Open-Label Extension to Evaluate the Long-Term Safety, Tolerability and Analgesic Efficacy

Inclusion Criteria: * The subject enrolled in this study must have been previously randomized and dosed in the double-blind treatment period of study EN3202-016 or EN3202-019 and completed the exit visits for these studies. * Males or females, age 18 and 75 years, inclusively. * The subject continues to have chronic moderate to severe back or cancer pain that requires opioid medication. Women must continue to be of non-childbearing potential. Exclusion Criteria: * The subject experienced any serious drug related adverse events in studies EN3202-016 or EN3202-019. * The subject withdrew from EN3202-016 or EN3202-019 for a reason other than lack of efficacy. * Subjects with known allergies to opiate-class narcotic agents (morphine, codeine, hydrocodone, propoxyphene, meperidine, oxycodone, etc.) or naproxen, or subjects who have any medical condition in which opiates are medically contraindicated.