The pharmacokinetics of oxybutynin in man

First-in-human study to assess the pharmacokinetics, tolerability, and safety of single-dose oxybutynin hydrochloride administered via a microprocessor-controlled intravaginal ring

Polymeric drug-releasing vaginal rings are useful for both local and systemic administration of drugs via the intravaginal route. Typically, they provide continuous sustained or controlled release of drug(s) over extended time periods, thereby avoiding overdose and improving adherence. This first-in-human study (EudraCT number: 2020-0050044-30) evaluated the pharmacokinetics, safety, and tolerability of a single dose of oxybutynin administered by a novel microprocessor-controlled vaginal ring (MedRing). Eight healthy female subjects received an electronically controlled single intravaginal dose of 3 mg oxybutynin hydrochloride (100 mg/mL) dissolved in 1:1 water/propylene glycol administered via MedRing. Following dosing, MedRing was kept in situ for up to 6 h. Blood samples were collected 1 h prior to oxybutynin dosing and subsequently at regular intervals post-dose for the assessment of plasma concentrations of oxybutynin and its active metabolite N-desethyloxybutynin. The results showed that MedRing efficiently administered oxybutynin via the intravaginal route, resulting in plasma oxybutynin levels comparable to orally administered oxybutynin. The mean ± standard deviation pharmacokinetic parameters for oxybutynin were Cmax 5.4 ± 2.7 ng/mL, AUCinf 34.9 ± 17.4 h ng/mL, t1/2 8.5 ± 3.5 h and for N-desethyloxybutynin were Cmax 3.9 ± 2.5 ng/mL, AUCinf 51.1 ± 43.1 h ng/mL, t1/2 7.7 ± 5.9 h. No serious adverse events were reported. The study demonstrates that intravaginal administration of oxybutynin hydrochloride using the MedRing device was well tolerated.

An evidence-based approach to the recovery of bladder and bowel function after pediatric spinal cord injury

INTRODUCTION

Bladder dysfunction and associated complications of the urinary system negatively impact the quality of life in children living with spinal cord injury (SCI). Pediatric lower urinary tract deficits include bladder over-activity, inefficient emptying, decreased compliance, and incontinence. Recent evidence in adults with SCI indicates significant improvements in bladder capacity and detrusor pressure following participation in an activity-based recovery locomotor training (ABR-LT) rehabilitative program. Additionally, anecdotal self-reports from parents in our Pediatric NeuroRecovery Program reference changes in bladder function, ranging from awareness of bladder fullness to gains in voluntary control while undergoing ABR-LT.

CASE PRESENTATION

In a within-subjects repeated measures study, we investigated the effect of ABR-LT on bladder function in three children (ages: 2.5, 3, and 6 years) who sustained upper motor neuron SCI. Each child received at least 60 sessions of ABR-LT (5x/week) for 12-14 weeks. Bladder function was assessed via urodynamics and the Common Data Elements questionnaires. Awareness of bladder filling during cystometry was present in all children and detrusor leak point pressure (LPP) was reduced post-ABR-LT relative to pre-training. A decrease in LPP after locomotor training was observed in all three participants. One out of the three participants had substantial improvements in bladder capacity post-ABR-LT and experienced less bowel incontinence following training.

DISCUSSION

Like our evidence in adults, the changes in bladder function suggest an interaction between lumbosacral networks regulating spinal reflex control of bladder filling, voiding, and afferent input (including potential descending supraspinal commands) and those activated by ABR-LT. Locomotor training may be associated with increased bladder capacity, enhanced perception of bladder filling, and decreased LPP as well as improvements in bowel control in pediatric SCI. This research suggests that children and adolescents with traumatic SCI could experience dynamic improvements in bladder and bowel function with the aid of various therapies. Studies assessing the durability of training on the recovery of bladder and bowel dysfunction in children with SCI are needed.

Treating Lower Urinary Tract Symptoms in Older Adults: Intravesical Options

This article provides an overview of the diagnosis and the treatment of lower urinary tract symptoms in older adults complicated by the neurodegenerative changes in the micturition reflex and further confounded by age-related decline in hepatic and renal clearance raising the propensity of adverse drug reactions. The first-line drug treatment for lower urinary tract symptoms, orally administered antimuscarinics, fails to reach the equilibrium dissociation constant of muscarinic receptors even at their maximum plasma concentration and tends to evoke a half-maximal response at a muscarinic receptor occupancy of just 0.206% in the bladder with a minimal difference from exocrine glands, which raises the adverse drug reaction risk. On the contrary, intravesical antimuscarinics are instilled at concentrations 1000-fold higher than the oral maximum plasma concentration and the equilibrium dissociation constant erects a downhill concentration gradient that drives passive diffusion and achieves a mucosal concentration around ten-fold lower than the instilled concentration for a long-lasting occupation of muscarinic receptors in mucosa and sensory nerves. A high local concentration of antimuscarinics in the bladder triggers alternative mechanisms of action and is supposed to engage retrograde transport to nerve cell bodies for neuroplastic changes that underlie a long-lasting therapeutic effect, while an intrinsically lower systemic uptake of the intravesical route lowers the muscarinic receptor occupancy of exocrine glands to lower the adverse drug reaction relative to the oral route. Thus, the traditional pharmacokinetics and pharmacodynamics of oral treatment are upended by intravesical antimuscarinics to generate a dramatic improvement (~ 76%) noted in a meta-analysis of studies enrolling children with neurogenic lower urinary tract symptoms on the primary endpoint of maximum cystometric bladder capacity as well as the secondary endpoints of filling compliance and uninhibited detrusor contractions. The therapeutic success of intravesical multidose oxybutynin solution or oxybutynin entrapped in the polymer for sustained release in the pediatric population bodes well for patients with lower urinary tract symptoms at the other extreme of the age spectrum. Though generally used to predict oral drug absorption, Lipinski's rule of five can also explain the ten-fold lower systemic uptake from the bladder of positively charged trospium over oxybutynin, a tertiary amine. Chemodenervation by an intradetrusor injection of onabotulinumtoxinA is merited for patients with idiopathic overactive bladder discontinuing oral treatment because of a lack of efficacy. However, age-related peripheral neurodegeneration potentiates the adverse drug reaction risk of urinary retention that motivates the quest of liquid instillation, delivering larger fraction of onabotulinumtoxinA to the mucosa as opposed to muscle by an intradetrusor injection can also probe the neurogenic and myogenic predominance of idiopathic overactive bladder. Overall, the treatment paradigm of lower urinary tract symptoms in older adults should be tailored to individual's overall health status and the risk tolerance for adverse drug reactions.

