Efficacy, moderators and mediators of cognitive behavioural analysis system of psychotherapy (CBASP) versus behavioural activation (BA) in persistently depressed treatment-resistant inpatients: study protocol for the multicentre, randomised controlle

INTRODUCTION: Up to 30% of individuals with depression develop persistent depressive disorder (PDD), an often disabling and difficult to treat condition. The Cognitive Behavioural Analysis System of Psychotherapy (CBASP) is the only psychotherapy developed specifically for treating individuals with PDD. While several randomised controlled trials (RCTs) have demonstrated its efficacy in outpatient settings, evidence for its use in inpatient settings remains limited. Pilot studies of CBASP inpatient programmes in Germany have shown promising feasibility and effectiveness; however, no RCTs to date have systematically evaluated their outcomes. This study represents the first RCT to compare the short- and long-term efficacy and safety of CBASP with Behavioural Activation (BA), a first-line psychotherapy for depression, within an intensive multimodal inpatient setting. METHODS AND ANALYSIS: In this prospective, multicentre, rater-blinded RCT with an active control group, we aim to recruit 396 adults (aged 18-70 years) with treatment-resistant PDD at eight German university hospitals. Participants will be randomly assigned to receive either (1) CBASP or (2) BA within an intensive treatment programme consisting of 10 weeks acute treatment in an inpatient and/or day clinic setting, followed by 6 weeks of outpatient continuation treatment. Primary and secondary outcome assessments will be conducted at multiple time points: baseline (T0), treatment onset (T1), after 5 and 10 weeks of acute treatment (T2, T3), at the end of continuation treatment (T4, week 16) and every 2 months up to week 64 (T5, naturalistic follow-up).The primary outcome measure will be the change in depression severity, as assessed by the Hamilton Depression Rating Scale (24-item version), after 16 weeks of treatment (from T0 to T4). Secondary outcomes will include response, remission, deterioration and relapse rates, self-reported depression and anxiety symptoms and additional psychological variables. A cost-benefit analysis will evaluate the health-economic benefits of both interventions. Additionally, this RCT will explore personalised treatment selection and mechanisms of change, including potential moderators and mediators of treatment effects. The findings from this trial are expected to provide clinicians with evidence-based guidance on choosing CBASP versus BA for inpatients with treatment-resistant PDD. ETHICS AND DISSEMINATION: This study has received ethical approval from the ethics committees of all participating university hospitals. All participants will provide written informed consent before enrolment. Study findings will be published in peer-reviewed journals and presented at national and international conferences. We have involved people with lived experience from the earliest pilots onward, using their feedback to refine our study design. Ongoing consultation at conferences and public events has further ensured that our research remains grounded in patient perspectives. TRIAL REGISTRATION NUMBER: NCT04996433.