M(3) muscarinic receptors mediate contraction of human urinary bladder

Darifenacin, an M3 selective receptor antagonist, is an effective and well-tolerated once-daily treatment for overactive bladder

OBJECTIVES

To evaluate the efficacy, tolerability and safety of darifenacin, a once-daily M3) selective receptor antagonist (M3 SRA), in patients with overactive bladder (OAB).

METHODS

This multicentre, double-blind, placebo-controlled, parallel-group study enrolled 561 patients (19-88 years; 85% female) with OAB symptoms for >6 months, and included some patients with prior exposure to antimuscarinic agents. After washout and a 2-week placebo run-in, patients were randomised (1:4:2:3) to once-daily oral darifenacin controlled-release tablets (3.75 mg [n=53], 7.5 mg [229] or 15 mg [n=115]) or matching placebo (n=164) for 12 weeks. Patients recorded daily incontinence episodes, micturition frequency, bladder capacity (mean volume voided), frequency of urgency, severity of urgency, incontinence episodes resulting in change of clothing or pads and nocturnal awakenings due to OAB using an electronic diary during weeks 2, 6 and 12 (directly preceding clinic visits). Tolerability data were evaluated from adverse event reports.

RESULTS

Darifenacin 7.5 mg and 15 mg had a rapid onset of effect, with significant improvement compared with placebo being seen for most parameters at the first clinic visit (week 2). This effect was sustained through week 12. At this time the number of incontinence episodes per week was reduced from baseline by 67.7% with darifenacin 7.5 mg and 72.8% with darifenacin 15 mg compared with 55.9% with placebo (p=0.010 and p=0.017, respectively, versus placebo). The 3.75 mg group (null dose arm) was included for proof of concept of dose flexibility, therefore formal sample sizing and statistical analysis were not performed for this group. Darifenacin 7.5 mg and 15 mg, respectively, were significantly superior to placebo for improvements in micturition frequency (p<0.001, p<0.001), bladder capacity (p<0.040, p<0.001), frequency of urgency (p<0.001, p=0.005), severity of urgency (p<0.001, p=0.002) and number of incontinence episodes leading to a change in clothing or pads (p<0.001, p=0.002). There was no significant reduction in nocturnal awakenings due to OAB. The most common adverse events were mild-to-moderate dry mouth and constipation. However, no patients withdrew from the study as a result of dry mouth and discontinuation related to constipation was rare (0.6% placebo versus 0.9% darifenacin). In addition, there was a low need for laxative use, with no difference between the darifenacin groups and those taking placebo. There were no reports of blurred vision and the CNS and cardiac safety profile was comparable to placebo.

CONCLUSIONS

Darifenacin significantly improves the major symptoms of OAB. No significant CNS (primarily M1-receptor mediated) adverse events or cardiac (primarily M2-receptor mediated) adverse events were identified in this study, as may be predicted from the M3 selective receptor profile of darifenacin.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

An essential role of Cav1.2 L-type calcium channel for urinary bladder function

Mice deficient in the smooth muscle Cav1.2 calcium channel (SMACKO, smooth muscle alpha1c-subunit calcium channel knockout) have a severely reduced micturition and an increased bladder mass. L-type calcium current, protein, and spontaneous contractile activity were absent in the bladder of SMACKO mice. K+ and carbachol (CCh)-induced contractions were reduced to 10-fold in detrusor muscles from SMACKO mice. The dihydropyridine isradipine inhibited K+- and CCh-induced contractions of muscles from CTR but had no effect in muscles from SMACKO mice. CCh-induced contraction was blocked by removing extracellular Ca2+ but was unaffected by the PLC inhibitor U73122 or depletion of intracellular Ca2+ stores by thapsigargin. In muscles from CTR and SMACKO mice, CCh-induced contraction was partially inhibited by the Rho-kinase inhibitor Y27632. These results show that the Cav1.2 Ca2+ channel is essential for normal bladder function. The Rho-kinase and Ca2+-release pathways cannot compensate the lack of the L-type Ca2+ channel.

