention-deficit hyperactivity disorder : clinical approaches and review of current available evidence

This review provides practical information on and clinical reasons for switching children and young people with attention-deficit hyperactivity disorder (ADHD) from neurostimulants to atomoxetine, detailing currently available evidence, and switching options. The issue is of particular relevance following recent guidance from the National Institute for Health and Clinical Excellence and European ADHD guidelines endorsing the use of atomoxetine, along with the stimulants methylphenidate and dexamphetamine, in the management of ADHD in children and adolescents in the UK. The selective norepinephrine (noradrenaline) reuptake inhibitor, atomoxetine, is a non-stimulant drug licensed for the treatment of ADHD in children and adolescents, and in adults who have shown a response in childhood. Following the once-daily morning dose, its therapeutic effects extend through the waking hours, into late evening, and in some patients, through to early the next morning. Atomoxetine may be considered for patients who are unresponsive or incompletely responsive to stimulant treatment, have co-morbid conditions (e.g. tics, anxiety, depression), and have sleep disturbances or eating problems, for patients in whom stimulants are poorly tolerated, and for situations where there is potential for drug abuse or diversion. Atomoxetine has been shown to be effective in relapse prevention and there is suggestion that atomoxetine may have a positive effect on global functioning; specifically health-related quality of life, self-esteem, and social and family functioning. According to one study, approximately 50% of non-responders to methylphenidate will respond to atomoxetine therapy and approximately 75% of responders to methylphenidate will also respond to atomoxetine. Atomoxetine may be initiated by a schedule of dose increases and cross-tapering with methylphenidate. A slow titration schedule with divided doses minimizes the impact of adverse events within the first several weeks of treatment. Atomoxetine may be co-administered with methylphenidate during the switching period without undue concern for adverse events, such as cardiovascular effects (although monitoring of blood pressure and heart rate is necessary). Atomoxetine may be discontinued abruptly and patients may miss the occasional dose without rebound effects or discontinuation syndrome. A trial period of at least 6-8 weeks, perhaps longer, is recommended before evaluation of the overall tolerability and efficacy of atomoxetine. We conclude that patients with ADHD can be switched from neurostimulants, specifically methylphenidate, to atomoxetine, and may benefit from symptom improvement.

Determination of atomoxetine in human plasma by a high performance liquid chromatographic method with ultraviolet detection using liquid–liquid extraction

A HPLC method with UV detection (210 nm) was developed and validated for the quantification of atomoxetine, a new medication for the treatment of attention deficit/hyperactivity disorder, in human plasma. Following a two-step liquid-liquid extraction with diethyl ether, the analyte and internal standard (maprotiline) were separated using an isocratic mobile phase of acetonitrile/phosphate buffer (39/61, v/v, pH 6.6) on a reverse phase Inertsil C(18) column. Linearity was verified over the range of 3.12-200 ng/mL atomoxetine in plasma. The lowest limit of detection is 2.5 ng/mL (S/N=10). This HPLC method was validated with within- and between-batch precisions of 4.9-14.4% and 4.7-13.1%, respectively. The within- and between-batch biases were -1.9 to 1.4% and 0.1-13.8%, respectively. Commonly used psychotropic drugs and frequently coadministered drugs did not interfere with the drug and internal standard. This method is simple, economical and specific, and has been used successfully in a pharmacokinetic study of atomoxetine.

CYP2D6 and clinical response to atomoxetine in children and adolescents with ADHD

BACKGROUND

Atomoxetine, a selective norepinephrine reuptake inhibitor effective in the treatment of attention-deficit/hyperactivity disorder (ADHD), is metabolized through the cytochrome P-450 2D6 (CYP2D6) enzyme pathway, which is genetically polymorphic in humans. Variations in plasma atomoxetine exposures can occur because of genetic variation or as a consequence of coadministration with drugs that inhibit CYP2D6.

METHOD

We examined the effects of CYP2D6 on the efficacy, safety, and tolerability of atomoxetine in children and adolescents using pooled data from atomoxetine clinical trials.

RESULTS

At endpoint, poor metabolizers had markedly greater reductions in mean symptom severity scores compared with extensive metabolizers (p < .05). Poor metabolizers had greater increases in heart rate and diastolic blood pressure (p < .001) and smaller increases in weight (p < .05) than extensive metabolizers. Several adverse events, including decreased appetite and tremor, were more frequent in poor metabolizers (p < .05).

