Introduction

Overactive bladder (OAB) is a highly prevalent disorder characterized as a syndrome consisting of 'urgency with or without urgency incontinence, usually accompanied by frequency and nocturia' (see Wein & Rovner, 2002; Abrams et al., 2003). The process of micturition is critically dependent on the operation of a spinal–bulbospinal reflex, which comprises afferent (sensory) and efferent (motor) pathways that are integrated and coordinated at spinal and supraspinal centers (see de Groat et al., 1993). The lumbosacral parasympathetic outflow provides the excitatory motor input to the urinary bladder smooth muscle (detrusor). It is generally accepted that activation of muscarinic receptors on the detrusor is one of the mechanisms driving abnormal detrusor overactivity in the diseased bladder (see de Groat, 1997). Recent evidence, however, has raised the possibility of a potential role of urothelial/suburothelial muscarinic receptors in the etiology of detrusor overactivity or sensory urgency (see Andersson & Yoshida, 2003; de Groat, 2004). Regardless of the precise locus of action of antimuscarinic drugs (i.e., afferent or efferent), their widespread use for the treatment of OAB over the past three decades underscores the critical role of muscarinic receptors in the pathophysiology of this disorder. Recent advances in our understanding of muscarinic receptor pharmacology in the lower urinary tract have guided the discovery and development of new medicines for OAB (see Chapple et al., 2002; Chess-Williams et al., 2002; Hegde et al., 2004). The objective of this review is to summarize our current understanding of the functional role of muscarinic receptor subtypes in the bladder and to analyze the clinical and pharmacokinetic–pharmacodynamic (PK–PD) profile of marketed antimuscarinic drugs.

Muscarinic receptor pharmacology of the bladder

Pharmacological studies

Muscarinic receptors are classified, based on molecular and pharmacological criteria, into five subtypes (M₁–M₅) (see Caulfield & Birdsall, 1998). Radioligand saturation and competition-binding studies have shown that the bladder of most species, including man, is endowed principally with M₂ and M₃ receptors, with the former predominating in number (see Hegde & Eglen, 1999). It is pertinent to note that these studies are not sensitive enough to detect the muscarinic receptor population in nondetrusor components (e.g., nerves, urothelium) of the bladder, which make up a small fraction of the total tissue mass but nevertheless may be important from a functional standpoint.

Until recently, our understanding of the relative contribution of muscarinic receptor subtypes towards bladder function was derived primarily from pharmacological studies using molecules such as pirenzepine, 4-DAMP, p-F-HHSid, darifenacin and methoctramine, which have preferential affinity for one or more subtypes. These studies have suggested that the M₃ receptor primarily mediates direct contractile responses to agonists in the normal bladder (Wang et al., 1995; Hegde et al., 1997; Chess-Williams et al., 2001; Fetscher et al., 2002). It was originally thought that the M₃ contractile response in the bladder is mediated by intracellular calcium that is mobilized by inositol phosphates that are generated following activation of phospholipase C (Harriss et al., 1995). This assumption has been re-examined by a recent study that concluded, based on the inhibitory effects of a range of molecules that selectively target different transductional pathways, that M₃-mediated contraction of the human bladder depends on entry of extracellular calcium through L-type calcium channels and activation of a rho kinase (Schneider et al., 2004). Pharmacological results have failed to demonstrate a role of M₂ muscarinic receptors in mediating direct contractile responses, suggesting that this receptor either plays little or no role, or that the presence of the M₃ receptor is obligatory for its functional role (see the discussion below on findings from gene knockout (KO) studies). It has been suggested that the relative contribution of M₂ receptors toward the overall direct contractile response becomes more important in diseased states (Pontari et al., 2004), although this finding has recently been challenged (Stevens et al., 2004a, 2004b).

A key functional role of the dominant M₂ receptors appears to be in mediating an indirect contractile response by reversing -adrenoceptor-mediated relaxation through a cAMP-dependent mechanism (Hegde et al., 1997; Yamanishi et al., 2000). This effect can be demonstrated by first inactivating the M₃ receptor with a nitrogen mustard derivate of 4-DAMP and then measuring the contractile response to the muscarinic agonist in the presence of heterologous contractile (KCl) and relaxant (isoprenaline) agents. It should be noted, however, that the physiological role of the sympathetic nervous system and -adrenoceptors in bladder relaxation is controversial (Andersson, 1999) and there are no studies that have attempted to study the indirect role of M₂ receptors in the human bladder.

Parasympathetic nerves innervating the bladder are enriched with pre-junctional inhibitory and facilitatory muscarinic receptors that are differentially activated, depending on the frequency of nerve stimulation. Using pharmacological tools, the prejunctional faciliatory receptor has been classified as M₁ (Somogyi et al., 1994; Tobin & Sjogren, 1995), whereas the prejunctional inhibitory receptor has been classified as either M₂ (Tobin & Sjogren, 1995) or M₄ (D'Agostino et al., 2000).

