BackgroundThe objectives of this study were to assess the maintenance of effect of duloxetine 60mg once-daily (QD) in Chinese patients with chronic pain due to osteoarthritis (OA) of the knee or hip and to provide additional long-term safety data.MethodsThis was anopen-label, extension phase of a randomized, double-blind, placebo-controlled clinical trial. Eligible patients were outpatients who met the American College of Rheumatology clinical and radiographic criteria for OA with a rating4 on Brief Pain Inventory (BPI) 24-h average pain. After completing the 13-week placebo-controlled phase, patients originally assigned to placebo were titrated to duloxetine 60mg QD (PLA_DLX), whereas patients originally assigned to duloxetine 60mg QD remained on the same dose of duloxetine (DLX_DLX) for another 13weeks. The maintenance effect of duloxetine 60mg QD during the extension phase was evaluated by a 1-sided 97.5% confidence interval (CI) of the baseline-to-endpoint change in the extension phase for patients who took duloxetine and reported 30% reduction in BPI average pain at the end of placebo-controlled phase (placebo-controlled phase duloxetine responders). Other BPI severity and interference items, as well as safety and tolerability, were assessed.ResultsOf 342 patients entering the extension phase, 162 (97.6%) DLX_DLX-treated patients and 157 (89.2%) PLA_DLX-treated patients completed this phase. Most patients (76.0%) were female. Mean age was 60.6years. Mean BPI average pain was 5.5 at baseline of the placebo-controlled phase. Among 113 placebo-controlled phase duloxetine responders, mean change in BPI average pain during the extension phase was -0.59 (from 2.47 to 1.88); the upper bound of the 1-sided 97.5% CI was -0.31 and less than the pre-specified non-inferiority margin of a 1.5-point increase (p<0.001). Significant within-group improvements in all BPI items were observed for both PLA_DLX and DLX_DLX groups during the extension phase (all p<0.01). No deaths or suicide-related events occurred. Seven (4.0%) PLA_DLX-treated patients and no DLX_DLX-treated patients discontinued due to an adverse event.ConclusionThe analgesic effect of duloxetine 60mg QD among treatment responders was maintained for the entire duration of the extension phase. Duloxetine 60mg QD was well tolerated during the extension phase.Trial registrationClinicalTrials.gov identification number NCT01931475. Registered 29 August 2013.