CONCLUSIONS

These results suggest that CYP2D6 poor metabolizers taking atomoxetine in doses up to 1.8 mg/kg/day are likely to have greater efficacy, greater increases in cardiovascular tone, and some differences in tolerability compared with CYP2D6 extensive metabolizers taking similar doses.

Potential interactions of methylphenidate and atomoxetine with dextromethorphan

OBJECTIVE

To examine the potential for drug-drug interactions to influence drug metabolism between the attention-deficit/hyperactivity disorder (ADHD) dl-methylphenidate and atomoxetine with dextromethorphan, a probe for interactions involving cytochrome P450 (CYP) 2D6 isoenzyme.

DESIGN

In vitro and ex vivo analysis of changes in metabolism of study drugs.

SETTING

Laboratory.

PATIENTS

Not applicable.

INTERVENTIONS

Pooled human liver microsomal fractions prepared at CEDRA Corporation (now CellzDirect, Austin, Tex.) by the standard differential centrifugation method (lot 821-1). Human liver microsomes were pooled from 15 donors. Recombinant CYP 2D6-containing microsomes (Supersomes; lots 20 and 24 BD Gentest; Woburn, Mass.) were prepared from a baculovirus-infected insect cell line that expressed only the human CYP 2D6 isoform. Dextromethorphan, with and without effector, was incubated with pooled human liver and recombinant CYP 2D6-containing microsomes. Atomoxetine and dl-methylphenidate were tested at 0.1x, 1x, and 10x their reported therapeutic concentrations. Paroxetine, a known inhibitor of CYP 2D6, was used as a reference agent, and quinidine was used as a positive control inhibitor of CYP 2D6.

MAIN OUTCOME MEASURES

Changes in substrate metabolism indicative of CYP 2D6-mediated interactions.

RESULTS

Atomoxetine and paroxetine inhibited the formation of dextrorphan by about 50% in human liver microsomes and by more than 80% in recombinant microsomes; the profiles of atomoxetine and the known 2D6 inhibitor paroxetine were similar. High concentrations of dextromethorphan reversed the inhibition of its metabolism, indicating a competitive mechanism of the interaction. Conversely, dextromethorphan and dextrorphan only modestly inhibited atomoxetine and paroxetine metabolism. dl-Methylphenidate did not inhibit dextrorphan formation in either microsome preparation, and dl-methylphenidate metabolism was unaffected by dextromethorphan or dextrorphan.

CONCLUSION

These results demonstrate the potential for in vivo interactions between dextromethorphan and atomoxetine in patients with ADHD. However, they do not support the plausibility of an in vivo interaction between dextromethorphan and dl-methylphenidate.

Sensitive quantification of atomoxetine in human plasma by HPLC with fluorescence detection using 4-(4,5-diphenyl-1H-imidazole-2-yl) benzoyl chloride derivatization

The first HPLC-fluorescence method for the determination of atomoxetine in human plasma was developed and validated. Atomoxetine was derivatized with 4-(4,5-diphenyl-1H-imidazol-2-yl) benzoyl chloride (DIB-Cl) under mild conditions, and separated isocratically on a C18 column using a HPLC system with fluorescence detection (lambdaex: 318 nm, lambdaem: 448 nm). A linear calibration curve was obtained over the concentration range 1-1000 ng/mL (r=0.999). The limit of detection (S/N=3) was 0.3 ng/mL. The relative standard deviations of intra-day and inter-day variations were < or =8.30% and 7.47%, respectively. This method is rapid, sensitive, and suitable for both basic and clinical studies of atomoxetine.

Stochastic prediction of CYP3A-mediated inhibition of midazolam clearance by ketoconazole

Conventional methods to forecast CYP3A-mediated drug-drug interactions have not employed stochastic approaches that integrate pharmacokinetic (PK) variability and relevant covariates to predict inhibition in terms of probability and uncertainty. Empirical approaches to predict the extent of inhibition may not account for nonlinear or non-steady-state conditions, such as first-pass effects or accumulation of inhibitor concentration with multiple dosing. A physiologically based PK model was developed to predict the inhibition of CYP3A by ketoconazole (KTZ), using midazolam (MDZ) as the substrate. The model integrated PK models of MDZ and KTZ, in vitro inhibition kinetics of KTZ, and the variability and uncertainty associated with these parameters. This model predicted the time- and dose-dependent inhibitory effect of KTZ on MDZ oral clearance. The predictive performance of the model was validated using the results of five published KTZ-MDZ studies. The model improves the accuracy of predicting the inhibitory effect of increasing KTZ dosing on MDZ PK by incorporating a saturable KTZ efflux from the site of enzyme inhibition in the liver. The results of simulations using the model supported the KTZ dose of 400 mg once daily as the optimal regimen to achieve maximum inhibition by KTZ. Sensitivity analyses revealed that the most influential variable on the prediction of inhibition was the fractional clearance of MDZ mediated by CYP3A. The model may be used prospectively to improve the quantitative prediction of CYP3A inhibition and aid the optimization of study designs for CYP3A-mediated drug-drug interaction studies in drug development.