The urothelium was traditionally viewed as a passive membrane barrier between the urinary tract and its contents. However, emerging evidence suggests that the urothelium and suburothelial interstitial cells, which express a range of receptors, including muscarinic (Gillespie et al., 2003), actively participate in sensory function in the bladder by releasing neurotransmitters in response to distention and receptor activation (de Groat, 2004). Muscarinic receptors on the urothelial lining of the bladder have been shown to evoke the release of a diffusible mediator that inhibits contraction of the underlying detrusor muscle (Fovaeus et al., 1999; Hawthorn et al., 2000). Upon activation of muscarinic receptors, urothelial cells have been shown to release ATP, which in turn may activate afferent nerves to alter its excitability (see de Groat, 2004). The source of acetylcholine (ACh) (neuronal/extraneuronal) mediating the urothelial responses and its role in the pathophysiology of OAB remains to be elucidated and is the subject of extensive ongoing basic research.

Gene KO studies

In recent years, mutant mice deficient in one or more muscarinic receptor subtype have been constructed and used to assess the contribution of the five subtypes in bladder function. The findings from these gene KO studies have essentially confirmed the conclusions derived from pharmacological studies and also added new insights to our understanding of the role of muscarinic receptors in the physiology of the lower urinary tract.

In M₃ KO mice, bladder contractile responses to carbachol are attenuated by 95%, consistent with findings from pharmacological studies that have ascribed a major role of M₃ receptors in mediating direct contraction of the bladder (Matsui et al., 2002; Ehlert et al., 2005). Interestingly, cystometric studies have shown that, although M₃ KO animals have longer voiding intervals and larger micturition volumes and bladder capacity than wild-type controls, the bladder residual volume in these animals is unchanged, indicating no major functional impairment of bladder emptying (Igawa et al., 2004). A possible explanation for this finding is that chronic absence of the M₃ receptor is compensated by either another muscarinic receptor or noncholinergic mechanisms.

The residual (5%) direct contractile responses that persist in M₃ KO mice are completely lost in mice lacking both M₂ and M₃ receptors, implying that the M₂ receptor is capable of mediating small direct contractile responses (Matsui et al., 2002). Interestingly, the contractile responses to oxotremorine-M were more greatly inhibited by 4-DAMP mustard (M₃ inactivation) in the bladder tissue of M₂ KO than in wild-type mice, implying that M₂ receptors can also operate by enhancing M₃-mediated contractions and this effect is manifested only in wild-type mice (Ehlert et al., 2005). Cystometric studies have demonstrated small increases in voiding intervals and micturition volumes in M₂ KO animals (Igawa et al., 2004). Given the potential interaction between M₂ and M₃ receptors, it seems relevant to conduct cystometric studies in animals lacking M₂ and M₃ receptors, particularly as atropine's effects on cystometric parameters are more marked than that observed in M₃ or M₂ KO animals (Igawa et al., 2004). The biochemical mechanisms underlying the potential crosstalk between M₂ and M₃ receptors are unclear at present. The indirect c-AMP-related contractile effects of M₂ receptor activation have also been confirmed using M₂ and M₃ KO animals (Matsui et al., 2003; Ehlert et al., 2005). In bladders from M₃ KO mice that were precontracted with PGF2alpha and relaxed with isoprenaline or forskolin, oxotremorine produced concentration-dependent contractile responses that exhibited an M₂ profile in competitive antagonism studies and were completely abolished in M₂/M₃ KO mice.

As stated previously, pharmacological studies have been unable to discern whether the prejunctional inhibitory autoreceptors in the bladder belong to the M₂ or M₄ subtypes, owing to the paucity of molecules that discriminate between M₂ and M₄ receptors. This ambiguity has been clarified by use of M₂ and M₄ KO animals. In electrically stimulated bladder segments, pre-incubated with [³H]choline and superfused with oxotremorine, ipratropium stimulated tritium outflow in wild-type and M₂ KO bladders, but had no effect in M₄ KO bladders (Zhou et al., 2002). These studies clearly implicate a key role of M₄ receptors in regulating ACh release from post-ganglionic cholinergic nerves via a negative feedback mechanism. The existence of facilitatory M₁ receptors, whose presence has been demonstrated pharmacologically, has yet to be confirmed by gene KO studies.

Figure 1 summarizes our current understanding of the pharmacological role of muscarinic receptor subtypes in control of bladder function. The collective evidence from pharmacological and gene KO studies have added immense confidence to our knowledge base and may potentially guide the discovery of novel medicines for the treatment of OAB.

Figure 1.

Schematic showing the role of muscarinic receptor subtypes in modulation of detrusor contractility. The micturition reflex is dependent on the operation of the spinal–bulbospinal reflex. Detrusor contractile tone is dependent on the phosphorylation state of myosin filaments. The lumbosacral parasympathetic outflow provides the major excitatory efferent input to the detrusor. ACh release from post-ganglionic parasympathetic nerves is regulated by prejunctional inhibitory M₄ and facilitatory M₁ receptors. ACh agonizes detrusor M₃ receptors to produce 'direct contraction' by a mechanism that relies on entry of extracellular calcium (Ca²⁺) through L-type calcium channels. Interaction of Ca²⁺ with calmodulin (CaM) activates myosin light chain kinase (MLCK), resulting in contraction. M₃ receptor activation can also activate a rho kinase that inactivates myosin light chain phosphatase (MLCP) and promotes detrusor contraction. M₂ receptors can modulate M₃-mediated 'direct contractions' by a mechanism that is not clearly understood. Activation of M₂ receptors by ACh inhibits AC to produce 'indirect contractions' by inhibiting cAMP-dependent -adrenoceptor mediated relaxation. ACh, from neuronal or non-neuronal sources, can potentially activate muscarinic receptors (Mx, subtype unknown) on urothelial/sub-urothelial cells to release ATP, which in turn can alter excitability of afferent nerves via P2X₃ receptors. Activation of muscarinic receptors (Mx, subtype unknown) can also evoke the release of diffusible factors from urothelial/sub-urothelial cells to modulate detrusor contractility indirectly.

