This study is testing a new drug, KYN-5356, to help adults with memory and thinking problems related to schizophrenia. Schizophrenia is a mental illness that affects how a person thinks, feels, and behaves. The study lasts for about 42 days, where participants will stay at the clinic for 32 days. During this time, they will receive either a placebo (a fake treatment) or different doses of KYN-5356 for 28 days. The study is "double-blind," which means neither the participants nor the doctors know who gets the real drug or the placebo. They will check the drug’s effects on the body and mind. Some participants will also have tests to see how the drug affects the brain.

• Participants must return for a follow-up visit on Day 42 after being discharged on Day 29.
• The study is for adults aged 18 to 55 who have stable schizophrenia and are taking one antipsychotic drug.
• Participants need to be willing to stay in the clinic and follow all study procedures.