• The primary efficacy outcome for this study is the SPID24 (i.e., the sum of pain intensity difference at 24 hours relative to the first dose [ Time Frame: 24 hours ] [ Designated as safety issue: No ]
  

• Secondary effectiveness outcomes include, among others, the effect of CG5503 IR on the time to the need for the first rescue pain medication during the double-blind treatment period, and the SPID at 12, 48 and 72 hours relative to first dose. [ Time Frame: 48 hours ] [ Designated as safety issue: No ]
  

Subjects undergoing abdominal hysterectomy often experience moderate to severe acute pain post-surgery. Normally such pain is controlled when subjects receive repeated doses of opioid analgesics. However, opioid therapy is commonly associated with side effects such as nausea, vomiting, sedation, constipation, addiction, tolerance, and respiratory depression. CG5503, a newly synthesized drug with an immediate release (IR) formulation, also acts as a centrally acting pain reliever but has a dual mode of action. The aim of this study is to investigate the effectiveness (level of pain control) and safety (side effects) of 3 dose levels of CG5503 IR compared with no drug (placebo) or one dose of morphine (an opioid commonly used to treat post-surgical pain). This study is a randomized, double-blind (neither investigator nor patient will know which treatment was received), active- and placebo-controlled, parallel-group, multicenter study to evaluate the treatment of acute pain after abdominal hysterectomy. The study will include a blinded 72 hour inpatient phase immediately following hysterectomy, during which subjects will be treated with either 50-, 75-, or 100-mg CG5503 IR, a placebo, or 20-mg morphine, and pain relief will be periodically assessed. Assessments of pain relief include the pain intensity numeric rating scale (PI), pain relief numeric rating scale (PAR), and patient global impression of change scale (PGIC). Safety evaluations include monitoring of adverse events, physical examinations, and clinical laboratory tests. Venous blood samples will be collected for the determination of serum concentrations of CG5503 and morphine. The alternative study hypothesis is that at least 1 dose strength of CG5503 will be different from placebo in controlling pain at 24 hours.