Study was terminated for business reasons.

To evaluate the efficacy of ACP-044 compared with placebo in the treatment of pain associated with osteoarthritis of the knee

Inclusion Criteria:

• Male or female subjects ≥18 and <65 years of age at the time of Screening
• Has a body mass index (BMI) ≤39 kg/m2 at Screening
• Confirmed history of pain associated with OA in the index knee
• Willing to maintain current activity and exercise levels throughout the study
• Willing and able to comply with clinic visits and study-related procedures
• Consent to allow all radiographs and medical/surgical/hospitalization records of care received elsewhere to be shared with the Investigator and third parties who will examine the images (i.e., central x-ray reader)

Exclusion Criteria:

• Pain anywhere else in the body which is greater than or equal to OA pain in the index knee, or likely to interfere with subject's assessment of pain throughout the study, as judged by the Investigator
• History or presence on imaging of knee arthropathy (osteonecrosis, subchondral insufficiency fracture, rapidly progressive OA type 1 or type 2), recent fall, injury, or trauma affecting the index knee, ligament tear, neuropathic joint arthropathy, knee dislocation (patella dislocation is eligible), extensive subchondral cysts, Baker's cyst, evidence of bone fragmentation or collapse, or primary metastatic tumor with the exception of chondromas, or pathological fractures during the Screening Period
• History or presence at Screening of non-OA inflammatory joint disease (e.g., rheumatoid arthritis, lupus erythematosus, psoriatic arthritis, pseudo-gout, gout, spondyloarthropathy, joint infections within the past 5 years, Paget's disease of the spine, pelvis, or femur, neuropathic disorders, multiple sclerosis, fibromyalgia, tumors or infections of the spinal cord, or renal osteodystrophy) or any condition that would interfere with the rating of OA pain
• Recent arthroscopic surgery within 1 month of Screening; or has any planned surgery or procedure during the study
• Use of monoamine reuptake inhibitors, tricyclic antidepressants, anticonvulsants, and/or serotonin norepinephrine reuptake inhibitors within 4 weeks prior to Screening
• Unwilling to discontinue current use of analgesic medication following Screening and to adhere to study requirements for rescue treatments (study-provided acetaminophen to be taken as needed with a maximum daily dose of 2500 mg), including, but not limited to, nonsteroidal anti-inflammatory drugs (NSAIDs), opioids, selective cyclooxygenase 2 inhibitors, acetaminophen, or combinations thereof, within 7 days or five half-lives of the drug prior to the Baseline Pain Assessment Period, whichever is longer
• Use of immediate- or extended-release or controlled-release opioids (e.g., oxycontin), transdermal fentanyl, or methadone within 3 months prior to Screening
• Use of opioids, for the treatment of pain other than OA of the knee, with a morphine equivalent dose of ≥30 mg per day for more than 2 days per week within 1 month prior to Screening
• Use of systemic (i.e., oral) corticosteroids or intra-articular corticosteroids in any joint within 30 days prior to the screening visit (topical, intranasal, and inhaled corticosteroids are permitted)
• Intra-articular injection of any approved (i.e., hyaluronic acid and corticosteroids) or unapproved treatments (e.g., platelet-rich plasma, capsaicin) into the index knee within 3 months of Screening
• Physical/occupational/chiropractic therapy for the lower extremities or acupuncture for the lower extremities within 30 days of Screening, or the need for such therapy during the study
• Has current evidence, or history within the previous 12 weeks prior to Screening, of a serious and/or unstable psychiatric, neurologic, cardiovascular, respiratory, gastrointestinal, renal, hepatic, hematologic, endocrinologic, or other medical disorder, that in the judgment of the Investigator and/or Medical Monitor would jeopardize the safe participation of the subject in the study
• Had a malignancy in the last year, with the exception of nonmetastatic basal cell of the skin or localized carcinoma in situ of the cervix

Additional inclusion/exclusion criteria apply. Patients will be evaluated at screening to ensure that all criteria for study participation are met. Patients may be excluded from the study based on these assessments (and specifically, if it is determined that their baseline health and psychiatric condition do not meet all pre-specified entry criteria).

Investigator may publish the study results, relative to their own patients, only after review, comment and approval by the sponsor. No publication of confidential information shall be made without the sponsor's prior written consent. At least 60 days prior to submitting a manuscript or prior to any public presentation, a copy of the manuscript or presentation will be provided to the Sponsor for review and comment. The sponsor has 60 days to review and comment.