[Effect of 4-ethylamino-2-butynyl(2-cyclohexyl-2-phenyl) glycolate, metabolite of oxybutynin, on intra-artery administered acetylcholine-induced urinary bladder contraction in anesthetized dogs]

Oxybutynin has been used for neurogenic bladder disorders in clinic and known to have anti-cholinergic and spasmolytic properties. Metabolite of oxybutynin, 4-ethylamino-2-butynyl(2-cyclohexyl-2-phenyl) glycolate (N-desethyloxybutynin: DEOB) has been known to have similar anti-cholinergic and spasmolytic properties. However, the effect of DEOB on the urinary bladder has not been clarified in situ. Therefore, in the present study, we studied the effect of DEOB on acetylcholine-induced urinary bladder contraction in comparison with oxybutynin in anesthetized dogs. Intravenously administered DEOB dose-dependently inhibited acetylcholine-induced contractions. Oxybutynin also showed similar efficacy. From the Schild plot, it was found that the slope of DEOB and oxybutynin were 0.78 (95% confidence limit: 0.45-1.11) and 1.49 (95% confidence limit: 0.91-2.08), respectively. The dose of DEOB or oxybutynin needed to shift the concentration-dependent curve of acetylcholine rightward to a two times higher dose was calculated. The doses of DEOB and oxybutynin were 6.4 micro g/kg (95% confidence limit: 1.7-12.8 micro g/kg) and 13.9 micro g/kg (95% confidence limit: 6.3-24.5 micro g/kg), respectively. From the above results, it was found that DEOB has the same anti-cholinergic property as oxybutynin and that its activity was almost equipotent to that of oxybutynin. Therefore, DEOB was suggested to play an important role during oxybutynin therapy for neurogenic bladder disorder.

The M2 muscarinic receptor mediates in vitro bladder contractions from patients with neurogenic bladder dysfunction

Bladder muscle specimens from seven patients with neurogenic bladder dysfunction were analyzed to determine whether the muscarinic receptor subtype mediating contraction shifts from M(3) to the M(2) subtype as found in the denervated, hypertrophied rat bladder. Seven bladder specimens were analyzed from six female and one male patients. Six of the patients had traumatic cervical spinal cord injuries (C(4)-C(7)), and the other patient had an L(1) congenital myelomeningocele. This was compared with results from bladder specimens obtained from eight organ transplant donors. The affinities of three subtype-selective muscarinic receptor antagonists for inhibition of carbachol-induced contractions were determined. The affinity of the M(3) selective antagonists darifenacin or p-fluoro-hexahydrosiladifenadol (p-F-HHSiD) was determined in six of the seven spinal injury patient specimens. The affinity was consistent with M(2)-mediated contractions in four of these six specimens, intermediate between M(2) and M(3) in one specimen, and within the M(3) range in one specimen. The other specimen, tested only with the M(2) selective antagonist methoctramine, showed an M(3) affinity. In the organ donors, the affinity of p-F-HHSiD was within the M(2) range for six of seven specimens, whereas the affinity of darifenacin was within the M(3) range for five of six and intermediate between M(2) and M(3) for the other specimen tested. The affinity of methoctramine in both organ donor specimens tested was within the M(3) range. Whereas normal detrusor contractions are mediated by the M(3) receptor subtype, in patients with neurogenic bladder dysfunction as well as certain organ transplant donors, contractions can be mediated by the M(2) muscarinic receptor subtype.

Muscarinic receptors mediating contraction of female mouse urinary bladder: effects of oestrogen

Muscarinic receptors mediating contraction of bladder detrusor muscle from female mice were examined. Mice were untreated (A) or treated with oestradiol cypionate (200 microg/kg) 24 h (B) or 96 h (C) before experimentation, or were pregnant (day 17) (D). Saturation radioligand binding experiments using [(3)H]quinuclidinyl benzilate ([(3)H] QNB) indicated similar muscarinic receptor densities and affinities in bladders from groups A and B. Neither oestrogen treatment nor pregnancy altered pD(2) estimates for methacholine. Maximum responses to methacholine and high-K(+) physiological salt solution (KPSS) were significantly greater (P<0.05) in tissues from groups C and D than in A and B. Potencies of other muscarinic receptor agonists were similar in groups A and B with an order of acetylcholine plus physostigmine (10 microM) approximately methacholine plus physostigmine (10 microM)>methacholine approximately acetylcholine>bethanechol. Antagonist pK(B) estimates were similar in bladders from groups A and B with a rank order of: atropine>/=4-diphenyl acetoxy-N-methyl piperidine methiodide>parafluorohexahydrosiladifenidol approximately pirenzepine>himbacine, implicating muscarinic M(1) and/or M(5) as well as muscarinic M(3) receptors in mediating methacholine-induced bladder contraction.

