To test the ability of mirabegron to acutely stimulate human BAT, we wanted to evaluate its efficacy in subjects who were already known to have detectable BAT, thereby reducing the likelihood of a false-negative finding (). Fifteen eligible subjects were screened first with cold exposure, and twelve had detectable BAT ( Supplemental Experimental Procedures Figure S1 A available online). The amount of BAT activity, based on the standard uptake value (SUV), was comparable to that reported in other studies using lower doses of radiotracer (). In these twelve subjects ( Table 1 ), cold exposure increased RMR (128 ± 32 kcal/day, or +8%) (p = 0.001), systolic blood pressure (BP), and diastolic BP; decreased heart rate ( Figures S1 B–S1E); and led to metabolite concentrations consistent with what we have previously reported (). Safety considerations limited the subjects to three PET/CT scans each, so we could evaluate only one dose of mirabegron in comparison to placebo. We selected 200 mg since this dose has a higher efficacy than the currently approved dose of 50 mg for reducing the symptoms of overactive bladder and was therefore more likely to have a detectable effect on BAT metabolic activity. In addition, the 200 mg dosage has been well-tolerated even after 12 weeks of daily oral administration ().

Metabolites were drawn 3.5 hr after dosing of mirabegron and placebo, which was the reported T). Mirabegron was detectable in the plasma with a mean concentration of 310 ± 73 ng/mL (p = 0.001), values comparable to other studies (). Administration of mirabegron was notable for inducing higher levels of glucose, nonesterified fatty acids (NEFAs), β-hydroxybutyrate, insulin, C-peptide, and significant elevations in HOMA-IR (p = 0.002) ( Table S1 ). Maximal glucose uptake (SUVmax) was lower in the myocardium, though it did not achieve significance after applying Bonferroni’s correction. No apparent differences were seen in the subcutaneous white adipose tissue (WAT), skeletal muscle, or liver ( Figures 2 A–2D).

18 F-FDG uptake in the twelve volunteers when given placebo or 200 mg mirabegron is shown for different tissues. Each circle represents a single subject.

Compared to placebo, mirabegron significantly increased RMR (203 ± 40 kcal/d, or +13%) (p = 0.001), HR (14 ± 3 bpm) (p = 0.002), and systolic BP (11 ± 2 mmHg) (p = 0.002), but not diastolic BP (2 ± 1 mmHg) (p = 0.07) ( Figures 1 C–1F). This drug-induced stimulation of the cardiovascular system was considerably lower than what has been reported for broadly acting sympathomimetics such as ephedrine or isoproterenol, particularly for the extent of changes seen in RMR (). There were no unanticipated adverse effects.

Compared to placebo, for all twelve subjects BAT glucose uptake was significantly higher after treatment with mirabegron (median 132, interquartile range 70–253 ml⋅SUVmean⋅g/ml, p = 0.001) ( Figures 1 A, 1B, and S2 ). The principal sites of detectable BAT were the cervical-supraclavicular-axillary adipose tissue depots, but in some subjects, glucose uptake was seen even in the paraspinal, periaortic, perihepatic, perirenal, and perisplenic regions. The correlation between drug- and cold-induced BAT glucose uptake was not significant ( Figure S3 ), indicating that it is not consistently reliable to use only one method of stimulation—either cold or mirabegron—to accurately measure whole-body BAT mass or activity.

Each circle represents a single subject, and the numbers correspond to subject identification number in Figure S2 . The dashes represent the mean.

(A) PET images of a 21-year-old man who was given placebo (left) or 200 mg of the β3-AR agonist mirabegron (right). Twelve male subjects were given placebo or 200 mg mirabegron.

