Jennifer Witcher

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-hydroxy-atomoxetine 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.

An open-label, dose-ranging study of atomoxetine in children with attention deficit hyperactivity disorder.

OBJECTIVE The goal of this study was to evaluate the tolerability and effectiveness of the experimental, noradrenergic specific reuptake inhibitor atomoxetine in the treatment of children with attention deficit hyperactivity disorder (ADHD). METHODS This was an open, prospective, dose-ranging study of atomoxetine monotherapy in the treatment of 30 children with ADHD between the ages of 7 and 14 years. Atomoxetine was started at 10-20 mg/day and titrated weekly up to 90 mg over 11 weeks, depending on response and adverse effects. Twenty-two children completed the full 11 weeks. We assessed efficacy with weekly clinician and parent ratings of ADHD and oppositional symptoms and monitored adverse effects, laboratory findings, and cardiovascular parameters. RESULTS Treatment with atomoxetine (mean final, total daily dose of 1.9 mg/kg/day) was very well tolerated without meaningful adverse effects. Atomoxetine significantly reduced core symptoms of ADHD (ADHD-Rating Scale-IV; 38.6% decrease vs. baseline, p < 0.001) with significant improvement (p < 0.05) in all but 1 of the 18 individual items in the ADHD-Rating Scale-IV. More than 75% of subjects who completed 10 weeks of treatment showed > 25% decrease in ADHD symptoms. CONCLUSIONS These findings extend to children the positive results previously reported in adults diagnosed with ADHD who were treated with atomoxetine. These results support additional controlled trials of atomoxetine in cases of pediatric ADHD.

Atomoxetine Pharmacokinetics in Children and Adolescents with Attention Deficit Hyperactivity Disorder

OBJECTIVE Atomoxetine is indicated for the treatment of attention deficit hyperactivity disorder in children, adolescents, and adults. This study was conducted, in part, to evaluate the single-dose and steady-state pharmacokinetics of atomoxetine in pediatric patients. METHODS This was an open-label, dose-titration study in pediatric patients with attention deficit hyperactivity disorder. Eligible patients could elect to participate in a single-dose or steady-state discontinuation pharmacokinetic evaluation including serial plasma sample collection over 24 hours. Plasma concentrations of atomoxetine, 4-hydroxyatomoxetine, and N-desmethylatomoxetine were determined using an atmospheric pressure chemical ionization liquid chromatography/mass spectrometry/mass spectrometry assay. Pharmacokinetic parameters were calculated using noncompartmental analysis. RESULTS Twenty-one cytochrome P450 2D6 extensive metabolizer patients participated in these single-dose and steady-state pharmacokinetic evaluations. Atomoxetine was rapidly absorbed, with peak plasma concentrations occurring 1 to 2 hours after dosing. Half-life averaged 3.12 and 3.28 hours after a single dose and at steady state, respectively. Minimal accumulation occurred in plasma after multiple twice-daily dosing in extensive metabolizer pediatric patients, as expected based on single-dose pharmacokinetics. As the dose (in mg/kg) increased, proportional increases in area under the curve were observed. CONCLUSIONS The pharmacokinetics of atomoxetine in extensive metabolizer patients were well characterized over a wide range of doses in this study. Atomoxetine pharmacokinetics in pediatric patients and adult subjects were similar after adjustment for body weight.

Effect of potent CYP2D6 inhibition by paroxetine on atomoxetine pharmacokinetics.

The purpose of this study was to characterize the effect of potent CYP2D6 inhibition by paroxetine on atomoxetine disposition in extensive metabolizers. This was a single‐blind, two‐period, sequential study in 22 healthy individuals. In period 1, 20 mg atomoxetine bid was administered to steady state. In period 2, 20 mg paroxetine was administered qd for 17 days. On days 12 through 17, 20 mg atomoxetine bid were coadministered. Plasma pharmacokinetics of atomoxetine, 4‐hydroxyatomoxetine, and N‐desmethylatomoxetine was determined at steady state in each treatment period. Plasma pharmacokinetics of paroxetine were determined after the 11th and 17th doses. Paroxetine increased Css,max, AUC0–12, and t1/2 of atomoxetine by approximately 3.5‐, 6.5‐, and 2.5‐fold, respectively. After coadministration with paroxetine, increases in N‐desmethylatomoxetine and decreases in 4‐hydroxyatomoxetine concentrations were observed. No changes in paroxetine pharmacokinetics were observed after coadministration with atomoxetine. It was concluded that inhibition of CYP2D6 by paroxetine markedly affected atomoxetine disposition, resulting in pharmacokinetics similar to poor metabolizers of CYP2D6 substrates.