Signal Transduction Underlying Carbachol-Induced Contraction of Human Urinary Bladder

The present study was designed to reexamine the muscarinic acetylcholine receptor subtype mediating carbachol-induced contraction of human urinary bladder and to investigate the underlying signal transduction. Based upon the nonselective tolterodine, the highly M(2)-selective (R)-4-[2-[3-(4-methoxy-benzoylamino)-benzyl]-piperidin-1-ylmethyl]piperidine-1-carboxylic acid amide (Ro-320-6206), and the highly M(3)-selective darifenacin and 3-(1-carbamoyl-1,1-diphenylmethyl)-1-(4-methoxyphenylethyl)pyrrolidine (APP), contraction occurs via M(3) receptors. The phospholipase C inhibitor 1-(6-[([17beta]-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl)-1H-pyrrole-2,5-dione (U 73,122) (1-10 microM) did not significantly affect carbachol-stimulated bladder contraction. The phospholipase D inhibitor butan-1-ol relative to its negative control butan-2-ol (0.3% each) caused small but detectable inhibition of carbachol-induced bladder contraction. The Ca(2+) entry blocker nifedipine (10-100 nM) strongly inhibited carbachol-induced bladder contraction. In contrast, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole HCl (SK&F 96,365) (1-10 microM), an inhibitor of store-operated Ca(2+) channels, caused little inhibition. The protein kinase C inhibitor bisindolylmaleimide I (1-10 microM) did not significantly affected carbachol-induced bladder contraction. In contrast, trans-4-[(1R)-1-aminoethyl]-N-4-pyridinylcyclohexanecarboxamide (Y 27,632) (1-10 microM), an inhibitor of rho-associated kinases, concentration dependently and effectively attenuated the carbachol responses. We conclude that carbachol-induced contraction of human urinary bladder via M(3) receptors largely depends on Ca(2+) entry through nifedipine-sensitive channels and activation of a rho kinase, whereas phospholipase D and store-operated Ca(2+) channels contribute only in a minor way. Surprisingly, phospholipase C or protein kinase C do not seem to be involved to a relevant extent.

Effects of 4-Ethylamino-2-butynyl(2-cyclohexyl-2-phenyl)glycolate Hydrochloride, a Metabolite of Oxybutynin, on Bladder Specimens and Rhythmic Bladder Contraction in Rats in Comparison With Oxybutynin

Oxybutynin has been used for neurogenic bladder disorders and is known to have anti-cholinergic and antispasmodic properties. However, the anti-cholinergic and antispasmodic properties of 4-ethylamino-2-butynyl(2-cyclohexyl-2-phenyl)glycolate hydrochloride (N-desethyloxybutynin: DEOB), a metabolite of oxybutynin, have not been clarified. Therefore, in the present study, we studied these properties by using rat urinary bladder specimens in comparison with oxybutynin. Moreover, the effect of DEOB on rhythmic urinary bladder contraction was also evaluated using anesthetized rats. DEOB and oxybutynin concentration-dependently inhibited the carbachol-induced contraction, the pA(2) values being 7.19 and 7.11, respectively. DEOB and oxybutynin also concentration-dependently inhibited the 100 mM KCl-induced contraction, the ED(50) values being 12.1 and 10.4 microM, respectively. Intravenously administered DEOB and oxybutynin dose-dependently (0.03 - 0.3 mg/kg) inhibited the amplitude of the rhythmic bladder contraction to similar degrees, but had no affect on the frequency. From the above results, it was determined that DEOB has anti-cholinergic and antispasmodic properties and that these activities were almost equal to those of oxybutynin. Therefore, DEOB may play an important role during oxybutynin therapy for neurogenic bladder disorder.

Antimuscarinics for treatment of overactive bladder

For many years, antimuscarinic drugs have been the first-line pharmacological treatment for urgency, frequency, and urge incontinence, all symptoms of the disorder termed overactive bladder. Antimuscarinic treatment is not always effective and is associated with side-effects that limit its clinical use. The clinical significance of the effects of antimuscarinic drugs has been questioned lately. In this review, the rationale for the use of these drugs in the management of overactive bladder is re-examined and the results of treatment are discussed. I conclude that these drugs are the only treatment with undisputed effectiveness in the treatment of overactive bladder. They may not be the perfect treatment for all patients with this disorder, but their value for individual patients should not be underestimated. Further clinical trials with improvement in quality of life as the primary endpoint are needed and may give a fair reflection of the clinical value of antimuscarinic drugs.