Full figure and legend (127K)

Implications for drug discovery

A muscarinic antagonist used for the treatment of OAB should ideally normalize bladder function without causing secondary antimuscarinic adverse effects, chiefly dry mouth. Given the critical role of M₃ receptors in mediating direct bladder contraction, antagonism of this receptor would be expected to result in some level of efficacy in OAB patients. Unfortunately, both pharmacological and gene KO studies have shown that M₃ receptors mediate, at least in part, the sialogogue response to muscarinic agonists (Nakamura et al., 2004). This paradigm poses considerable challenges to the discovery of an efficacious muscarinic antagonist that is devoid of dry mouth. Given the potential crosstalk between M₂ and M₃ receptors and the important indirect role of M₂ receptors in mediating bladder contraction, the strategy of selectively targeting the M₂ receptor for OAB has some merit, but is clinically unprecedented and carries the risk of cardiac adverse effects.

Comparative analysis of approved antimuscarinic drugs

Pharmacological profile

Currently, oxybutynin, tolterodine, solifenacin, darifenacin and trospium are the five major antimuscarinic drugs that are employed for the treatment of OAB. These molecules differ in their pharmacological profile at the five human recombinant muscarinic receptors (Table 1). Tolterodine (including its 5-hydroxymethyl metabolite) and trospium essentially do not discriminate between the five subtypes. Oxybutynin (and its N-desethyl metabolite) and solifenacin do possess marginal selectivity ( 10-fold) for M₃ over the M₂/M₅ subtypes, but do not distinguish between M₃ and M₁/M₄ subtypes. In contrast, darifenacin, which behaves as an insurmountable antagonist (Fetscher et al., 2002), has a high degree of selectivity for M₃ over the M₂/M₄ subtypes and modest selectivity for M₃ over the M₁/M₅ subtypes. In animal models, greater bladder-to-salivary gland selectivity ratios have been reported for tolterodine ((Nilvebrant et al., 1997a; Gillberg et al., 1998), darifenacin (Gupta et al., 2002) and solifenacin (Ikeda et al., 2002) compared to oxybutynin.

Table 1 - Affinity of antimuscarinics for human muscarinic receptor subtypes.

Full table

Clinical efficacy and adverse effect profile

The effectiveness of antimuscarinic drugs in OAB has been well established with a number of molecules, but the magnitude of efficacy is modest and its clinical significance has been questioned (see Herbison et al. (2003) for review). Data from large controlled trials have been published for most of the marketed antimuscarnic drugs, including the five commonly used oral products (tolterodine-ER, oxybutynin-ER, solifenacin, darifenacin and trospium) and one transdermal product (oxybutynin-TDS) (see Chapple et al. (2005c) for a systematic review and meta-analysis of all trials). The data from the vast majority of these studies cannot, however, be used to make relative comparisons of efficacy and tolerability, given the inconsistent placebo response and lack of standardization of enrollment criteria and measurement instruments. Taking this caveat into consideration, it is worth noting, however, that the efficacy of these drugs falls within a narrow range with respect to reduction in incontinence episode frequency (-43 to -77%) or voiding frequency (-16 to -28%) (Table 2). Likewise, with the exception of oxybutynin-TDS, the incidence of dry mouth for these molecules is roughly in the range of 22–35% (Table 2). Although dry mouth is not an issue with oxybutynin-TDS, pruritis at the application site is a limitation of this product. Compared to the other drugs, darifenacin does appear to produce a higher incidence of constipation, especially at the higher dose.

Table 2 - Clinical efficacy and adverse effects of marketed antimuscarinics for OAB (12-week trials).

Full table

Head-to-head comparative studies have been conducted in a few instances and offer the opportunity of making a cogent scientific assessment of relative efficacy and tolerability. Two such trials have been published thus far using tolterodine ER as the common comparator drug. In the first case (OPERA Trial), the efficacy and tolerability of tolterodine-ER (4 mg, qd) was compared to oxybutynin-ER (10 mg, qd) in a randomized, double-blind, 12-week study in 790 women (Diokno et al., 2003). Both medications produced similar reductions in the weekly frequency of urge urinary incontinence (primary end point) and total incontinence episodes. However, the percent reduction in weekly micturition frequency was significantly greater and the percent of patients reporting complete control of continence was statistically higher in the oxybutynin-ER group compared to the tolterodine-ER group. Dry mouth was reported by a greater number of patients on oxybutynin-ER (30%) than on tolterodine ER (22%). In the second case (STAR Trial), solifenacin (flexible dosing, 5 or 10 mg, qd) was compared to tolterodine ER (4 mg, qd) in a double-blind, double-dummy, 12-week study in 1355 patients (Chapple et al., 2005b). Solifenacin produced statistically significantly greater reductions in episodes of urgency, urge incontinence and overall incontinence compared to tolterodine-ER. More solifenacin-treated patients became continent and reported improvements in perception of their bladder condition. Adverse effect profiles of the two drugs were roughly comparable.

