On the mode of action of mirabegron

Mirabegron has been shown to cause relaxation of the isolated mouse urethra, partly through blockade of α_1A-adrenoceptors and α_1D-adrenoceptors. However, although this effect might be an interesting pharmacological in vitro observation, it seems to have no relevance for the clinical use of mirabegron in the treatment of the overactive bladder.

Refers to Alexandre, E.C. et al. Mirabegron relaxes urethral smooth muscle by a dual mechanism involving β3-adrenoceptor activation and α1-adrenoceptor blockade. Br. J. Pharmacol. http://dx.doi.org:10.1111/bph.13367

The clinical success of mirabegron for treatment of the overactive bladder (OAB) syndrome has resulted in substantial interest in its mechanism of action. β₃-Adrenoceptor agonists, including mirabegron, have generally been considered to relieve OAB symptoms through direct relaxation of detrusor muscle, inhibition of spontaneous contractile activity in the bladder (microcontractions in vitro and nonvoiding contractions in vivo), and reductions in bladder afferent activity. On the molecular level, β₃-adrenoceptor agonists increase the generation of cyclic AMP (cAMP) and thus activate K⁺ channels, causing membrane hyperpolarization¹. For example, β₃-adrenoceptor agonists have been shown to inhibit microcontractions in strips of human detrusor muscle in a concentration-dependent manner, and several investigators have shown that, in vivo, β₃-adrenoceptor agonists can decrease nonvoiding contractions in the obstructed bladder¹. Effects on afferent bladder activity have been demonstrated, with mirabegron dose-dependently decreasing single-unit afferent activities of both Aδ-fibres and C-fibres in response to bladder filling. However, in a series of studies, Gillespie and colleagues questioned the accepted view on the mode and site of action of β₃-adrenoceptor agonists in the bladder². These authors suggested that with the concentrations obtained in vivo after administration of clinically used doses of mirabegron, neither the effects on spontaneous microcontractions nor the effects on nonvoiding contractions (in bladder-outlet-obstructed rats, for example) can be fully explained. Supporting the view that mechanisms other than direct effects on detrusor muscle might contribute, recent evidence showed that activation of prejunctional β₃-adrenoceptors might result in downregulation of acetylcholine release from cholinergic terminals, thereby exerting an additional inhibitory control of parasympathetic activity³.

So far, discussion of the site/s and mechanisms of action of mirabegron have assumed that the drug acts exclusively on β₃-adrenoceptors. However, the results by Alexandre et al. question this assumption⁴. These researchers conclude that mirabegron is a competitive antagonist of α_1A-adrenoceptors and α_1D-adrenoceptors, in addition to its well-known actions as a β₃-adrenoceptor agonist. Their comprehensive studies, carried out in isolated mouse and rat tissues, included the use of functional assays and competition assays for the specific binding of [³H]prazosin to membrane preparations of HEK-293 cells expressing each of the human α₁-adrenoceptors, as well as β-adrenoceptor mRNA expression studies and cAMP measurements in mouse urethra. The functional data showed that mirabegron produced concentration-dependent urethral relaxations that were right-shifted by the selective β₃-adrenoceptor antagonist L-748,337, but unaffected by β₁-adrenoceptor and β₂-adrenoceptor antagonists (atenolol and ICI 118,551, respectively). Mirabegron stimulated cAMP synthesis, and the mirabegron-induced relaxations were enhanced by the PDE-4 inhibitor rolipram. Mirabegron also produced rightward shifts in urethral contractions induced by the α₁-adrenoceptor agonist phenylephrine. Schild regression analysis revealed that mirabegron behaved as a competitive antagonist of α₁-adrenoceptors in rat vas deferens (α_1A-adrenoceptors), prostate (α_1A-adrenoceptors), and aorta (α_1D-adrenoceptors), but not in spleen (α_1B-adrenoceptors). The affinities estimated for mirabegron in functional assays were consistent with those estimated by radioligand binding studies with human recombinant α_1A-adrenoceptors and α_1D-adrenoceptors.

These findings raise a number of important questions. Are the results really correct? Were there any methodological problems? Why has such a potentially important effect not been detected during the preclinical development of the drug? Do these findings have any translational impact? Do they have implications for the clinical use of mirabegron?

Considering the first three questions, the experimental approaches seem difficult to criticise — the methods are appropriate, the experiments are comprehensive and they have been carefully performed. The experimental data provide rather convincing evidence that under the experimental conditions used, mirabegron, at high concentrations, is an α₁-adrenoceptor antagonist. Of interest is that this action seems to be selective for α_1A-adrenoceptors and α_1D-adrenoceptors over α_1B-adrenoceptor subtypes, given that α_1A-adrenoceptors and α_1D-adrenoceptors are the predominating subtypes in the female human urethra⁵.

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Considering the questions regarding translational relevance and clinical implications, the concentrations of mirabegron required for α_1A-adrenoceptor and α_1D-adrenoceptor blockade seem to be much higher than those for β₃-adrenoceptor agonist effects. To explain why the α-adrenoceptor blocking effect could still be important; the authors refer to preclinical experiments in rats showing that, in vivo, the drug becomes concentrated in tissues and that the tissue:plasma concentration ratios might be up to 20-fold. However, it should be emphasized that the studies referred to were single-dose studies using ¹⁴C-labelled mirabegron. Such studies do not reflect what happens during the steady state. High tissue levels require that mirabegron is in some way 'trapped' in the tissues, but so far there seem to be no experimental data showing this to be the case.

If the α_1A-adrenoceptor and α_1D-adrenoceptor antagonistic effect of mirabegron had any clinical implications, a blood-pressure-lowering effect would also be expected. By contrast, mirabegron has been shown to increase blood pressure at higher doses than those used therapeutically. In clinical trials with recommended doses (25–50 mg), the increase was negligible⁶. If mirabegron had any effects on the human urethra, stress incontinence, particularly in females, would be anticipated, but this finding has not been observed in clinical studies. Furthermore, an effect on bladder outlet resistance and urethral pressure would be expected, possibly reflected in urodynamic studies of voiding. However, Nitti et al. found that mirabegron (versus placebo) did not affect voiding urodynamic parameters (maximum urinary flow and detrusor pressure at maximum urinary flow) after 12 weeks of treatment of men with lower urinary tract symptoms and bladder outlet obstruction⁷. In females with OAB syndrome, Matsukawa et al. clearly demonstrated that mirabegron promoted storage function by increasing bladder capacity and inhibiting detrusor overactivity without affecting voiding functions⁸. This finding is interesting as it has previously been shown that α_1A-adrenoceptor (and α_1D-adrenoceptor) receptor blockade with tamsulosin had no effect on women with OAB syndrome⁹. Thus, there seems to be no clinical evidence for a relevant blockade of lower urinary tract α_1A-adrenoceptors and α_1D-adrenoceptors by mirabegron. By contrast, tamsulosin showed a significant relaxing effect on the resting urethral tone in healthy human females¹⁰.

Thus, it seems that the α_1A-adrenoceptor-blocking and α_1D-adrenoceptor-blocking effect of mirabegron is an interesting pharmacological in vitro observation in mouse and rat lower urinary tract tissues. However, such an effect seems to have no relevance for the clinical effects of mirabegron on the human lower urinary tract, or the use of the drug for the treatment of OAB. It would be of interest to determine whether an α_1A-adrenoceptor-blocking and α_1D-adrenoceptor-blocking effect of mirabegron can also be demonstrated in human lower urinary tract tissues.