Pharmacological effect of TRK-380, a novel selective human β3-adrenoceptor agonist, on mammalian detrusor strips

The β3-Adrenergic Receptor: Structure, Physiopathology of Disease, and Emerging Therapeutic Potential

The discovery and characterization of the signal cascades of the β-adrenergic receptors have made it possible to effectively target the receptors for drug development. β-Adrenergic receptors are a class A rhodopsin type of G protein-coupled receptors (GPCRs) that are stimulated mainly by catecholamines and therefore mediate diverse effects of the parasympathetic nervous system in eliciting "fight or flight" type responses. They are detectable in several human tissues where they control a plethora of physiological processes and therefore contribute to the pathogenesis of several disease conditions. Given the relevance of the β-adrenergic receptor as a molecular target for many pathological conditions, this comprehensive review aims at providing an in-depth exploration of the recent advancements in β3-adrenergic receptor research. More importantly, we delve into the prospects of the β3-adrenergic receptor as a therapeutic target across a variety of clinical domains.

Inside the Biology of the β3-Adrenoceptor

Since the first discovery in 1989, the β3-adrenoceptor (β3-AR) has gained great attention because it showed the ability to regulate many physiologic and metabolic activities, such as thermogenesis and lipolysis in brown and white adipose tissue, respectively (BAT, WAT), negative inotropic effects in cardiomyocytes, and relaxation of the blood vessels and the urinary bladder. The β3-AR has been suggested as a potential target for cancer treatment, both in adult and pediatric tumors, since under hypoxia its upregulation in the tumor microenvironment (TME) regulates stromal cell differentiation, tumor growth and metastases, signifying that its agonism/antagonism could be useful for clinical benefits. Promising results in cancer research have proposed the β3-AR being targeted for the treatment of many conditions, with some drugs, at present, undergoing phase II and III clinical trials. In this review, we report the scientific journey followed by the research from the β3-Ars' discovery, with focus on the β3-Ars' role in cancer initiation and progression that elects it an intriguing target for novel antineoplastic approaches. The overview highlights the great potential of the β3-AR, both in physiologic and pathologic conditions, with the intention to display the possible benefits of β3-AR modulation in cancer reality.

Adrenoceptors in the Lower Urinary Tract

Adrenoceptors importantly contribute to the physiological regulation of lower urinary tract (LUT) function and have become a target of several clinically successful treatments for major LUT diseases. In the bladder dome, β-adrenoceptor subtypes are found in multiple cell types and mediate relaxation of detrusor smooth muscle, perhaps partly indirectly by acting on afferent nerves and cells of the mucosa. β3-adrenoceptor agonists such as mirabegron and vibegron are used to treat overactive bladder syndrome. In the bladder trigone and urethra, α1-adrenoceptors cause contraction and thereby physiologically contribute to bladder outlet resistance. α1-adrenoceptors in the prostate also cause contraction and pathophysiologically elevate bladder outlet resistance leading to voiding dysfunction in benign prostatic hyperplasia. α1-adrenoceptor antagonist such as tamsulosin is widely used as a first-line option to treat LUT symptoms in men, but it remains unclear to which extent and how smooth muscle relaxation contributes to symptom relief.

Antagonism of α1-adrenoceptors by β3-adrenergic agonists: Structure-function relations of different agonists in prostate smooth muscle contraction

