NKTR-181 is being developed as an analgesic compound for the treatment of moderate to severe chronic pain - active as a mu agonist, but with inherent molecular properties designed to provide a unique clinical profile, including most notably, reduced CNS side effects and an attenuated attractiveness as a target of abuse.

Inclusion Criteria:

• Willing and able to give written informed consent;
• Willing and able to understand the study procedures, and comply with all study procedures;
• Females or males, age ≥ 18 years old;
• Body mass index 18-39, inclusive;
• In good general health;
• Clinical diagnosis of OA in one or both knees;
• Have been on a stable regimen of pain medication for the management of OA knee pain;
• Not experiencing adequate pain relief with their current dosing regimen;
• Women of childbearing potential (WCBP) must agree to use highly effective methods of birth control. Male subjects must agree to use contraception.

Exclusion Criteria:

• Females who are pregnant or lactating;
• Known history of hypersensitivity, intolerance, or allergy to opioids;
• Diagnosed as having any chronic pain symptom that in the Investigator's opinion would interfere with the assessment of pain and other symptoms of OA;
• Presence of any medical condition that would preclude study participation in the opinion of the investigator;
• Clinically significant abnormalities of vital signs or clinical laboratory results;
• Clinically significant electrocardiographic abnormalities;
• Received systemic corticosteroids within 30 days prior to signing the consent form;
• Subjects who are known or suspected to be currently abusing alcohol or drugs;
• Positive urine drug screen, or alcohol breath test during Screening Period testing;
• Positive serology for the surface antigen of Hepatitis B (HBsAg) or Hepatitis C (anti-HCV) during Screening Period testing;
• Known to be human immunodeficiency virus (HIV) positive;
• Donation of blood or plasma within 30 days prior to signing the consent form;
• Participation in another drug or biologic study within 30 days prior to signing the consent form;
• Any other reason that, in the opinion of the Investigator or Medical Monitor, would render the subject unsuitable for participation in the study.