CYP2D6 metabolizer status and atomoxetine dosing in children and adolescents with ADHD.

To determine whether physicians can adequately titrate atomoxetine without knowing genotype status for hepatic cytochrome P450 2D6, we pooled data from two open-label studies of atomoxetine in children and adolescents with attention-deficit/hyperactivity disorder. Patients were assessed weekly up to 10 weeks and doses titrated for efficacy and tolerability at the discretion of investigators (max. 1.8 mg/kg/d). Mean dose was 0.1 mg/kg/d lower in poor metabolizer (PM) patients (n=87) than extensive metabolizers (EMs, n=1239). PMs demonstrated marginally better efficacy on the ADHDRS-IV-Parent:Inv and had comparable safety profiles, except for a 4.0-bpm greater increase in mean pulse rate and a 1.0-kg greater weight loss. Changes from baseline in Fridericia QTc did not differ between groups or correlate with dose in PMs. Results suggest genotyping is unnecessary during routine clinical management, because investigators were able to dose atomoxetine to comparable efficacy and safety levels in EMs and PMs without knowledge of genotype metabolizer status.

Safety, tolerability, and pharmacokinetic evaluation of single- and multiple-ascending doses of a novel kappa opioid receptor antagonist LY2456302 and drug interaction with ethanol in healthy subjects.

Accumulating evidence indicates that selective antagonism of kappa opioid receptors may provide therapeutic benefit in the treatment of major depressive disorder, anxiety disorders, and substance use disorders. LY2456302 is a high‐affinity, selective kappa opioid antagonist that demonstrates >30‐fold functional selectivity over mu and delta opioid receptors. The safety, tolerability, and pharmacokinetics (PK) of LY2456302 were investigated following single oral doses (2–60 mg), multiple oral doses (2, 10, and 35 mg), and when co‐administered with ethanol. Plasma concentrations of LY2456302 were measured by liquid chromatography‐tandem mass spectrometry method. Safety analyses were conducted on all enrolled subjects. LY2456302 doses were well‐tolerated with no clinically significant findings. No safety concerns were seen on co‐administration with ethanol. No evidence for an interaction between LY2456302 and ethanol on cognitive‐motor performance was detected. LY2456302 displayed rapid oral absorption and a terminal half‐life of approximately 30–40 hours. Plasma exposure of LY2456302 increased proportionally with increasing doses and reached steady state after 6–8 days of once‐daily dosing. Steady‐state PK of LY2456302 were not affected by coadministration of a single dose of ethanol. No clinically important changes in maximum concentration (Cmax) or AUC of ethanol (in the presence of LY2456302) were observed.

Pharmacokinetics, Safety, and Tolerability of Atomoxetine and Effect of CYP2D6*10/*10 Genotype in Healthy Japanese Men

Atomoxetine is a cytochrome P4502D6 (CYP2D6) substrate. The reduced‐activity CYP2D6*10 allele is particularly prevalent in the Japanese population and may contribute to known ethnic differences in CYP2D6 metabolic capacity. The purpose of this study was to examine atomoxetine pharmacokinetics, safety, tolerability, and the effect of the CYP2D6*10/*10 genotype after single‐stepped dosing (10, 40, 90, or 120 mg) and at steady state (40 or 60 mg twice a day for 7 days) in 49 healthy Japanese adult men. Dose proportionality was shown and tolerability confirmed at all doses studied. Comparison of pharmacokinetics, safety, and tolerability between Japanese and US subjects showed no clinically meaningful ethnic differences. The CYP2D6*10/*10 subjects had 2.1‐ to 2.2‐fold and 1.8‐fold higher area under the plasma concentration—time curve values relative to the CYP2D6*1/*1 and *1/*2 subjects and the CYP2D6*1/*10 and *2/*10 subjects, respectively. The adverse events reported by CYP2D6*10/*10 subjects were indistinguishable from those of other Japanese participants. The higher mean exposure in CYP2D6*10/*10 subjects is not expected to be clinically significant.