Effects of β₃-adrenergic agonists on prostate smooth muscle contraction are poorly characterized, although mirabegron is used for treatment of lower urinary tract symptoms. Off-target effects of several β₃-adrenergic agonists include antagonism of α₁-adrenoceptors. Proposed, but unconfirmed explanations include phenylethanolamine backbones, found in some β₃-adrenergic agonists and imparting interaction with catecholamine binding pockets of adrenoceptors. Here, we examined effects of β₃-adrenergic agonists on contractions of human prostate tissues, including ZD7114 (without phenylethanolamine moiety), ZD2079 (phenylethanolamine backbone), BRL37344 and CL316243 (chloride-substituted phenylethanolamine deriatives). Prostate tissues were obtained from radical prostatectomy. Contractions by α₁-adrenergic agonists and electric field stimulation (EFS) were studied in an organ bath. ZD7114 (10 µM) right-shifted concentration responses curves for α₁-adrenergic agonists, resulting in increased EC₅₀ values for phenylephrine, methoxamine and noradrenaline up to one magnitude, without affecting E_max values. ZD7114 (10 µM) inhibited EFS-induced contractions, resulting in reduced E_max values. All effects of ZD7114 were resistant to the β₃-adrenergic antagonist L-748337, including increases in EC₅₀ values for α₁-adrenergic agonists, up to more than two magnitudes. Using 10 µM, neither ZD2079, BRL37344 or CL316243 affected α₁-adrenergic or EFS-induced contractions. At escalated concentrations, BRL37344 (200 µM) right-shifted concentration response curves for phenylephrine, increased EC₅₀ values for phenylephrine, and inhibited EFS-induced contractions, while CL316243 (300 µM) did not affect phenylephrine- or EFS-induced contractions. In conclusion, phenylethanolamine backbones are not decisive to impart α₁-adrenoceptor antagonism to β₃-agonists. Effects of β₃-adrenergic agonists on prostate smooth muscle contraction are limited to off-target effects, including α₁-adrenoceptor antagonism by ZD7114 and BRL37344.

Does coupling to ADP ribosylation factor 6 explain differences between muscarinic and other receptors in interaction with β-adrenoceptor-mediated smooth muscle relaxation?

Numerous studies in airways, ileum, and urinary bladder have demonstrated that relaxation by β-adrenoceptor agonists has lower potency and/or efficacy when contraction was elicited by muscarinic receptor agonists as compared to other G-protein-coupled receptors, KCl, or basal tone, but the molecular mechanisms behind this relative resistance remain unclear. A paper by Huang et al. in this issue demonstrates that NAV2729, an inhibitor of ADP ribosylation factor 6, inhibits contraction of isolated blood vessels elicited by muscarinic receptor agonists, but not by α₁-adrenoceptor agonists or KCl. Against this background, we discuss the role of ADP ribosylation factor 6 in cellular responses to G-protein-coupled receptor stimulation. While ADP ribosylation factor 6 apparently is the only promising molecular explanation for the relative resistance of smooth muscle contraction elicited by muscarinic agonists, the existing data are insufficient for a robust conclusion.

Validation of Fenoterol to Study β2-Adrenoceptor Function in the Rat Urinary Bladder

Fenoterol is a β2-adrenoceptor (AR)-selective agonist that is commonly used to investigate relaxation responses mediated by β2-AR in smooth muscle preparations. Some data have questioned this because fenoterol had low potency in the rat urinary bladder when a muscarinic agonist was used as a pre-contraction agent and because some investigators proposed that fenoterol may act in part via β3-AR. We designed the present study to investigate whether fenoterol is a proper pharmacological tool to study β2-AR-mediated relaxation responses in the rat urinary bladder. Firstly, we have compared the effect of pre-contraction agents on fenoterol potency and found that fenoterol potency was about 1.5 log units greater against KCl than carbachol (pEC50 7.19 ± 0.66 and 5.62 ± 1.09 of KCl and of carbachol, respectively). To test the selectivity of fenoterol, we have determined the effects of the β2-AR antagonist ICI 118,551 and the β3-AR antagonist L 748,337 on relaxation responses to fenoterol. While 300 nM L 748,337 had little effect on the potency of fenoterol (pEC50 6.56 ± 0.25 and 6.33 ± 0.61 in the absence and presence of L 748,337, respectively), the relaxation curve for fenoterol was right-shifted in the presence 300 nM ICI 118,551 (pEC50 5.03 ± 0.18). Thus, we conclude that fenoterol is a proper pharmacological tool to assess β2-AR-mediated responses in the rat urinary bladder and most likely in other smooth-muscle preparations containing multiple subtypes of the β-AR.

TAS-303 Ameliorates Carbachol-Induced Detrusor Overactivity in Rats, Revealing Its Therapeutic Potential for Overactive Bladder