PK–PD

A major attribute of recently introduced antimuscarinic drugs is their convenient dosing flexibility. With the exception of trospium, which is dosed twice daily, the remaining four oral drugs (tolterdoine-ER, oxybutynin-ER, solifenacin and darifenacin) are dosed once daily. Table 3 summarizes the human exposure (C_max, AUC, Cavg) of the five molecules at their therapeutic doses. The C_avg (free fraction and total drug) was used to estimate % M₂ and M₃ receptor occupancies at therapeutic doses (Figure 2). As shown in Figure 2a, the 'free fraction' receptor occupancy estimates appear to be unrealistically low for all drugs, with the exception of solifenacin. This could imply that the pharmacodynamic effects of some of the drugs are driven by the local detrusor concentrations, as opposed to plasma drug levels, possibly as a result of partitioning of molecules into tissues. An alternate explanation is that protein binding has a marginal effect on drug concentrations at the receptor compartment. It is pertinent to note that the affinity of most drugs for plasma protein-binding sites is usually in the low micromolar level. Consequently, for drugs which have nanomolar affinity for their therapeutic targets, protein binding would be expected to have a negligible effect on the drug–receptor equilibrium. If one reflects on the 'total drug' receptor occupancy estimates (Figure 2b), it is striking that the five molecules possess roughly similar clinical efficacy despite producing divergent occupancy of the M₃ receptor, the key target that has been implicated in modulation of bladder function. This suggests that the antimuscarinic mechanism by which the drugs produce their pharmacodynamic effects may be more complex than previously envisaged. For example, it is possible that the efficacies of darifenacin and solifenacin result solely from high and preferential M₃ receptor antagonism, whereas those of pan-selective molecules (tolterodine, trospium) result from antagonism of multiple receptors including M₂. This is not unreasonable to postulate, since inhibition of bladder function can theoretically result from antagonism of post-junctional M₂/M₃ receptors or prejunctional facilitatory M₁ receptors (Figure 1). There does appear to be, however, some relationship between the estimates of M₃ receptor occupancy and incidence of dry mouth. Both darifenacin and oxybutynin-ER, which produce 92–96% M₃ receptor occupancy, tend to produce a higher incidence of dry mouth when compared to tolterodine, which occupies approximately 65% of M₃ receptors at therapeutic doses. The selective occupation of the M₃ receptor in the gastrointestinal tract by darifenacin may also be the mechanism for its greater propensity to cause constipation, but it is unclear as why solfenacin does not share this property.

Figure 2.

Estimated M₂ and M₃ receptor occupancy at free (a) and total (b) plasma drug levels of antimuscarinic drugs. Hill–Langmuir equation was used to compute receptor occupancy by using binding affinity constants of the drugs at the human M₂ and M₃ recombinant receptors (Table 1) and average plasma drug levels (C_avg, total drug) achieved in humans at therapeutic doses of tolterodine-ER (4 mg, qd), oxybutynin-ER (10 mg, qd), darifenacin (15 mg, qd), solifenacin (10 mg, qd) and trospium (20 mg, bid) (Table 3). In the case of oxybutynin and tolterodine, the contributions of both parent and active metabolite were taken into consideration to estimate the net receptor occupancy.

Full figure and legend (31K)

Table 3 - Steady-state exposure of oral antimuscarinics at therapeutic doses.

Full table

Compliance and persistence on antimuscarinic drugs

Given the chronic nature of OAB, long-term compliance and persistence on antimuscarinic drugs is essential for engendering meaningful improvements in patient's quality of life, reduction in comorbidities and lowering of healthcare costs (see Noe et al., 2004; Haab & Castro-Diaz, 2005). The convenience of once-daily dosing with newer antimuscarinic medicines has led to expected improvements in patient's compliance on therapy. The likelihood of patients to persist on a given therapy for a protracted period (>1 year), however, is dependent on their own assessment of the medicine's overall effectiveness and tolerability. In long-term, open-label studies, the percentage of patients who persist on therapy for the duration of the study (approximately 10–12 months) was 46, 76, 81 and 81% for oxybutynin-ER (Diokno et al., 2002), tolterodine-ER (Kreder et al., 2002), trospium (Halaska et al., 2003) and solifenacin (Haab et al., 2005), respectively. It should be recognized, however, that the observed persistence rates in controlled trials might not be reflective of 'real-life' clinical settings (see Andrade et al., 1995). A recent study tracked prescription data at monthly intervals to assess the percentage of patients who continue on therapy during a 12-month period and noted that less than 20% of patients were still on their medication (oxybutynin and tolterodine IR and ER) at the end of 12 months (Chui et al., 2004). This remarkably low persistence rate, attributable to poor efficacy and adverse effects, implies that we are still a long way from achieving our goal of delivering a medicine for OAB patients that offers a satisfactory balance of efficacy and tolerability over a chronic treatment period.

