This study tests a new medicine, ATX101, for pain relief after knee replacement surgery. The goal is to see if ATX101 works better than a saline placebo (a saltwater solution that doesn't have any active medicine) and bupivacaine (a medicine that numbs the area to relieve pain). The study also looks at how much opioids (strong painkillers) people need after taking ATX101. Osteoarthritis is a condition where the knee joint becomes worn out and sore. The study is for people having their first knee replacement surgery for knee pain due to osteoarthritis. They must be healthy enough (classified as ASA class 1, 2, or 3) and able to attend all study visits. People who are pregnant, have had certain knee surgeries before, or use certain medicines can't join. The trial uses an e-diary to track pain and other information.

• The study compares ATX101 to a placebo and bupivacaine.
• Participants must be scheduled for their first knee replacement.
• Exclusions include certain surgeries and medicine use.