Urinary incontinence is defined as an involuntary leakage of urine and is categorized into three types: stress urinary incontinence (SUI), urge urinary incontinence (UUI), and mixed urinary incontinence, which includes symptoms of SUI and UUI. As the underlying mechanisms of SUI and UUI are different, no drug is approved to treat all three types of urinary incontinence. TAS-303 is a selective norepinephrine reuptake inhibitor and has therapeutic potential for patients with SUI. In this report, we describe newly discovered pharmacological properties of TAS-303 and its effects on bladder function. Radioligand binding studies showed that TAS-303 inhibits M3 muscarinic receptor binding, with a Ki value of 547 nM. TAS-303 at 1, 3, and 10 mg/kg dose-dependently prolonged the intercontraction interval of carbachol-induced detrusor overactivity in rats, exhibiting a maximal effect that was comparable to tolterodine. These effects may result from coordinated regulation of bladder afferent activity via M3 muscarinic inhibition and β3 adrenoreceptor activation by norepinephrine elevation due to norepinephrine transporter inhibition. Moreover, TAS-303 at the effective dose for bladder function did not induce dry mouth or constipation in rats, showing that this compound may have a lower risk of antimuscarinic side effects. Thus, TAS-303 is expected to be a new profile agent with therapeutic potential for all types of urinary incontinence. SIGNIFICANCE STATEMENT: Urinary incontinence is categorized into stress, urge, and mixed urinary incontinence, but because the underlying mechanisms of each differ, no drugs are available that treat all three. TAS-303 has therapeutic potential for stress urinary incontinence. This study describes newly discovered pharmacological properties of TAS-303, which ameliorated bladder afferent activity partly via M3 muscarinic inhibition, indicating improvement in urge urinary incontinence, and highlights the potential of TAS-303 as a new therapeutic agent for all types of urinary incontinence.

Desensitization of cAMP Accumulation via Human β3-Adrenoceptors Expressed in Human Embryonic Kidney Cells by Full, Partial, and Biased Agonists

β₃-Adrenoceptors couple not only to cAMP formation but, at least in some cell types, also to alternative signaling pathways such as phosphorylation of extracellular signal-regulated kinase (ERK). β₃-Adrenoceptor agonists are used in long-term symptomatic treatment of the overactive bladder syndrome; it is only poorly understood which signaling pathway mediates the clinical response and whether it undergoes agonist-induced desensitization. Therefore, we used human embryonic kidney cells stably transfected with human β₃-adrenoceptors to compare coupling of ligands with various degrees of efficacy, including biased agonists, to cAMP formation and ERK phosphorylation, particularly regarding desensitization. Ligands stimulated cAMP formation with a numerical rank order of isoprenaline ≥ L 755,507 ≥ CL 316,243 > solabegron > SR 59,230 > L 748,337. Except for the weakest agonist, L 748,337, pretreatment with any ligand reduced cAMP responses to freshly added isoprenaline or forskolin to a similar extent. On the other hand, we were unable to detect ERK phosphorylation despite testing a wide variation of conditions. We conclude that a minor degree of efficacy for cAMP formation may be sufficient to induced full desensitization of that response. Transfected human embryonic kidney cells are not suitable to study desensitization of ERK phosphorylation by β₃-adrenoceptor stimulation.

Is pretreatment cystometry important in predicting response to mirabegron in women with overactive bladder symptoms?

INTRODUCTION AND HYPOTHESIS

The aim of this study was to evaluate for any association between pretreatment cystometry results and outcome of treatment with mirabegron in women with overactive bladder (OAB) symptoms.

METHODS

This was a prospective observational study of women with OAB symptoms that proved refractory to conservative management. All women underwent filling and voiding subtraction cystometry prior to further treatment. Women were treated with mirabegron 50 mg once daily, and outcomes were evaluated after 6 weeks' treatment. The primary outcome measure was change in symptoms as indicated by response to the Patient Global Impression of Improvement (PGI-I) scale. The presence of detrusor overactivity (DO), the highest detrusor pressure recorded during the filling phase, the presence of urodynamic stress incontinence (USI), cystometric capacity, voided volume, maximum flow rate and detrusor pressure at maximum flow were all compared between responders and nonresponders.

RESULTS

The study population consisted of 169 women; response rate to mirabegron was 69.8 %. There was no association between the presence of DO or maximum detrusor pressure during filling and USI, cystometric capacity, maximum flow rate and detrusor pressure at maximum flow and treatment response. In a subgroup with OAB symptoms refractory to previous treatment with antimuscarinics, there was an association between the presence of DO and a positive treatment response (p = 0.02).

CONCLUSIONS

Overall, there is no association between urodynamic findings and response to treatment with mirabegron. This may reflect the fact that mirabegron's mode of action mechanisms are not measurable using cystometry. In women with refractory symptoms, however, the presence of DO is associated with a positive response to treatment.

