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 without congenital knee pathology * 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 laboratory tests, and 12-lead electrocardiogram (ECG) findings * Body Mass Index of 18-40 kg/m2 * Stable medical regimen for at least 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 (e.g., rheumatoid arthritis, lupus, ankylosing spondylitis, psoriatic arthritis), with the exception of clinically stable/non-active gout that does not affect the knee and does not interfere with walking * Operative arthroscopy in the surgical knee in the last 4 months (except for meniscal repair which will be no shorter than 6 months before randomization) or in the contralateral knee in the last 2 months (or 3 months for meniscal repair), 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 anesthesia using potent inhalational agents, peripheral nerve block (i.e., femoral nerve block), neuroaxial (intrathecal or epidural) opioids postoperatively, or knee capsule injections * Received aspirin or any nonsteroidal anti-inflammatory drug (NSAID) prior to randomization within the washout period required by the surgical team for surgery with spinal anesthetic, 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 (e.g., gabapentin, pregabalin) within 2 weeks 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-operatively through Day 28 * Treatment with immunosuppressives, antipsychotics, anticholinergics, or anticonvulsants within 1 month of randomization, with the exception of aspirin for cardiac prophylaxis (as long as discontinued within 1 week of randomization) * Current neurologic disorder, which could confound the assessment of pain (e.g., 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 * Severe pulmonary disease; if symptomatic sleep apnea is currently treated with continuous positive airway pressure (CPAP) or bi-level positive airway pressure (BIPAP), use must be continued postoperatively at least during the inpatient period. Patients with sleep apnea associated with a history of postoperative delirium are excluded. * Current evidence of alcohol abuse or history of alcohol-related complications within 1 year of randomization including, but not limited to, alcoholic withdrawal seizures, hallucinations, delirium tremens or detoxification treatment * 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 * Previous participation in any study involving AYX1 Injection