Urinary Excretion Contributes to Long-Lasting Blockade of Bladder Muscarinic Receptors by Imidafenacin: Effect of Bilateral Ureteral Ligation

Predicting muscarinic receptor occupancy in human bladder mucosa from urinary concentrations of antimuscarinic agents for overactive bladder

To assess the pharmacologically relevant and selective muscarinic receptor occupancy in the bladder mucosa, we considered not only plasma drug concentrations but also urinary drug concentrations. The purpose of this study was to predict muscarinic receptor occupancy in the human bladder mucosa based on urinary concentrations in response to clinical dosages of antimuscarinic agents used to treat overactive bladder. The calculated mean plasma or serum unbound steady state concentrations were 0.06-11 nM in clinical dosages of five antimuscarinic agents. Urinary concentrations calculated from the mean plasma or serum and renal clearance ranged between 19 nM and 2 μM, which were >10-fold higher than the Ki values for bladder muscarinic receptors excluding propiverine. Bladder mucosal muscarinic receptor occupancy estimated from the urinary concentrations and the Ki values was >90 % at a steady state in clinical dosages of five antimuscarinic agents. The bladder muscarinic receptor occupancy was higher than that in the parotid gland calculated based on the mean plasma or serum unbound concentrations and Ki values for muscarinic receptors in the parotid gland. These results suggest that sufficient and selective muscarinic receptor occupancy by antimuscarinic agents, to exert pharmacological effects, in the bladder mucosa can be predicted using urinary concentrations.

The large part German medicine has played in the development of experimental pharmacology in Japan

The history of hitherto existing pharmacology in Japan presented here is authored in commemoration of the 150th anniversary of Naunyn-Schmiedeberg's Archives of Pharmacology. After the publication of the new book of anatomy "Anatomische Tabellen" translated into Japanese in 1774, the foundation of understanding the medical science was gradually formed in Japan under seclusion policy, and, since the Meiji Restoration of 1868, the modernization of Japanese medicine was rapidly fostered on the basis of German medicine. Thus, the Japanese government officially adopted German medicine, and the philosophy and practice of German medical schools were incorporated. Most of the medical texts used in Japan were of German origins, often in Dutch translations, and many Japanese physicians and medical researchers studied abroad in Germany. The start of experimental pharmacology in Japan was also made up by Japanese disciples of Oswald Schmiedeberg, who was the one of founders of the Archives in 1873. Additionally, it was customary for professor candidates in charge of pharmacology in medical faculties in Japan to go to Germany and study pharmacology. Through such historical circumstances, the Japanese Pharmacology Society has been established to fulfill the responsibility for contributing internationally to world-class research achievements in the field of medical sciences by supplying numerous talented pharmacologists. During the course of the development of experimental pharmacology in Japan, the Archives has provided an excellent stage for many Japanese pharmacologists to publish their research outcomes to proliferate them internationally. Without German medicine influence, Japanese pharmacology would not have been what it is today.

Muscarinic receptor binding activity in rat tissues by vibegron and prediction of its receptor occupancy levels in the human bladder

OBJECTIVES

To examine the effects of vibegron, a selective β₃ -adrenoceptor agonist, used to treat overactive bladder, on muscarinic receptors in the rat bladder, and to predict the occupancy levels of muscarinic receptors by vibegron in the bladders of humans orally administered a clinical dose.

METHODS

Muscarinic receptors in the rat bladder and other tissues were examined by a radioligand binding assay using [N-methyl-³ H]scopolamine chloride. The occupancy levels of muscarinic receptors by vibegron in bladders of humans after its oral administration were predicted from the estimation of unbound concentrations in human plasma and urine in the literature.

RESULTS

Vibegron (0.1-100 μmol/L) inhibited specific [N-methyl-³ H]scopolamine chloride binding in the bladder and other tissues of rats in a concentration-dependent manner. The 50% inhibitory concentration value of vibegron in the bladder was approximately twofold higher than that in the heart, and approximately 315- and 3.5-fold lower than those in the submaxillary gland and brain, respectively. Therefore, the binding affinity of vibegron for muscarinic receptors was higher in the heart and bladder than in the submaxillary gland and brain. By using the rat bladder receptor binding affinity, occupancy levels of muscarinic receptors in the human bladder were predicted to be 51-91% until 24 h after its oral administration at 50 mg of vibegron.