Atomoxetine and fesoterodine combination improves obstructive sleep apnoea severity in patients with milder upper airway collapsibility

BACKGROUND AND OBJECTIVE

The combination of the noradrenergic atomoxetine plus the anti-muscarinic oxybutynin acutely increased genioglossus activity and reduced obstructive sleep apnoea (OSA) severity. However, oxybutynin has shorter half-life than atomoxetine and side effects that might discourage long-term usage. Accordingly, we aimed to test the combination of atomoxetine and fesoterodine (Ato-Feso), a newer anti-muscarinic with extended release formulation, on OSA severity and endotypes.

METHODS

Twelve subjects with OSA underwent a randomized, double-blind, crossover trial comparing one night of atomoxetine plus fesoterodine (80-4 mg) to placebo. Parameters of OSA severity (e.g., apnoea-hypopnoea index [AHI], nadir oxygen desaturation and hypoxic burden) were calculated from two clinical, in-lab polysomnographic studies. OSA endotypes (including collapsibility per VMIN and arousal threshold) were derived from validated algorithms.

RESULTS

Compared to placebo, Ato-Feso did not reduce the AHI (34.2 ± 19.1 vs. 30.1 ± 28.2 events/h, p = 0.493), but reduced the apnoea index (12.9 [28.8] vs. 1.8 [9.1] events/h, median [interquartile range], p = 0.027) and increased nadir desaturation (76.8 [8.0] vs. 82.2 [8.8] %, p = 0.003); a non-significant trend for improved hypoxic burden was observed (52.4 [50.5] vs. 29.7 [78.9] %min/h, p = 0.093). Ato-Feso lowered collapsibility (raised VMIN ; 43.7 [29.8-55.7] vs. 56.8 [43.8-69.8] %VEUPNOEA , mean [CI], p = 0.002), but reduced the arousal threshold (129.3 [120.1-138.6] vs. 116.7 [107.5-126] %VEUPNOEA , p = 0.038). In post hoc analysis, 6/6 patients with milder collapsibility (VMIN  > 43%) exhibited OSA resolution (drop in AHI > 50% and residual AHI < 10 events/h) and improved hypoxaemia.

CONCLUSION

While inefficacious in unselected patients, Ato-Feso administered for one night suppressed OSA in patients with milder collapsibility. Ato-Feso may hold some promise as an alternative OSA treatment in certain subgroups of individuals.

Dosing Variability and Clinical Outcomes of Oxybutynin: A Pediatric Cohort of Patients With Neurogenic Bladder

Background

Despite the therapeutic advancements of the last several decades, neurogenic bladder remains a significant source of morbidity for patients with a spinal pathology. Oxybutynin is a mainstay of treatment in pediatric populations despite significant side effects and highly variable bioavailability.

Objectives

To characterize the use of oxybutynin in a cohort of pediatric patients with neurogenic bladder.

Methods

Retrospective data were collected of dosing, drug interactions, and urodynamics parameters in the 100 consecutive patients in a spinal differences clinic who had an appointment between October 7, 2015, and December 30, 2015. In addition to descriptive statistics, a linear regression model of oxybutynin dose versus age and sex was developed to examine the impact of age on dosing variability.

Results

One hundred patients (52% female) with a median age of 6.8 years were included. The median daily dose of oxybutynin was 0.36 mg/kg (interquartile range, 0.28-0.54 mg/kg). Of the 48 patients with a recent urodynamics study, 13 had a detrusor leak point pressure (DLPP) greater than the typical cutoff of 40 cm H₂O, indicating a need for management escalation. However, of these 13 patients, 38% were already on or exceeding oxybutynin's maximum recommended dose.

Conclusion

The wide dosing variability and high DLPPs despite maximal dosing indicate a need for further investigation of oxybutynin's bioavailability in this population compared to its side effects and clinical outcomes. If variability in response to the medication is due to differences in bioavailability, then a precision-dosing model based on patient genomics could be developed for oxybutynin.

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.

Improvements in Bladder Function Following Activity-Based Recovery Training With Epidural Stimulation After Chronic Spinal Cord Injury

Spinal cord injury (SCI) results in profound neurologic impairment with widespread deficits in sensorimotor and autonomic systems. Voluntary and autonomic control of bladder function is disrupted resulting in possible detrusor overactivity, low compliance, and uncoordinated bladder and external urethral sphincter contractions impairing storage and/or voiding. Conservative treatments managing neurogenic bladder post-injury, such as oral pharmacotherapy and catheterization, are important components of urological surveillance and clinical care. However, as urinary complications continue to impact long-term morbidity in this population, additional therapeutic and rehabilitative approaches are needed that aim to improve function by targeting the recovery of underlying impairments. Several human and animal studies, including our previously published reports, have documented gains in bladder function due to activity-based recovery strategies, such as locomotor training. Furthermore, epidural stimulation of the spinal cord (scES) combined with intense activity-based recovery training has been shown to produce volitional lower extremity movement, standing, as well as improve the regulation of cardiovascular function. In our center, several participants anecdotally reported improvements in bladder function as a result of training with epidural stimulation configured for motor systems. Thus, in this study, the effects of activity-based recovery training in combination with scES were tested on bladder function, resulting in improvements in overall bladder storage parameters relative to a control cohort (no intervention). However, elevated blood pressure elicited during bladder distention, characteristic of autonomic dysreflexia, was not attenuated with training. We then examined, in a separate, large cross-sectional cohort, the interaction between detrusor pressure and blood pressure at maximum capacity, and found that the functional relationship between urinary bladder distention and blood pressure regulation is disrupted. Regardless of one's bladder emptying method (indwelling suprapubic catheter vs. intermittent catheterization), autonomic instability can play a critical role in the ability to improve bladder storage, with SCI enhancing the vesico-vascular reflex. These results support the role of intersystem stimulation, integrating scES for both bladder and cardiovascular function to further improve bladder storage.

A novel multi-parametric high content screening assay in ciPTEC-OAT1 to predict drug-induced nephrotoxicity during drug discovery

Drug-induced nephrotoxicity is a major concern in the clinic and hampers the use of available treatments as well as the development of innovative medicines. It is typically discovered late during drug development, which reflects a lack of in vitro nephrotoxicity assays available that can be employed readily in early drug discovery, to identify and hence steer away from the risk. Here, we report the development of a high content screening assay in ciPTEC-OAT1, a proximal tubular cell line that expresses several relevant renal transporters, using five fluorescent dyes to quantify cell health parameters. We used a validation set of 62 drugs, tested across a relevant concentration range compared to their exposure in humans, to develop a model that integrates multi-parametric data and drug exposure information, which identified most proximal tubular toxic drugs tested (sensitivity 75%) without any false positives (specificity 100%). Due to the relatively high throughput (straight-forward assay protocol, 96-well format, cost-effective) the assay is compatible with the needs in the early drug discovery setting to enable identification, quantification and subsequent mitigation of the risk for nephrotoxicity.

