The disposition and metabolism of I.C.I. 58,834 (viloxazine) in animals

Viloxazine in the Management of CNS Disorders: A Historical Overview and Current Status

Viloxazine has a long history of clinical use in Europe as an antidepressant, and has recently been repurposed into an extended-release form for the treatment of attention-deficit/hyperactivity disorder in the USA. An immediate-release formulation was approved for the treatment of depression in the UK in 1974, and was subsequently marketed there and in several European countries for 30 years with no major safety concerns. In contrast to first-generation antidepressants (e.g., tricyclic antidepressants, monoamine oxidase inhibitors), viloxazine was associated with a relatively low risk for cardiotoxicity. Gastrointestinal symptoms were the most commonly reported side effects. The therapeutic effects of viloxazine are thought to be primarily the result of its action as a norepinephrine reuptake inhibitor, although in vitro and preclinical in vivo animal data suggest that viloxazine may also impact the serotoninergic system. This review summarizes the evolving knowledge of viloxazine based on information from previously published preclinical and clinical investigations, and acquired unpublished historical study reports from both open-label and blinded controlled clinical trials. We review the chemical properties, mechanism of action, safety, and tolerability across these studies, and discuss the contemporary rationale for the development of this agent as an extended-release oral formulation for the treatment of attention-deficit/hyperactivity disorder.

Metabolism and in vitro drug-drug interaction assessment of viloxazine

Viloxazine is currently being developed as a treatment for attention deficit/hyperactivity disorder (ADHD). The aim of these studies is to update the understanding of the rat and human metabolism and the in vitro drug-drug interaction profile of viloxazine to a degree where it meets current regulatory standards for such investigations. In vivo absorption-distribution-metabolism-excretion (ADME) studies demonstrated that in humans 5-hydroxylation followed by glucuronidation is the major metabolic route. This route was also seen as a minor route in rats where the major route is O-deethylation with subsequent sulfation. In humans, the 5-hydoxylation pathway is mediated by CYP2D6. An estimate for the fraction of the metabolism via this pathway suggests a PM/EM difference of <2-fold, making it highly unlikely that this will be an issue of clinical significance. Viloxazine forms a unique N-carbamoyl glucuronide in humans. The chemical reactivity characteristics of this metabolite are similar to stable glucuronide conjugates and dissimilar from acyl glucuronides; therefore, it is regarded as a stable Phase II conjugate. In vitro drug-drug interaction (DDI) testing indicates that viloxazine is not a significant inhibitor or inducer of CYPs and transporters with the exception of CYP1A2.

Evaluation of rat intestinal absorption data and correlation with human intestinal absorption

The absorption of 111 drug and drug-like compounds was evaluated from 111 references based on the ratio of urinary excretion of drugs following oral and intravenous administration to intact rats and biliary excretion of bile duct-cannulated rats. Ninety-eight drug compounds for which both human and rat absorption data were available were selected for correlation analysis between the human and rat absorption. The result shows that the extent of absorption in these two species is similar. For 94% of the drugs the absorption difference between humans and rats is less than 20% and for 98% of drugs the difference is less than 30%. There is only one drug for which human absorption is significantly different from rat absorption. The standard deviation is 11% between human and rat absorption. The linear relationship between human and rat absorption forced through the origin, as determined by least squares regression, is %Absorption (human)=0.997%Absorption (rat) (n=98, SD=11). It is suggested that the absorption in rats could be used as an alternative method to human absorption in pre-clinical oral absorption studies.

Metabolism of some "second"- and "fourth"-generation antidepressants: iprindole, viloxazine, bupropion, mianserin, maprotiline, trazodone, nefazodone, and venlafaxine

1. This review summarizes the major known aspects of the metabolism of second-generation (iprindole, viloxazine, bupropion, mianserin, maprotiline, and trazodone) and fourth-generation (nefazodone and venlafaxine) antidepressants. 2. Discussions about specific enzymes involved and about possible pharmacokinetic drug-drug interactions, particularly as they relate to cytochrome P450 enzymes, are provided.

Comparison of the disposition and of the metabolic pattern of Reboxetine, a new antidepressant, in the rat, dog, monkey and man

The purpose of this study was to compare the disposition and the metabolic pattern of Reboxetine in several species, including man. [14C]-Reboxetine was given orally to the rat, the dog, the monkey (5 mg/kg) and man (2 and 4 mg/kg). Radioactivity was eliminated both by the renal and faecal route in the rat and the dog, mainly in urine in the monkey and man. Reboxetine was extensively metabolized. A number of urinary metabolites were quantified by radio-HPLC and tentatively identified by comparison with the retention times of reference compounds. Suggested routes of metabolic transformation are: 2-O-dealkylation; hydroxylation of the ethoxyphenoxy ring; oxidation of the morpholine ring; morpholine ring-opening; and combinations of these. Metabolites were partially or completely conjugated with glucuronic acid and/or sulphuric acid.

