Plasma protein binding study of oxybutynin by high-performance frontal analysis

A Population Pharmacokinetic Model of (R)- and (S-) Oxybutynin and Its Active Metabolites After Oral and Intravesical Administration to Healthy Volunteers

Oxybutynin is a racemic anticholinergic drug used for the symptomatic treatment of detrusor overactivity. The formation of active metabolites related to tolerability problems depends on the route of administration. The objective of this evaluation was to develop a pharmacokinetic model for oral/intravesical administration as the basis for simulations with different dosages. Data from a published changeover clinical study with 18 healthy adults receiving a single oral dose of 5 mg immediate-release oxybutynin and single and multiple intravesical doses of 10 mg oxybutynin solution was evaluated. Enantioselective plasma concentrations of oxybutynin and N-desethyloxybutynin (NDO) were used to establish a population pharmacokinetic model using nonlinear mixed-effects modeling with NONMEM 7.4.1. For both enantiomers, the data were described well by a 2-compartment model for oxybutynin with an additional compartment for NDO. Oxybutynin absorption was modeled by transit compartments for oral and first-order absorption for intravesical application. Bioavailability of the more active (R)-enantiomer was 7% for oral and 10%-22% for intravesical administration. In simulations, intravesical doses of 5 to 15 mg (R)-oxybutynin administered 2 to 3 times daily decreased peak-trough fluctuations of NDO to 8% compared with 24% after oral administration. The NDO/oxybutynin ratio was reduced from 17 after oral administration to unity. Chronic intravesical versus oral administration of (R)-oxybutynin generates distinctly lower and less variable concentrations of (R)-NDO. Pharmacokinetic simulations suggest that exposure for 12.5 mg (R)-oxybutynin administered twice daily might not compromise efficacy and tolerability compared with exposure for standard thrice-daily administrations. This assumption needs to be assessed in clinical studies.

Development of a Novel Simplified PBPK Absorption Model to Explain the Higher Relative Bioavailability of the OROS® Formulation of Oxybutynin

A new minimal Segmented Transit and Absorption model (mSAT) model has been recently proposed and combined with intrinsic intestinal effective permeability (P _eff,int ) to predict the regional gastrointestinal (GI) absorption (f _abs ) of several drugs. Herein, this model was extended and applied for the prediction of oral bioavailability and pharmacokinetics of oxybutynin and its enantiomers to provide a mechanistic explanation of the higher relative bioavailability observed for oxybutynin's modified-release OROS® formulation compared to its immediate-release (IR) counterpart. The expansion of the model involved the incorporation of mechanistic equations for the prediction of release, transit, dissolution, permeation and first-pass metabolism. The predicted pharmacokinetics of oxybutynin enantiomers after oral administration for both the IR and OROS® formulations were in close agreement with the observed data. The predicted absolute bioavailability for the IR formulation was within 5% of the observed value, and the model adequately predicted the higher relative bioavailability observed for the OROS® formulation vs. the IR counterpart. From the model predictions, it can be noticed that the higher bioavailability observed for the OROS® formulation was mainly attributable to differences in the intestinal availability (F _G ) rather than due to a higher colonic f _abs , thus confirming previous hypotheses. The predicted f _abs was almost 70% lower for the OROS® formulation compared to the IR formulation, whereas the F _G was almost eightfold higher than in the IR formulation. These results provide further support to the hypothesis of an increased F _G as the main factor responsible for the higher bioavailability of oxybutynin's OROS® formulation vs. the IR.

Pretreatment of plasma samples by a novel hollow fiber centrifugal ultrafiltration technique for the determination of plasma protein binding of three coumarins using acetone as protein binding releasing agent

