Efficacy and safety of semaglutide versus placebo for people with schizophrenia on clozapine with obesity (COaST): a phase 2, multi-centre, participant and investigator- blinded, randomised controlled trial in Australia.

BACKGROUND: People with schizophrenia have a 16-20-year reduction in life expectancy, primarily due to cardiometabolic disease. Clozapine, the most efficacious antipsychotic for treatment-resistant schizophrenia, is associated with weight gain and metabolic dysfunction. Glucagon-like peptide-1 receptor agonists, including semaglutide, contribute to substantial weight loss in the general population, but their effect and safety profile in people with schizophrenia remain unknown. We evaluated the efficacy and safety of semaglutide for weight reduction in individuals with schizophrenia who were prescribed clozapine.

METHODS: COaST was an Australian randomised, placebo-controlled, multi-site trial, independent of pharmaceutical industry support, with methods informed by people with lived experience. Adults (aged 18-64 years) across six sites were randomly assigned (1:1) to once weekly subcutaneous semaglutide titrated to 2·0 mg or placebo for 36 weeks. Participants were included if they fulfilled criteria for schizophrenia or schizoaffective disorder, were prescribed clozapine for 18 weeks or more, had a BMI of at least 26 kg/m2, and had less than 5% bodyweight increase or loss in the previous 3 months. The primary outcome was percentage body weight change, analysed using a mixed model for repeated measures, at 36 weeks post-baseline assessment. All investigators and participants were masked to medication allocation. Secondary measures included clozapine and norclozapine concentrations and psychosis symptoms as measured by the Positive and Negative Syndrome Scale (PANSS). The protocol was prospectively registered with the Australia New Zealand Clinical Trials Registry (ACTRN12621001539820; recruitment finished, pending follow-up assessments).

FINDINGS: 166 individuals were screened for eligibility, 135 were excluded, and the remaining 31 were randomly assigned to either the semaglutide group (n=15) or the control group (n=16). 21 males and ten females were included in the study, with a mean age of 38·9 years (range 21-58). All participants assigned to each group were included in the analysis of the primary outcome. 84% of participants were White, 7% were from the Indian Subcontinent, 3% were Asian (not including from the Indian Subcontinent), 3% were Australian Aboriginal or Torres Strait Islanders, and 3% were New Zealand Māori or Pacific Islanders. Recruitment commenced on Aug 30, 2022 and was suspended in June, 2024 before achieving the intended number of 80 participants due to non-availability of the investigational product. At week 36, semaglutide yielded a 13·88% (SE 0·90) body weight reduction compared with 0·42% (SE 0·93) for placebo (between-group difference: -13·46%; p<0·0001). No differences were observed in clozapine or norclozapine concentrations or PANSS scores. Semaglutide was well tolerated, with no serious adverse events that were deemed to be related to the treatment, and low rates of constipation.

INTERPRETATION: Semaglutide led to significantly greater weight loss than placebo in this small trial without affecting psychotic symptoms or clozapine concentrations. Semaglutide appears to be safe and well tolerated in this population. These encouraging findings highlight the need for larger confirmatory trials.

FUNDING: National Health and Medical Research Council, Qld Advancing Clinical Research Fellowship, and Metro South Health Research Support Scheme Program.