New insights in the metabolism of oxybutynin: evidence of N-oxidation of propargylamine moiety and rearrangement to enaminoketone

1. Oxybutynin hydrochloride is an antimuscarinic agent prescribed to patients with an overactive bladder (OAB) and symptoms of urinary urge incontinence. Oxybutynin undergoes pre-systemic metabolism, and the N-desethyloxybutynin (Oxy-DE), is reported to have similar anticholinergic effects. 2. We revisited the oxidative metabolic fate of oxybutynin by liquid chromatography-tandem mass spectrometry analysis of incubations with rat and human liver fractions, and by measuring plasma and urine samples collected after oral administration of oxybutynin in rats. This investigation highlighted that not only N-deethylation but also N-oxidation participates in the clearance of oxybutynin after oral administration. 3. A new metabolic scheme for oxybutynin was delineated, identifying three distinct oxidative metabolic pathways: N-deethylation (Oxy-DE) followed by the oxidation of the secondary amine function to form the hydroxylamine (Oxy-HA), N-oxidation (Oxy-NO) followed by rearrangement of the tertiary propargylamine N-oxide moiety (Oxy-EK), and hydroxylation on the cyclohexyl ring. 4. The functional activity of Oxy-EK was investigated on the muscarinic receptors (M_1-3) demonstrating its lack of antimuscarinic activity. 5. Despite the presence of the α,β-unsaturated function, Oxy-EK does not react with glutathione indicating that in the clearance of oxybutynin no reactive and potentially toxic metabolites were formed.

Analysis of the variability of the pharmacokinetics of multiple drugs in young adult and elderly subjects and its implications for acceptable daily exposures and cleaning validation limits

The elderly constitute a significant, potentially sensitive, subpopulation within the general population, which must be taken into account when performing risk assessments including determining an acceptable daily exposure (ADE) for the purpose of a cleaning validation. Known differences in the pharmacokinetics of drugs between young adults (who are typically the subjects recruited into clinical trials) and the elderly are potential contributors affecting the interindividual uncertainty factor (UF_H) component of the ADE calculation. The UF_H values were calculated for 206 drugs for young adult and elderly groups separately and combined (with the elderly assumed to be a sensitive subpopulation) from published studies where the pharmacokinetics of the young adult and elderly groups were directly compared. Based on the analysis presented here, it is recommended to use a default UF_H value of 10 for worker populations (which are assumed to be approximately equivalent to the young adult groups) where no supporting pharmacokinetic data exist, while it is recommended to use a default UF_H value of 15 for the general population, to take the elderly into consideration when calculating ADE values. The underlying reasons for the large differences between the exposures in the young adult and elderly subjects for the 10 compounds which show the greatest separation are different in almost every case, involving the OCT2 transporter, glucuronidation, hydrolysis, CYP1A2, CYP2A6, CYP2C19, CYP2D6, CYP3A4 or CYP3A5. Therefore, there is no consistent underlying mechanism which appears responsible for the largest differences in pharmacokinetic parameters between young adult and elderly subjects.

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.

Korean clinical practice guideline for benign prostatic hyperplasia

In 2014, the Korean Urological Association organized the Benign Prostatic Hyperplasia Guideline Developing Committee composed of experts in the field of benign prostatic hyperplasia (BPH) with the participation of the Korean Academy of Family Medicine and the Korean Continence Society to develop a Korean clinical practice guideline for BPH. The purpose of this clinical practice guideline is to provide current and comprehensive recommendations for the evaluation and treatment of BPH. The committee developed the guideline mainly by adapting existing guidelines and partially by using the de novo method. A comprehensive literature review was carried out primarily from 2009 to 2013 by using medical search engines including data from Korea. Based on the published evidence, recommendations were synthesized, and the level of evidence of the recommendations was determined by using methods adapted from the 2011 Oxford Centre for Evidence-Based Medicine. Meta-analysis was done for one key question and four recommendations. A draft guideline was reviewed by expert peer reviewers and discussed at an expert consensus meeting until final agreement was achieved. This evidence-based guideline for BPH provides recommendations to primary practitioners and urologists for the diagnosis and treatment of BPH in men older than 40 years.

Treatment of Overactive Bladder in the Elderly Female: The Case for Trospium, Oxybutynin, Fesoterodine and Darifenacin

Overactive bladder (OAB) is a common constellation of lower urinary tract storage symptoms that causes a significant impact on a person's quality of life. The elderly may be disproportionally impacted by these symptoms due to concomitant poor mobility, comorbid conditions such as diabetes and heart failure, and polypharmacy. While behavioral modification and pelvic floor muscle training should be considered first-line treatment options, pharmacotherapy remains the backbone of the therapeutic regimen. Trospium, oxybutynin, fesoterodine, and darifenacin all have unique properties that may confer certain advantages in the elderly population. The hydrophilicity and quaternary amine structure of trospium may limit its ability to cross the blood-brain barrier and thus minimize impact on cognition in the elderly. In its oral form, oxybutynin may have the most significant effect on cognition; however, the transdermal preparations may be favorable in the elderly population due to the ability to avoid first-pass metabolism and its limited antimuscarinic adverse effects. Fesoterodine may be the most extensively studied OAB medication in the elderly population. Darifenacin has a strong affinity for the M3 receptor in the bladder, while having a weak affinity for the M1 receptor commonly found in the brain. It must be noted that all muscarinic receptor antagonists are associated with common adverse effects to some degree, and frequent re-evaluation of the elderly patient is necessary to confirm the proper benefit-to-risk profile.

Transdermal patches: history, development and pharmacology

Transdermal patches are now widely used as cosmetic, topical and transdermal delivery systems. These patches represent a key outcome from the growth in skin science, technology and expertise developed through trial and error, clinical observation and evidence-based studies that date back to the first existing human records. This review begins with the earliest topical therapies and traces topical delivery to the present-day transdermal patches, describing along the way the initial trials, devices and drug delivery systems that underpin current transdermal patches and their actives. This is followed by consideration of the evolution in the various patch designs and their limitations as well as requirements for actives to be used for transdermal delivery. The properties of and issues associated with the use of currently marketed products, such as variability, safety and regulatory aspects, are then described. The review concludes by examining future prospects for transdermal patches and drug delivery systems, such as the combination of active delivery systems with patches, minimally invasive microneedle patches and cutaneous solutions, including metered-dose systems.

