This prospective single-arm paired comparison trial was conducted at a tertiary referral center, Yonsei University Severance Hospital, between January 2016 and March 2023. The study enrolled β3-adrenoceptor agonist-naïve patients with BC ≤20 mL/cm H₂O [8], in urodynamic studies performed while they were taking anticholinergics for more than 1 month.

The patient inclusion/exclusion criteria are provided in Supplementary Table 1.

To eliminate potential additive effects of anticholinergics and assess mirabegron’s pure therapeutic effect, a 2-week washout period was implemented. Subsequently, patients received 50 mg of mirabegron daily for 8 weeks. Then, anticholinergics treatment with the same regimen used in the baseline assessment was performed for another 8 weeks. Given mirabegron’s pharmacokinetics (~99% eliminated within 11–14 days), no additional washout period was implemented at this stage [9, 10]. Urodynamic studies and patient evaluations were performed at baseline, 8 weeks after mirabegron, and 8 weeks after anticholinergics treatment (Fig. 1).

Urodynamic studies were performed in accordance with the “Good Urodynamic Practices” suggested by the International Continence Society [11]. Using body-temperature saline, bladder instillation was performed at a mean rate of 20–30 (range, 5–40) mL/min [11]. BC was defined as the change in bladder volume divided by the change in detrusor pressure (Pdet) [12]. The change in bladder volume was defined as the maximum cystometric capacity (MCC) when there was no terminal detrusor overactivity; the volume at detrusor leak point pressure, when autonomic dysreflexia occurred, or when the patient complained of discomfort. When there was terminal detrusor overactivity, cystometric capacity was determined at the point immediately before involuntary detrusor contraction [12]. At the start of the urodynamic study, Pdet was set to 0 cm H₂O; therefore, the BC was determined by dividing the cystometric capacity by the Pdet at cystometric capacity (Pdet end-filling).

We set two primary endpoints: a comparison of the proportion of patients with improved BC higher than 20 mL/cm H₂O after mirabegron and anticholinergics treatment (McNemar test) and the change in BC during each treatment period (paired t-test). The secondary endpoints were changes in the MCC and Pdet end-filling during each treatment period (paired t-test). Additionally, changes in the International Consultation on Incontinence Questionnaire (ICIQ) score and functional capacity on a bladder diary were evaluated (paired t-test). To explore potential relationships between the duration of the neurological deficit and treatment outcomes, Pearson correlation analysis was performed.

The sample size was established using retrospective data of patients who received anticholinergics and were then switched to mirabegron or add-on mirabegron therapy to anticholinergics in an unplanned manner. A sample size of 9 pairs achieved 80% power for a 1-sided McNemar test with a significance level of 0.025, with a proportion of discordant pairs of 0.75, specifically found in cell 1,2. Considering a 30% dropout rate, the target number of patients was set to 13. Data are expressed as mean with 95% confidence interval (CI) or range. Statistical analyses were performed using IBM SPSS Statistics ver. 27.0 (IBM Co., USA) and SAS 9.4 (SAS Institute Inc., USA). Statistical significance was set at P<0.05.

Seventeen candidates met the inclusion criteria, but 3 were excluded because of irregular anticholinergics medication at the time of baseline assessment. Fourteen patients were enrolled. Three patients dropped out after 8 weeks of mirabegron because of protocol violations; 1 refused to resume anticholinergics for fear of suffering severe dry mouth; 1 arbitrarily switched to mirabegron 7 days after returning to anticholinergics due to aggravated urinary incontinence; and 1 missed follow-up because of impaired ambulation caused by a fall. Finally, 11 patients completed all study sessions. Analysis included all 14 enrolled patients, including the 3 patients who dropped out after completion of mirabegron treatment (Fig. 2).

