In vitro activity: Mirabegron concentration-dependently increases the accumulation of cAMP in CHO cells expressing human 3-adrenoceptors (ARs) with I.A. of 0.8. Mirabegron has little agonistic effect on 1- and 2-ARs. Mirabegron concentration-dependently relaxes rat and Human bladder smooth muscle strips precontracted with 10-6 M or 10-7 M carbachol with EC50 values of 5.1 μM and 0.78 μM, respectively. The maximal relaxant effects of Mirabegron are 94.0 % and 89.4% that of carbachol, respectively. Mirabegron is a time-dependent inhibitor of CYP2D6 in the presence of NADPH as the IC50 value in human liver microsomes decreased from 13 to 4.3 μM after 30 min preincubation. Mirabegron acts partly as an irreversible or quasi-irreversible metabolism-dependent inhibitor of CYP2D6,

Cell Assay: CHO cells (105) are seeded in each well of a 24-well culture plate and subcultured. Three days later, the medium is exchanged with 250 μL/well Hanks' balanced salt solution containing 0.1 mM 3-isobutyl-1-methylxanthine, pH 7.4. The cells are incubated with each compound (isoproterenol, Mirabegron, BRL37344, and CL316,243 at final concentrations of 10-10 to 10-4 M) for 10 min at 37°C, after which incubation is stopped by the addition of 250 μL of 0.2 M HCl. cAMP concentration in the reaction mixture is measured by radioimmunoassay using an 125I-cAMP assay system using a gamma counter. Fifty microliters of reaction mixture is incubated with 50 μL of succinyl agent for 10 min at room temperature, after which the reaction is stopped by the addition of 400 μL of buffer solution. Fifty microliters of succinylated sample is incubated with 50 μL of 125I-cAMP and 50 μL of anti-cAMP antibody for 24 h at 4°C. At the end of the incubation period, 250 μL of charcoal suspension is added and centrifuged for 10 min at 2800g at 4°C. Two hundred and fifty microliters of supernatant is transferred into a tube and counted for 1 min using a gamma counter. The intrinsic activity (I.A.) relative to isoproterenol for each β-adrenoceptor agonist is calculated using the maximal response of each compound.