Atomoxetine alone or combined with fluoxetine for treating ADHD with comorbid depressive or anxiety symptoms

OBJECTIVE

Symptoms of depression and anxiety are commonly comorbid with attention-deficit/hyperactivity disorder (ADHD). The authors assessed the safety and effectiveness of atomoxetine monotherapy compared with combined atomoxetine/fluoxetine therapy in a population of children and adolescents with ADHD and concurrent symptoms of depression or anxiety.

METHOD

Patients were randomized to treatment with fluoxetine (n = 127) or placebo (n = 46) under double-blind conditions for 8 weeks, with concomitant atomoxetine use the last 5 weeks.

RESULTS

At end point, reductions in ADHD, depressive, and anxiety symptoms were marked for both treatment groups (p < .001 for the relevant scale in each symptom cluster). Some differences between treatment groups for depressive symptoms were significant, but the magnitudes of the differences were small and likely of limited clinical importance. Completion rates for the two groups were similar, as were discontinuation rates for adverse events. The combination group had greater increases in blood pressure and pulse than did the monotherapy group.

CONCLUSIONS

In pediatric patients with ADHD and comorbid symptoms of depression or anxiety, atomoxetine monotherapy appears to be effective for treating ADHD. Anxiety and depressive symptoms also improved, but the absence of a placebo-only arm does not allow us to conclude that these effects are specifically the result of treatment with atomoxetine. Combined atomoxetine/fluoxetine therapy was well tolerated.

Attention-deficit/hyperactivity disorder pharmacogenomics

Although the efficacy of medications for attention-deficit/hyperactivity disorder (ADHD) is well demonstrated in clinical trials, substantial numbers of patients fail to remain on therapy, and there is tremendous variability in tolerability and treatment acceptance. The emerging science of pharmacogenomics seeks to identify patterns of genetic variation that will direct individually tailored treatment regimens and enhance long-term adherence. For this review, existing studies in ADHD pharmacogenomics were reviewed to assess current knowledge and provide a basis for planning future research. Twelve studies were identified. The majority investigated the role of candidate genes in predicting clinical response to methylphenidate. The most frequently studied is DAT1, although findings are inconsistent, with the 10-repeat polymorphism predicting both increased and decreased symptom reduction in various reports. Other candidates include DRD4, DRD5, DBH, 5HTT, SNAP-25, and COMT. One study was based on quantitative trait analyses in a genome-wide scan. Although interest in ADHD pharmacogenomics is encouraging, preliminary studies have been limited by small sample sizes, inconsistent research designs, retrospective reports, and a focus on symptom response. Future studies should emphasize large, prospective trials, assess multiple medications in individual subjects, and consider a full range of pharmacodynamic and pharmacokinetic outcomes.

Atomoxetine: a new pharmacotherapeutic approach in the management of attention deficit/hyperactivity disorder

Atomoxetine is a novel, non-stimulant, highly selective noradrenaline reuptake inhibitor that has been studied for use in the treatment of attention deficit/hyperactivity disorder (ADHD). Data from clinical trials show it to be well tolerated and effective in the treatment of ADHD in children, adolescents, and adults. Improvements were seen not only in core symptoms of ADHD, but also in broader social and family functioning and self esteem. Once-daily dosing of atomoxetine has been shown to be effective in providing continuous symptom relief. Atomoxetine does not appear to have abuse potential and is associated with a benign side effect profile. The development of atomoxetine thus represents an important advance in the pharmacological management of ADHD.