Overactive bladder - 18 years - Part II

Traditionally, the treatment of overactive bladder syndrome has been based on the use of oral medications with the purpose of reestablishing the detrusor stability. The recent better understanding of the urothelial physiology fostered conceptual changes, and the oral anticholinergics - pillars of the overactive bladder pharmacotherapy - started to be not only recognized for their properties of inhibiting the detrusor contractile activity, but also their action on the bladder afference, and therefore, on the reduction of the symptoms that constitute the syndrome. Beta-adrenergic agonists, which were recently added to the list of drugs for the treatment of overactive bladder, still wait for a definitive positioning - as either a second-line therapy or an adjuvant to oral anticholinergics. Conservative treatment failure, whether due to unsatisfactory results or the presence of adverse side effects, define it as refractory overactive bladder. In this context, the intravesical injection of botulinum toxin type A emerged as an effective option for the existing gap between the primary measures and more complex procedures such as bladder augmentation. Sacral neuromodulation, described three decades ago, had its indication reinforced in this overactive bladder era. Likewise, the electric stimulation of the tibial nerve is now a minimally invasive alternative to treat those with refractory overactive bladder. The results of the systematic literature review on the oral pharmacological treatment and the treatment of refractory overactive bladder gave rise to this second part of the review article Overactive Bladder - 18 years, prepared during the 1st Latin-American Consultation on Overactive Bladder.

The expression of β3-adrenoceptors and their function in the human prostate

BACKGROUND

Little is known about β3-adrenoceptor (AR) expression and function in human prostate. We examined the expression and distribution of β-AR subtypes in normal prostate and benign prostatic hyperplasia (BPH) tissues, and investigated which selective β-AR subtype agonist was most involved in the relaxation of isolated human prostate strips.

METHODS

Messenger RNA (mRNA) expression for β1-, β2-, and β3 -ARs was investigated using reverse transcriptase-polymerase chain reactions (RT-PCR). Quantitative analysis of mRNA expression of β-AR subtypes between normal prostate and BPH tissues was performed using quantitative RT-PCR (qPCR). Distributions were examined by immunohistochemistry (IHC). Strips of human normal prostate or BPH were suspended in organ baths and exposed to isoproterenol, dobutamine, procaterol, and TRK-380 to investigate their relaxant effects on KCl-induced contractions, and their inhibitory effects on electrical field stimulation (EFS)-induced contractions.

RESULTS

We confirmed the presence of mRNA for β1-, β2-, and β3-ARs both in normal prostate and in BPH tissues. For β3-AR, mRNA expression in BPH tissues was significantly higher than in normal prostate tissues, but there was no significant difference in β1- and β2-AR expression between normal and BPH tissues. IHC revealed differences in staining intensity between smooth muscle cells and glandular cells, with different proportions for different β-AR subtypes. Staining of β3-AR was particularly intense in smooth muscle cells as opposed to glandular cells. Isoproterenol and TRK-380 significantly decreased the tone of KCl-induced contractions of the normal prostate strips. The rank order of relaxant effects was isoproterenol > TRK-380 > procaterol > dobutamine. All selective β-AR agonists significantly decreased the amplitude of EFS-induced contractions of the normal prostate strips. The rank order of inhibitory effects was isoproterenol > dobutamine >TRK-380 > procaterol. In BPH strips, all selective β-AR agonists showed no significant relaxant or inhibitory effects on KCl- or EFS-induced contractions.

CONCLUSIONS

β3 -AR is abundant in human prostate smooth muscle, whose relaxation is mediated by β1- and β3-AR stimulation. β3-AR agonists may have clinical use in the treatment of male non-BPH patients or neurogenic bladder patients with voiding dysfunction.

Oral pharmacotherapy for overactive bladder in older patients: mirabegron as a potential alternative to antimuscarinics

OBJECTIVE

Overactive bladder (OAB) is a particular challenge to treat in older adults with co-morbid conditions taking multiple medications. Antimuscarinics (e.g., solifenacin, fesoterodine) and β3-adrenergic receptor agonists (mirabegron) are similarly efficacious; however, antimuscarinics may be associated with side effects that result in poor persistence and contribute to anticholinergic burden, particularly in those taking other medications with anticholinergic properties. With a mechanism of action distinct from antimuscarinics, mirabegron has a different tolerability profile and does not contribute to anticholinergic burden. The objective of this review was to compare and contrast the tolerability profiles of antimuscarinics and mirabegron in older patients to inform practice.