A novel and practical sample pretreatment method based on hollow fiber centrifugal ultrafiltration (HFCF-UF) was developed to determine plasma protein binding by using HPLC. The samples for analyzing unbound and total concentrations could be prepared in parallel simultaneously by the same device. It only required centrifugation for a short time and the filtrate could be injected directly for HPLC analysis without further treatment. Coumarins were selected as the model drugs. Acetone was chosen as the releasing agent to free the binding drug from the drug-protein complex for the total drug concentration determination. Non-specific bindings (NSBs) between the analytes and hollow fiber membrane materials were investigated. The type and volume of protein binding releaser were optimized. Additionally, centrifugal speed and centrifugal time were considered. Under the optimized conditions, the absolute recovery rates of the unbound and total concentrations were in the range of 97.5-100.9% for the three analytes. The limits of detection were in the range of 0.0135-0.0667μgmL(-1). In vitro plasma protein binding of the three coumarins was determined at three concentrations using the validated method and the relative standard deviations (RSDs) were less than 3.4%. Compared with traditional method, the HFCF-UF method is simple to run, no specialized equipment requirement and is a more accurate plasma pretreatment procedure with almost excellent drug-protein binding equilibrium. Therefore, this method can be applied to determine the plasma protein binding in clinical practice. It also provides a reliable alternative for accurate monitoring of unbound or total drug concentration in therapeutic drug monitoring (TDM).

Stereoselective binding of chiral drugs to plasma proteins

Chiral drugs show distinct biochemical and pharmacological behaviors in the human body. The binding of chiral drugs to plasma proteins usually exhibits stereoselectivity, which has a far-reaching influence on their pharmacological activities and pharmacokinetic profiles. In this review, the stereoselective binding of chiral drugs to human serum albumin (HSA), α1-acid glycoprotein (AGP) and lipoprotein, three most important proteins in human plasma, are detailed. Furthermore, the application of AGP variants and recombinant fragments of HSA for studying enantiomer binding properties is also discussed. Apart from the stereoselectivity of enantiomer-protein binding, enantiomer-enantiomer interactions that may induce allosteric effects are also described. Additionally, the techniques and methods used to determine drug-protein binding parameters are briefly reviewed.

Specific pharmacokinetic aspects of the urinary tract

This chapter reviews the evidence for "specific" pharmacokinetics playing a role in currently marketed drugs intended to treat lower urinary tract (LUT) symptoms. Principles of drug targeting include intrinsic properties of drugs or organs as well as drug formulations to modify drug release or to create confinement of drug presence. Prodrugs and specific formulations to deliver high drug concentrations at the site(s) of action as well as other ways to manipulate drug distribution to achieve enrichment in target tissues are considered. In overactive bladder (OAB), specific formulations for oxybutynin have been introduced to reduce the level of side effects of the active drug. Extended release tablet formulations and a topical gel formulation have been introduced, with efficacy similar to immediate release (IR) tablets, but with a reduction in anticholinergic adverse effects. However, these modifications have not led to outstanding performance parameters compared to other anticholinergic drugs marketed as IR formulations. Urinary excretion is discussed as potential mechanism for targeting LUT symptoms, but no strong indications appear to exist that this mechanism would contribute for currently available drugs. Intravesical administration of drugs is not a preferred option and only considered for drugs like botulinum toxin, where the inconvenient application compensates for a reasonable degree of long-term efficacy in severe refractory OAB. Alpha acid glycoprotein binding is discussed as a potential factor to influence drug tissue distribution, and it is concluded that there is reasonable evidence that for tamsulosin this mechanism is responsible for the difference in free fraction of the drug observed in plasma and prostate, which could contribute to its relative absence of blood pressure effects in patients with LUT symptoms related to benign prostate hyperplasia (LUTS-BPH). The principle of irreversible inhibition of type II 5α-reductase as a tool to develop drugs to reduce prostatic levels of dihydrotestosterone is employed by both dutasteride and finasteride for treatment of LUTS-BPH. Of the mechanisms discussed, the principles employed for the 5α-reductase blockers and tamsulosin in this respect can be considered relatively specific for its urological indication.

