Supplementary Material for: Clinical and Neuroimaging Effects of Mindfulness-Based Cognitive Therapy for Symptomatic OCD Patients after First-Line Treatments: A Randomised Controlled Trial

Introduction: Obsessive-compulsive disorder (OCD) is a chronic condition where many patients remain symptomatic despite first-line treatments such as Cognitive Behavioural Therapy and selective serotonin reuptake inhibitors. This randomised controlled trial evaluated Mindfulness-Based Cognitive Therapy (MBCT) efficacy as an augmentation strategy and its impact on brain functional connectivity. Methods: Sixty-eight participants with moderately symptomatic OCD were randomised into MBCT or Treatment as Usual (TAU). Clinical outcomes were evaluated using the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) and the Obsessive-Compulsive Inventory, alongside other relevant secondary outcomes. Data were analysed using repeated measures ANOVA to assess time * group effects. Neuroimaging functional measures (resting-state network–connectivity), were collected before and after the intervention and analysed using independent component analysis. Results: Primary outcome: MBCT significantly reduced OCD symptoms compared to TAU (31.73% vs. 8.07% Y-BOCS reduction). Secondary outcomes: MBCT group also experienced reductions in depressive symptoms, rumination, perceived stress and quality of life. No significant post-treatment changes were observed in resting-state connectivity. However, baseline connectivity demonstrated significant predictive value, with lower connectivity in pre-selected networks of interest, including the fronto-striatal, salience, and default-mode networks, associated with greater reductions in Y-BOCS scores. Conclusion: MBCT is an effective strategy for individuals with moderately symptomatic OCD who continue to experience symptoms despite prior gold-standard treatments. While no post-treatment changes in brain functional connectivity were observed, baseline connectivity patterns predicted symptom reduction, suggesting a neural basis for MBCT response. Trial name: Mindfulness-Based Cognitive Therapy: Efficacy and fMRI-based Response Predictors in a Group of OCD Patients. ID number: NCT03128749.