Itraconazole moderately increases serum concentrations of oxybutynin but does not affect those of the active metabolite

Adsorptive stripping voltammetric behavior and determination of anticholinergic agent oxybutynin chloride on a mercury electrode

Oxybutynin chloride is an antispasmodic, anticholinergic agent indicated for the treatment of overactive bladder with symptoms of urge urinary incontinence, urgency, and frequency. Its electrochemical behavior in phosphate buffers of pH range 2-10 at a hanging mercury drop electrode has been investigated using cyclic voltammetry, differential pulse cathodic adsorptive stripping voltammetry (DPCAdSV), and squarewave cathodic adsorptive stripping voltammetry (SWCAdSV). Voltammograms of the drug in phosphate buffer of pH 2-10 exhibited a single two-electron wave and it may be attributed to the reduction of the C[triple bond]C center. Based on the high adsorptive character of oxybutynin chloride onto the mercury electrode, a validated direct squarewave cathodic adsorptive stripping voltammetric and differential pulse cathodic adsorptive stripping voltammetric procedure has been developed for the determination of drug in bulk form and pharmaceutical formulation. The proposed SWCAdS and DPCAdS voltammetric methods allow quantitation over the range 1-18 and 1-17.6 microg mL(-1) with detection limits of 0.1 and 0.23 microg mL(-1), respectively. Precision and accuracy were also checked and were within the limits.

Transdermal oxybutynin: a review

Overactive bladder is a common and disabling problem. The mainstay of pharmacological treatment is with oral anticholinergic drugs. Anticholinergic side effects are common and include dry mouth and constipation. Compliance is limited by these side effects. Transdermal administration of oxybutynin has been shown to be as effective as oral treatment while minimising the anticholinergic side effects. Skin reactions occur frequently, necessitating changes of application site. Despite this, the preparation is a useful element in the armamentarium to treat overactive bladder. It is likely to be particularly useful in those in whom side effects of oral medication are intolerable or in whom oral administration of drug is not possible. Here, the pharmacokinetics, pharmacodynamics, efficacy and safety of transdermal oxybutynin are reviewed.

Treatment of the overactive bladder syndrome with muscarinic receptor antagonists - a matter of metabolites?

Antagonists of muscarinic acetylcholine receptors, such as darifenacin, oxybutynin, propiverine, solifenacin, tolterodine, and trospium, are the mainstay of the treatment of the overactive bladder syndrome. Fesoterodine is a newer drug awaiting regulatory approval. We briefly review the pharmacological activity of their metabolites and discuss how active metabolites may contribute to their efficacy and tolerability in vivo. Except for trospium, and perhaps solifenacin, all of the above drugs form active metabolites, and their presence and activity need to be taken into consideration when elucidating relationships between pharmacokinetics and pharmacodynamics of these drugs. Moreover, the ratios between parent compounds and metabolites may differ depending on genotype of the metabolizing enzymes, concomitant medication, and/or drug formulation. Differential generation of active metabolites of darifenacin or tolterodine are unlikely to influence the overall clinical profile of these drugs in a major way because the active metabolites exhibit a similar pharmacological profile as the parent compound. In contrast, metabolites of oxybutynin and propiverine may behave quantitatively or even qualitatively differently from their parent compounds and this may have an impact on the overall clinical profile of these drugs. We conclude that more comprehensive studies of drug metabolites are required for an improved understanding of their clinical effects.

Drug-Drug Interaction of Antifungal Drugs

This article reviews the in vitro metabolic and the in vivo pharmacokinetic drug-drug interactions with antifungal drugs, including fluconazole, itraconazole, micafungin, miconazole, and voriconazole. In the in vitro interaction studies, the effects of antifungal drugs on specific activities of cytochrome P450s (CYPs), including CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4, in human liver microsomes are compared to predict the possibility of drug interactions in vivo. Fluconazole, micafungin, and voriconazole have lower inhibitory effects on CYP3A4 activities than itraconazole and miconazole, and IC(50) and/or K(i) values against CYP2C9 and CYP2C19 activities are the lowest for miconazole, followed by voriconazole and fluconazole. In in vivo pharmacokinetic studies, it is well known that itraconazole is a potent clinically important inhibitor of the clearance of CYP3A4 substrates, and fluconazole and voriconazole are reported to increase the blood or plasma concentrations of not only midazolam and cyclosporine (CYP3A4 substrates) but also of phenytoin (CYP2C9 substrate) and/or omeprazole (CYP2C19/CYP3A4 substrate). On the other hand, no inhibition of CYP activities except for CYP3A4 activity by micafungin is observed in vitro, and the blood concentrations of cyclosporine and tacrolimus are not affected by coadministration of micafungin in vivo, suggesting that micafungin would not cause clinically significant interactions with drugs that are metabolized by CYPs via the inhibition of metabolism. Miconazole is a potent inhibitor of all CYPs investigated in vitro, although there are few detailed studies on the clinical significance of this except for CYP2C9. Therefore the differential effects of these antifungal drugs on CYP activities must be considered in the choice of antifungal drugs in patients receiving other drugs.