Clinical Pharmacokinetics of Atomoxetine

Atomoxetine (Strattera, a potent and selective inhibitor of the presynaptic norepinephrine transporter, is used clinically for the treatment of attention-deficit hyperactivity disorder (ADHD) in children, adolescents and adults. Atomoxetine has high aqueous solubility and biological membrane permeability that facilitates its rapid and complete absorption after oral administration. Absolute oral bioavailability ranges from 63 to 94%, which is governed by the extent of its first-pass metabolism. Three oxidative metabolic pathways are involved in the systemic clearance of atomoxetine: aromatic ring-hydroxylation, benzylic hydroxylation and N-demethylation. Aromatic ring-hydroxylation results in the formation of the primary oxidative metabolite of atomoxetine, 4-hydroxyatomoxetine, which is subsequently glucuronidated and excreted in urine. The formation of 4-hydroxyatomoxetine is primarily mediated by the polymorphically expressed enzyme cytochrome P450 (CYP) 2D6. This results in two distinct populations of individuals: those exhibiting active metabolic capabilities (CYP2D6 extensive metabolisers) and those exhibiting poor metabolic capabilities (CYP2D6 poor metabolisers) for atomoxetine. The oral bioavailability and clearance of atomoxetine are influenced by the activity of CYP2D6; nonetheless, plasma pharmacokinetic parameters are predictable in extensive and poor metaboliser patients. After single oral dose, atomoxetine reaches maximum plasma concentration within about 1-2 hours of administration. In extensive metabolisers, atomoxetine has a plasma half-life of 5.2 hours, while in poor metabolisers, atomoxetine has a plasma half-life of 21.6 hours. The systemic plasma clearance of atomoxetine is 0.35 and 0.03 L/h/kg in extensive and poor metabolisers, respectively. Correspondingly, the average steady-state plasma concentrations are approximately 10-fold higher in poor metabolisers compared with extensive metabolisers. Upon multiple dosing there is plasma accumulation of atomoxetine in poor metabolisers, but very little accumulation in extensive metabolisers. The volume of distribution is 0.85 L/kg, indicating that atomoxetine is distributed in total body water in both extensive and poor metabolisers. Atomoxetine is highly bound to plasma albumin (approximately 99% bound in plasma). Although steady-state concentrations of atomoxetine in poor metabolisers are higher than those in extensive metabolisers following administration of the same mg/kg/day dosage, the frequency and severity of adverse events are similar regardless of CYP2D6 phenotype.Atomoxetine administration does not inhibit or induce the clearance of other drugs metabolised by CYP enzymes. In extensive metabolisers, potent and selective CYP2D6 inhibitors reduce atomoxetine clearance; however, administration of CYP inhibitors to poor metabolisers has no effect on the steady-state plasma concentrations of atomoxetine.

Atomoxetine, a Novel Treatment for Attention-Deficit–Hyperactivity Disorder

Atomoxetine is the first nonstimulant drug approved by the United States Food and Drug Administration (FDA) for the treatment of attention-deficit-hyperactivity disorder (ADHD), and the only agent approved by the FDA for the treatment of ADHD in adults. Atomoxetine is a norepinephrine transport inhibitor that acts almost exclusively on the noradrenergic pathway. Its mechanism of action in the control and maintenance of ADHD symptoms is thought to be through the highly specific presynaptic inhibition of norepinephrine. Clinical trials to evaluate the short-term effects of atomoxetine in children and adults have shown that atomoxetine is effective in maintaining control of ADHD. Likewise, long-term trials have determined that atomoxetine is effective in preventing relapse of ADHD symptoms without an increase in adverse effects. A comparative trial of atomoxetine with methylphenidate in school-aged children indicated similar safety and efficacy without the abuse liability associated with some psychostimulants. The most commonly reported adverse effects in children and adolescents are dyspepsia, nausea, vomiting, decreased appetite, and weight loss. The rates of adverse events in the trials were similar for both the once- and twice-daily dosing regimens. The discontinuation rate was 3.5% in patients treated with atomoxetine versus 1.4% for placebo and appeared to be dose dependent, wit a higher percentage of discontinuation at dosages greater than 1.5 mg/kg/day. In clinical trials involving adults, the emergence of clinically significant or intolerable adverse events was low. The most common adverse events in adults were dry mouth, insomnia, nausea, decreased appetite, constipation, urinary retention or difficulties with micturition, erectile disturbance, dysmenorrhea, dizziness, and decreased libido. Sexual dysfunction occurred in approximately 2% of patients treated with atomoxetine. Atomoxetine should be used with caution in patients who have hypertension or any significant cardiovascular disorder. Overall, atomoxetine therapy in patient with ADHD appears to be effective in controlling symptoms and maintaining remission, with the advantages being comparable efficacy with that of methylphenidate, a favorable safety profile, and non-controlled substance status. Additional long-term studies are needed to determine its continued efficacy for those who require lifelong treatment, and comparative trials against other stimulant and nonstimulant agents.