This is a Phase 2, randomized, double-blind, placebo-controlled study to evaluate the efficacy, safety, Pharmacokinetic (PK) and Pharmacodynamic (PD) effects of 3 different dose regiments of KYN-5356 and placebo for 28 days.

Participants will be randomized to one of 4 treatment groups: placebo, KYN-5356 low dose, KYN-5356 medium dose, KYN-5356 high dose.

Participants will be admitted to the clinic on Day -3 and will remain in residence at the clinic for 32 days, from baseline through the treatment period.

Participants will be administered investigational medicinal product on Days 1 through 28. Efficacy, safety, PK and exploratory PD assessments will be performed throughout the dosing period. Participants will be discharged on Day 29 after safety assessments are completed and return for a follow-up visit on Day 42.

A subset of participants from selected sites will undergo electrophysiological assessments to evaluate the effect of KYN-5356 on neurophysiological measures of brain function.

Inclusion Criteria:

• Has an established primary psychiatric diagnosis of schizophrenia
• Clinically stable and in the residual (nonacute) phase of their illness for at least 8 weeks before screening in the judgement of the investigator
• Diagnosis of schizophrenia for at least 1 year before screening
• Must be in ongoing maintenance antipsychotic monotherapy on a stable medication treatment regimen for ≥ 2 months before screening, including concomitant psychotropic medications.
• Male or female, aged ≥18 and ≤55 years
• Has a body mass index (BMI) between 18 and 40 kg/m2 Inclusive
• Must exhibit capability to comply with all protocol procedures in the judgement of the investigator
• Signed and dated written informed consent before screening in accordance with Good Clinical Practice.

Exclusion Criteria:

• Currently being treated with more than 1 antipsychotic at the time of screening,
• A score of 2 or higher on any of the individual items of the Modified Simpson-Angus Scale (mSAS) at screening
• Moderate to severe substance use disorder (other than nicotine or caffeine, but including alcohol)
• Evidence of unstable medical condition