Stereoselective pharmacokinetics of oxybutynin andN-desethyloxybutyninin vitroandin vivo

In vitro studies and the multiple applications of an oxybutynin (OXY) transdermal delivery system to Japanese healthy volunteers were conducted to characterize the stereoselectivity in the pharmacokinetics of OXY and its metabolite, N-desethyloxybutynin (DEOB). In human liver microsomes, (R)-OXY and (R)-DEOB were eliminated slightly slower than the corresponding (S)-enantiomers. The production of DEOB from OXY for the (R)-enantiomer was also slower than that for the (S)-enantiomer. In human P450-expressing liver microsomes, OXY was metabolized mainly by CYP3A4 among five cytochrome P450s (CYPs) tested (CYP2C9, CYP2C19, CYP2D6, CYP3A4 and CYP3A5) and the kinetics were slightly different for the enantiomer. The unbound fraction of (R)-OXY in plasma was almost two times higher than that of (S)-OXY, whereas (R)-DEOB was bound to plasma protein more than (S)-DEOB. No differences were observed in the blood-plasma concentration ratios for the enantiomers. After multiple applications of the transdermal delivery system, the plasma concentrations of (R)-OXY were lower than those of (S)-OXY. These data indicate that for the stereoselectivity of OXY, the unbound fraction of each OXY enantiomer was a major factor and the metabolism in liver had a minimal effect.

Pharmacokinetics/Pharmacodynamics Analysis of the Relationship between the in Vivo Micturition Pressure and Receptor Occupancy of (R)-Oxybutynin and Its Metabolite in Rats

To elucidate the relationships between the pharmacokinetics and pharmacological effects of oxybutynin ((R/S)-OXY), the micturition pressure and the plasma concentration profiles of (R)-OXY and (R)-N-desethyloxybutynin ((R)-DEOB), a pharmacologically active metabolite, after administration by three different routes (i.v., p.o. and transdermal) in rats were measured and analyzed using an inhibitory effect E(max) model with their in vitro pharmacological effects. The plasma exposure ratios of (R)-DEOB to (R)-OXY calculated from the AUCs were somewhat different among the routes administered. (R)-OXY and (R)-DEOB equally inhibited the acetylcholine-induced contractions in vitro. The micturition pressure, measured using the cystometric method in vivo, exhibited saturation against the dose administered. The inhibitory effect E(max) model well described the relationship between the micturition pressure and the receptor occupancy calculated from the plasma concentrations and pA(2) values and resulted in an extremely small receptor occupancy (0.206%) to exhibit half of the maximum effect. The estimated receptor occupancy profiles suggested a sufficient and long-lasting receptor occupation after transdermal administration of (R/S)-OXY, while the receptor occupancy diminished rapidly after the i.v. and p.o. administration. These data indicate that transdermal administration of (R/S)-OXY would be useful to achieve suitable pharmacological effects without excess plasma concentrations.

Bioanalytical interaction studies executed by preincubation affinity capillary electrophoresis

The versatility of CE is beneficial for the study of many types of molecular interactions, because different experimental designs can be made to suit the characteristics of a particular interaction. A very versatile starting point is the preequilibration type of affinity CE that has been used extensively for characterizing biomolecular interactions in the last 15 years. We review this field here and include a comprehensive overview of the existing preincubation ACE modes including their advantages and limitations as well as the methodological developments and applications within the bioanalytical field.

Biantennary Glycans as Well as Genetic Variants of α1-Acid Glycoprotein Control the Enantioselectivity and Binding Affinity of Oxybutynin

PURPOSE

The purpose of this study was to investigate the role of biantennary branching glycans of alpha1-acid glycoprotein (AGP) and its genetic variants in the enantioselective binding of oxybutynin (OXY).

METHOD

Human native AGP was separated using imminodiacetate-copper (II) affinity chromatography into two fractions, the A variant and a mixture of the F1 and S variants (F1-S). These fractionated AGPs were further separated by concanavalin A affinity chromatography into two fractions, with and without biantenarry glycans. An on-line high-performance liquid chromatography (HPLC) system consisting of a high-performance frontal analysis column, an extraction column, and an analytical HPLC column was developed to determine the binding affinities of OXY enantiomers for respective AGP species.

RESULTS

The total binding affinity as well as the enantiomeric selectivity of OXY in the F1-S mixed variant was significantly higher than that for the A variant, indicating that the chiral recognition ability of native AGP for the OXY enantiomers highly depends on the F1-S mixed variant. Furthermore, not only the genetic variants but also bianntenary glycans of AGP affect the binding affinity of OXY and are also responsible for the enantioselectivity.

CONCLUSIONS

Both genetic variants and glycan structures significantly contribute to the enantioselectivity and the binding affinity of OXY.

