Objectives: To characterize the effects of adjunctive brexpiprazole on patient life engagement and depressive symptoms in patients with major depressive disorder (MDD) using patient-reported outcomes., Methods: An 8-week, Phase 4, open-label, interventional study was conducted at 15 Canadian trial sites between April 2021 and May 2022. Adult outpatients with MDD (at least moderately severe) and inadequate response to 1-2 antidepressants continued their current antidepressant and received oral adjunctive brexpiprazole 0.5-2 mg/day. Co-primary endpoints were change from baseline to Week 8 in Inventory of Depressive Symptomatology Self-Report (IDS-SR) 10-item Life Engagement subscale score, and IDS-SR 30-item total score. Safety was assessed by standard variables., Results: Of 122 enrolled patients, 120 (98.4%) were treated (mean [ SD ] dose: 1.2 [0.4] mg/day) and analyzed, and 111 (91.0%) completed the study. Statistically significant least squares mean improvements to Week 8 were observed on IDS-SR 10 Life Engagement subscale score (baseline mean [ SD ]: 16.1 [4.7]; change [95% confidence interval]: -8.11 [-9.34, -6.88]; p < 0.001) and IDS-SR total score (baseline mean [ SD ]: 41.3 [9.8]; change [95% confidence interval]: -17.38 [-20.08, -14.68]; p < 0.001). Improvements were observed from Week 2, onwards. Treatment-emergent adverse events with incidence ≥5% were fatigue ( n = 13, 10.8%), headache ( n = 13, 10.8%), insomnia ( n = 12, 10.0%), nausea ( n = 9, 7.5%), tremor ( n = 8, 6.7%), and weight increase ( n = 7, 5.8%). Six patients (5.0%) discontinued due to adverse events. Mean ( SD ) change in body weight from baseline to last visit was +1.9 (3.4) kg., Conclusions: Using an exploratory patient-reported outcome measure, patients with MDD and inadequate response to antidepressants who received open-label adjunctive brexpiprazole showed early and clinically meaningful improvement in patient life engagement, which should be further assessed in a prospective randomized controlled trial. Patient-rated depressive symptoms (on the validated 30-item IDS-SR) also improved. Adjunctive brexpiprazole was well tolerated, and no new safety signals were observed., Clinical Trial Registration: ClinicalTrials.gov identifier: NCT04830215., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: FT is a full-time employee of Otsuka Canada Pharmaceutical Inc.; CW is a full-time employee of Otsuka Pharmaceutical Development & Commercialization Inc.; PC has served as speaker/received consultant fees from AbbVie, Lundbeck, Janssen, Otsuka, Purdue, and Takeda, and has received research/grant support from AbbVie, Lundbeck, Otsuka, Takeda, Janssen, Telus, and Mitacs; JH has served as a speaker/ad board participant or performed scientific planning committee work for Pfizer, Amgen, AbbVie, GSK, Bayer, Valeo, Boehringer, Eli-Lilly, Elvium, Takeda, Bausch, AstraZeneca, Novartis, Lundbeck, Novo Nordisk, Janssen, Eisai, HLS, Otsuka, Idorsia, MDBriefcase, Liv, MedPlan, Master Clinician Alliance, Academy, Bridge, PeerVoice, Seacourses, Thrombosis Canada, Meducom, CHRC, CTC, STA, CCRN, CPD Network, Telus Health, EOCI, AgenceUnik, Humber Hospital, ABPHE, and CSEM; ZI has received grant support from CIHR, NIH, Brain Canada, and Weston Foundation, and has served as a consultant for Eisai, Lundbeck, Novo Nordisk, Otsuka, and Roche. Additionally, his institution has received funds on his behalf from Biogen and Roche; RSM has received research grant support from CIHR/GACD/National Natural Science Foundation of China (NSFC) and the Milken Institute; speaker/consultation fees from Lundbeck, Janssen, Alkermes, Neumora Therapeutics, Boehringer Ingelheim, Sage, Biogen, Mitsubishi Tanabe, Purdue, Pfizer, Otsuka, Takeda, Neurocrine, Neurawell, Sunovion, Bausch Health, Axsome, Novo Nordisk, Kris, Sanofi, Eisai, Intra-Cellular, NewBridge Pharmaceuticals, Viatris, Abbvie, and Atai Life Sciences. He is a CEO of Braxia Scientific Corp; EMM was a full-time employee of Lundbeck Canada Inc. at the time of this work.