The patients’ age was 54.4 (range, 23.9–78.3) years; 57.4% (8 of 14) had neurological disorders of spinal cord origin, 14.3% (2 of 14) had diseases of brain origin, and 7.1% (1 of 14) had neurofibromatosis involving both the brain and spine. 21.4% (3 of 14) had bladder dysfunction following radical pelvic surgery (1 cervical cancer, 2 rectal cancer). Both rectal cancer patients underwent pelvic radiation. The estimated duration of neurological deficit—defined as the time from symptom onset (for neurological diseases), date of trauma (for spinal cord or brain injury), or date of surgery (for patients who underwent radical pelvic surgery)—was 16.7 (range, 1.7–37.0) years. The body surface area-adjusted estimated glomerular filtration rate was 102.6 (range, 55.0–153.0) mL/min. At the baseline urodynamic study, the anticholinergics treatment duration was 10.1 (range, 2.1–72.1) months. No participants received any urological drugs other than the study medications that might affect bladder function (Table 1 and Fig. 3).

After 8 weeks of mirabegron treatment, 71.43% (10 of 14) achieved BC >20 mL/cm H₂O (95% CI, 45.35%–88.28%). Eight weeks after returning to the previously administered anticholinergics, 54.55% (6 of 11) achieved BC >20 mL/cm H₂O (95% CI, 28.01%–78.73%). The absolute effect size (risk difference) was 16.88% (95% CI, -20.88% to 54.64%), and the relative effect size (relative risk) was 1.31 (95% CI, 0.70–2.47), indicating a 31% higher likelihood of achieving BC >20 mL/cm H₂O with mirabegron compared to anticholinergics. However, this difference between the two treatment sessions was not statistically significant (P=0.317).

After 8 weeks of mirabegron treatment, BC increased significantly from 12.02 mL/cm H₂O (95% CI, 9.52–14.52) to 39.67 mL/cm H₂O (95% CI, 21.60–57.73), with a significant mean difference of +27.65 mL/cm H₂O (95% CI, 8.78–46.51; P=0.007) (N=14). Eight weeks after returning to the previously administered anticholinergics, BC decreased from 39.63 mL/cm H₂O (95% CI, 17.04–62.21) to 20.94 mL/cm H₂O (95% CI, 15.78–26.10), with a mean difference of -18.68 mL/cm H₂O (95% CI, -39.63 to 2.26; P=0.075) (N=11) (Fig. 4).

After 8 weeks of mirabegron, MCC increased from 352.21 mL (95% CI, 282.78–421.65) to 442.71 mL (95% CI, 348.95–536.48), with a mean difference of +90.50 mL (95% CI, -16.69 to 197.69; P=0.091) (N=14). After returning to the previously administered anticholinergics for 8 weeks, MCC decreased from 447.64 mL (95% CI, 332.72–562.56) to 402.00 mL (95% CI, 315.92–488.08), with a mean difference of -45.64 mL (95% CI, -122.97 to 31.69; P=0.218) (N=11). There was no statistically significant difference in MCC for either treatment session (Fig. 5).

After 8 weeks of mirabegron, a significant decrease in Pdet end-filling was observed, from 30.50 cm H₂O (95% CI, 25.61–35.39) to 14.43 cm H₂O (95% CI, 10.79–18.06), with a significant mean difference of -16.07 cm H₂O (95% CI, -21.73 to -10.41; P<0.001) (N=14). After returning to the previously administered anticholinergics for 8 weeks, Pdet end-filling increased from 14.18 cm H₂O (95% CI, 10.69–17.67) to 20.36 cm H₂O (95% CI, 16.26–24.46), with a mean difference of +6.18 cm H₂O (95% CI: -0.18 to 12.55; P=0.056) (N=11), approaching conventional significance (Fig. 6).

ICIQ scores were assessed in 13 patients, excluding one patient who was using a urethral clamp for true urinary incontinence. After 8 weeks of mirabegron, the ICIQ score decreased significantly from 10.92 (95% CI, 7.70–14.14) to 6.62 (95% CI, 2.70–10.53), with a mean difference of -4.30 (95% CI, -7.88 to -0.74; P=0.022) (N=13). Subsequently, the ICIQ score increased from 5.30 (95% CI, 0.74–9.86) to 8.90 (95% CI, 3.34–14.46), with a mean difference of +3.60 (95% CI, -1.76 to 8.96; P=0.163) (N=10) after returning to anticholinergics.