CONCLUSIONS

This is the first study to suggest that vibegron binds to muscarinic receptors in the rat bladder and other tissues, with a potentially higher affinity for the M₂ subtype than the M₁ and M₃ subtypes. These results might be clinically relevant for pharmacotherapy with vibegron for overactive bladder.

Possible Involvement of Muscarinic Receptor Blockade in Mirabegron Therapy for Patients with Overactive Bladder

The selective β ₃-adrenoceptor agonist mirabegron, an established alternative to antimuscarinic therapy for patients with overactive bladder, induces additional effects against receptors, transporters, and hepatic enzymes. The present study aimed to elucidate the effects of mirabegron on muscarinic receptors in the rat bladder using radioligand binding and functional assays. Mirabegron (0.1-100 μM) inhibited specific [N-methyl-³H]scopolamine methyl chloride binding in the bladder and other tissues of rats in a concentration-dependent manner. Binding affinity in the bladder was similar to that in the heart and significantly higher than those in the submaxillary gland and brain. Mirabegron induced the concentration-dependent relaxation of carbachol-induced contractions in the rat isolated bladder. Further analyses using a two-site model revealed that the relative quantities of high- and low-affinity components for mirabegron were 44.5% and 55.5%, respectively. Respective pEC₅₀ values were 7.06 and 4.97. Based on the receptor binding affinity and pharmacokinetics of mirabegron, muscarinic receptor occupancy in the human bladder for 24 hours after the administration of a single oral dose of 50 mg mirabegron was 37%-76%. The present results demonstrate for the first time that mirabegron may relax the detrusor smooth muscle not only by β ₃-adrenoceptor activation but also muscarinic receptor blockade. SIGNIFICANCE STATEMENT: Mirabegron, the first selective β ₃-adrenoceptor agonist, represents an alternative to antimuscarinic agents for management of overactive bladder (OAB). The present study aimed to clarify whether mirabegron directly binds to muscarinic receptors and affects cholinergic agonist-induced contractions in rat urinary bladder and to predict muscarinic receptor occupancy in human bladder after oral administration of mirabegron. The results demonstrated that mirabegron therapy for patients with OAB may be due not only to β ₃-adrenoceptor activation but also muscarinic receptor blockade.

Additive effects of intravenous and intravesical application of vibegron, a β3-adrenoceptor agonist, on bladder function in rats with bladder overactivity

To examine the effects of intravenous and intravesical application of vibegron, a new β₃-adrenoceptor (β₃-AR) agonist, on bladder function in rats with oxotremorine methiodide (oxo-M: a nonselective muscarinic receptor agonist)-induced bladder overactivity. Cystometry was performed in conscious female rats with intravesical instillation of oxo-M (200 μM). In oxo-M-treated rats, vehicle or vibegron (1 and 10 mg/kg) was cumulatively applied intravenously at 30-min intervals. In other groups of rats, oxo-M + vehicle or oxo-M + vibegron (10, 100 μM, and 1 mM) was cumulatively instilled intravesically at 60-min intervals followed by intravenous application of vibegron (10 mg/kg). Expression of β₃-ARs in the bladder was also evaluated using immunohistochemical staining. Intravenous application of vibegron (10 mg/kg) significantly increased bladder capacity (1.3 times) and decreased baseline, threshold, and maximal voiding pressure compared with vehicle. Next, intravesical application of vibegron (1 mM) significantly increased threshold pressure and bladder capacity (1.2 times) compared with vehicle. Combined treatments of intravesical (1 mM) and intravenous (10 mg/kg) application of vibegron induced a significantly larger degree of increases in bladder capacity (1.4 times) compared with vehicle. In addition, β₃-ARs were expressed throughout the rat bladder, mainly in the urothelium. These results suggest that vibegron excreted in urine as an unchanged compound can induce the additive inhibitory effects on bladder overactivity possibly through urothelial β₃-AR activation, which inhibits the afferent limb of micturition reflex rather than the efferent function as evidenced by the increases in threshold pressure and bladder capacity without affecting bladder contractile function after intravesical vibegron application.