Individualization of treatments with drugs metabolized by CES1: combining genetics and metabolomics

CES1 is involved in the hydrolysis of ester group-containing xenobiotic and endobiotic compounds including several essential and commonly used drugs. The individual variation in the efficacy and tolerability of many drugs metabolized by CES1 is considerable. Hence, there is a large interest in individualizing the treatment with these drugs. The present review addresses the issue of individualized treatment with drugs metabolized by CES1. It describes the composition of the gene encoding CES1, reports variants of this gene with focus upon those with a potential effect on drug metabolism and provides an overview of the protein structure of this enzyme bringing notice to mechanisms involved in the regulation of enzyme activity. Subsequently, the review highlights drugs metabolized by CES1 and argues that individual differences in the pharmacokinetics of these drugs play an important role in determining drug response and tolerability suggesting prospects for individualized drug therapies. Our review also discusses endogenous substrates of CES1 and assesses the potential of using metabolomic profiling of blood to identify proxies for the hepatic activity of CES1 that predict the rate of drug metabolism. Finally, the combination of genetics and metabolomics to obtain an accurate prediction of the individual response to CES1-dependent drugs is discussed.

Refractory overactive bladder: a common problem?

INTRODUCTION AND HYPOTHESIS

Unsatisfactory treatment outcome sometimes is described as frequently occurring in patients treated with first-line therapy for overactive bladder (OAB). The present article reviews the different circumstances which may result in failure to respond to lifestyle interventions, behavioral therapy, and/or antimuscarinic treatment.

METHODS

An extensive literature search was conducted to identify relevant articles on pathophysiological, clinical, and pharmacological aspects of refractory OAB.

RESULTS

Missing definition, unrealistic individual expectation of treatment outcomes, lack of communication between physician and patient as well as pathophysiological and pharmacological processes were identified as relevant for failure to respond to first-line OAB treatment. Increase of patient's motivation to adhere to the prescribed treatment, critical examination of the patient in regard to the initial diagnosis, and individual adjustment of antimuscarinic therapy may be appropriate tools to improve treatment outcome in OAB patients.

CONCLUSIONS

Overall, the incidence of refractory OAB seems to be overestimated. There are several approaches to improve therapy results.

Analysis of the impact of controlled release formulations on oral drug absorption, gut wall metabolism and relative bioavailability of CYP3A substrates using a physiologically-based pharmacokinetic model

Controlled release (CR) formulations are usually designed to achieve similar exposure (AUC) levels as the marketed immediate release (IR) formulation. However, the AUC is often lower following CR compared to IR formulations. There are a few exceptions when the CR formulations have shown higher AUC. This study investigated the impact of CR formulations on oral drug absorption and CYP3A4-mediated gut wall metabolism. A review of the current literature on relative bioavailability (Frel) between CR and IR formulations of CYP3A substrates was conducted. This was followed by a systematic analysis to assess the impact of the release characteristics and the drug-specific factors (including metabolism and permeability) on oral bioavailability employing a physiologically-based pharmacokinetic (PBPK) modelling and simulation approach. From the literature review, only three CYP3A4 substrates showed higher Frel when formulated as CR. Several scenarios were investigated using the PBPK approach; in most of them, the oral absorption of CR formulations was lower as compared to the IR formulations. However, for highly permeable compounds that were CYP3A4 substrates the reduction in absorption was compensated by an increase in the fraction that escapes from first pass metabolism in the gut wall (FG), where the magnitude was dependent on CYP3A4 affinity. The systematic simulations of various interplays between different parameters demonstrated that BCS class 1 highly-cleared CYP3A4 substrates can display up to 220% higher relative bioavailability when formulated as CR compared to IR, in agreement with the observed data collected from the literature. The results and methodology of this study can be employed during the formulation development process in order to optimize drug absorption, especially for CYP3A4 substrates.

The effect of oxybutynin on cardiac autonomic modulation in healthy subjects

PURPOSE

The aim of this study was to assess the various doses of oral oxybutynin on cardiac autonomic modulation by measuring short-term heart rate variability (HRV) indexes during supine rest position.

METHODS

Eight male healthy subjects (20-23 years) participated in the double-blind crossover randomized study. The single dose of oxybutynin (2.5, 5 and 10 mg) or placebo was given to the volunteers in four sessions within 5-day intervals. Before and minutes of 30, 60, 90 and 120 after administration, lead II electrocardiogram (ECG) was recorded for 5 min. ECG extracted RR intervals data became the base of the calculation of time domain and frequency domain HRV parameters, which indicate cardiac autonomic activity. Statistical analysis was done by using the nonparametric Wilcoxon and Kruskal-Wallis tests.

RESULTS

The data analysis has revealed that MNN (P < 0.001), SDNN (P < 0.05), PNN50% (P < 0.01), RMSSD (P < 0.001), HFnu (P < 0.05) and LF/HF ratio (P < 0.05) values were significantly increased relative to baseline at various time points in all the groups except in placebo group. LFnu (P < 0.05) values were significantly increased relative to baseline at various time points in all the groups except in placebo group.

CONCLUSIONS

Our findings have revealed that acute consumption of 2.5, 5 and 10 mg oxybutynin (an anticholinergic compound) in the juvenile healthy male subjects produces a cholinergic effect according to time and frequency domain of HRV indexes.

Simultaneous analysis of oxybutynin and its active metabolite N-desethyl oxybutynin in human plasma by stable isotope dilution LC-MS/MS to support a bioequivalence study

An isotope dilution high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed for the simultaneous determination of oxybutynin and its pharmacologically active metabolite N-desethyl oxybutynin in human plasma. Extraction of oxybutynin, its metabolite and their deuterated analogs as internal standards (ISs) from 300 μL human plasma was carried out by liquid-liquid extraction with methyl tert-butyl ether-ethyl acetate solvent mixture. Chromatographic separation of analytes was performed on Cosmosil C18 (150 mm × 4.6 mm, 5 μm) column under isocratic conditions with acetonitrile-1.0mM ammonium acetate (90:10, v/v) as the mobile phase. Six endogenous plasma phospholipids (496.3/184.0, 524.3/184.0, 758.5/184.0, 786.5/184.0, 806.5/184.0 and 810.5/184.0) were monitored to determine the extraction efficiency under different extraction conditions. The precursor→product ion transition for both the analytes and ISs were monitored on a triple quadrupole mass spectrometer, operating in the multiple reaction monitoring and positive ionization mode. The method was validated over a concentration range of 0.050-10.0 ng/mL for oxybutynin and 0.500-100 ng/mL for N-desethyl oxybutynin. The mean extraction recovery for analytes (80.4%) and ISs (76.9%) was consistent across five QC levels. Bench top, wet and dry extract, freeze-thaw and long term stability was evaluated for both the analytes. The method was applied to support a bioequivalence study of 5mg tablet formulation in 74 healthy Indian subjects. Assay reproducibility was demonstrated by reanalysis of 344 incurred samples.

