Previous studies found that some NMDA-enhancing agents were able to improve clinical symptoms of patients with schizophrenia. In addition, several drugs with antioxidant properties have been tested in clinical trials for the treatment of schizophrenia too. Whether combined treatment of an NMDA-enhancing agent and a drug with antioxidant property can be better than an NMDA-enhancing agent alone deserves study.

Several lines of evidence suggest that both NMDA and oxidative stress hypotheses have been implicated in schizophrenia. Previous studies found that some NMDA-enhancing agents were able to augment efficacy of antipsychotics in the treatment of schizophrenia. In addition, several drugs with antioxidant properties have been tested in clinical trials for the treatment of schizophrenia too. Whether a drug with antioxidant property can strengthen the efficacy of an NMDA-enhancer (NMDAE) in the treatment of schizophrenia remains unknown. Therefore, this study aims to compare NMDAE plus a drug with antioxidant property and NMDAE plus placebo in the treatment of schizophrenia. The subjects are the schizophrenia patients who remain symptomatic while having been stabilized with antipsychotic treatment. They keep their original treatment and are randomly, double-blindly assigned into two treatment groups for 12 weeks: (1) NMDAE plus Antioxidant agent (AO), or (2) NMDAE plus placebo. Clinical performances and side effects are measured at weeks 0, 2, 4, 6, 9, and 12. Cognitive functions are assessed at baseline and at endpoint of treatment by a battery of tests. The efficacies of NMDAE plus AO and NMDAE plus placebo will be compared.

Chi-square (or Fisher's exact test) will be used to compare differences of categorical variables and t-test (or Mann-Whitney test if the distribution is not normal) for continuous variables between treatment groups. Mean changes from baseline in repeated-measure assessments will be assessed using the generalized estimating equation (GEE). All p values for clinical measures will be based on two-tailed tests with a significance level of 0.05.

Inclusion Criteria:

• Have a DSM-5 (American Psychiatric Association) diagnosis of schizophrenia
• Remain symptomatic but without clinically significant fluctuation, while their antipsychotic doses are unchanged for at least 3 months and will be maintained during the period of the 12-week trial
• PANSS total score ≥ 60
• Agree to participate in the study and provide informed consent

Exclusion Criteria:

• DSM-5 diagnosis of intellectual disability or substance (including alcohol) use disorder
• History of epilepsy, head trauma, stroke, or serious medical or central nervous system diseases (other than schizophrenia) which may interfere with the study
• Clinically significant laboratory screening tests (including blood routine, biochemical tests)
• Pregnancy or lactation
• Inability to follow protocol