Based on bladder diaries, functional capacity increased significantly from 375.83 mL (95% CI, 248.57–503.09) to 522.50 mL (95% CI, 335.44–709.56), with a mean difference of +146.67 mL (95% CI, 29.31–264.02; P=0.019) (N=12) after 8 weeks of mirabegron. Subsequently, functional capacity decreased from 489.00 mL (95% CI, 267.63–710.37) to 453.00 mL (95% CI, 279.66–626.34), with a mean difference of -36.00 mL (95% CI, -164.64 to 92.64; P=0.542) (N=10) after returning to anticholinergics.

After 8 weeks of mirabegron treatment, significant changes in BC and Pdet end-filling were observed. However, the amount of change varied among patients. Correlation analyses revealed a significant negative correlation between the duration of the neurological deficit and the amount of decrease in Pdet endfilling (Pearson r=-0.566; 95% CI, -0.843 to -0.051; P=0.035), indicating that patients with a longer duration of neurological deficit experienced a smaller decrease in Pdet end-filling. However, there was no significant correlation between the duration of neurological deficit and the amount of increase in BC (Pearson r=0.009; 95% CI, -0.524 to 0.537; P=0.977) or MCC (Pearson r=0.128; 95% CI, -0.432 to 0.617; P=0.664).

Two patients experienced treatment-emergent adverse events (palpitations, skin rash) after starting mirabegron. Both were mild, resolved without intervention, and had no lasting effects. Consequently, both patients completed the planned 8-week mirabegron treatment and subsequent anticholinergic protocol as scheduled.

Unlike the 13 patients whose BC increased with mirabegron, one spina bifida patient showed decreased BC after mirabegron, with a notable decrease in MCC (patient ID 12; Figs. 3–6). Eight months after study completion, pelvic ultrasonography and magnetic resonance imaging revealed a large uterus (724.4 mL), with myomas, the largest of which was 11 cm in diameter on the anterior uterine wall.

As researches progress to date, it is known that there are many common aspects in the distribution of receptors and the ultimate mechanisms of action through each unique pathway of anticholinergics and β3-adrenoceptor agonist [6, 13-15]. Here, detrusor contraction during the storage phase and detrusor muscle tone should be considered separately [2, 14, 16-18]. Both anticholinergics and β3-adrenoceptor agonists are believed to act on detrusor contraction during the storage phase. By suppressing afferent Aδ or C fiber excitation [6, 14, 15], both agents increase the bladder volume where ‘bladder filling awareness’ occurs, inhibit involuntary detrusor contraction, and finally increase MCC [16, 19]. Therefore, among the two functions that comprise BC, both anticholinergics and β3-adrenoceptor agonists might have effect in improving the bladder capacity. The lack of a statistically significant difference in MCC between anticholinergics and mirabegron observed in this study may be attributed to these phenomena. However, these two groups of agents are believed to have different effects on detrusor muscle tone, which is believed to be induced by the intramural movement of the bladder, known as ‘micromotion’ [13, 16, 17, 20, 21].

Micromotion is a disinhibited, autonomous, and localized movement inherent to the bladder—observed in animals, including humans—and is well-controlled in a normally innervated bladder [16, 22]. Microdistortion of the bladder wall caused by micromotion is a basic component for activating mechanosensitive afferents [16, 23], and is assumed to prevent stretching of the microvasculature during bladder distention, thereby producing bladder wall tension [24].

Micromotion is different from the nonvoiding fluctuation of intravesical pressure, which can be observed on cystometrograms in neurologically intact or partially denervated bladders. To detect pure micromotion, reflected as intravesical pressure oscillations on a cystometrogram, the bladder should be completely denervated, theoretically [16, 22, 25]. However, micromotion of the human bladder is difficult to observe on cystometrograms because the terminal neural ganglia are believed to exist inside the bladder wall [26, 27]. In the human bladder, micromotion has been confirmed using electrodes, demonstrating higher activity in patients with hypersensitive bladder [23].

Unlike human, rodents have extravesical terminal neural ganglia known as the major pelvic ganglion (MPG) [28, 29]. We previously developed a living micromotion rat model whose nonvoiding oscillation of intravesical pressure was the closest to micromotion by excising bilateral MPGs. A systemically administered β3-adrenoceptor agonist significantly suppressed micromotion, but nonselective anticholinergics oxybutynin could not control micromotion significantly [30].