Characterization of bladder selectivity of antimuscarinic agents on the basis of in vivo drug-receptor binding

The in vivo muscarinic receptor binding of antimuscarinic agents (oxybutynin, solifenacin, tolterodine, and imidafenacin) used to treat urinary dysfunction in patients with overactive bladder is reviewed. Transdermal administration of oxybutynin in rats leads to significant binding of muscarinic receptors in the bladder without long-term binding in the submaxillary gland and the abolishment of salivation evoked by oral oxybutynin. Oral solifenacin shows significant and long-lasting binding to muscarinic receptors in mouse tissues expressing the M₃ subtype. Oral tolterodine binds more selectively to muscarinic receptors in the bladder than in the submaxillary gland in mice. The muscarinic receptor binding of oral imidafenacin in rats is more selective and longer-lasting in the bladder than in other tissues such as the submaxillary gland, heart, colon, lung, and brain, suggesting preferential muscarinic receptor binding in the bladder. In vivo quantitative autoradiography with (+)N-[¹¹C]methyl-3-piperidyl benzilate in rats shows significant occupancy of brain muscarinic receptors with the intravenous injection of oxybutynin, solifenacin, and tolterodine. The estimated in vivo selectivity in brain is significantly greater for solifenacin and tolterodine than for oxybutynin. Imidafenacin occupies few brain muscarinic receptors. Similar findings for oral oxybutynin were observed with positron emission tomography in conscious rhesus monkeys with a significant disturbance of short-term memory. The newer generation of antimuscarinic agents may be advantageous in terms of bladder selectivity after systemic administration.

Oxybutynin: past, present, and future

Oxybutynin chloride is primarily indicated for the treatment of overactive bladder syndrome (OAB). It remains the most widely prescribed compound for OAB in the world. OAB is defined as the presence of urinary urgency, usually accompanied by daytime urinary frequency and nocturia, with or without urgency urinary incontinence, in the absence of infection or other identifiable etiology. This is a significant problem for men and women said to affect over 33 million adults in the USA, with the prevalence increasing with age. These symptoms can alter quality of life, with both physical and psychological impairment, as well as cause significant financial burden including the cost of sanitary supplies and decreased work productivity. Both pharmacological and non-pharmacological methods may be employed to aid in the treatment of OAB. The mainstay of treatment for OAB relies on pharmacological management, most specifically treatment with antimuscarinic medications. These medications are thought to prevent involuntary bladder contractions and/or urgency by inhibiting the muscarinic receptors within the urothelium and detrusor muscle. Currently, there are six different medications approved by the US Food and Drug Administration (FDA) for the treatment of OAB, with more than nine formulations including immediate- and extended-release tablets, transdermal patch and gel, vaginal ring, and suppository. This review will focus specifically on oxybutynin chloride, which has been used to treat OAB for four decades in numerous formulations.

Interindividual variation in transdermal and oral drug deliveries

It is generally assumed that the topical absorption of drugs is subject to more interindividual variation than the oral absorption of drugs. To date, we are unaware of any clinical studies or meta-analyses that compare the interindividual variation of transdermal and oral drug deliveries for a large number of medications. In this research article, the absorption data for 10 medications that can be used as an oral medication or a transdermal patch were compiled, and from the collected data, the interindividual variance was calculated for topical and oral absorption as an overall average and by drug. This research article also briefly reviews the pharmacokinetics and pharmacodynamics of transdermal and oral drug absorption. Our results indicate that there is considerable interindividual variation in topical and oral absorption for the 10 medications investigated. Yet, surprisingly, the calculated overall mean and median coefficient of variation (CV) for topical and oral absorption were comparable (within 10% of each other). Therefore, the interindividual variation in topical and oral absorption may not be as divergent as assumed previously. In a drug-by-drug comparison, certain medications demonstrated considerably more variation when absorbed orally versus topically and vice versa. It is unclear why certain drugs had less variation in absorption when delivered topically versus orally (or vice versa). However, patterns in drug molecular weight (MW) or octanol partition coefficient (log K(OCT) ) could not totally explain these findings. In our analysis, the previously reported correlation between MW or log K(OCT) and interindividual variation in absorption could only be replicated when plotting the topical absorption CV and MW. What became clear from our analysis is that the drug itself is an important variable when considering which route of delivery (oral or topical) will provide the least amount of interindividual variation. Our study had many limitations because of study design, which may have affected our calculations and conclusions. Further experimentation is needed to support and reveal the basic science of skin or drug chemistry that can further explain these findings.

Pharmacokinetics and toxicity of antimuscarinic drugs for overactive bladder treatment in females

INTRODUCTION

Antimuscarinics (AMs) are the mainstay of pharmacological treatment of overactive bladder (OAB), a symptom complex defined by the presence of urinary urgency, usually associated with frequency and nocturia, with or without urgency urinary incontinence. The AMs used to treat OAB differ in their pharmacological profiles, which may affect their potential for causing adverse effects (AEs).

AREAS COVERED

The present article aims to review the literature about pharmacokinetics (PK) of the different AMs used in the treatment of OAB. Furthermore, the AEs related to the use of these drugs and their incidence are presented. This systematic review is based on material searched and obtained via Medline, Pubmed and EMBASE up to March 2012 using the search terms "adverse events, pharmacokinetics, tolerability" in combination with "darifenacin, fesoterodine, imidafenacin, oxybutynin, propiverine, solifenacin, tolterodine, and trospium."

EXPERT OPINION

Antimuscarinics are the first-line pharmacological treatment for OAB. Despite the development of new molecules that improve their efficacy/safety profile, there are some drugs that are pharmacokinetically more appropriate to be prescribed in specific populations such as patients with neurological disease or the elderly. Moreover, research should be encouraged in evaluating antimuscarinics in conjunction with other drugs such as estrogens or beta-agonists. The identification of prognostic criteria for pharmacological therapy would be helpful.

Oxybutynin gel for the treatment of overactive bladder

INTRODUCTION

Overactive bladder (OAB) is a common condition that has a profound impact on an individual's overall health and quality of life. Muscarinic receptor antagonists are the mainstay of oral pharmacotherapy for OAB. While all of the medications in this class are significantly more effective than placebo, they are also associated with more adverse events that may limit their overall use. Transdermal application of oxybutynin has been shown to avoid first-pass metabolism and, thus, may be associated with fewer antimuscarinic side effects.

AREAS COVERED

This paper reviews the pharmacology of transdermal oxybutynin gel and summarizes the available data regarding this product in the treatment of OAB. It also discusses the role of this product in the OAB treatment armamentarium.

EXPERT OPINION

Oxybutynin transdermal gel has been shown to have significant advantages over placebo, in terms of urgency incontinence episodes, urinary frequency and voided volume in a Phase III study. Application site effects were higher in the gel group, but the incidence of antimuscarinic side effects were lower than those seen with oral preparations. The lower incidence of skin side effects, as compared with the transdermal patch, may confer a theoretical advantage toward the gel product. While promising, unanswered questions remain regarding persistence with treatment after this mode of therapy, and head-to-head comparisons with other antimuscarinics are absent.

