Background: Duloxetine provides an analgesic effect of patients with OA. The mode of action of duloxetine is partly believed to act through modulating the descending inhibitory pain pathways from the brainstem towards the spinal cord thereby dampening pain by gating the afferent pain signals from the periphery during their passage to the brain. This study aims to investigate if the analgesic effect of duloxetine is due to modulation of pain mechanisms. Study Rationale: The present study will utilize a set of quantitative pain biomarkers developed to assess peripheral and central manifestations in OA and the influence of duloxetine on those manifestations. Treatment: Patients will be randomized to one of two treatment sequences: 1. Sequence 1: 20 mg duloxetine QD for 1 week, 40 mg Duloxetine QD for 1 week, 60 mg duloxetine QD for 10 weeks, 40 mg duloxetine QD for 1 week, 20 mg duloxetine QD for 1 week, followed by 14 weeks of corresponding placebo 2. Sequence 2: 14 weeks of placebo followed by 20 mg Duloxetine QD for 1 week, 40 mg duloxetine QD for 1 week, 60 mg duloxetine QD for 10 weeks, 40 mg duloxetine QD for 1 week and 20 mg duloxetine QD for 1 week. The two treatment periods of 14 weeks each are separated by a washout period of two weeks and include a two-week titration period. Primary Objective: To assess the effect of 60 mg daily maintenance dose administration of Duloxetine for 10 weeks compared with placebo on pain mechanisms. Sample Size Justification/Statistics: The sample size was calculated to 32 patients providing a power of 85% with a significant level of 0.05 to detect a group difference of 1 point in the change from baseline of the week 12 mean of 24-hour worst pain between duloxetine and placebo treatment. Patient Selection: Up to 40 patients with osteoarthritic knee pain will be enrolled in this study in order to complete 32 patients. Study sites: Mech-Sense Aalborg University Hospital, DK-9000 Aalborg, Denmark Study Assessments: As the primary objective of this study is the assessment of which pain mechanisms are modulated by administration of the study drug, the primary endpoints will be Experimental Mechanism Based Pain Measures (EPMs) including 1) Pressure Pain Thresholds (PPTs), Temporal Summation, Conditioned Pain Modulation (CPM) and Offset Analgesia. In addition, efficacy will be evaluated using 1) pain severity (worst daily pain and night pain), 2) Pittsburgh Sleep Quality Index (PSQI), 3) Brief Pain Inventory (BPI), 4) Investigator and Patient Global Assessment of Changes (IGIC and PGAC), 5) Western Ontario and MacMaster (WOMAC) OA physical function, 6) PainDetect, and 7) Central Sensitization Index (CSI). Safety: Discontinuation rates and Treatment Emergent Adverse Events (TEAEs). Key Inclusion/Exclusion Criteria: Males or females between 40 and 75 years of age, who are postmenopausal or using allowed contraception methods, and have a Body Mass Index (BMI) between 20-35 kg/m2 inclusive Patient with unilateral or bilateral OA of knee diagnosed according to the American College of Rheumatology (ACR) criteria based on clinical and radiographic evidence with pain severity equal to or higher than 5 on a Visual Analogue Scale (VAS) assessed as the worst pain within the last 24 hours.
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Pressure Pain Threshold.
Timeframe: Difference between baseline (prior to treatment) compared with 10 weeks of 60 mg/daily duloxetine compared with placebo