The purpose of this study is to evaluate the effectiveness (level of pain control) and safety of Tapentadol (CG5503) extended release (ER) (base) compared to placebo in patients with moderate to severe pain from diabetic peripheral neuropathy.

The primary objective of this randomized-withdrawal (randomized means study medication assigned to patients by chance and withdrawal means to stop using), multicenter, double-blind (neither patient nor investigator knows the study medication), placebo-controlled, Phase 3 study is to determine the effectiveness and safety of orally administrated Tapentadol (CG5503) extended release (ER) (base) at doses of 100-250 mg twice daily in patients with moderate to severe pain from diabetic peripheral neuropathy. The study is being conducted for registration and approval of CG5503 in the US and outside US. The trial will consist of two phases: Phase I is open-label and Phase 2 is double- blind. In the Open-Label Phase 1 there will be four periods: screening (to assess eligibility), washout (dependent on the pain medication the patient was previously taking), pain intensity perctoris evaluation (over a 3 day period), and open-label titration (study drug will be titrated to an optimal dose starting with 50 mg twice a day and adjusted to an optimal dose for a period of 3 weeks). Patients with at least a 1-point reduction in the average pain intensity score at the end of the open-label titration period, as compared with pre-titration will be eligible for randomization in the double-blind phase. In the double-blind Phase 2, there will be two periods: double-blind maintenance period (take drug for 12 weeks), and follow-up (a visit within 4 days of the intake of the last dose of study drug and a follow-up call approximately 10-14 days after the intake of the last dose of the study drug). The patient will maintain the dose level achieved at the end of the Phase 1 during the double-blind treatment phase. The study hypothesis is that the study drug will be different from placebo in the change in pain intensity. Titrate Tapentadol (CG5503) extended release (ER) 50 mg to patient's optimal dose ranging between 100mg and 250mg twice a day; Placebo (no active ingredients). All doses of trial treatment will be taken orally with or without food, for a maximum timeframe of 15 weeks.

Inclusion Criteria:

• Patients with Type 1 or Type 2 diabetes mellitus must have a documented clinical diagnosis of painful diabetic peripheral neuropathy with symptoms and signs for at least 6 months, and pain present at the time of screening
• The investigator considers the patient's blood glucose to be controlled by diet, or hypoglycemics, or insulin for at least 3 months prior to enrolling in the study (this control should be documented by figures of glycated hemoglobin [HbA1c] no greater than 11% at screening)
• Patients have been taking analgesic medications for the condition for at least 3 months prior to screening (patients taking opioid analgesics must be dissatisfied with current treatment, and patients taking non-opioid analgesics must be dissatisfied with current analgesia)
• Patients currently requiring opioid treatment must be taking daily doses of an opioid-based analgesic equivalent to <=160 mg of oral morphine

Exclusion Criteria:

• No significant pulmonary, gastrointestinal, endocrine, metabolic (except diabetes mellitus), neurological, psychiatric disorders (resulting in disorientation, memory impairment or inability to report accurately as in schizophrenia, Alzheimer's disease), or any other clinically significant disease that in the Investigator's opinion may affect efficacy or safety assessments or may compromise patient's safety during trial participation
• no history of moderate to severe hepatic impairment such as chronic hepatitis B or C, presence of active hepatitis B or C within the last 3 months or impaired hepatic function with ALT or AST greater than 3-fold ULN
• No patients with severely impaired renal function
• No laboratory values above or below limits of normal unless considered not clinically relevant by the Investigator
• No significant cardiac disease (e.g., unstable angina pectoris, angina pectoris Canadian Cardiovascular Society (CCS) class III-IV, acute myocardial infarction within the last 3 months, cardiac insufficiency New York Heart Association (NYHA) class III-IV) or significant vascular disease (e.g., peripheral arterial occlusive disease (PAOD) Fontaine class IIb-IV)
• no life-long history of seizure disorders or epilepsy

All adverse events were reported from the time a signed and dated informed consent was obtained throughout the follow-up phase of the study. Serious adverse events were collected for 30 days after the last dose of study drug.