Rapid and selective UV spectrophotometric and RP-HPLC methods for dissolution studies of oxybutynin immediate-release and controlled-release formulations

A new UV spectrophotometric method and a reversed-phase HPLC method were developed for quantitative evaluation of oxybutynin hydrochloride (OXB) formulations. Determination of OXB by UV spectroscopic method was based on complexation of OXB with picric acid to form picrate, which was extracted to chloroform. The picrate complex showed quantifiable absorbance at 344nm. Chromatography was carried out at 25 degrees C on a 4.6mm x 250mm 5microm cyano column that contained USP packing L10 with water:methanol:acetonitrile::48:12:40 (v/v), as mobile phase. UV detector was set at 203nm. Both methods were found to be selective, linear, accurate and precise in the specified ranges. The LOD and LOQ of HPLC method were 0.5 and 1.65microg/ml, respectively. Intra-day and inter-day variability for both methods were <2% RSD. These methods were successfully used for quantification of OXB in drug-release studies from immediate-release tablets and controlled-release (CR) formulations.

Clinical pharmacokinetics of drugs used to treat urge incontinence

Urge incontinence (also known as overactive bladder) is a common form of urinary incontinence, occurring alone or as a component of mixed urinary incontinence, frequently together with stress incontinence. Because of the pathophysiology of urge incontinence, anticholinergic/antispasmodic agents form the cornerstone of therapy. Unfortunately, the pharmacological activity of these agents is not limited to the urinary tract, leading to systemic adverse effects that often promote nonadherence. Although the pharmacokinetics of flavoxate, propantheline, scopolamine, imipramine/desipramine, trospium chloride and propiverine are also reviewed here, only for oxybutynin and tolterodine are there adequate efficacy/tolerability data to support their use in urge incontinence. Oxybutynin is poorly absorbed orally (2-11% for the immediate-release tablet formulation). Controlled-release oral formulations significantly prolong the time to peak plasma concentration and reduce the degree of fluctuation around the average concentration. Significant absorption occurs after intravesical (bladder) and transdermal administration, although concentrations of the active N-desethyl metabolite are lower after transdermal compared with oral administration, possibly improving tolerability. Food has been found to significantly affect the absorption of one of the controlled-release formulations of oxybutynin, enhancing the rate of drug release. Oxybutynin is extensively metabolised, principally via N-demethylation mediated by the cytochrome P450 (CYP) 3A isozyme. The pharmacokinetics of tolterodine are dependent in large part on the pharmacogenomics of the CYP2D6 and 3A4 isozymes. In an unselected population, oral bioavailability of tolterodine ranges from 10% to 74% (mean 33%) whereas in CYP2D6 extensive metabolisers and poor metabolisers mean bioavailabilities are 26% and 91%, respectively. Tolterodine is metabolised via CYP2D6 to the active metabolite 5-hydroxymethyl-tolterodine and via CYP3A to N-dealkylated metabolites. Urinary excretion of parent compound plays a minor role in drug disposition. Drug effect is based upon the unbound concentration of the so-called 'active moiety' (sum of tolterodine + 5-hydroxymethyl-tolterodine). Terminal disposition half-lives of tolterodine and 5-hydroxymethyl-tolterodine (in CYP2D6 extensive metabolisers) are 2-3 and 3-4 hours, respectively. Coadministration of antacid essentially converts the extended-release formulation into an immediate-release formulation. Knowledge of the pharmacokinetics of these agents may improve the treatment of urge incontinence by allowing the identification of individuals at high risk for toxicity with 'usual' dosages. In addition, the use of alternative formulations (controlled-release oral, transdermal) may also facilitate adherence, not only by reducing the frequency of drug administration but also by enhancing tolerability by altering the proportions of parent compound and active metabolite in the blood.