Conclusions

Despite impressive advances in our comprehension of the pathophysiology of the lower urinary tract, muscarinic receptor antagonism remains the only clinically proven mechanism for alleviating the symptoms of OAB. The recent regulatory approval of a number of new antimuscarinic molecules has expanded the therapeutic armamentarium for OAB. Emerging data from head-to-head clinical trials are revealing subtle differences in the clinical profile of these drugs. There is also greater clarity on the PK–PD relationship of these drugs. Nevertheless, modest efficacy and high incidence of dry mouth remain a common shortcoming of all drugs. Indeed, low persistence with therapy is a significant issue that limits the long-term benefit of antimuscarinic drugs. The combination of pharmacological and gene KO studies has solidified our understanding of the functional role of muscarinic receptor subtypes in the bladder. The findings from these studies, however, have not yet offered a clear discovery path for improving the therapeutic index of antimuscarinic molecules. There is still an unmet need for an antimuscarinic medicine with superior clinical effectiveness that can translate into better persistence on therapy.

References

1. ABRAMS, P., CARDOZO, L., FALL, M., GRIFFITHS, D., ROSIER, P., ULMSTEN, U., VAN KERREBROECK, P., VICTOR, A. & WEIN, A. (2003). Standardisation Sub-Committee of the International Continence Society. Related Articles. The standardisation of terminology in lower urinary tract function: report from the standardisation sub-committee of the International Continence Society. Urology, 61, 37–49. | Article | PubMed | ISI |
2. ANDERSSON, K.E. (1999). Changes in bladder tone during filling: pharmacological aspects. Scand. J. Urol. Nephrol. Suppl., 201, 67–72. | PubMed | ChemPort |
3. ANDERSSON, K.E. & YOSHAIDA, M. (2003). Antimuscarinics and the overactive detrusor – which is the main mechanism of action? Eur. Urol., 43, 1–5. | Article | PubMed | ISI | ChemPort |
4. ANDRADE, S.E., WALKER, A.M., GOTTLIEB, L.K., HOLLENBERG, N.K., TESTA, M.A., SAPERIA, G.M. & PLATT, R. (1995). Discontinuation of antihyperlipidemic drugs: Do rates reported in clinical trials reflect rates in primary care settings? New Engl. J. Med., 332, 1125–1131. | Article | PubMed | ISI | ChemPort |
5. CARDOZO, L., LISEC, M., MILLARD, R., VAN VIERSSEN TRIP, O., KUZMIN, I., DROGENDIJK, T.E., HUANG, M. & RIDDER, A.M. (2004). Randomized, double-blind placebo controlled trial of the once daily antimuscarinic agent solifenacin succinate in patients with overactive bladder. J. Urol., 172 (Part 1), 1919–1924. | Article | PubMed | ISI | ChemPort |
6. CAULFIELD, M.P. & BIRDSALL, N.J. (1998). International Union of Pharmacology. XVII. Classification of muscarinic acetylcholine receptors. Pharmacol. Rev., 50, 279–290. | PubMed | ISI | ChemPort |
7. CHAPPLE, C., STEERS, W., NORTON, P., MILLARD, R., KRALIDIS, G., GLAVIND, K. & ABRAMS, P. (2005a). A pooled analysis of three phase III studies to investigate the efficacy, tolerability and safety of darifenacin, a muscarinic M₃ selective receptor antagonist, in the treatment of overactive bladder. B.J.U. Int., 95, 993–1001. | Article | PubMed | ISI | ChemPort |
8. CHAPPLE, C.R., MARTINEZ-GARCIA, R., SELVAGGI, L., TOOZS-HOBSON, P., WARNACK, W., DROGENDIJK, T., WRIGHT, D.M., BOLODEOKU, J. & for the STAR study group (2005b). A comparison of the efficacy and tolerability of solifenacin succinate and extended release tolterodine at treating overactive bladder syndrome: results of the STAR trial. Eur. Urol., 48, 464–470. | ISI | ChemPort |
9. CHAPPLE, C.R., YAMANISHI, T. & CHESS-WILLIAMS, R. (2002). Muscarinic receptor subtypes and management of the overactive bladder. Urology, 60 (Suppl 1), 82–88. | Article | PubMed | ISI |
10. CHAPPLE, C., KHULLAR, V., GABRIEL, Z. & DOOLEY, J.A. (2005c). The effects of antimuscarinic treatments in overactive bladder: a systematic review and meta-analysis. Eur. Urol., 48, 5–26. | Article | ISI | ChemPort |
11. CHESS-WILLIAMS, R. (2002). Muscarinic receptors of the urinary bladder: detrusor, urothelial and prejunctional. Auton. Autacoid Pharmacol., 22, 133–145. | Article | PubMed | ChemPort |