Identification of the biliary metabolites of (+/-)-3-dimethylamino-1,1-diphenylbutane HCl (recipavrin) in rats

1. The in vivo biliary metabolites of (+/-)-3-dimethylamino-1,1-diphenylbutane hydrochloride (recipavrin) isolated from Wistar rats have been characterized by g.l.c.-mass spectrometry. 2. Non-conjugated metabolites include recipavrin (1), norrecipavrin (2), diphenylbutanone (3), diphenylbutanone oxime (4), diphenylbutanone phenol (12), diphenylbutanone oxime phenol (14), recipavrin phenol (19), diphenylbutanone O-methylcatechol (16) and diphenylbutanone oxime O-methylcatechol (18). 3. Following beta-glucuronidase hydrolysis and extraction from pH 10 solution, diphenylbutanone (3), diphenylbutanone oxime (4), an unidentified compound (6), primary amine (8), norrecipavrin (2), recipavrin (1), phenols (12, 14, 15), norrecipavrin phenol (13), O-methylcatechols (16, 18), diphenylbutanol O-methylcatechol (17), recipavrin O-methylcatechol (19) and a secondary formamide (5) were identified by g.l.c.-mass spectrometry. 4. Various extraction solvents were employed in sample workup. The formamide (5) was present regardless of solvent used, while the trace presence of secondary acetamide (7) may be associated with the use of ethyl acetate. 5. Metabolites isolated after beta-glucuronidase hydrolysis were characterized by g.l.c.-mass spectrometry of the underivatized form, and as the trimethylsilyl (TMS) derivatives, or following methylation with diazomethane or trimethylanilinium hydroxide (TMAH).

Implications of chirality and geometric isomerism in some psychoactive drugs and their metabolites

Many drugs contain a chiral centre, or such a centre is introduced during metabolism of the drug in man and in animals. If a single chiral centre is present, the drug will normally exist as a mixture of two enantiomers, of which one may have quite different pharmacologic and/or toxic effects than the other. Chiral drugs that are used in psychiatry, and some other pharmacologically related drugs are identified, and the implications of the presence of one or two chiral centres in these drugs are discussed. Differences in pharmacologic properties of drug and metabolite enantiomers are identified and discussed. Also reviewed are the properties of some drugs used in psychiatry that both are chiral and display geometric isomerism.

Disposition and metabolism of indeloxazine hydrochloride, a cerebral activator, in rats

1. The disposition and metabolism of indeloxazine hydrochloride ((+/-)-2-[(inden-7-yloxy)methyl]morpholine hydrochloride) were studied in male Sprague-Dawley rats. 2. After oral administration of 14C-indeloxazine hydrochloride, the plasma concentration of total radioactivity reached a maximum at 15 min and declined with an apparent half-life of 2.2 h in the first 6 h period and declined more slowly thereafter. Unchanged drug in the plasma represented 13.5%, 5.9% and 0.4% of the total radioactivity at 15 min, 1 h and 6 h respectively after administration and levels decayed with a half-life of 0.9 h. 3. After oral and i.v. administration of the labelled compound, the urinary and faecal excretion of radioactivity in 72 h were 61-65% and 31-36% of the dose, respectively. Biliary excretion in bile duct-cannulated animals amounted to 49% of the dose in 72 h. 4. Seven metabolites have been isolated from the plasma or urine and characterized by i.r., n.m.r. and mass spectrometry. They were derived through dihydrodiol formation in the indene ring, hydroxylation of the indene ring and N-acetylation, oxidation and oxidative degradation of the morpholine ring. Some metabolites were excreted as their glucuronic acid or glucose conjugates. The major metabolite appeared to the trans-indandiol analogue of indeloxazine. 5. Possible metabolic pathways of degradation of the morpholine ring are discussed.

The metabolism of aromatic amines

Aromatic amines are of general interest in drug metabolism and some are a health hazard, particularly as bladder carcinogens. Conditions for the biological ring- and N-oxidation of aniline and its derivatives are reviewed. The metabolism of 2-naphthylamine and aminobiphenyls and the involvement of metabolites of aromatic amines in bladder cancer is discussed.

