Study to Evaluate Safety/Efficacy of a Single Pre-Op Dose of AYX1 Injection to Treat Pain After Knee Replacement Surgery

Inclusion Criteria: * Scheduled to undergo primary unilateral TKA for painful osteoarthritis * American Society of Anesthesiologists Physical Status Classification System ≤ 3 * Medically stable as determined by the Investigator based on pre-study medical history, physical/neurological examination, clinical lab tests, and 12-lead ECG findings * Body mass index of 18-40 kg/m2 * Stable medical regimen for ≥ 1 month before randomization * Able to read and understand study instructions in English, and willing and able to comply with all study procedures Exclusion Criteria: * More than 2 other current focal areas of pain, none greater in intensity than the target knee and no other active chronic pain conditions that would compromise operative knee pain evaluation * Inflammatory arthridities (i.e., rheumatoid arthritis, lupus, ankylosing spondylitis, psoriatic arthritis, gout) * Operative arthroscopy in the surgical knee in the last 6 months, or other prior surgery in either knee in the last 12 months, except for diagnostic arthroscopy * Planned use of any of the following for TKA: general endotracheal anesthesia (GETA), peripheral nerve block (i.e., femoral nerve block), neuroaxial (intrathecal or epidural) opioids postoperatively, or knee capsule injections * Received aspirin within 1 week of randomization, or any nonsteroidal anti-inflammatory drug (NSAID) within 5 half-lives prior to randomization, or planned use of NSAIDS post-operatively through Day 28 * Use of more than 20 mg per day (on average) of oral morphine or its equivalent within 1 month prior to randomization * Use of adjuvant analgesics for chronic pain control (i.e., gabapentin, pregabalin) during the month prior to randomization or planned use post-operatively through Day 28 * Daily use of benzodiazepines in the 3 months prior to randomization (unless used for sleep and dosage will be consistent after surgery) * Use of systemic corticosteroids (does not include inhaled steroids) within 3 months or intra-articular steroid injections within 1 month prior to randomization, or planned use of either post-op through Day 28 * Treatment with antibiotics or antivirals (EXCEPTION: topical treatments), immunosuppressives, antipsychotics, anticholinergics, or anticonvulsants within 1 month of randomization, with the exception of prescribed pre-surgical prophylactic antibiotics, and aspirin for cardiac prophylaxis (as long as discontinued within 1 week of randomization * Current neurologic disorder, which could confound the assessment of pain (i.e., Parkinson's, Multiple Sclerosis) * Current active depression symptoms * Has had a change in dose or regimen of SSRIs or SNRIs for depression within 1 month of randomization * Mini Mental State Exam score \< 24 at screening * Current history of insulin dependent diabetes mellitus, or autoimmune conditions * Severe chronic obstructive or restrictive pulmonary disease, current diagnosis of obstructive sleep apnea documented by a sleep laboratory study, or current home use of continuous positive airway pressure (CPAP) or bi-level positive airway pressure (BIPAP) * Current evidence of alcohol abuse or history of alcohol-related complications within 1 year of randomization * Known or suspected history of illicit drug abuse within 1 year before randomization, current or planned use of medical marijuana, or history of opioid dependence within 2 years before randomization * Any malignancy within the past year, with the exception of basal cell carcinoma or uncomplicated or stable skin cancers documented to not require further or immediate treatment * Women who are pregnant or nursing