Sensitive determination of oxybutynin and desethyloxybutynin in dog plasma by LC-ESI/MS/MS

A sensitive and selective liquid chromatographic method coupled with tandem mass spectrometry (LC-MS/MS) was developed for the quantification of oxybutynin and desethyloxybutynin in dog plasma. Diazepam was used as internal standard, with plasma sample extracted using n-hexane and back-extracted using hydrochloric acid. A centrifuged lower layer (aqueous layer) was injected into a C(18) XTerra MS column (2.1 x 30 mm(2)) with 3.5 microm particle size. The analytical column lasted for at least 500 injections. The mobile phase was composed of 90% methanol, with flow rate at 200 microl/min. The mass spectrometer was operated in positive ion mode using electrospray ionization. Nitrogen was used as the nebulizer gas and argon was used as the collision gas. Using MS/MS with multiple reaction monitoring (MRM) mode, oxybutynin and desethyloxybutynin were detected without severe interferences from plasma matrix. Oxybutynin produced a protonated precursor ion ([M+H](+)) at m/z 358 and a corresponding product ion at m/z 142. Desethyloxybutynin produced a protonated precursor ion ([M+H](+)) at m/z 330 and a corresponding product ion at m/z 96. And internal standard (diazepam) produced a protonated precursor ion ([M+H](+)) at m/z 285 and a corresponding product ion at m/z 193. Detection of oxybutynin and desethyloxybutynin in dog plasma were accurate and precise, with detection limit at 0.1 ng/ml. This method has been successfully applied to a study of oxybutynin and desethyloxybutynin in dog plasma.

Oxybutynin does not affect cyclosporin blood levels

Cyclosporin is an important immunosuppressive medication used to prevent organ rejection. Drug interactions that alter its blood levels can cause serious problems with toxicity or transplant rejection. Current evidence indicates that both cyclosporin and oxybutynin, which is used to treat bladder dysfunction, are metabolized by the cytochrome P450 3A enzyme system, raising the possibility of an adverse interaction between these medications. However, a study of two children receiving cyclosporin with and without oxybutynin revealed no significant changes in trough blood cyclosporin concentrations.

Differences in the effects of urinary incontinence agents S-oxybutynin and terodiline on cardiac K(+) currents and action potentials

1. The cardiac electrophysiological effects of S-oxybutynin, a single-enantiomer drug under evaluation for the management of urinary incontinence, have been investigated and compared with those of terodiline, an incontinence agent withdrawn following reports of QT lengthening and ventricular tachyarrhythmia. Membrane currents were recorded from whole-cell configured guinea-pig and rabbit ventricular myocytes, and action potentials were recorded from guinea-pig and rabbit papillary muscles. 2. L-type Ca(2+) current (I:(Ca,L)), rapidly-activating K(+) current (I:(Kr)) and slowly-activating K(+) current (I:(Ks)) were unaffected by submicromolar S-oxybutynin and inhibited by higher concentrations; IC(50) values were 17.8 microM for I:(Ca,L), 12 microM for I:(Kr), and 41 microM for I:(Ks). Terodiline IC(50) values were somewhat lower for I:(Ca,L) (15.2 microM) and I:(Ks) (30 microM), but 24 fold lower in the case of I:(Kr) (0.5 microM). 3. The durations of action potentials in guinea-pig and rabbit papillary muscles driven at 1 Hz were unaffected or moderately shortened by 0.1 - 100 microM S-oxybutynin, but lengthened by terodiline. Terodiline (< or =10 microM) also depressed maximal upstroke velocity. 4. The action potential plateau shortened by an average of 23% when control rabbit papillary muscles were driven at 0.4 Hz instead of 1 Hz. Plateau shortening was significantly smaller in the presence of drugs (30 microM S-oxybutynin, 3 and 30 microM terodiline), suggesting that they suppress the transient outward current (I:(to)) involved in rate-dependent shortening. In experiments on rabbit ventricular myocytes, 3 and 30 microM S-oxybutynin inhibited I:(to) by 9+/-2% and 35+/-3%, respectively, whereas 3 and 30 microM terodiline inhibited the current by 31+/-3% and 87+/-3%, respectively. 5. The results indicate that S-oxybutynin has relatively weak non-specific effects on cardiac ion channels, and that clinically relevant submicromolar concentrations are unlikely to have terodiline-like proarrhythmic actions on the myocardium.

