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

Inclusion Criteria: * Scheduled to undergo primary unilateral TKA with spinal anesthesia 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 examination, clinical laboratory tests, and 12-lead electrocardiogram (ECG) findings * Body Mass Index of 18-45 kg/m2 * Stable medical regimen for at least 1 week 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, any pain areas greater in intensity than the target knee, or any 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 * Surgery in either knee within 3 months prior to randomization, or surgery in either knee greater than 3 months prior to randomization with residual symptoms that would interfere with post-TKA study assessments; use of cryoneurolysis (including Iovera) on the current operative knee region within the 6 months prior to randomization and/or at any time through the duration of the study * Planned use of general anesthesia or potent inhalational agents, peripheral nerve block (e.g., femoral nerve block), neuroaxial (intrathecal or epidural) opioids, preoperative extended release/long acting opioids, or any use of ketamine preoperatively and/or at any time through the duration of the study * Use of more than 40 mg per day (on average) of oral morphine or its equivalent within 1 month prior to randomization * Use of gabapentin or pregabalin within 1 week prior to randomization or planned use post-operatively through Day 28 * Use of systemic corticosteroids (IV or oral) within 3 months prior to randomization through Day 28; planned use of intra-articular steroid injections in the operative knee from the time of randomization through Day 28 * Current neurologic disorder, which could confound the assessment of pain (e.g., Parkinson's, Multiple Sclerosis) * Untreated or inadequately treated (in the opinion of the Investigator) active depression * Mini Mental State Exam score \< 24 at screening * 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 use within 1 year before randomization, or current or planned use of marijuana (including medical approved use) within 1 month before randomization and/or through the duration of the study * Any confirmed 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