METHODS

Prospective trials or retrospective subgroup analyses of antimuscarinics for the treatment of OAB in older patients were identified through a search of PubMed. Tolerability data and results of subgroup analyses of mirabegron in patients aged ≥65 and ≥75 years from a pooled analysis of three trials each of 12 weeks and a 1 year trial are described.

RESULTS

Anticholinergic adverse events (AEs) including dry mouth and constipation were more frequent with antimuscarinics versus mirabegron. In patients aged ≥65 years, dry mouth occurred with a six-fold higher incidence with tolterodine extended-release (ER) 4 mg than with mirabegron 25 mg or 50 mg over 12 weeks, and a three-fold higher incidence with tolterodine ER than mirabegron 50 mg over 1 year. Mirabegron had a low incidence of central nervous system effects. A systematic review of the cardiovascular safety profile of mirabegron has not identified any clinically significant effects on blood pressure or pulse rate at therapeutic doses amongst patients aged ≥65 years.

CONCLUSIONS

Mirabegron has a more favorable tolerability profile than antimuscarinics amongst older patients and may provide an improved benefit-to-risk ratio and therefore be considered as an alternative to antimuscarinics for older patients.

Innovative pharmacotherapies for women with overactive bladder: where are we now and what is in the pipeline?

INTRODUCTION AND HYPOTHESIS

The impressive prevalence of overactive bladder (OAB) and the relevant limitations of current treatments urge the need for novel therapeutic approaches.

METHODS

A systematic literature and web search was performed to identify investigational drugs that entered the early and late phases of clinical development for women with OAB symptoms.

RESULTS

Approved pharmacological therapies for OAB (antimuscarinics, beta-3 agonists, and botulinum toxin) are evolving with the development of alternative administration methods, combination strategies, and novel compounds, expected to improve effectiveness, bladder selectivity, and dose flexibility. A wealth of investigational compounds, developed with both public and companies' indoor nonclinical disease-oriented studies, entered the early and late stages of clinical development in the last decade. Most non-anticholinergic compounds in ongoing clinical trials target central and peripheral neurotransmitter receptors involved in neurological modulation of micturition, nonadrenergic-noncholinergic mechanisms, cyclic nucleotide metabolism, different subtypes of ion channels or peripheral receptors of prostaglandins, vanilloids, vitamin D3, and opioids. Fascinating advances are ongoing also in the field of genetic therapy.

CONCLUSIONS

New pharmaceutical formulations and drug combinations are expected to be available in the next decade in order to overcome the limitations of current drugs for OAB. Although proof-of-concept, patient-oriented studies yielded disappointing results for several tentative drugs, a lot of clinical research is ongoing that is expected to provide clinicians with novel therapeutic agents in the near future.

The pharmacological rationale for combining muscarinic receptor antagonists and β-adrenoceptor agonists in the treatment of airway and bladder disease

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Muscarinic receptors increase smooth muscle tone in airways and urinary bladder.

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β-Adrenoceptors relax smooth muscle tone and oppose muscarinic contraction.

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Opposition involves transmitter release, signal transduction and receptor expression.

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This supports the combined use of muscarinic antagonists and β-adrenoceptor agonists.

Muscarinic receptor antagonists and β-adrenoceptor agonists are used in the treatment of obstructive airway disease and overactive bladder syndrome. Here we review the pharmacological rationale for their combination. Muscarinic receptors and β-adrenoceptors are physiological antagonists for smooth muscle tone in airways and bladder. Muscarinic agonism may attenuate β-adrenoceptor-mediated relaxation more than other contractile stimuli. Chronic treatment with one drug class may regulate expression of the target receptor but also that of the opposing receptor. Prejunctional β₂-adrenoceptors can enhance neuronal acetylcholine release. Moreover, at least in the airways, muscarinic receptors and β-adrenoceptors are expressed in different locations, indicating that only a combined modulation of both systems may cause dilatation along the entire bronchial tree. While all of these factors contribute to a rationale for a combination of muscarinic receptor antagonists and β-adrenoceptor agonists, the full value of such combination as compared to monotherapy can only be determined in clinical studies.