A challenge to studying human thermogenesis is determining the contribution of each organ. Mirabegron increased BAT glucose uptake, which correlates directly with tissue thermogenesis (), and the degree of activation accounted for 50% of the variability in drug-induced changes in RMR. The amount of BAT thermogenesis in absolute terms must still be determined, particularly as there may be an important contribution from nonselective activation of other β-AR’s on other tissues. Nevertheless, our findings indicate that human BAT may play a role in the thermogenesis associated with β3-AR agonist treatment as well as exposure to mild cold (). An intriguing issue is the proportional contribution by the two different brown adipocyte lineages: the constitutive “brown” adipocytes in the cervical and supraclavicular depots and the recruitable “beige/brite” adipocytes in the supraclavicular, abdominal, and other sites (). Based on the wide distribution of detectable glucose uptake, it is likely that mirabegron stimulates both kinds of BAT, though more detailed in vitro studies are required to quantify what functional differences may exist between these adipocytes.

Since both BAT and WAT express the β3-AR (), as exploratory analyses, we compared the changes in RMR with BAT activity and WAT mass. We found a close association between change in RMR and BAT activity stimulated by mirabegron (p = 0.006) but not placebo (p = 0.17) ( Figures 3 A and 3B ). There was also a positive relationship between changes in RMR and body fat mass in response to mirabegron (p = 0.02) but not placebo (p = 0.64) ( Figures 3 C and 3D). In contrast, there were no associations between the change in RMR and the change in heart rate (R= 0.00, p = 0.97), systolic BP (R= 0.01, p = 0.14), or fat-free mass (R= 0.00, p = 0.96).

Potential Therapeutic Applications of Chronic Treatment with β3-AR Agonists

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Villarroya F. An endocrine role for brown adipose tissue?. However, it is important to consider that in rodent models, treatment with β3-AR agonists chronically can improve glucose disposal and increase RMR before a reduction in body weight is seen (). This distinction between the acute and chronic effects of treatment with mirabegron applies to the current study. In the acute setting, we saw a significant increase in HOMA-IR, a measure of insulin resistance. As with the changes in heart rate, it is not known if this effect is from off-target binding to other β-AR’s, and future studies are needed to determine which metabolic changes are attributable specifically to β3-AR agonists. In the present study, in addition to the effects on BAT, there was evidence for β3-AR agonist stimulation of WAT lipolysis (), which was reflected by the non-significantly higher levels of serum NEFAs and lower myocardial glucose uptake that is associated with this change in fuel availability (). Thus, chronic treatment with a β3-AR agonist in humans may improve multiple facets of metabolism even in the absence of weight loss through consumption of lipids and glucose and also the release of beneficial adipokines ().

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Cheng C.P. β3-Adrenergic receptor antagonist improves exercise performance in pacing-induced heart failure. This study is limited by its small size and duration. Also, the young, lean, healthy male subjects with detectable BAT were selected deliberately for the purpose of a proof-of-concept study, so the efficacy of mirabegron needs to be evaluated in women and other populations, such as those with different ages and BMIs. In addition, the changes in RMR and metabolism were measured after administration of a single dose of mirabegron, so we do not know if chronic treatment would yield the metabolic benefits seen with other β3-AR agonists. Finally, the decision to use the more potent 200 mg dose of mirabegron was advantageous in that all twelve subjects had detectable BAT activity that was higher than placebo. For this small sample size, the wide range of BAT activities we observed was crucial for learning meaningful aspects about human BAT physiology. However, the 200 mg dose was also higher than necessary to selectively activate only the β3-AR (). As a result, we observed binding to the β1-AR with resultant tachycardia, a treatment-emergent adverse event noted in several clinical trials of mirabegron (). Given the acute stimulation of the cardiovascular system, the long-term safety of this approach must be established. It remains to be determined if the approved daily dose of 50 mg for overactive bladder, which was associated with smaller effects on heart rate (), will sufficiently stimulate BAT growth and thermogenesis. Given that activation of the myocardial β3-AR has limited chronotropic effects (), concomitant administration of 200 mg mirabegron with a β1-AR blocker could mitigate the undesirable cardiovascular stimulation. Alternatively, it is possible that the off-target effects may be satisfactorily reduced by newly designed members of the class having greater selectivity for the β3-AR.

In summary, we demonstrate that the β3-AR agonist mirabegron acutely stimulates human BAT thermogenesis and increases RMR. Given that mirabegron is already approved for treatment of overactive bladder, we anticipate that these findings will accelerate the development of pharmacological strategies designed to increase energy expenditure and treat obesity and metabolic disease.