Analysis of the electrophysiologic effects of short-term oxybutynin on guinea pig and rabbit ventricular cells

The objective of this study was to investigate the cardioactive properties of oxybutynin, a drug that is widely prescribed for management of voiding dysfunction. Membrane currents were recorded from whole-cell-configured guinea pig ventricular myocytes, and action potentials were recorded from guinea pig and rabbit papillary muscles. L-type Ca2+ current (I(Ca),L), inward-rectifier K+ current (I(K1)), and delayed-rectifier K+ current (I(K)) were unaffected by < or = 1 microM oxybutynin, and inhibited by higher concentrations. The concentrations that reduced the currents to one-half of predrug control amplitude (K0.5) were as follows: 1(Ca),L, 16.1 microM, I(K1), 18.2 microM, rapidly activating I(K)(I(Kr)), 11.4 microM, and slowly activating I(K)(I(Ks)), 28.7 microM. Action-potential durations at 20 and 90% repolarization (APD20, APD90) were unaffected by oxybutynin < or =3 microM in guinea pig papillary muscles driven at 1 Hz; higher concentrations selectively shortened the APD20 by as much as 25% (100 microM), and caused moderate reductions in maximal upstroke velocity. Changes in the action potentials of rabbit papillary muscles were even smaller than in the guinea pig muscles. Because the peak therapeutic plasma concentration of oxybutynin is in the 0.01-0.1 microM range, the results suggest that the drug is highly unlikely to have adverse effects on cardiac electrical activity.

Drug interactions with itraconazole, fluconazole, and terbinafine and their management

A drug interaction develops when the effect of a drug is increased or decreased or when a new effect is produced by the prior, concurrent, or subsequent administration of the other. Before prescribing a drug, it is important to obtain a thorough drug history of the prescription and nonprescription medications taken by the patient. The nonprescription medications may include items such as nutritional supplements and herbal medications. The risk of side effects is an inevitable consequence of drug use. The frequency of adverse reactions is increased in those patients receiving multiple medications. Drug interactions reported in animal or in vitro studies may not necessarily develop in humans. When drug interactions are observed with a particular agent, it cannot be automatically assumed that all closely related drugs will necessarily produce the same interaction. However, caution is advised until sufficient experience accrues. The prescriber should not overestimate or underestimate the potential for a given drug interaction on the basis of personal experience alone. Drug interactions will not necessarily occur in every patient who is given a particular combination of drugs known to produce an interaction. For a clinically significant drug interaction to be manifest, several other factors may be relevant other than just using the two drugs. In many instances drug interactions can be predicted and therefore avoided if the pharmacodynamic effects, the pharmacokinetic properties, and the mechanisms of action of the 2 drugs in question are known. In the case of contraindicated drugs, it may be possible to use an alternative agent.

Cytochrome P450 specificity of metabolism and interactions of oxybutynin in human liver microsomes

Oxybutynin has an extensive first pass metabolism after oral administration, the main active metabolite being N-desethyloxybutynin. The purpose of this study was to investigate the CYP isoform specificity of oxybutynin N-deethylation and possible interactions. Oxybutynin N-deethylation in human liver microsomes in vitro was potently inhibited by ketoconazole (IC50 4.5 microM), less and variably by itraconazole and not by quinidine or several other reference inhibitors, suggesting that CYP3A enzymes are predominant catalysts of the reaction. Recombinant CYP3A5 enzyme had higher activity in oxybutynin N-deethylation than recombinant CYP3A4. Ketoconazole inhibited oxybutynin N-deethylation by the recombinant CYP3A4 and CYP3A5 almost completely, whereas itraconazole inhibited the activity of CYP3A4 more potently than that of CYP3A5. Oxybutynin inhibited CYP3A4- and CYP2D6- associated activities (testosterone 6 beta-hydroxylase and dextromethorphan O- demethylase, respectively) in human liver microsomes. CYP1A1/2-, CYP2A6-, CYP2C9- and CYP2E1-associated activities were inhibited less potently or not at all by oxybutynin when compared with reference inhibitors. Although the reasons for the weak and variable inhibition by itraconazole remain to be studied, it seems that oxybutynin is predominantly metabolized by CYP3A4 and CYP3A5 but not by CYP2D6. However, it seems to have some affinity also to the latter enzyme.