A Study to Assess the Safety and Efficacy of ASP4345 as Add-on Treatment for Cognitive Impairment in Subjects With Schizophrenia on Stable Doses of Antipsychotic Medication

Inclusion Criteria: * Subject has a diagnosis of schizophrenia or schizoaffective disorder according to the Diagnostic and Statistical Manual of Mental Disorders, 5th edition criteria and confirmed by the Mini-International Neuropsychiatric Interview version 7.02 * Subject has a stable clinical course as suggested by the following: * no psychiatric hospitalization within the last 4 months, * no symptom-related changes in psychotropic medications (as defined in the concomitant medication section) within 4 weeks prior to baseline for oral medications and within 2 months for depot medications, * and core positive symptoms no worse than moderate in severity and no evidence of a current severe major depressive episode (moderately severe depression is allowed) * Subject has a stable living situation * Subject's extrapyramidal symptoms are no worse than mild in severity * Subject must be in ongoing maintenance (i.e., at least 4 weeks prior to day 1 for oral medications and within 2 months for depot medications) on up to 2 antipsychotic therapies (oral or depot) other than clozapine * Subject has a body mass index range of 18.5 to 45.0 kg/m2 * Female subject must either: * Be of nonchildbearing potential: * Postmenopausal (defined as at least 1 year without menses) prior to screening or * Documented as surgically sterile * Or, if of childbearing potential * Agrees not to try to become pregnant during the study and for 28 days after the final study drug administration * And has a negative blood pregnancy test at screening and a negative urine pregnancy test at day 1, * and if heterosexually active, agrees to consistently use 1 form of highly effective birth control starting at screening and throughout the study period and for 28 days after the final study drug administration * Female subjects must agree not to breastfeed starting at screening and throughout the study period, and for 28 days after the final study drug administration * Female subject must not donate ova starting at screening and throughout the study period, and for 28 days after the final study drug administration * A sexually active male subject with female partner(s) who is of childbearing potential is eligible if: * Agrees to use male condom starting at screening and throughout the study period, and for 28 days after the final study drug administration * Male subject must not donate sperm starting at screening and throughout the study period, and for 28 days after the final study drug administration * Male subject with a pregnant or breastfeeding partner(s) must agree to remain abstinent or use a condom for the duration of the pregnancy or time partner is breastfeeding throughout the study period and for 90 days after the final study drug administration * Subject agrees not to participate in another interventional study while participating in the present study, defined as signing the informed consent form until completion of the last study visit * Subject has a negative urine drug screen for drugs of abuse at screening and day 1, excluding cannabis and documented prescribed benzodiazepines Exclusion Criteria: * Subject has a known or suspected hypersensitivity to ASP4345 or any components of the formulation * Subject has had previous exposure with ASP4345 * Subject has a history of suicide attempt or suicidal behavior within 1 year prior to screening or has any suicidal ideation that meets criteria at a level of 4 or 5 by using the Columbia Suicide Severity Rating Scale (C-SSRS) or who is at significant risk to commit suicide * Subject has any clinically significant liver chemistry test result (aspartate aminotransferase \[AST\], alanine aminotransferase \[ALT\], total bilirubin \[TBL\]) or a result \> 1.5 times above the upper limit of normal (ULN) at screening or repeated within 1 week prior to potential randomization (day 1). In such a case, the assessment may be repeated once * Subject has any history or evidence of any clinically significant allergic, cardiovascular, gastrointestinal, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, history of seizure disorder, renal and/or other major disease or malignancy * Subject has any clinically significant abnormality of the physical examination, electrocardiogram (ECG) and clinical laboratory tests at screening or at admission to the study (day 1) * Subject has known kidney disease and a glomerular filtration rate (GFR) \< 60 mL/min per meter squared at screening and subjects will be discontinued from treatment only for decreases in the GFR that are clinically relevant * Subject has a resting systolic blood pressure \> 180 mmHg or \< 90 mmHg, and a resting diastolic blood pressure \> 100 mmHg at screening. These assessments may be repeated once, after a reasonable time period, at the investigator's discretion (but within the screening period) * Subject has a mean corrected QTcF \> 450 msec (for male subjects) and \> 470 msec (for female subjects) at screening or at randomization. If the mean QTcF exceeds the limits above, one additional triplicate ECG can be taken on day 1 * Subject has a history in the 6 months prior to screening of consuming more than 14 units of alcoholic beverages per week for males and more than 7 units of alcoholic beverages per week for females. (Note 1 unit = 12 ounces of beer, 4 ounces of wine, or 1 ounce of spirits) * Subject is currently using prohibited medications and is unable to washout, including over-the-counter products and agrees not to consume grapefruit and/or grapefruit juice * Subject is currently using clozapine for treatment of schizophrenia * Subject has a positive test for hepatitis B surface antigen (HBsAg), hepatitis A virus antibodies (immunoglobulin M) (anti-HAV \[IgM\]) or hepatitis C virus antibodies (anti- HCV) at Screening or has history of a positive test for human immunodeficiency virus type 1(HIV-1) and/or type 2 (HIV-2) * Subject who has had electroconvulsive therapy within the 6 months prior to screening. * Subject has a history of head injury with clinically significant sequelae, including loss of consciousness for 1 hour or greater * Subject has received investigational study drug within 28 days or 5 half-lives, whichever is longer, prior to screening