Résumé : Background: New minimally invasive treatments are vital to delay joint replacement surgery in patients with knee osteoarthritis. This study was designed to select the most effective among three formulations of an enhanced protein solution containing clonidine, hyaluronic acid, and human plasma (JTA-004), and compare the safety and efficacy of intra-articular administration of the selected formulation with a reference treatment (hyaluronic acid) in symptomatic knee osteoarthritis patients. Methods: In this two-stage, double-blind, phase II/III study conducted in 12 Belgian centers, 50–79-year-old patients with primary knee osteoarthritis were randomized (1:1:1:1) to receive one dose of one of three JTA-004 formulations (differing in clonidine concentration [50 or 100 μg/ml] and volume [2 or 4 ml]) or the reference treatment (hylan G-F 20). Patients were evaluated using Western Ontario McMaster Universities (WOMAC®) Scores and the Short-Form health survey up to 6 months post-injection (Month 6). Drug consumption and safety were evaluated. Results: Among 164 treated patients, 147 completed the study. The JTA-004 formulation containing 200 μg clonidine and 20 mg hyaluronic acid in 2 ml (JTA-200/2) was selected based on interim results at Month 6. The difference in adjusted mean change in WOMAC Pain Subscale Score from baseline (JTA-200/2 minus reference group) at Month 6 was − 9.49 mm; statistical superiority of JTA-200/2 over the reference was not demonstrated. No statistically significant differences in adjusted mean changes from baseline between JTA-200/2 and reference groups were observed for Pain, Physical Function and Stiffness Subscales WOMAC Scores, Total WOMAC Score, and Well-being Score at any timepoint, although JTA-200/2 induced larger improvements in WOMAC Scores than the reference. Statistically significantly larger improvements in WOMAC Pain Subscale Scores for JTA-004 versus the reference were observed in post-hoc analyses on pooled data from all JTA-004 formulations at Month 6 (p = 0.030) and Month 3 (p = 0.014). All JTA-004 formulations had clinically acceptable safety profiles. Conclusions: This study provided preliminary evidence of the safety of intra-articular injection of JTA-004 in knee osteoarthritis patients. Phase III randomized controlled trials with larger sample sizes are needed to evaluate the efficacy of JTA-004 in knee osteoarthritis. Trial registration: Clinicaltrials.gov/identifier NCT02740231; clinicaltrialsregister.eu/identifier 2015–002117-30. Retrospectively registered 13/4/2016.