Pharmacokinetics of oxybutynin chloride topical gel: effects of application site, baths, sunscreen and person-to-person transference

BACKGROUND

Oxybutynin chloride topical gel (OTG; Gelnique®) is an approved formulation for the transdermal administration of oxybutynin, an established antimuscarinic therapy for overactive bladder (OAB). Transdermal administration of oxybutynin minimizes plasma concentrations of the active metabolite N-desethyloxybutynin (N-DEO), which can have anticholinergic adverse effects.

OBJECTIVES

In four phase I studies, we separately assessed the effects of OTG application site selection on oxybutynin bioavailability (site-to-site study); the effects of post-application showering on oxybutynin steady-state pharmacokinetics (showering study); the effects of sunscreen application on oxybutynin absorption (sunscreen study); and the person-to-person transfer of oxybutynin through skin-to-skin contact at the application site (transference study).

METHODS

All four studies were open-label, randomized, phase I studies. The site-to-site and showering studies involved repeated administration of OTG to establish steady-state plasma concentrations of oxybutynin and N-DEO; the other two studies involved single doses. Clinical visits were required for pharmacokinetic sampling, supervision of OTG self-application on pharmacokinetic sampling days, showering, sunscreen application and transference experiments. The study included healthy subjects aged 18-45 years. Subjects with conditions requiring medical therapy or interfering with the application of OTG or the interpretation of pharmacokinetic results were excluded. Participants applied OTG (1 g containing oxybutynin chloride 10%, 1.14 mL/dose) once daily to the abdomen, upper arm/shoulder or thigh. Showering occurred 1-6 hours after dosing. Sunscreen was applied 30 minutes before or after OTG application. Abdomen-to-abdomen contact with movement for 15 minutes between treated and untreated participants was conducted 1 hour after dosing. Time points of serial blood sampling for pharmacokinetic analyses varied among studies. Plasma concentrations of oxybutynin and N-DEO (except transference study) were measured. Bioequivalence was tested with ANOVA models for log(e)-transformed plasma exposure (area under the plasma concentration-time curve [AUC]) and maximum plasma concentration (C(max)) to generate 90% confidence intervals (CIs).

RESULTS

Oxybutynin and N-DEO exposures (AUCs) from time zero to 24 hours (AUC(24)) were similar for the three application sites, with N-DEO/oxybutynin mean AUC(24) ratios of approximately 0.9. The 90% CIs for thigh-to-abdomen ratios of oxybutynin AUC(24) (0.93, 1.23) and C(max) (0.85, 1.16) were within the interval required for bioequivalence (0.8, 1.25); the other application site ratios for oxybutynin had boundaries slightly outside this interval. Showering 1-6 hours and sunscreen application 30 minutes before or after OTG application had minor effects on oxybutynin concentrations. After vigorous skin contact between treated and untreated participants at the application site, the mean ± SD AUC from time zero to 48 hours (AUC(48)) of oxybutynin in 12 untreated participants was 29.8 ± 24.5 ng · h/mL, approximately one-quarter of the exposures generally seen in subjects treated with a single dose of OTG. Oxybutynin AUC(48) after clothing-to-skin contact was undetectable in 12 of 14 untreated participants and very low (mean ± SD 0.4 ± 0.8 ng · h/mL) in two untreated female participants.

CONCLUSION

The bioavailability of oxybutynin and its pharmacokinetic profile are not greatly affected by application site selection, post-application showering or sunscreen use shortly before or after dosing with OTG. Oxybutynin transference to untreated persons is essentially prevented by avoiding direct skin-to-skin contact with the application site.

Update on the management of overactive bladder: patient considerations and adherence

Overactive bladder (OAB) is a common condition that causes a profound impact on an individual’s overall health and quality of life. Muscarinic receptor antagonists are the mainstay of oral pharmacotherapy for OAB. Although all of the medications in this class are significantly more effective than placebo, they are also associated with more adverse events that may limit their overall use. Although newer preparations of these medications have sought to improve tolerability and efficacy through alternative routes of delivery and once-daily dosing, improved adherence to treatment and treatment persistence continue to be an ongoing challenge. An improved understanding of the factors involved in persistence of medical OAB therapy is imperative in efforts to optimize therapeutic benefits in this chronic and potentially morbid condition.

Correlation of male overactive bladder with intravesical prostatic protrusion

Purpose

Male overactive bladder (OAB) may be caused by prostatic pathologies such as bladder outlet obstruction (BOO). Intravesical prostatic protrusion (IPP) has been found to correlate with BOO and acute urinary retention. We investigated the interrelation between male OAB symptoms and IPP for estimating anatomical changes to the prostate.

Materials and Methods

We assessed 179 consecutive men aged >40 years who presented with lower urinary tract symptoms. The initial evaluation included International Prostate Symptom Score (IPSS) and quality of life assessments, transrectal ultrasonography (TRUS), uroflowmetry, and postvoid residual urine volume. The degree of IPP was determined by the distance from the tip of the protrusion to the circumference of the bladder at the base of the prostate gland. Patients with IPP <0.5 cm were defined as group A (n=114), and patients having 0.5 cm≤IPP<1 cm were defined as group B (n=38). The others were defined as group C (n=27).

Results

A total of 51 patients complained of urgency in group A, 38 patients in group B, and 27 patients in group C. Likewise, 14 patients had a history of acute urinary retention in group A, 8 patients in group B, and 16 patients in group C. IPP grade had a statistically significant relation with both OAB and a history of acute urinary retention.

Conclusions

The results of our study have shown that male OAB is correlated with IPP. However, larger scale studies are needed to confirm these results.