High performance liquid chromatographic determination of oxeladin citrate and oxybutynin hydrochloride and their degradation products

Two high performance liquid chromatographic (HPLC) methods are presented for the determination of oxeladin citrate (OL) and oxybutynin hydrochloride (OB) and their degradation products. The first method was based on HPLC separation of OL from its degradation product using a Nucleosil C(18) column with a mobile phase consisting of acetonitrile -0.1% phosphoric acid (60:40 v/v). The second method was based on HPLC separation of OB from its degradation product using a VP-ODS C(18) column with a mobile phase consisting of acetonitrile/0.01 M potassium dihydrogen phosphate/diethylamine (60:40:0.2). Quantitation was achieved with UV detection at 220 nm based on peak area. The two HPLC methods were applied for the determination of OL or OB, their degradation products, methylparaben and propylparaben in pharmaceutical preparations. The proposed methods were used to investigate the kinetics of acidic and alkaline degradation processes of OL and OB at different temperatures and the apparent pseudofirst-order rate constant, half-life and activation energy were calculated. The pH-rate profiles of degradation of OL and OB in Britton-Robinson buffer solutions within the pH range 2-12 were studied.

High-Performance Frontal Analysis of the Binding of Thyroxine Enantiomers to Human Serum Albumin Binding of Thyroxine Enantiomers to Human Serum Albumin Kimura

PURPOSE

The aim of this study was to characterize the binding property between thyroxine and human serum albumin (HSA) qualitatively and enantioselectively using high-performance frontal analysis (HPFA).

METHODS

An on-line HPLC system consisting of an HPFA column, an extraction column, and an analytical HPLC column was developed to be used to determine the unbound concentrations of thyroxine enantiomers.

RESULTS

Both enantiomers were bound to human serum albumin at two high-affinity sites with similar affinities. The binding constant (K) and the number of binding sites on an HSA molecule (n) evaluated from Scatchard plot analysis were K = 1.01 x 10(6)m(-1) and n = 1.90 for L: -thyroxine, and K = 9.71 x 10(5) m(-1) and n = 1.97 for D: -thyroxine. The binding sites were identified using phenylbutazone and diazepam as site-specific probes for sites I and II, respectively, and each enantiomer was found to bind to both sites. Incorporation of a chiral HPLC column into the on-line system permitted the investigation of enantiomer-enantiomer interactions, which revealed that both enantiomers competitively bind to the same binding sites without significant allosteric effects.

Lipid vesicles in capillary electrophoretic techniques: characterization of structural properties and associated membrane-molecule interactions

This paper reviews the use of lipid vesicles as model membranes in capillary electrophoresis (CE). The history and utility of CE in the characterization of microparticles is summarized, focusing on the application of colloidal electromigration theories to lipid vesicles. For instance, CE experiments have been used to characterize the size, surface properties, enclosed volumes, and electrophoretic mobilities of lipid vesicles and of lipoprotein particles. Several techniques involving small molecules or macromolecules separated in the presence of lipid vesicles are discussed. Interactions between the analytes and the lipid vesicles - acting as a pseudostationary phase or coated stationary phase in electrokinetic chromatography (EKC) - can be used to obtain additional information on the characteristics of the vesicles and analytes, and to study the biophysical properties of membrane-molecule interactions in lipid vesicles and lipoproteins. Different methods of determining binding constants by EKC are reviewed, along with the relevant binding constant calculations and a discussion of the application and limitations of these techniques as they apply to lipid vesicle systems.

Protein binding study of clozapine by capillary electrophoresis in the frontal analysis mode

We have used capillary electrophoresis in the frontal analysis mode (CE-FA) to determine the unbound concentration of clozapine in human serum albumin (HSA), human plasma, rabbit serum and plasma sample. The unbound clozapine concentration was directly measured from the height of the frontal peak. Samples were injected directly into an uncoated fused silica capillary (0.65 m (LC) x 75 microm i.d.; LE = 0.35 m) and separation was accomplished within 11 min without extensive sample pretreatment. The most suitable running buffer to separate unbound clozapine peak from the other peaks due to endogenous substances was found to contain 1 mmol1(-1) EDTA, 0.5 mol1(-1) glycine, and 67 mmol1(-1) phosphate with pH 7.4. The concentrations of unbound clozapine agreed well with those determined by the conventional ultrafiltration method. The methodology is validated and good correlation and precision are obtained. It was found that clozapine is strongly bound to protein in human plasma, rabbit plasma, and serum, while hardly bound to HSA. The present method enables the determination of the unbound drug concentration in multiple equilibrium system with less than ultra-micro injection volume, and would be hence especially useful for the protein binding study in biological samples that are only available in minute quantities.