Consistent with the differential effects of β3-adrenoceptor agonists and anticholinergics on micromotion, prior studies have reported similar distinctions in their effects on detrusor muscle tone, which is thought to be micromotion-related. One study performed with live dogs suggested little relationship between cholinergic components and detrusor tone [18]. A later study performed with isolated whole bladders of mice implied the possibility that the cholinergic pathway acts on bladder afferents through a mechanism independent of changes in detrusor tone [14]. Contrary to anticholinergics, an experiment performed with live rats denervated at the L6 dorsal root level showed a decrease in afferent firing simultaneously with amelioration of microcontraction and a decrease in intravesical pressure after β3-adrenoceptor agonist administration, whereas these effects were not observed with anticholinergics [31]. Several in vitro studies have shown that β3-adrenoceptor agonists can ameliorate detrusor tone via various mechanisms unique to the β3-adrenergic pathway [32].

Caution should be exercised when interpreting the results of individual experiments conducted with different animals and under various conditions, as the response may vary depending on the species and pathological condition, even when the same agent is used [33]. Further research is required to verify the relationships between β3-adrenoceptor agonists and micromotion, detrusor muscle tone, and BC for application to the human bladder.

The exploratory analysis revealed a significant negative correlation between the duration of neurological deficit and mirabegron’s effect on reducing Pdet end-filling, suggesting that longer disease duration may reduce responsiveness. However, as these analyses were not prespecified, the findings should be interpreted cautiously. Additionally, one patient with a large pelvic mass showed a paradoxical decrease of BC after mirabegron. While the mass may have influenced MCC, its direct relationship with BC remains unclear, warranting cautious interpretation. Prespecified hypotheses-driven studies may validate these findings.

An unplanned finding outside the original study design revealed that BC and Pdet end-filling remained significantly improved compared to baseline after the reintroduction of anticholinergics, although the effect was weaker than during mirabegron treatment. Considering mirabegron’s pharmacokinetics, the improvement observed 8 weeks after discontinuation—far beyond complete elimination from the bloodstream—is unlikely to be attributable to mirabegron’s effect on the neurofunctional (active) components of BC, such as the dynamic modulation of micromotion or detrusor tone [9, 10]. However, the possibility of longer-lasting effects of mirabegron on the structural (passive) components of BC, potentially mediated through partial reversal of fibrosis, cannot be excluded [34]. Whether such changes persist after drug elimination and contribute to sustained improvement in BC warrants further investigation.

This study has limitations, including the use of a 20 mL/cm H₂O cutoff as an endpoint. Although 20 mL/cm H₂O is commonly used to define low BC, a critical dichotomization bias may exist because as BC approaches 20 mL/cm H₂O, there can be an increased risk that the results may be misjudged [35]. This study found no significant difference in the proportion of patients with BC >20 mL/cm H₂O after mirabegron and anticholinergics, but the findings warrant cautious interpretation.

As the study included only mirabegron-naive patients who exhibited BC ≤20 mL/cm H₂O despite anticholinergics, patients who were anticholinergics-naive or who might have responded favorably to anticholinergics were not represented, reflecting a limitation in the spectrum of the study population.

When determining the sample size, 9 pairs were sufficient to differentiate the effects of the two drug classes; therefore, this study was performed with a confined number of 14 patients. Additional studies with a large number of patients are required to support these results, more robustly.

Despite these limitations, this study provides valuable insights into the potentially different effects of the two drug classes on detrusor muscle tone. It indicates that the regulation of detrusor muscle tone via the modulation of bladder micromotion, which is believed to be predominantly mediated through the β3-adrenergic pathway, may play a crucial role in improving BC. To the best of our knowledge, this study is the first to prospectively compare a β3-adrenoceptor agonist and anticholinergics regarding BC.

A β3-adrenoceptor agonist, mirabegron, was more effective than anticholinergics in improving BC. Among the two components of improved BC—increased bladder volume and reduced detrusor filling pressure—the β3-adrenoceptor agonist showed a more pronounced effect on lowering detrusor filling pressure, compared to anticholinergics. These findings suggest that β3-adrenoceptor agonists might play an important role in reducing the tension of the bladder wall by controlling detrusor muscle tone, and this may be an important target for future research.