The purpose of the trial is to determine whether GRT6005 is effective in patients with pain due to osteoarthritis of the knee.

The purpose of this study is to evaluate the safety, tolerability and efficacy of 3 doses of GRT6005 taken once daily for 4 weeks in outpatients with moderate-to-severe painful osteoarthritis of the knee.

Inclusion Criteria:

• painful Osteoarthritis of the knee based on American College of Rheumatology Criteria
• signed informed consent
• on stable analgesic medications for painful osteoarthritis with regular analgesic intake for at least 3 months
• pain intensity score of 4 or greater on the 11-point numeric rating scale (where 0 indicates "no pain" and 10 indicates "pain as bad as you can imagine").

Exclusion Criteria:

• Substance Abuse
• Significant cardiac disease
• Presence of risk factors for Torsades de Pointes and chronic severe cardiac disease
• History of seizure disorder
• Chronic gastrointestinal disease
• Conditions that contribute and confound to the assessment of pain
• Surgery or painful procedure during or within 3 months of enrollment
• Cancer
• Subjects with impaired renal function
• Subjects with impaired hepatic function
• Female subjects who are breastfeeding
• History of chronic hepatitis B or C or human immunodeficiency virus infection, or presence of acute hepatitis A, B, or C within the past 3 months.
• Clinically relevant history of hypersensitivity or allergy to paracetamol, opioids or the excipients.