['low back pain', 'chronic pain']
['nktr-181 bid tablets', 'placebo to match nktr-181 bid tablets']

        Inclusion Criteria:

          -  Male or non-pregnant, non-nursing female aged 18 to 75 years old

          -  Clinical diagnosis of moderate to severe, chronic non-neuropathic low back pain for at
             least six months

          -  Not experiencing adequate pain relief or have failed previous treatment with
             non-opioid analgesics

          -  Opioid analgesia is necessary

          -  Currently taking no more than 10 mg morphine sulfate equivalents per day of short
             acting opioids for 14 days prior to entry

          -  Females of child bearing potential must be using a highly effective form of birth
             control. All subjects must agree to use double-barrier contraception during
             participation in this study and for at least 2 months after the last dose of the study
             drug.

          -  Willing and able to provide informed consent

        Exclusion Criteria:

          -  Taking extended release or long-acting opioids within 6 months

          -  History of hypersensitivity, intolerance, or allergy to opioids

          -  Compression of spinal nerve root; spinal fracture, tumor, or abscess

          -  Surgical procedures on the low back in the last 12 months or facet nerve root block or
             radiofrequency ablation in the last 3 months

          -  Untreated moderate to severe sleep apnea
      