Receptor Occupancy of the κ-Opioid Antagonist LY2456302 Measured with Positron Emission Tomography and the Novel Radiotracer 11C-LY2795050

The κ-opioid receptor (KOR) is thought to play an important therapeutic role in a wide range of neuropsychiatric and substance abuse disorders, including alcohol dependence. LY2456302 is a recently developed KOR antagonist with high affinity and selectivity and showed efficacy in the suppression of ethanol consumption in rats. This study investigated brain penetration and KOR target engagement after single oral doses (0.5–25 mg) of LY2456302 in 13 healthy human subjects. Three positron emission tomography scans with the KOR antagonist radiotracer 11C-LY2795050 were conducted at baseline, 2.5 hours postdose, and 24 hours postdose. LY2456302 was well tolerated in all subjects without serious adverse events. Distribution volume was estimated using the multilinear analysis 1 method for each scan. Receptor occupancy (RO) was derived from a graphical occupancy plot and related to LY2456302 plasma concentration to determine maximum occupancy (rmax) and IC50. LY2456302 dose dependently blocked the binding of 11C-LY2795050 and nearly saturated the receptors at 10 mg, 2.5 hours postdose. Thus, a dose of 10 mg of LY2456302 appears well suited for further clinical testing. Based on the pharmacokinetic (PK)-RO model, the rmax and IC50 of LY2456302 were estimated as 93% and 0.58 ng/ml to 0.65 ng/ml, respectively. Assuming that rmax is 100%, IC50 was estimated as 0.83 ng/ml.

Determining Pharmacological Selectivity of the Kappa Opioid Receptor Antagonist LY2456302 Using Pupillometry as a Translational Biomarker in Rat and Human

Background: Selective kappa opioid receptor antagonism is a promising experimental strategy for the treatment of depression. The kappa opioid receptor antagonist, LY2456302, exhibits ~30-fold higher affinity for kappa opioid receptors over mu opioid receptors, which is the next closest identified pharmacology. Methods: Here, we determined kappa opioid receptor pharmacological selectivity of LY2456302 by assessing mu opioid receptor antagonism using translational pupillometry in rats and humans. Results: In rats, morphine-induced mydriasis was completely blocked by the nonselective opioid receptor antagonist naloxone (3mg/kg, which produced 90% mu opioid receptor occupancy), while 100 and 300mg/kg LY2456302 (which produced 56% and 87% mu opioid receptor occupancy, respectively) only partially blocked morphine-induced mydriasis. In humans, fentanyl-induced miosis was completely blocked by 50mg naltrexone, and LY2456302 dose-dependently blocked miosis at 25 and 60mg (minimal-to-no blockade at 4–10mg). Conclusions: We demonstrate, for the first time, the use of translational pupillometry in the context of receptor occupancy to identify a clinical dose of LY2456302 achieving maximal kappa opioid receptor occupancy without evidence of significant mu receptor antagonism.

Population pharmacokinetic analysis of atomoxetine in pediatric patients

Atomoxetine is a novel treatment of ADHD in children, adolescents, and adults. The purpose of this analysis was to characterize atomoxetine pharmacokinetics and the potential influence of patient factors in pediatric patients. A population pharmacokinetic model was developed using the combined sparse and serial plasma data of 420 patients with 2354 observations from 5 pediatric studies. Patient factors with clinical and demographic significance were identified a priori and evaluated on model parameters. The final model was a 1‐compartment model and was validated using several methods. Covariates retained in the final model included CYP2D6 genotype, body weight, and food consumption. The results demonstrate linear pharmacokinetics across the dose range evaluated of 5 to 45 mg twice‐daily. CYP2D6 poor metabolizers had a 9‐fold lower clearance compared to extensive metabolizers. Clearance and volume of distribution increased nearly proportional to increased body weight, indicating dosing based on body weight is appropriate. Simulations showed that weight‐based dosing provides a more narrow and predictable range of exposures in patients. Food consumption decreased the rate of atomoxetine absorption, however the decrease (9% lower Cmax) was deemed clinically insignificant. Age, gender, ethnic origin, and caffeine consumption did not influence atomoxetine disposition. This analysis provided valuable data concerning these patient factors in the target patient population.