The purpose of this study is to evaluate whether CG5503 prolonged-release (PR) tablets at doses of 100-250 mg twice daily provide a better pain relief in patients with moderate to severe chronic pain due to osteoarthritis of the knee than a placebo (a medication without active substance). In addition the tolerability of CG5503 PR will be assessed. One third of the patients will receive CG5503 and one third will receive placebo. For further comparison one third of the patients will receive oxycodone controlled release (CR) at doses of 20-50 mg twice daily which is an active approved pain medication.

This is a randomized (study medication assigned to patients by chance), double-blind (neither patient nor investigator knows which patient gets which study medication, i.e. CG5503, placebo, oxycodone), placebo and active control study. The primary objective is to evaluate the efficacy and safety of orally administered CG5503 prolonged-release (PR) at doses of 100-250 mg (base) twice daily in patients with moderate to severe chronic pain from osteoarthritis (OA) of the knee. The study is being conducted for registration and approval of CG5503 in Europe and outside Europe. The study will consist of five periods: screening (to assess eligibility), washout (3-7 days with determination of a baseline pain intensity), titration (of dose over 3 weeks to the optimal individual level), maintenance (investigational drug intake for 12 weeks with adjustments allowed), and follow-up (2 weeks after end of treatment). The study hypothesis is that the study drug will be more effective than placebo in reducing patients' pain intensity. The secondary objectives include the collection of pharmacokinetic (related to how the body absorbs, distributes, changes and excretes the drug) information for dose verification. The efficacy objectives will be assessed by comparing the baseline pain level to the pain level during the maintainence period. This will be done by looking at the patients' pain diary information (electronic diaries).

• Pain
• Knee Osteoarthritis

• Drug: CG5503, centrally acting analgesic
• Drug: Placebo
• Drug: CG5503
• Drug: Oxycodone

Inclusion Criteria:

• Patients diagnosed with osteoarthritis of the knee based on the American College of Rheumatology (ACR) criteria and functional capacity class of I- III;
• patients taking analgesic medications for at least 3 months prior to screening and dissatisfied with their current therapy;
• Patients requiring opioid treatment must be taking daily doses of opioid- based analgesic, equivalent to <160 mg of oral morphine;
• baseline score of >=5 on an 11-point numeric rating scale, calculated as the average pain intensity during the last 3 days prior to randomization.

Exclusion Criteria:

• History of alcohol and/or drug abuse in Investigator's judgment;
• chronic hepatitis B or C, or HIV, presence of active hepatitis B or C within the past 3 months;
• life-long history of seizure disorder or epilepsy;
• history of malignancy within past 2 years, with exception of basal cell carcinoma that has been successfully treated;
• uncontrolled hypertension;
• patients with severely impaired renal function;
• patients with moderate to severely impaired hepatic function or with laboratory values reflecting inadequate hepatic function, treatment with neuroleptics, monoamine oxidase inhibitors, serotonin norepinephrine reuptake inhibitor (SNRI), tricyclic antidepressants, anticonvulsants, or anti-parkinsonian drugs, treatment with any other analgesic therapy than investigational medication or rescue medication during the trial