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Background: Brexpiprazole is a dopamine/serotonin receptor partial agonist (D2, 5-HT1A) and antagonist (5-HT2A) approved for treatment of schizophrenia and major depressive disorder (adjunct to antidepressants). Aims: This study aimed to investigate brexpiprazole as monotherapy in acute mania (bipolar I disorder) in two short-term (ST) studies (study 080 and study 081) and one open-label (OL) extension (study 083). Methods: ST studies were three-week randomized, double-blind, flexible dose (2–4 mg/day), placebo-controlled studies. The primary endpoint was mean change in Young Mania Rating Scale (YMRS) total score from baseline to day 21. The OL study was a 26-week flexible dose (2–4 mg/day) study for patients completing the ST studies. Results: A total of 164 and 158 (study 080) and 170 and 162 (study 081) inpatients with DSM-5 mania with/without mixed features were randomized to placebo or brexpiprazole, respectively. The primary analyses did not show a statistically significant difference between brexpiprazole and placebo: study 080: least squares mean difference (95% confidence limits): 0.14 (−1.74, 2.03), p = 0.8797; study 081: −1.62 (−3.56, 0.32), p = 0.1011. OL study patients (n = 381) demonstrated a gradual improvement in YMRS total score. Akathisia was the only adverse event, with an incidence of ⩾5% with brexpiprazole and more than placebo in the ST studies, or ⩾5% in the OL study. Brexpiprazole was more efficacious in patients with impaired or no insight (predominantly EU patients) than in patients with excellent insight (predominantly US patients). Conclusions: Further studies are necessary to address the potential efficacy of brexpiprazole in acute mania, which should ensure that the study sample is severe enough (especially with regard to insight), and that the dose/titration schedule is not too modest. [ABSTRACT FROM AUTHOR]