12. CHESS-WILLIAMS, R., CHAPPLE, C.R., YAMANISHI, T., YASUDA, K. & SELLERS, D.J. (2001). The minor population of M₃-receptors mediates contraction of human detrusor muscle in vitro. J. Auton. Pharmacol., 21, 243–248. | Article | PubMed | ISI | ChemPort |
13. CHUI, M.A., WILLIAMSON, T., ARCINIEGA, J., THOMPSON, C. & BENECKE, H. (2004). Patient persistency with medications for overactive bladder (abstract). Value Health, 7, 366. | ISI |
14. D'AGOSTINO, G., BOLOGNESI, M.L., LUCCHELLI, A., VICINI, D., BALESTRA, B., SPELTA, V., MELCHIORRE, C. & TONINI, M. (2000). Prejunctional muscarinic inhibitory control of acetylcholine release in the human isolated detrusor: involvement of the M₄ receptor subtype. Br. J. Pharmacol., 129, 493–500. | Article | PubMed | ChemPort |
15. DE GROAT, W.C. (1997). A neurologic basis for the overactive bladder. Urology, 50 (Suppl), 36–52. | Article | PubMed | ISI | ChemPort |
16. DE GROAT, W.C. (2004). The urothelium in overactive bladder: passive bystander or active participant? Urology, 64 (Suppl 1), 7–11. | PubMed | ISI |
17. DE GROAT, W.C., BOOTH, A.M. & YOSHIMURA, N. (1993). Neurophysiology of micturition and its modification in animal models of human disease. In: Nervous Control of the Urogenital System, ed. Maggi, C.A. pp. 227–290. New York: Harwood Acedemic Publishers.
18. DIOKNO, A., SAND, P., LABASKY, R., SIEBER, P., ANTOCI, J., LEACH, G., ATKINSON, L. & ALBRECHT, D. (2002). Long-term safety of extended release oxybutynin chloride in a community-dwelling population of participants with overactive bladder: a one-year study. Int. Urol. Nephrol., 34, 43–49. | Article | PubMed | ChemPort |
19. DIOKNO, A.C., APPELL, R.A., SAND, P.K, DMOCHOWSKI, R.R, GBUREK, B.M., KLIMBERG, I.W., KELL, S.H. & OPERA STUDY GROUP. (2003). Prospective, randomized, double-blind study of the efficacy and tolerability of the extended-release formulations of oxybutynin and tolterodine for overactive bladder: results of the OPERA trial. Mayo Clin. Proc., 78, 687–695. | PubMed | ISI | ChemPort |
20. DMOCHOWSKI, R.R., SAND, P.K., ZINNER, N.R., GITTELMAN, M.C., DAVILA, G.W., SANDERS, S.W. & TRANSDERMAL OXYBUTYNIN STUDY GROUP. (2003). Comparative efficacy and safety of transdermal oxybutynin and oral tolterodine versus placebo in previously treated patients with urge and mixed urinary incontinence. Urology, 62, 237–242. | Article | PubMed | ISI |
21. DOROSHYENKO, O., JETTER, A., ODENTHAL, K.P. & FUHR, U. (2005). Clinical pharmacokinetics of trospium chloride. Clin. Pharmacokinet., 44, 701–720. | Article | PubMed | ISI | ChemPort |
22. EHLERT, F.J., GRIFFIN, M.T., ABE, D.M., VO, T.H., TAKETO, M.M., MANABE, T. & MATSUI, M. (2005). The M₂ muscarinic receptor mediates contraction through indirect mechanisms in mouse urinary bladder. J. Pharmacol. Exp. Ther., 313, 368–378. | PubMed | ISI | ChemPort |
23. FETSCHER, C., FLEICHMAN, M., SCHMIDT, M., KREGE, S. & MICHEL, M.C. (2002). M₃ muscarinic receptors mediate contraction of human urinary bladder. Br. J. Pharmacol., 136, 641–643. | Article | PubMed | ISI | ChemPort |
24. FOVAEUS, M., FUJIWARA, M., HOGESTATT, E.D., PERSSON, K. & ANDERSSON, K.E. (1999). A non-nitrergic smooth muscle relaxant factor released from rat urinary bladder by muscarinic receptor stimulation. J. Urol., 161, 649–653. | Article | PubMed | ISI | ChemPort |
25. GILLBERG, P.G., SUNDQUIST, S. & NILVEBRANT, L. (1998). Comparison of the in vitro and in vivo profiles of tolterodine with those of subtype-selective muscarinic receptor antagonists. Eur. J. Pharmacol., 349, 285–292. | Article | PubMed | ISI | ChemPort |
26. GILLESPIE, J.I., HARVEY, I.J. & DRAKE, M.J. (2003). Agonist- and nerve-induced phasic activity in the whole isolated bladder of the guinea pig: evidence for two types of bladder activity. Exp. Physiol., 88, 343–357. | Article | PubMed | ISI | ChemPort |
27. GUPTA, P., ANDERSON, C., CARTER, A., CASEY, J. & NEWGREEN, D. (2002). In vivo bladder selectivity of darifenacin, a new M₃ antimuscarinic agent, in the anesthetised dog. Eur. Urol., 1 (Suppl. 1), 131.
28. HAAB, F. & CASTRO-DIAZ, D. (2005). Persistence with antimuscarinic therapy in patients with overactive bladder. Int. J. Clin. Pract., 59, 931–937. | Article | PubMed | ISI | ChemPort |