TRK-380, a novel selective human β3-adrenoceptor agonist, ameliorates formalin-induced pollakiuria in rats and carbachol-induced bladder contraction in dogs

OBJECTIVE

To investigate the effects of TRK-380, a selective β3-adrenoceptor (β3-AR) agonist, on voiding behavior in rats with pollakiuria and on carbachol (CCh)-induced bladder contraction in dogs.

METHODS

The voiding behavior of female Sprague Dawley rats was recorded continuously with a balance. Rats were intravesically pretreated with 2.5% formalin under isoflurane anesthesia. The next day, the effect of TRK-380 (7.5-30 mg/kg, orally) or tolterodine, an antimuscarinic drug (3.75-15 mg/kg, orally), on the voiding frequency was evaluated. In another experiment, male beagle dogs were anesthetized with pentobarbital, CCh (3 μg/kg, intravenously) was administered to them, and the effect of TRK-380 (0.1 or 0.3 μg/kg/minute, intravenously infusion) on CCh-induced bladder contraction was evaluated.

RESULTS

Rats treated with formalin showed a significant increase in the voiding frequency compared with the sham group, and the increase in it was significantly and dose-dependently suppressed by TRK-380 at doses of ≥15 mg/kg. In contrast, tolterodine did not lead to a significant change in the voiding frequency even at the highest dose. In dogs, CCh-induced bladder contraction was dose-dependently suppressed by TRK-380; the plasma concentration required for 30% suppression of the CCh-induced bladder contraction (30% relaxation) was 4.90 ng/mL.

CONCLUSION

This study indicated that TRK-380 ameliorated pollakiuria, which was resistant to an antimuscarinic drug, and that it also suppressed the bladder contraction induced by cholinergic stimulation in dogs, whose bladder relaxation is known to be predominantly mediated by β3-ARs, as in humans. These data strengthen the therapeutic potential of β3-AR for the treatment of overactive bladder.

Do β-adrenoceptor agonists induce homologous or heterologous desensitization in rat urinary bladder?

β3-Adrenoceptor agonists have recently been introduced for the symptomatic treatment of the overactive bladder syndrome. As such treatment is not curative, long-term treatment is anticipated to be required. As the susceptibility of β3-adrenoceptors to undergo agonist-induced desensitization is cell type- and tissue-dependent, we have explored whether pre-treatment with a β-adrenoceptor agonist will attenuate subsequent relaxation responses to freshly added agonist using rat urinary bladder as a model. We have used the prototypical β-adrenoceptor agonist isoprenaline, the β2-selective fenoterol and the β3-selective CL 316,243 and mirabegron as well as the receptor-independent bladder relaxant forskolin. We show that a 6-h pre-treatment with agonist can significantly reduce subsequent relaxation against KCl-induced smooth muscle tone, but agonist-induced desensitization was also observed with longer pre-treatments or against passive tension. The agonist-induced desensitization was prominent for the β2 component of rat bladder relaxation but much weaker or even absent for the β3 component. Moreover, β-adrenoceptor agonist pre-treatment reduced contractile responses to the muscarinic agonist carbachol and the receptor-independent stimulus KCl. Taken together these data do not support the hypothesis that the long-term clinical efficacy of β3-adrenoceptor agonists in the treatment of the overactive bladder syndrome will be limited by receptor desensitization. Rather they raise the possibility that such treatment may not only cause smooth muscle relaxation but also may attenuate hyper-contractility of the bladder.

Mirabegron in overactive bladder: a review of efficacy, safety, and tolerability

AIMS

Mirabegron, the first β3 -adrenoceptor agonist to enter clinical practice, has a different mechanism of action from antimuscarinic agents. This review presents data on the efficacy, safety, and tolerability of mirabegron in studies conducted to date.

METHODS

All clinical data on mirabegron that are currently in the public domain are included, including some in-press manuscripts.