The clinical pharmacology of the medical treatment for overactive bladder in adults

INTRODUCTION

Overactive bladder is a prevalent symptom complex that affects the patient's quality of life. Any disruption between the neuronal micturition pathway can lead to bladder overactivity. Neurogenic causes, myogenic causes, aging, bladder outlet obstruction, sex, and psychological factors are some of the factors contributing to bladder overactivity. The complaint of any symptoms of OAB, which is highly prevalent and affects overall QOL, often needs therapeutic interventions. When conservative therapy methods fail, the addition of medications is recommended. The most commonly used agents for the treatment of OAB are antimuscarinic drugs. New classes of drugs, such as beta-3 agonists, have enriched our pharmacologic armamentarium.

AREAS COVERED

In this review, with a special focus on oral pharmacological treatments, we discussed the definition, etiology, symptoms, diagnosis, and management of OAB.

EXPERT OPINION

OAB is a multifactorial condition with every patient presenting with a different collection of symptoms and signs. Medical therapies should be given in conjunction with behavioral therapies. Using high or low doses, flexible doses, and stopping or changing the medications are interchangeable strategies based on the level of treatment efficacy and patient satisfaction.

Direct in vitro and in vivo demonstration of muscarinic receptor binding by the novel radioligand, [3H]5-hydroxymethyltolterodine, in the bladder and other tissues of rats

In vitro and in vivo binding sites of [³H]-labeled 5-hydroxymethyltolterodine (5-HMT), a new radioligand for labeling muscarinic receptors in rat tissues were characterized. Specific [³H]5-HMT binding in rat tissues was saturable and of high affinity in each tissue. The dissociation constant (K_d) was significantly lower in bladder and heart than in submaxillary gland. Significant levels of in vivo specific [³H]5-HMT binding by intravenous injection of the radioligand were detected in tissues, except for cerebral cortex. Thus, [³H]5-HMT was shown to specifically label muscarinic receptors in rat tissues, suggesting a useful radioligand for labeling muscarinic receptors with high affinity.

Basic and clinical aspects of antimuscarinic agents used to treat overactive bladder

Antimuscarinic agents are now widely used as the pharmacological therapy for overactive bladder (OAB) because neuronal (parasympathetic nerve) and non-neuronal acetylcholine play a significant role for the bladder function. In this review, we will highlight basic and clinical aspects of eight antimuscarinic agents (oxybutynin, propiverine, tolterodine, solifenacin, darifenacin, trospium, imidafenacin, and fesoterodine) clinically used to treat urinary dysfunction in patients with OAB. The basic pharmacological characteristics of these eight antimuscarinic agents include muscarinic receptor subtype selectivity, functional bladder selectivity, and muscarinic receptor binding in the bladder and other tissues. The measurement of drug-receptor binding after oral administration of these agents allows for clearer understanding of bladder selectivity by the integration of pharmacodynamics and pharmacokinetics under in vivo conditions. Their central nervous system (CNS) penetration potentials are also discussed in terms of the feasibility of impairments in memory and cognitive function in elderly patients with OAB. The clinical aspects of efficacy focus on improvements in the daytime urinary frequency, nocturia, bladder capacity, the frequency of urgency, severity of urgency, number of incontinence episodes, OAB symptom score, and quality of life (QOL) score by antimuscarinic agents in patients with OAB. The safety of and adverse events caused by treatments with antimuscarinic agents such as dry mouth, constipation, blurred vision, erythema, fatigue, increased sweating, urinary retention, and CNS adverse events are discussed. A dose-dependent relationship was observed with adverse events, because the risk ratio generally increased with elevations in the drug dose of antimuscarinic agents. Side effect profiles may be additive to or contraindicated by other medications.