Clinical pharmacokinetics and pharmacodynamics of solifenacin

The succinate salt of solifenacin, a tertiary amine with anticholinergic properties, is used for symptomatic treatment of overactive bladder. Solifenacin peak plasma concentrations of 24.0 and 40.6 ng/mL are reached 3-8 hours after long-term oral administration of a 5 or 10 mg solifenacin dose, respectively. Studies in healthy adults have shown that the drug has high absolute bioavailability of about 90%, which does not decrease with concomitant food intake. Solifenacin has an apparent volume of distribution of 600 L, is 93-96% plasma protein bound, and probably crosses the blood-brain barrier. Solifenacin is eliminated mainly through hepatic metabolism via cytochrome P450 (CYP) 3A4, with about only 7% (3-13%) of the dose being excreted unchanged in the urine. Solifenacin metabolites are unlikely to contribute to clinical solifenacin effects. In healthy adults, total clearance of solifenacin amounts to 7-14 L/h. The terminal elimination half-life ranges from 33 to 85 hours, permitting once-daily administration. Urinary excretion plays a minor role in the elimination of solifenacin, resulting in renal clearance of 0.67-1.51 L/h. Solifenacin does not influence the activity of CYP1A1/2, 2C9, 2D6 and 3A4, and shows a weak inhibitory potential for CYP2C19 and P-glycoprotein in vitro; however, clinical drug-drug interactions with CYP2C19 and P-glycoprotein substrates are very unlikely. Exposure to solifenacin is increased about 1.2-fold in elderly subjects and about 2-fold in subjects with moderate hepatic and severe renal impairment, as well as by coadministration of the potent CYP3A4 inhibitor ketoconazole 200 mg/day. The full therapeutic effects of solifenacin occur after 2-4 weeks of treatment and are maintained upon long-term therapy. Although solifenacin pharmacokinetics display linearity at doses of 5-40 mg, no obvious dose dependency was observed in efficacy and tolerability studies. The efficacy of solifenacin (5 or 10 mg/day) is at least equal to that of extended-release (ER) tolterodine (4 mg/day) in reducing the mean number of micturitions per 24 hours and urgency episodes, and in increasing the volume voided per micturition. Solifenacin (5 mg/day) appears to be superior to ER tolterodine (4 mg/day) in reducing incontinence episodes (mean -1.30 vs -0.90, p = 0.018) and is superior to propiverine (20 mg/day) at the dose of 10 mg/day in reducing urgency (-2.30 vs -2.78, p = 0.012) and nocturia episodes. Based on withdrawal rates due to adverse effects during the 52-week treatment period, solifenacin appears to have better tolerability than immediate-release (IR) oxybutynin 10-15 mg/day and IR tolterodine 4 mg/day. With regard to the pharmacokinetics of solifenacin, and for safety reasons, doses exceeding 5 mg/day are not recommended for patients with moderate hepatic impairment (Child-Pugh score 7-9), patients with severe renal impairment (creatinine clearance <30 mL/min) and subjects undergoing concomitant therapy with CYP3A4 inhibitors.

Oxybutynin chloride topical gel: a new formulation of an established antimuscarinic therapy for overactive bladder

BACKGROUND

Oxybutynin, a cholinergic-muscarinic receptor antagonist, is established as a safe and effective pharmacological treatment for patients with overactive bladder syndrome (OAB). Oxybutynin is available in multiple immediate- and extended-release oral and two transdermal formulations. Oxybutynin chloride topical gel (OTG) (Gelnique), Watson Pharmaceuticals, Corona, CA, USA) was approved in January 2009 by the US FDA. OTG was designed to provide consistent plasma oxybutynin levels with daily application, favorably altering the circulating N-desethyloxybutynin metabolite:oxybutynin ratio, and to utilize a biocompatible delivery system, thus minimizing both the anticholinergic adverse effects of oral formulations and the application-site skin reactions associated with other available forms of transdermal delivery.

OBJECTIVE/METHODS

This review summarizes the pharmacological properties and the clinical efficacy and safety profile of OTG based on the published literature and unpublished data provided by the manufacturer upon request.

RESULTS/CONCLUSION

OTG represents an efficacious, safe, and convenient alternative to other oxybutynin formulations and other oral anticholinergic medications for the treatment of OAB. Future studies and broad clinical experience should confirm the promising early experience observed with this formulation.

The forefront for novel therapeutic agents based on the pathophysiology of lower urinary tract dysfunction: bladder selectivity based on in vivo drug-receptor binding characteristics of antimuscarinic agents for treatment of overactive bladder

We have reviewed the binding of antimuscarinic agents, used to treat urinary dysfunction in patients with overactive bladder, to muscarinic receptors in target and non-target tissues in vivo. Transdermal administration of oxybutynin in rats led to significant binding in the bladder without long-term binding in the submaxillary gland and the abolishment of salivation evoked by oral oxybutynin. Oral solifenacin showed significant and long-lasting binding to muscarinic receptors in mouse tissues expressing the M(3) subtype. Oral tolterodine bound more selectively to muscarinic receptors in the bladder than in the submaxillary gland in mice. The muscarinic receptor binding activity of oral darifenacin in mice was shown to be pronounced and long-lasting in the bladder, submaxillary gland, and lung. In vivo quantitative autoradiography using (+)N-[(11)C]methyl-3-piperidyl benzilate in rats showed significant occupancy of brain muscarinic receptors on intravenous injection of oxybutynin, propiverine, solifenacin, and tolterodine. The estimated in vivo bladder selectivity compared to brain was significantly greater for solifenacin and tolterodine than oxybutynin. Darifenacin occupied few brain muscarinic receptors. Similar findings were also observed with positron emission tomography in conscious rhesus monkeys. The newer generation of antimuscarinic agents may be advantageous in the bladder selectivity after systemic administration.

Urothelial effects of oral agents for overactive bladder

The cholinergic system of the bladder includes muscarinic receptors distributed to detrusor myocytes and structures within mucosa including bladder afferent (sensory) nerves. The receptors have been shown to be involved in afferent signaling from the bladder, but it has not been established to what extent effects on this mucosal signaling pathway contribute to the therapeutic efficacy of the clinically used antimuscarinics. Mucosa can be influenced by antimuscarinics via the bloodstream. However, some antimuscarinics and their active metabolites are excreted in urine in amounts that may affect the mucosal muscarinic receptors from the luminal side. This has not yet been demonstrated to imply superior clinical efficacy. Nevertheless, mucosal afferent signaling pathways are therapeutically interesting targets that should be further explored.

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.

Formulation and optimization of porous osmotic pump-based controlled release system of oxybutynin

The aim of the current study was to design a porous osmotic pump-based drug delivery system for controlled release of oxybutynin. The porous osmotic pump contains pore-forming water-soluble additives in the coating membrane, which after coming in contact with water, dissolve, resulting in an in situ formation of a microporous structure. The dosage regimen of oxybutynin is one 5-mg tablet 2 to 3 times a day. The plasma half-life ranges from approximately 2 to 3 hours. Hence, oxybutynin was chosen as a model drug with an aim to develop a controlled release system for a period of 24 hours. Linear and reproducible release similar to that of Ditropan XL was achieved for optimized formulation (f2 >50) independent of hydrodynamic conditions. The effect of different formulation variables, namely, ratio of drug to osmogent, membrane weight gain, and level of pore former on the in vitro release was studied. Cellulose acetate (CA) was used as the semipermeable membrane. It was found that drug release rate increased with the amount of osmogent because of the increased water uptake, and hence increased driving force for drug release. Oxybutynin release was inversely proportional to the membrane weight gain; however, directly related to the level of pore former, sorbitol, in the membrane. This system was found to deliver oxybutynin at a zero-order rate for 20 hours. The effect of pH on drug release was also studied. The optimized formulations were subjected to stability studies as per International Conference on Harmonisation (ICH) guidelines and formulations were stable after a 3 month study.

Comparative evaluation of central muscarinic receptor binding activity by oxybutynin, tolterodine and darifenacin used to treat overactive bladder

PURPOSE

We characterized muscarinic receptor binding in the mouse cerebral cortex after oral administration of anticholinergic agents used to treat overactive bladder.