Separation methods applicable to the evaluation of enzyme-inhibitor and enzyme-substrate interactions

Enzymes catalyze a rich variety of metabolic transformations, and do so with very high catalytic rates under mild conditions, and with high reaction regioselectivity and stereospecificity. These characteristics make biocatalysis highly attractive from the perspectives of biotechnology, analytical chemistry, and organic synthesis. This review, containing 128 references, focuses on the use of separation techniques in the elucidation of enzyme-inhibitor and enzyme-substrate interactions. While coverage of the literature is selective, a broad perspective is maintained. Topics considered include chromatographic methods with soluble or immobilized enzymes, capillary electrophoresis, biomolecular interaction analysis tandem mass spectrometry (BIA-MS), phage and ribosomal display, and immobilized enzyme reactors (IMERs). Examples were selected to demonstrate the relevance and application of these methods for determining enzyme kinetic parameters, ranking of enzyme inhibitors, and stereoselective synthesis and separation of chiral entities.

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.

Oxybutynin Extended-Release

The OROS-based oxybutynin extended-release (ER) formulation (Lyrinel XL; Ditropan XL) represents a new form of oral delivery for oxybutynin, a muscarinic receptor antagonist used in the treatment of overactive bladder (OAB). The release of oxybutynin from oxybutynin ER occurs in a sustained manner, resulting in a smoother plasma concentration-time profile and a lower maximum plasma concentration than those seen with oxybutynin immediate-release (IR). The ER formulation has been developed with the aim of improving the tolerability of oxybutynin therapy and facilitating once-daily administration. Moreover, oxybutynin ER offers greater flexibility in dosage (5-30 mg/day) than the other available treatment options. At dosages of 5-30 mg once daily, oxybutynin ER produced significant decreases from baseline in weekly urinary urge incontinence in patients with OAB. In addition, there were significant decreases in weekly total incontinence episodes and micturition frequency. In two randomised, double-blind studies in patients with OAB, the improvement in all the symptoms with once-daily oxybutynin ER 5-30 mg/day was similar to that produced by oxybutynin IR 5-20 mg/day given one to four times daily. Once-daily oxybutynin ER 10 mg was superior to tolterodine IR 4 mg/day given as two daily doses and as effective as once-daily tolterodine ER 4 mg/day in decreasing urinary incontinence; the decreases in micturition frequency with oxybutynin ER were significantly greater than those seen with either of tolterodine formulations. Oxybutynin ER was well tolerated in all the trials, with adverse events usually being mild to moderate and transient. In direct comparisons, the overall tolerability profile of oxybutynin ER was better than that of oxybutynin IR. Oxybutynin ER was similar to tolterodine (IR and ER) with respect to the incidence of clinically important dry mouth. A large 12-month tolerability study demonstrated no significant risks associated with the long-term use of oxybutynin ER. A few noncomparative studies have shown promising results with oxybutynin ER in the treatment of adult and paediatric patients with neurogenic bladder dysfunction secondary to neuronal injury. Long- and short-term studies have reported significant improvements in health-related quality of life with oxybutynin ER therapy. In addition, pharmacoeconomic studies have suggested that oxybutynin ER is more cost effective than oxybutynin IR and at least as cost effective as tolterodine IR. In conclusion, oxybutynin ER shows excellent efficacy in the treatment of symptoms associated with OAB in adults and the elderly with a good tolerability profile over a prolonged period of use (12 months). The ER formulation of oxybutynin provides a smooth plasma concentration profile over the 24-hour dosage interval, facilitating once-daily administration. Hence, given its overall efficacy/tolerability profile and dosage flexibility, oxybutynin ER provides an excellent treatment option in the first-line pharmacotherapy of OAB.

