The purpose of this study is to determine the efficacy, safety, tolerability and pharmacokinetics (PK) properties of CPL500036 compound (PDE10a inhibitor) in patients with an acute exacerbation of schizophrenia after 28 days of administration..

This is a double-blind, randomized, placebo controlled, parallel group, dose ranging study to explore the efficacy, safety, tolerability and PK of 2 different doses of CPL500036 (phosphodiesterase 10A [PDE10A] inhibitor) in patients with an acute exacerbation of schizophrenia. Approximately 165 patients will be randomized at a 1:1:1 ratio and will be dosed with 20 mg CPL500036, 40 mg CPL500036 or placebo once daily for 28 consecutive days (Day 1 to Day 28). Patients will remain in house for the duration of the Treatment Period. The study will comprise of a Screening Period (that will include a prior Medication Washout Period), a Treatment Period and a Follow-up Period. After discharge from the Clinical Unit, patients will return to the Clinical Unit for 2 once weekly Follow-up Visits. Approximately 30% of the patients (17 patients in each of the 3 treatment groups) will undergo extensive PK sampling during the Treatment Period, and the remaining 70% of the patients will only undergo sparse PK sampling.

Inclusion Criteria:

1. The patient has a primary diagnosis of schizophrenia confirmed by clinical interview [SCID-5-CT].
2. Male or female patient aged 18 to 65, inclusive, at Screening.
3. The patient's with exacerbation of psychotic symptoms
4. The patient has a score of 5 or higher in 3 or more items of the following PANSS items at Screening and Baseline
5. The patient has a PANSS Total Score of 80 or higher during Screening and on Baseline
6. The patient of childbearing potential willing to use acceptable forms of contraception.
7. The patient has a score in CGI-S scale of 4 or greater at Screening and on Baseline
8. The patient is able to and agrees to remain off prior antipsychotic medication and all excluded medications as outlined in the protocol for the duration of the Treatment Period.
9. The patient is able to sign informed consent after receiving information about the trial and has the ability and willingness to comply with the requirements and restrictions of the study protocol.

Exclusion Criteria:

1. The patient has a decrease in the PANSS Total Score at Baseline compared with the Total Score at Screening.
2. Patient who recently participated in another interventional clinical study with an Investigational Medicinal Product.
3. The patient has uncontrolled abnormality which may impact the ability of the patient to participate or potentially confound the study results.
4. The patient has a history of severe head injury, traumatic brain injury, myocardial infarction or stroke.
5. The patient has a moderate or severe substance use disorder for alcohol or other substances of abuse except nicotine or caffeine.
6. The patient is pregnant or lactating or intending to become pregnant or intending to donate ova.
7. The patient has a history of or known personality disorder or other psychiatric disorder that, in the opinion of the Investigator, would interfere with participation in the study.
8. The patient is considered by the Investigator to be at imminent risk of suicide or injury to self or others.
9. The patient has chronic movement disorder that may interfere with the interpretation of study results.
10. The patient has any existing or previous history of cancer or has newly diagnosed diabetes.
11. The patient has long QT syndrome or is under treatment with antiarrhythmic drugs.
12. The patient is considered to be treatment resistant. .
13. The patient has received electroconvulsive therapy.
14. The patient has any laboratory values outside the normal range that are considered by the Investigator to be clinically significant at Screening.