Muscarinic receptors: What we know

An understanding of muscarinic receptors is tantamount to an understanding of overactive bladder. The M(3) muscarinic receptor subtype is responsible for detrusor smooth muscle contraction and it exerts an exocrine function in the salivary glands. Alterations in the receptor's response to acetylcholine as a result of injury may lead to hypersensitivity and overactivity. The M(2) receptor subtype, which is mainly responsible for cardiac function, is the muscarinic receptor of highest proportion in the detrusor. M(2) also may play a role in detrusor contraction in injury and pathologic states. Muscarinic antagonists are the mainstay of pharmacotherapy for overactive bladder, but those that are available are not tissue specific. Growing knowledge of the nuances of receptor-ligand behavior and interaction between muscarinic receptors subtypes may provide novel targets for future drug development, improve efficacy, and reduce bothersome side effects.

Signal transduction underlying carbachol-induced contraction of rat urinary bladder. I. Phospholipases and Ca2+ sources

We have reexamined the muscarinic receptor subtype mediating carbachol-induced contraction of rat urinary bladder and investigated the role of phospholipase (PL)C, D, and A2 and of intra- and extracellular Ca2+ sources in this effect. Based on the nonsubtype-selective tolterodine, the highly M2 receptor-selective (R)-4-[2-[3-(4-methoxy-benzoylamino)-benzyl]-piperidin-1-ylmethyl]-piperidine-1-carboxylic acid amide (Ro-320-6206), and the highly M3 receptor-selective darifenacin and 3-(1-carbamoyl-1,1-diphenylmethyl)-1-(4-methoxyphenylethyl)pyrrolidine (APP), contraction occurs via M3 receptors. Carbachol stimulated inositol phosphate formation in rat bladder slices, and this was abolished by the phospholipase C inhibitor 1-(6-[([17beta]-3-methoxyestra-1,3,5[10]-trien-17-yl)-amino]hexyl)-1H-pyrrole-2,5-dione (U 73,122; 10 microM). Nevertheless, U 73,122 (1-10 microM) did not significantly affect carbachol-stimulated bladder contraction. Carbachol had only little effect on PLD activity in bladder slices, but the PLD inhibitor butan-1-ol, relative to its negative control butan-2-ol (0.3% each), caused detectable inhibition of carbachol-induced bladder contraction. The cytosolic PLA2 inhibitor arachidonyltrifluoromethyl ketone weakly inhibited carbachol-induced contraction at a concentration of 300 microM, but the cyclooxygenase inhibitor indomethacin (1-10 microM) remained without effect. The Ca2+ entry blocker nifedipine (10-100 nM) almost completely inhibited carbachol-induced bladder contraction. In contrast, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole HCl (SKF 96,365; 10 microM), an inhibitor of store-operated Ca2+ channels, caused little inhibition. We conclude that carbachol-induced contraction of rat bladder largely depends on Ca2+ entry through nifedipine-sensitive channels and, perhaps, PLD, PLA2, and store-operated Ca2+ channels, whereas cyclooxygenase and, surprisingly, also PLC are not involved to a relevant extent.

[New pharmacological treatment concepts for overactive bladder]

Muscarinic receptor antagonists such as oxybutynin, propiverine, tolterodine, or trospium are the basis of medical treatment for overactive bladder. While they are moderately efficacious, their use can be limited by adverse effects such as dry mouth. This has sparked the search for new treatment options. Vanilloid receptor agonists, tachykinin receptor antagonists, potassium channel openers, and beta(3)-adrenoceptor agonists are currently under investigation, but are unlikely to become clinically available in the next few years. Therefore, current attempts to optimize treatment focus on improvement of existing drugs by new pharmaceutical formulations. Indeed, extended release formulations of oxybutynin (not available in Germany) or tolterodine have demonstrated an improved tolerability in clinical studies which was accompanied by an efficacy at least equal to that of their standard formulations.

Gender comparison of muscarinic receptor expression and function in rat and human urinary bladder: differential regulation of M2 and M3 receptors?

