ABSTRACT:Studies were performed to determine the human enzymes responsible for the biotransformation of atomoxetine to its major metabolite, 4-hydroxyatomoxetine, and to a minor metabolite, N-desmethylatomoxetine. Utilizing human liver microsomes containing a full complement of cytochrome P450 (P450) enzymes, average K m and CL int values of 2.3 M and 103 l/min/mg, respectively, were obtained for 4-hydroxyatomoxetine formation. Microsomal samples deficient in CYP2D6 exhibited average apparent K m and CL int values of 149 M and 0.2 l/min/mg, respectively. In a human liver bank characterized for P450 content, formation of 4-hydroxyatomoxetine correlated only to CYP2D6 activity. Of nine expressed P450s examined, 4-hydroxyatomoxetine was formed at a rate 475-fold greater by CYP2D6 compared with the other P450s. These results demonstrate that CYP2D6 is the enzyme primarily responsible for the formation of 4-hydroxyatomoxetine. Multiple P450s were found to be capable of forming 4-hydroxyatomoxetine when CYP2D6 was not expressed. However, the efficiency at which these enzymes perform this biotransformation is reduced compared with CYP2D6. The formation of the minor metabolite Ndesmethylatomoxetine exhibited average K m and CL int values of 83 M and 0.8 l/min/mg, respectively. Utilizing studies similar to those outlined above, CYP2C19 was identified as the primary enzyme responsible for the biotransformation of atomoxetine to Ndesmethylatomoxetine. In summary, CYP2D6 was found to be the primary P450 responsible for the formation of the major oxidative metabolite of atomoxetine, 4-hydroxyatomoxetine. Furthermore, these studies indicate that in patients with compromised CYP2D6 activity, multiple low-affinity enzymes will participate in the formation of 4-hydroxyatomoxetine. Therefore, coadministration of P450 inhibitors to poor metabolizers of CYP2D6 substrates would not be predicted to decrease the clearance of atomoxetine in these individuals.Atomoxetine (Fig. 1) (formally known as tomoxetine; LY139603) is under development as a therapeutic agent for the treatment of attention deficit hyperactivity disorder in children and adults. Atomoxetine enhances norepinephrine function through a highly selective blockade of the presynaptic norepinephrine transporter and has low affinities for other neuronal transporters or neurotransmitter receptor sites (Wong et al., 1982;Gehlert et al., 1993). This is an interesting and potentially important new drug since it is likely to be the first approved treatment for attention deficit hyperactivity disorder that is not a psychostimulant. Studies with this compound in healthy human volunteers (Farid et al., 1985) showed that the clearance of atomoxetine exhibited a bimodal distribution, suggesting that an enzyme that exhibits a genetic polymorphism was involved in the metabolism of atomoxetine. The study further reported that in both extensive and poor metabolizers of atomoxetine, para-hydroxyatomoxetine (later definitively identified as 4-hydroxyatomoxetine) was the major oxidative metabo...

In vitro results suggested that tadalafil would have little effect on the pharmacokinetics of drugs metabolized by CYP3A. Clinical studies demonstrated that the pharmacokinetics of 2 different CYP3A substrates, midazolam and lovastatin, were virtually unchanged after tadalafil coadministration. Thus therapeutic concentrations of tadalafil do not produce clinically significant changes in the clearance of drugs metabolized by CYP3A.