RESULTS

In Phase III clinical trials in patients with overactive bladder (OAB), mirabegron at daily doses of 25, 50, and 100 mg demonstrated significant efficacy in treating the symptoms of OAB, including micturition frequency, urgency incontinence, and urgency. Significant improvements in micturition frequency, urgency incontinence, and mean volume voided/micturition were seen as early as the first assessment (week 4) for mirabegron 50 and 100 mg, and were maintained throughout treatment. Responder analyses showed a significant improvement with mirabegron 50 and 100 mg in terms of dry rates, ≥50% reduction in mean number of incontinence episodes/24 hr, and the proportion of patients with ≤8 micturitions/24 hr at final visit. The benefit of mirabegron 50 and 100 mg was also evident in patients ≥65 years of age, and in both treatment-naïve patients and those who previously discontinued antimuscarinic therapy. These data therefore demonstrate a clinically meaningful benefit with mirabegron in the objective endpoints of OAB. Assessment of measures of health-related quality of life and treatment satisfaction showed that patients perceived treatment with mirabegron as meaningful. In OAB clinical trials of up to 12 months mirabegron appeared to be well tolerated. The most common adverse events (AEs) observed with mirabegron in clinical trials of up to 12 months were hypertension, nasopharyngitis, and urinary tract infection. The incidence of dry mouth was similar to placebo, and was between three and fivefold less than for tolterodine extended release 4 mg. Since dry mouth is the most bothersome AE associated with antimuscarinic drugs and often a reason for treatment discontinuation, mirabegron may be a valuable treatment option for these patients.

CONCLUSIONS

In Phase III clinical trials, mirabegron at daily doses of 25, 50, and 100 mg demonstrated significant efficacy in treating symptoms of OAB and, at doses of 50 and 100 mg, demonstrated significant improvements versus placebo on key secondary endpoints, as early as the first assessment (week 4), and these were maintained throughout treatment. In OAB clinical trials of up to 12 months, mirabegron appeared to be well tolerated.

Functional BK channels facilitate the β3-adrenoceptor agonist-mediated relaxation of nerve-evoked contractions in rat urinary bladder smooth muscle isolated strips

The large-conductance voltage- and Ca(2+)-activated K(+) (BK) channel is a major regulator of detrusor smooth muscle (DSM) contractility thus facilitating urinary bladder function. Recent findings suggest that activation of β3-adrenoceptors causes DSM relaxation. However, it is unknown whether the β3-adrenoceptor-mediated DSM relaxation is BK channel-dependent during nerve-evoked contractions. To test this hypothesis, we induced nerve-evoked contractions in rat DSM isolated strips by using a tissue bath system equipped with platinum electrodes for electrical field stimulation (EFS). (±)-(R(*),R(*))-[4-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]amino]propyl]phenoxy] acetic acid sodium hydrate (BRL37344), a β3-adrenoceptor agonist, significantly decreased the amplitude and muscle force of the 20 Hz EFS-induced DSM contractions in a concentration-dependent manner. The BRL37344 inhibitory effect was significantly antagonized by 1-(2-ethylphenoxy)-3-[[(1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino]-(2S)-2-propanol hydrochloride (SR59230A), a β3-adrenoceptor antagonist. We further isolated the cholinergic from the purinergic component of the 0.5-50 Hz EFS-induced DSM contractions by using selective inhibitors, atropine as well as suramin and α,β-methylene-ATP. We found that BRL37344 inhibited both the purinergic and cholinergic components of the nerve-evoked contractions in rat DSM isolated strips. The pharmacological blockade of the BK channels with iberiotoxin, a selective BK channel inhibitor, increased the amplitude and muscle force of the 20 Hz EFS-induced contractions in rat DSM isolated strips. In the presence of iberiotoxin, there was a significant reduction in the BRL37344-induced inhibition of the 20 Hz EFS-induced contractions in rat DSM isolated strips. These latter findings suggest that BK channels play a critical role in the β3-adrenoceptor-mediated inhibition of rat DSM nerve-evoked contractions.

New developments in the management of overactive bladder: focus on mirabegron and onabotulinumtoxinA

In the last few years, much new information has been generated on the pathophysiology, possible therapeutic targets, and pharmacologic treatment of overactive bladder (OAB). Antimuscarinic drugs are still first-line pharmacologic treatment for OAB and often have good initial response rates, but adverse effects and decreasing efficacy cause long-term compliance problems, prompting a search for new therapeutic alternatives. Mirabegron and onabotulinumtoxinA, two drugs with different mechanisms of action, and with adverse effect profiles different from those of antimuscarinics, were recently approved for treatment of OAB. However, their place in the treatment of this disorder has not yet been established. In this short review, the mechanisms of action, clinical efficacy, and safety profiles of these drugs are discussed and compared with those of the current gold standard, antimuscarinic agents.