Since symptoms of bladder dysfunction occur more frequently in women than in men and since muscarinic receptors are the physiologically most important system to mediate bladder contraction, we have compared the number, subtype distribution and function of muscarinic receptors in bladders from male and female rats. Muscarinic receptor function was also assessed in bladder strips from male and female human bladder. Male and female rats expressed a similar number of muscarinic receptors (144+/-5 vs. 140+/-6 fmol/mg protein in saturation radioligand binding). While competition binding curves for the moderately M(2)-selective methoctramine were not consistently better fitted by a two-site model, most competition curves for the M(3)-selective darifenacin were biphasic and yielded 29+/-10% and 31+/-7% high affinity sites (corresponding to M(3) receptors) in male and females, respectively. Immunoreactivity of alpha-subunits of the G-proteins G(q/11), G(i1/2), G(i3) and G(s) did not significantly differ between both genders. The muscarinic receptor agonist carbachol similarly stimulated inositol phosphate accumulation in bladder slices from male and female rats with calculated maximum responses of 69+/-17 and 77+/-18% over basal and pEC(50) values of 4.90+/-0.45 and 4.40+/-0.46, respectively. While darifenacin inhibited carbachol-stimulated inositol phosphate formation approximately 100-fold more potently than methoctramine, each antagonist was similarly potent in both genders. Carbachol concentration-dependently contracted bladder strips with a pEC(50) of 5.66+/-0.05 and 5.72+/-0.06 and maximum effects of 4.3+/-0.1 and 4.2+/-0.2 mN/mg wet weight in male and female rats, respectively. The contractile effect of carbachol was concentration-dependently antagonised by the non-selective atropine (1-30 nM), the M(1)-selective pirenzepine (1-30 M), the M(2)-selective methoctramine (1-10 microM) and the M(3)-selective darifenacin (10-100 nM), with the latter exhibiting a partly unsurmountable antagonism. The overall potency of all four antagonists suggested that contraction was mediated predominantly if not exclusively by M(3) receptors with no appreciable differences between both male and female rats. Similarly, the maximum effects (4.4+/-0.6 vs. 4.4+/-2.4 mN/mg) and pEC(50) (6.07+/-0.05 vs. 6.32+/-0.14) of carbachol did not differ between genders in bladder samples from 25 consecutive patients. We conclude that number und function of muscarinic receptors and the relative roles of their M(2) and M(3) subtypes do not differ between urinary bladders of male and female rats; at least with regard to overall muscarinic responsiveness this situation appears to be similar in humans.

Antimuscarinics and the overactive detrusor--which is the main mechanism of action?

Contraction of the bladder, voluntary or involuntary, involves stimulation of the muscarinic receptors on the detrusor by acetylcholine, released from activated cholinergic nerves. Antimuscarinics are the drugs of choice for treatment of detrusor overactivity and the overactive bladder (OAB) syndrome. However, antimuscarinics at clinically recommended doses have little effect on voiding contractions, and may act mainly during the bladder storage phase, during which there is normally no parasympathetic outflow from the spinal cord. Supporting this, antimuscarinics have been shown to reduce bladder tone during storage, and to increase cystometric bladder capacity. A basal release of acetylcholine from non-neuronal (urothelial) as well as neuronal sources has been demonstrated in isolated human detrusor muscle. It is suggested that this release, which is increased by stretching the muscle and in the aging bladder, contributes to detrusor overactivity and OAB by eventually increasing bladder afferent activity during storage.

A benefit-risk assessment of extended-release oxybutynin

Oxybutynin is a muscarinic receptor antagonist, which has been available for a number of years in its original immediate-release (IR) formulation. While oxybutynin IR has proven effective for the treatment of overactive bladder, its extended use can be limited by adverse effects, particularly dry mouth. An extended-release (ER) formulation of oxybutynin based on the OROS system has recently become available, which allows once daily administration. In direct comparison to oxybutynin IR, oxybutynin ER has an increased oral bioavailability for the parent compound oxybutynin which is accompanied by a reduced bioavailability for the active metabolite N-desethyl-oxybutynin. The latter has been implicated in mediating a major part of the adverse effects of oxybutynin treatment. Two double-blind, placebo-controlled, randomised studies in patients with overactive bladder have demonstrated that oxybutynin ER has a similar efficacy as oxybutynin IR but with improved tolerability. This is in line with clinical pharmacological studies demonstrating a smaller impairment of saliva production with oxybutynin ER than with oxybutynin IR. Thus, the ER formulation of oxybutynin maintains the therapeutic benefits and concomitantly improves tolerability.