In the studies reported here, the ability of atomoxetine hydrochloride (Strattera) to inhibit or induce the metabolic capabilities of selected human isoforms of cytochrome P450 was evaluated. Initially, the potential of atomoxetine and its two metabolites, N-desmethylatomoxetine and 4-hydroxyatomoxetine, to inhibit the metabolism of probe substrates for CYP1A2, CYP2C9, CYP2D6, and CYP3A was evaluated in human hepatic microsomes. Although little inhibition of CYP1A2 and CYP2C9 activity was observed, inhibition was predicted for CYP3A (56% predicted inhibition) and CYP2D6 (60% predicted inhibition) at concentrations representative of high therapeutic doses of atomoxetine. The ability of atomoxetine to induce the catalytic activities of CYP1A2 and CYP3A in human hepatocytes was also evaluated; however, atomoxetine did not induce either isoenzyme. Based on the potential of interaction from the in vitro experiments, drug interaction studies in healthy subjects were conducted using probe substrates for CYP2D6 (desipramine) in CYP2D6 extensive metabolizer subjects and CYP3A (midazolam) in CYP2D6 poor metabolizer subjects. Single-dose pharmacokinetic parameters of desipramine (single dose of 50 mg) were not altered when coadministered with atomoxetine (40 or 60 mg b.i.d. for 13 days). Only modest changes (approximately 16%) were observed in the plasma pharmacokinetics of midazolam (single dose of 5 mg) when coadministered with atomoxetine (60 mg b.i.d. for 12 days). Although at high therapeutic doses of atomoxetine inhibition of CYP2D6 and CYP3A was predicted, definitive in vivo studies clearly indicate that atomoxetine administration with substrates of CYP2D6 and CYP3A does not result in clinically significant drug interactions.Atomoxetine hydrochloride (Strattera; formerly known as tomoxetine hydrochloride) is known chemically as (Ϫ)-N-methyl-3-phenyl-3-(o-tolyloxy)-propylamine hydrochloride. Atomoxetine is a potent inhibitor of the presynaptic norepinephrine transporter with minimal affinity for other monoamine transporters or receptors (Wong et al., 1982;Gehlert et al., 1993) and is used clinically for the treatment of attention-deficit/hyperactivity disorder in children, adolescents, and adults.Atomoxetine is rapidly and completely absorbed after oral administration . The plasma pharmacokinetics of atomoxetine are linear over the recommended therapeutic dosing range (0.5-1.4 mg/kg) with proportional increases in both mean atomoxetine maximum plasma concentration (C max ) and area under the plasma concentration time curve (AUC) with increasing dose . Atomoxetine is predominantly metabolized by CYP2D6 (Ring et al., 2002); therefore, its pharmacokinetics and metabolism are influenced by the polymorphic expression of this enzyme (Farid et al., 1985;Sauer et al., 2003). As a result, the systemic clearance values of atomoxetine seem to be distributed in a bimodal manner. The enzymatic activity of CYP2D6 is regulated by a genetic polymorphism resulting in two major populations of individuals with either active met...

ABSTRACT:Studies were performed to determine the cytochromes P450 (P450) responsible for the biotransformation of (S) Over the last several years, evidence has accumulated that implicates the hyperglycemia-induced activation of protein kinase C (PKC 1 ) as one of the mechanisms responsible for the development and/or progression of chronic complications of diabetes. Hyperglycemia-induced increases in diacylglycerol, a physiological activator of PKC, have been demonstrated in the organs that are susceptible to developing diabetic complications, including the retina, kidney, aorta, and heart (Craven and DeRubertis, 1989;Ayo et al., 1991;Inoguchi et al., 1992;Shiba et al., 1993). In diabetes, hyperglycemia-induced generation of diacylglycerol activates the beta 1 and 2 isoforms of the PKC gene family (Inoguchi et al., 1992;Ishii et al., 1996). (S)-9-((Dimethylamino)methyl)-6,7,10,11-tetrahydro-9H,18H-5,21:12,17-dimethenodibenzo(e,k)pyrrolo(3,4-h)(1,4,13)oxadiaza-cyclohexadecine-18,20(19H)-dione (LY333531) (Engel et al., 2000), is a selective inhibitor of PKC beta. LY333531 and its N-desmethyl metabolite, which is formed in animals and humans, inhibit PKC beta 1 and 2 isoforms with an approximate IC 50 of 5 nM . Therefore, LY333531 may be useful in the treatment of diabetic complications and is currently under clinical development for the treatment of diabetic microvascular complications including retinopathy, macular edema, and peripheral neuropathy.-In vitro methodologies using human liver tissue have been developed to aid in the prediction of possible variation in metabolic clearance in vivo and drug-drug interactions for a new molecular entity. The studies described herein used these in vitro techniques to identify the P450(s) responsible for the formation of the major (and equipotent) metabolite of LY333531, N-desmethyl LY333531. The initial step in the identification of these enzymes was a kinetic analysis of the formation of N-desmethyl LY333531 following incubations of the drug with human liver microsomes. The identification of the enzyme(s) involved in the formation of N-desmethyl LY333531 was then accomplished by correlating the rate of formation of the metabolite with immunoquantified levels and/or the associated formselective catalytic activities for the drug-metabolizing enzymes by a bank of human liver microsomes. The ability of specific cDNAexpressed cytochromes P450 (P450s) to form N-desmethyl LY333531 was used to corroborate the results of the correlation studies. Finally, P450-selective inhibitors were used to examine their effect on the formation of the metabolite in question.To predict interactions that may occur in the clinical setting between LY333531 and coadministered drugs, the ability of the PKC beta inhibitor, LY333531, and its N-desmethyl metabolite to inhibit metabolism mediated by CYP3A, CYP2D6, CYP1A2, and CYP2C9 was examined. Using in vitro metabolism of specific form-selective substrates as probes of metabolism, CYP2D6, CYP2C9, CYP1A2,