MATERIALS AND METHODS

Muscarinic receptors in the mouse cerebral cortex and bladder after oral administration of anticholinergic agents were measured using [(3)H]N-methylscopolamine.

RESULTS

In vitro binding affinities of tolterodine and its metabolite 5-hydroxymethyl metabolite in the mouse cerebral cortex and bladder were considerably greater than those of oxybutynin and darifenacin. Also, muscarinic receptor binding affinity of oxybutynin and its metabolite N-desethyl-oxybutynin in the cerebral cortex compared with that in the bladder was 2 to 3 times higher, whereas that of tolterodine and 5-hydroxymethyl metabolite was approximately 2 times lower. Oral administration of oxybutynin (76.1 micromol/kg), tolterodine (6.31 micromol/kg) and darifenacin (59.1 micromol/kg) showed binding activity that was approximately equal to that of bladder muscarinic receptors. Oral administration of oxybutynin (76.1 micromol/kg) showed significant binding of cerebral cortical muscarinic receptors in mice, as indicated by about a 2-fold increase in K(d) values for specific [(3)H]N-methylscopolamine binding 0.5 and 2 hours later. On the other hand, tolterodine and darifenacin given at oral doses that would exert a similar extent of bladder receptor binding activity as oxybutynin showed only a low level of binding activity of central muscarinic receptors in mice.

CONCLUSIONS

Significant binding of brain muscarinic receptors in mice was observed by the oral administration of oxybutynin but not tolterodine and darifenacin.

Pharmacotherapy for Nocturia in the Elderly Patient

Nocturia may be attributable to nocturnal polyuria (nocturnal urine overproduction), a diminished nocturnal bladder capacity or a combination of the two conditions.A disorder of the vasopressin (antidiuretic hormone) system with very low or undetectable levels of vasopressin at night, affecting some elderly people, may cause an increase in the nocturnal urine output, which in the most extreme cases accounts for 85% of the 24-hour diuresis. The increased urine output can be treated with desmopressin orally at bedtime, generally using low doses. Self-imposed fluid restrictions before bedtime are not effective in reducing the nocturnal urine output in this condition. Nocturia is also more prevalent in association with a reduced bladder capacity. Antimuscarinic drugs are used in attempts to depress involuntary bladder contractions. Decreased nocturnal voided volumes in men and consequent increased nocturia may suggest difficulty in emptying the bladder or detrusor overactivity. alpha(1)-Adrenoceptor antagonists and 5alpha-reductase inhibitors are often used in men with symptoms indicative of benign prostatic hyperplasia, and one of their effects is reduction of nocturia. In women, estrogen deficiency, a common consequence of the menopausal transition, causes atrophic changes within the urogenital tract. Consequently, such women are more disposed to having urogenital symptoms, among them nocturia. This review emphasises the importance of correctly diagnosing and treating nocturia in elderly patients. This will improve patients' sleep and, in turn, reduce their risk of fall injuries and the associated detrimental consequences, thereby improving patients' health and quality of life.

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.

Effect of the proton pump inhibitor omeprazole on the pharmacokinetics of extended-release formulations of oxybutynin and tolterodine

This study assessed the effect of the proton pump inhibitor omeprazole on the bioavailability of the extended-release formulations of oxybutynin and tolterodine. Forty-four healthy volunteers received each of 4 treatments in a 4-period crossover design. The treatments consisted of osmotically controlled extended-release oxybutynin chloride tablets at 10 mg/d or extended-release tolterodine tartrate capsules at 4 mg/d, with and without preceding treatment with 20 mg omeprazole daily for 4 days. Blood samples collected predose and at scheduled time points for 36 hours postdose were analyzed for oxybutynin and its active metabolite, N-desethyloxybutynin, or tolterodine and its active 5-hydroxymethyl metabolite, as appropriate. The AUCinfinity ratios for oxybutynin and its metabolite with and without prior omeprazole fell within the 80% to 125% range (accepted as the criterion for bioequivalence), as did those for tolterodine and its active moiety. The peak concentration ratios for oxybutynin and metabolite also conformed to this range; those for tolterodine did not. Increasing gastric pH with omeprazole does not substantially alter the pharmacokinetic properties of extended-release oxybutynin but may alter those of extended-release tolterodine.

Muscarinic receptors: their distribution and function in body systems, and the implications for treating overactive bladder

1. The effectiveness of antimuscarinic agents in the treatment of the overactive bladder (OAB) syndrome is thought to arise through blockade of bladder muscarinic receptors located on detrusor smooth muscle cells, as well as on nondetrusor structures. 2. Muscarinic M3 receptors are primarily responsible for detrusor contraction. Limited evidence exists to suggest that M2 receptors may have a role in mediating indirect contractions and/or inhibition of detrusor relaxation. In addition, there is evidence that muscarinic receptors located in the urothelium/suburothelium and on afferent nerves may contribute to the pathophysiology of OAB. Blockade of these receptors may also contribute to the clinical efficacy of antimuscarinic agents. 3. Although the role of muscarinic receptors in the bladder, other than M3 receptors, remains unclear, their role in other body systems is becoming increasingly well established, with emerging evidence supporting a wide range of diverse functions. Blockade of these functions by muscarinic receptor antagonists can lead to similarly diverse adverse effects associated with antimuscarinic treatment, with the range of effects observed varying according to the different receptor subtypes affected. 4. This review explores the evolving understanding of muscarinic receptor functions throughout the body, with particular focus on the bladder, gastrointestinal tract, eye, heart, brain and salivary glands, and the implications for drugs used to treat OAB. The key factors that might determine the ideal antimuscarinic drug for treatment of OAB are also discussed. Further research is needed to show whether the M3 selective receptor antagonists have any advantage over less selective drugs, in leading to fewer adverse events.

Comparison of dry mouth in women treated with extended-release formulations of oxybutynin or tolterodine for overactive bladder

The incidence, severity and tolerability of dry mouth was compared in 790 women with overactive bladder who were treated with extended-release oxybutynin chloride 10 mg/day or extended-release tolterodine tartrate 4 mg/day for 12 weeks in a multicenter, double-blind, parallel-group study. Dry mouth was the most common adverse event associated with treatment, with an incidence rate of 28.1% in the oxybutynin group and 21.6% in the tolterodine group (P = 0.039). The majority of dry mouth events were mild in both treatment groups. Severe dry mouth occurred in 1.5% and 0.5% of patients in the oxybutynin and tolterodine groups, respectively (P = 0.173). Seven patients on extended-release oxybutynin and 4 patients on extended-release tolterodine discontinued treatment due to dry mouth (P = 0.380). The results of this analysis showed that dry mouth was common with both treatments, but most events were mild; there was no difference in the rate of severe dry mouth or in the rate of withdrawal due to dry mouth.