29. HAAB, F., CARDOZO, L., CHAPPLE, C., RIDDER, A.M. & SOLIFENACIN STUDY GROUP. (2005). Long-term open-label solifenacin treatment associated with persistence with therapy in patients with overactive bladder syndrome. Eur. Urol., 47, 376–384. | Article | PubMed | ISI | ChemPort |
30. HALASKA, M., RALPH, G., WIEDEMANN, A., PRIMUS, G., BALLERING-BRUHL, B., HOFNER, K & JONAS, U. (2003). Controlled, double-blind, multicenter clinical trial to investigate long-term tolerability and efficacy of trospium chloride in patients with detrusor instability. World J. Urol., 20, 392–399. | PubMed | ISI | ChemPort |
31. HARRISS, D.R., MARSH, K.A., BIRMINGHAM, A.T. & HILL, S.J. (1995). Expression of muscarinic M₃-receptors coupled to inositol phospholipid hydrolysis in human detrusor cultured smooth muscle cells. J. Urol., 154, 1241–1245. | Article | PubMed | ISI | ChemPort |
32. HAWTHORN, M.H., CHAPPLE, C.R., COCK, M. & CHESS-WILLIAMS, R. (2000). Urothelium-derived inhibitory factor(s) influences on detrusor muscle contractility in vitro. Br. J. Pharmacol., 129, 416–419. | Article | PubMed | ISI | ChemPort |
33. HEGDE, S.S. & EGLEN, R.M. (1999). Muscarinic receptor subtypes modulating smooth muscle contractility in the urinary bladder. Life Sci., 64, 419–428. | Article | PubMed | ISI | ChemPort |
34. HEGDE, S.S., CHOPPIN, A., BONHAUS, D., BRIAUD, S, LOEB, M., MOY, T.M., LOURY, D. & EGLEN, R.M. (1997). Functional role of M₂ and M₃ muscarinic receptors in the urinary bladder of rats in vitro and in vivo. Br. J. Pharmacol., 120, 1409–1418. | Article | PubMed | ISI | ChemPort |
35. HEGDE, S.S., MAMMEN, M. & JASPER, J.R. (2004). Antimuscarinics for the treatment of overactive bladder: current options and emerging therapies. Curr. Opin. Investig. Drugs., 5, 40–49. | PubMed | ChemPort |
36. HERBISON, P., HAY-SMITH, J., ELLIS, G. & MOORE, K. (2003). Effectiveness of anticholinergic drugs compared with placebo in the treatment of overactive bladder: systematic review. BMJ, 326, 841–844. | Article | PubMed | ChemPort |
37. IGAWA, Y., ZHANG, X., NISHIZAWA, O., UMEDA, M., IWATA, A, TAKETO, M.M., MANABE, T., MATSUI, M. & ANDERSSON, K.E. (2004). Cystometric findings in mice lacking muscarinic M₂ or M₃ receptors. J. Urol., 172 (Part 1 of 2), 2460–2464. | Article | PubMed | ISI | ChemPort |
38. IKEDA, K., KOBAYASHI, S., SUZUKI, M., MIYATA, K., TAKEUCHI, M., YAMADA, T. & HONDA, K. (2002). M₃ receptor antagonism by the novel antimuscarinic agent solifenacin in the urinary bladder and salivary gland. Naunyn-Schmiedeberg's Arch. Pharmacol., 366, 97–103. | Article | ChemPort |
39. KERBUSCH, T., WAHLBY, U., MILLIGAN, P.A. & KARLSSON, M.O. (2003). Population pharmacokinetic modelling of darifenacin and its hydroxylated metabolite using pooled data, incorporating saturable first-pass metabolism, CYP2D6 genotype and formulation-dependent bioavailability. Br. J. Clin. Pharmacol., 56, 639–652. | Article | PubMed | ISI | ChemPort |
40. KREDER, K., MAYNE, C. & JONAS, U. (2002). Long-term safety, tolerability and efficacy of extended release tolterodine in the treatment of overactive bladder. Eur. Urol., 41, 588–595. | Article | PubMed | ISI | ChemPort |
41. MATSUI, M., GRIFFIN, M.T., SHEHNAZ, D., TAKETO, M.M. & EHLERT, F.J. (2003). Increased relaxant action of forskolin and isoprenaline against muscarinic agonist-induced contractions in smooth muscle from M₂ receptor knockout mice. J. Pharmacol. Exp. Ther., 305, 106–113. | Article | PubMed | ISI | ChemPort |
42. MATSUI, M., MOTOMURA, D., FUJIKAWA, T., JIANG, J., TAKAHASHI, S., MANABE, T. & TAKETO, M.M. (2002). Mice lacking M₂ and M₃ muscarinic acetylcholine receptors are devoid of cholinergic smooth muscle contractions but still viable. J. Neurosci., 22, 10627–10632. | PubMed | ISI | ChemPort |
43. NAKAMURA, T., MATSUI, M., UCHIDA, K., FUTATSUGI, A., KUSAKAWA, S., MATSUMOTO, N., NAKAMURA, K., MANABE, T., TAKETO, M.M. & MIKOSHIBA, K. (2004). M(3) muscarinic acetylcholine receptor plays a critical role in parasympathetic control of salivation in mice. J. Physiol., 558 (Part 2), 561–575. | Article | PubMed | ISI | ChemPort |
44. NAPIER, C.M. & GUPTA, P. (2002). Darifenacin is selective for the human recombinant M3 receptor subtype. Neurourol. Urodyn., 21, A445.
45. NILVEBRANT, L., ANDERSSON, K.E., GILLBERG, P.G., STAHL, M. & SPARF, B. (1997a). Tolterodine – a new bladder-selective antimuscarinic agent. Eur. J. Pharmacol., 327, 195–207. | Article | PubMed | ISI | ChemPort |