Species differences in oxidative drug metabolism: some basic considerations

Perhaps one of the single most important developments in the past 20 years in the understanding of chemical toxicity has been the realisation of the importance of metabolic transformation in this process. It is now widely appreciated that the toxic effects of many chemicals is a function of their metabolism rather than the substance itself. Of central interest to the toxicologist therefore is an understanding of the metabolism of a toxic chemical and the significance of this in the toxic process. The metabolic process itself however can be highly variable both between and within animal species. For this reason the toxicologist may have to consider both species and strain differences in metabolism when attempting to extrapolate findings to man in the safety evaluation process. For the past twenty years, work on species differences in metabolism has been largely of a descriptive nature and the cataloguing of differences. However, developments in the last few years in the understanding of the genetic diversity of species, including man, in terms of biotransformation and the nature and substrate preferences of the various multiple forms of the drug-metabolizing enzymes now give a better insight into the nature of species differences of metabolism. Furthermore, an understanding of this problem tempers expectations in terms of what may be hoped for in the extrapolation from other species. For example, the search for a species that metabolizes like man will be seen to be ill-conceived and ill-advised. The presentation deals with some of the fundamental aspects of species and strain differences in oxidative metabolism in particular and the implications that this has for the toxicologist in the safety evaluation process.

Pharmacokinetics and toxicity testing

The pharmacokinetic basis for the design of toxicity tests is discussed with reference to the absorption and clearance of drugs. The absorption and clearance of a wide range of drugs by laboratory animals and man has been examined and reviewed to provide a firm basis against which new drugs can be compared. Some pitfalls in either the empirical approach to toxicology or the incorrect interpretation of kinetic data are highlighted. An approach is outlined for the rational application of animal pharmacokinetic data in the assessment of the safety in man of a new therapeutic agent.

Urinary metabolites of oxapadol (MD720111), a new non-narcotic analgesic agent, in the rat, dog and man

1. A number of metabolites of oxapadol were isolated from urine of rat, dog and man after administration of a single dose of 14C-labelled compound. They were identified by direct inlet mass spectrometry and chromatographic comparison with reference compounds. 2. Oxapadol was extensively metabolized and the unchanged drug was undetectable in rat or human urine; only traces were found in dog urine. Nine metabolites were identified in rat and dog urine, and six in man. 3. The routes of biotransformation were: (a) aromatic hydroxylation, mainly in the benzimidazole ring, (b) scission of the heterocyclic ring following two different pathways, and (c) a combination of the two. Regioselectivity was observed for aromatic hydroxylation, as only three of the four possible monohydroxy oxazepinobenzimidazoles could be detected.

Relationship between blood and cerebrospinal levels of the antidepressant agent viloxazine

Viloxazine levels in blood and CSF have been measured following acute and chronic dosing in depressed patients. Blood profiles confirm previous findings that viloxazine is rapidly absorbed and eliminated with a half-life of 4.5 h. Viloxazine crosses the blood-brain barrier and concentrations in CSF remain virtually unchanged over a ten hour period post administration. Viloxazine does not accumulate in CSF on chronic administration. The fact that CSF levels do not reflect concentrations in blood has significant implications on any attempt to correlate the clinical efficacy and the pharmacokinetic behaviour of an antidepressant agent.

Metabolites of pipemidic acid in human urine

1. The urine of men dosed orally with pipemidic acid, contained the metabolites acetylpipemidic, formylpipemidic and oxopipemidic acids together with unchanged pipemidic acid. 2. Each metabolite was equivalent to less than 2% of the unchanged pipemidic acid present in human urine. 3. All metabolites showed a similar antibacterial spectrum to pipemidic acid, but their potency was about 10 times lower than that of pipemidic acid. The acute toxicity of the metabolites in mice was as low as the parent compound. 4. These results indicate that the role of the metabolites in the clinical efficacy and toxicity of the drug is likely to be small.

N-Formylation of an aromatic amine as a metabolic pathway

1. Feeding 2-aminoanthraquinone (2-AAQ) in the diet to Fischer rats led to nephrotoxicity in females, caused by deposits of crystalline material in the kidney tubules. 2. This material consisted of 2-AAQ, N-acetyl-2-AAQ and N-formyl-2-AAQ. N-Formyl-2-AAQ was also identified in the ether extract of urine of rats fed 2-AAQ. 3. This represents the first case of identification of the N-formyl derivative of a primary aromatic amine as a metabolite in vivo.

Incorporation into endogenous metabolic pathways of small fragments derived from I.C.I, 58,834(vioxazine)

1. Metabolic degradation of the tetrahydro-oxazine ring of 2-(2-ethoxyphenoxymethyl)-2,3,5,6-tetrahydro-1,4-oxazine (I.C.I. 58,834) gives rise to one- or two-carbon fragments which are utilized by endogenous metabolic pathways. 2. Evidence of this in dogs is shown by the 14C-labelled residues in tissues, 14C-labelled material in blood which has a half-life of three weeks, and elimination of [14C]urea in urine. 3. The same phenomenon occurs in rat, mouse and man, but to a smaller extent than in the dog. 4. Intravenous administration of [14C]ethanolamine to a dog gave rise to residual 14C blood levels with a half-life comparable to that produced by metabolic incorporation of 14C from 14C-I.C.I. 58,834.