Drug affinity to immobilized target bio-polymers by high-performance liquid chromatography and capillary electrophoresis

This review addresses the use of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) as affinity separation methods to characterise drugs or potential drugs-bio-polymer interactions. Targets for the development of new drugs such as enzymes (IMERs), receptors, and membrane proteins were immobilized on solid supports. After the insertion in the HPLC system, these immobilized bio-polymers were used for the determination of binding constants of specific ligands, substrates and inhibitors of pharmaceutical interest, by frontal analyses and zonal elution methods. The most used bio-polymer immobilization techniques and methods for assessing the amount of active immobilized protein are reported. Examples of increased stability of immobilized enzymes with reduced amount of used protein were shown and the advantages in terms of recovery for reuse, reproducibility and on-line high-throughput screening for potential ligands are evidenced. Dealing with the acquisition of relevant pharmacokinetic data, examples concerning human serum albumin binding studies are reviewed. In particular, papers are reported in which the serum carrier has been studied to monitor the enantioselective binding of chiral drugs and the mutual interaction between co-administered drugs by CE and HPLC. Finally CE, as merging techniques with very promising and interesting application of microscale analysis of drugs' binding parameters to immobilized bio-polymers is examined.

Evalution of capillary electrophoresis-frontal analysis for the study of low molecular weight drug-human serum albumin interactions

Capillary electrophoresis frontal analysis was applied to 12 low molecular weight compounds including 8 drug substances displaying a range of different properties with respect to binding affinity, binding location, structure, lipophilicity, charge at physiological pH, and electrophoretic mobility. It was found that capillary electrophoresis frontal analysis can be used as a general method to study and quantify drug-human serum albumin interactions. The binding parameters obtained were consistent with literature values. Dextran was in some cases added to the run buffer to improve separation of the drug and human serum albumin plateau peaks. Results indicate that mobility differences between free and complexed human serum albumin give rise to only minor errors. Capillary electrophoresis frontal analysis was also found applicable to the study of human serum albumin drug displacement reactions. Low sensitivity of the UV-detection system was found to be the major limitation of capillary electrophoresis frontal analysis. The method is simple, and minimal effort has to be put into method development, which makes it well suited for screening in early drug development.

Binding study of desethyloxybutynin using high-performance frontal analysis method

Plasma protein binding of N-desethyloxybytynin (DEOXY), a major active metabolite of oxybutynin (OXY), was investigated quantitatively and enantioselectively using high-performance frontal analysis (HPFA). An on-line HPLC system which consists of HPFA column, extraction column and analytical column was developed to determine the unbound concentrations of DEOXY enantiomers in human plasma, in human serum albumin (HSA) solutions, and in human alpha1-acid glycoprotein (AGP) solutions. DEOXY is bound in human plasma strongly and enantioselectively. The unbound drug fraction in human plasma samples containing 5 microM (R)- or (S)-DEOXY was 1.19 +/- 0.001 and 2.33 +/- 0.044%, respectively. AGP plays the dominant role in this strong and enantioselective plasma protein binding of DEOXY. The total binding affinity (nK) of (R)-DEOXY and (S)-DEOXY to AGP was 2.97 x 10(7) and 1.31 x 10(7) M(-1), respectively, while the nK values of (R)-DEOXY and (S)-DEOXY to HSA were 7.77 x 10(3) and 8.44 x 10(3) M(-1), respectively. While the nK value of (S)-DEOXY is weaker than that of (S)-OXY (1.53 x 10(7) M(-1)), the nK value of (R)-DEOXY is 4.33 times stronger than that of (R)-OXY (6.86 x I0(6) M(-1)). This suggests that the elimination of an ethyl group weakens the binding affinity of the (S)-isomer because of the decrease in hydrophobicity, while the binding affinity of the (R)-isomer is enhanced by the decrease in steric hindrance. The total binding affinity of DEOXY to HSA is much lower than that of DEOXY-AGP binding as well as OXY-HSA binding (2.64 x 10(4) and 2.19 x 10(4) M(-1) for (R)-OXY and (S)-OXY, respectively). The study on competitive binding between OXY and DEOXY indicated that DEOXY enantiomers and OXY enantiomers are all bound competitively at the same binding site of AGP molecule.