46. NILVEBRANT, L., GILLBERG, P.G. & SPARF, B. (1997b). Antimuscarinic potency and bladder selectivity of PNU-200577, a major metabolite of tolterodine. Pharmacol. Toxicol., 81, 169–172. | ISI | ChemPort |
47. NOE, L., SNEERINGER, R., PATEL, B. & WILLIAMSON, T. (2004). The implications of poor medication persistence with treatment for overactive bladder. Managed Care Interface, 17, 54–60. | PubMed |
48. OLSSON, B. & SZAMOSI, J. (2001). Multiple dose pharmacokinetics of a new once daily extended release tolterodine formulation versus immediate release tolterodine. Clin. Pharmacokinet., 40, 227–235. | Article | PubMed | ISI | ChemPort |
49. PONTARI, M.A., BRAVERMAN, A.S. & RUGGIERI, M.R. (2004). The M₂ muscarinic receptor mediates in vitro bladder contractions from patients with neurogenic bladder dysfunction. Am. J. Physiol. (Regul. Integr. Comp. Physiol.), 286, R874–R880. | PubMed | ISI | ChemPort |
50. SATHYAN, G., CHANCELLOR, M.B. & GUPTA, S.K. (2001). Effect of OROS controlled-release delivery on the pharmacokinetics and pharmacodynamics of oxybutynin chloride. Br. J. Clin. Pharmacol., 52, 409–417. | Article | PubMed | ISI | ChemPort |
51. SCHNEIDER, T., FETSCHER, C., KREGE, S. & MICHEL, M.C. (2004). Signal transduction underlying carbachol-induced contraction of human urinary bladder. J. Pharmacol. Exp. Ther., 309, 1148–1153. | Article | PubMed | ISI | ChemPort |
52. SMULDERS, R.A., KRAUWINKEL, W.J., SWART, P.J. & HUANG, M. (2004). Pharmacokinetics and safety of solifenacin succinate in healthy young men. J. Clin. Pharmacol., 44, 1023–1033. | PubMed | ISI | ChemPort |
53. SOMOGYI, G.T., TANOWITZ, M. & DE GROAT, W.C. (1994). M₁ muscarinic receptor-mediated facilitation of acetylcholine release in the rat urinary bladder. J. Physiol., 480 (Part 1), 81–89. | PubMed | ISI | ChemPort |
54. STEVENS, L., CHAPPLE, C.R., TOPHILL, P. & CHESS-WILLIAMS, R. (2004a). A comparison of muscarinic receptor-mediated function in the normal and the neurogenic overactive bladder. J. Urol., 171, 143 (abstract 535). | ISI |
55. STEVENS, L., CHESS-WILLIAMS, R. & CHAPPLE, C.R. (2004b). Muscarinic receptor function in the idiopathic overactive bladder. J. Urol., 171, 140–141. | ISI |
56. TOBIN, G. & SJOGREN, C. (1995). In vivo and in vitro effects of muscarinic receptor antagonists on contractions and release of [³H]acetylcholine in the rabbit urinary bladder. Eur. J. Pharmacol., 281, 1–8. | Article | PubMed | ISI | ChemPort |
57. VAN KERREBROECK, P., KREDER, K., JONAS, U., ZINNER, N. & WEIN, A. (2001). Tolterodine Study Group: Tolterodine once-daily: superior efficacy and tolerability in the treatment of the overactive bladder. Urology, 57, 414–421. | Article | PubMed | ISI | ChemPort |
58. WALDECK, K., LARSSON, B. & ANDERSSON, K.E. (1997). Comparison of oxybutynin and its active metabolite, N-desethyl-oxybutynin, in the human detrusor and parotid gland. J. Urol., 157, 1093–1097. | Article | PubMed | ISI | ChemPort |
59. WANG, P., LUTHIN, G.R. & RUGGIERI, M.R. (1995). Muscarinic acetylcholine receptor subtypes mediating urinary bladder contractility and coupling to GTP binding proteins. J. Pharmacol. Exp. Ther., 273, 959–966. | PubMed | ISI | ChemPort |
60. WEIN, A.J. & ROVNER, E.S. (2002). Definition and epidemiology of overactive bladder. Urology, 60 (Suppl 1), 7–12. | Article | PubMed | ISI |
61. YAMANISHI, T., CHAPPLE, C.R., YASUDA, K. & CHESS-WILLIAMS, R. (2000). The role of M₂-muscarinic receptors in mediating contraction of the pig urinary bladder in vitro. Br. J. Pharmacol., 131, 1482–1488. | Article | PubMed | ISI | ChemPort |
62. ZHOU, H., MEYER, A., STARKE, K., GOMEZA, J., WESS, J. & TRENDELENBURG, A.U. (2002). Heterogeneity of release-inhibiting muscarinic autoreceptors in heart atria and urinary bladder: a study with M₂- and M₄ -receptor-deficient mice. Naunyn-Schmiedeberg's Arch. Pharmacol., 365, 112–122. | Article | ChemPort |
63. ZINNER, N., GITTELMAN, M., HARRIS, R., SUSSET, J., KANELOS, A., AUERBACH, S. & TROSPIUM STUDY GROUP. (2004). Trospium chloride improves overactive bladder symptoms: a multicenter phase III trial. J. Urol., 171 (Part 1), 2311–2315. | Article | PubMed | ISI | ChemPort |