A Study to Investigate the Efficacy, Safety and Tolerability of Four Different Doses of BI 409306 Compared to Placebo Given for 12 Weeks in Patients With Schizophrenia on Stable Antipsychotic Treatment.

Inclusion criteria:

1. Patients with established diagnoses of schizophrenia (per Diagnostic and Statistical
Manual of Mental Disorders, Fifth Edition (DSM-5)) with the following clinical
features:

a) Clinically stable and are in the residual (non-acute) phase of their illness for at
least 8 weeks b) Current antipsychotic and concomitant psychotropic medications must
meet the criteria below: b)-1 Maintained on current atypical (second generation)
antipsychotic medications (in any approved dosage form) other than Clozapine and on
current dose for at least 8 weeks prior to randomisation, and/or b)-2 Maintained on
current typical (first generation) antipsychotic medications and on current dose for
at least 6 months, optionally combined with anticholinergics if treated with a stable
dose for at least 6 months prior to randomisation, and/or b)-3 Maintained on current
concomitant psychotropic medications other than anticholinergics, antiepileptics and
lithium, and on current dose for at least 8 weeks prior to randomisation.
Antiepileptics and lithium are allowed if initiated at least 6 months prior to
randomisation.

b)-4 Anticholinergics, antiepileptics and lithium have been washed out for at least 6
months prior to randomisation if the treatments that patients were using before
entering the clinical trial are discontinued.

c) Have no more than a "moderate" severity rating on hallucinations and delusions
(Positive and Negative Syndrome Scale (PANSS)-positive syndrome Hallucinatory Behavior
item score < =4 and Delusions item score < = 4) d) Have no more than a "moderate"
severity rating on positive formal thought disorder (PANSS-positive syndrome
Conceptual Disorganization item score < = 4) e) Have a minimal level of extrapyramidal
symptoms (Simpson-Angus Scale total score < 6) and depressive symptoms (PANSS-general
psychopathology syndrome Depression item score < = 4)

2. Male or female patients age 18 to 55 years

3. Patients must exhibit reliability, physiologic capability, and an educational level
sufficient to comply with all protocol procedures,in the investigator's opinion.

4. Signed and dated written informed consent by date of Visit 1 in accordance with GCP
and the local legislation. If the patient needs a legal representative, then this
legal representative must give written informed consent as well.

5. Patients must have an identified informant who will be consistent throughout the
study. The informant must interact with the subject at least 2 times a week.

Note: Informant ratings are needed for SCoRS global ratings at Randomisation Visit (Visit
2) and (early) End of Treatment Visit. In person informant ratings on the study visits are
preferred whenever possible. However, if the informant is not available for in person
ratings, telephone interview is acceptable. The informant must be available for a telephone
interview at Visit 2 and (e)EOT Visit.

Exclusion criteria:

1. Patient treated with more than two antipsychotic medications (including more than two
dosage forms)

2. Patient's cognitive impairment severity compromises the validity of the cognitive
outcome measures, in the clinical judgment of the investigator

3. Any suicidal behavior in the past 2 years (i.e. actual attempt, interrupted attempt,
aborted attempt, or preparatory acts or behavior)

4. Any suicidal ideation of type 4 or 5 in the Columbia Suicidal Severity Rating Scale
(C-SSRS) in the past 3 months (i.e. active suicidal thought with intent but without
specific plan, or active suicidal thought with plan and intent)

5. In the judgment of the investigator, any clinically significant finding of the medical
examination (including BP, PR and ECG) or laboratory value deviating from normal or
any evidence of a clinically significant concomitant disease or any other clinical
condition that would jeopardize a patient's safety while participating in the clinical
trial

6. History or diagnosis of symptomatic and unstable/uncontrolled gastrointestinal,
hepatic, renal, respiratory, cardiovascular, metabolic, immunological, hematological
or hormonal disorders

7. For female patients:

Pre-menopausal women (last menstruation < =1 year prior to informed consent) who:

- are nursing or pregnant or

- are of child-bearing potential and are not practicing an acceptable method of
birth control, or do not plan to continue using this method throughout the trial
until 28 days after the last treatment administration, and do not agree to submit
to periodic pregnancy testing during participation in the trial. Acceptable
methods of birth control include tubal ligation, vasectomized partner,
transdermal patch, intra uterine devices/systems (IUDs/IUSs), combined
estrogen-progestin oral contraceptives as well as implantable or injectable
hormonal contraceptives. Complete sexual abstinence (if acceptable by local
health authorities) is allowed when this is in line with the preferred and usual
lifestyle of the subject. Periodic abstinence (e.g., calendar, ovulation,
symptom-thermal, post-ovulation methods) and withdrawal are not acceptable
methods of contraception. Double barrier methods are permissible (if acceptable
by local health authorities, note that this is not an acceptable method in EU
countries).

For male patients:

Men who are able to father a child, unwilling to be abstinent or use adequate
contraception for the duration of study participation and for at least 28 days after
treatment has ended.

8. Known history of HIV infection

9. Diseases of the central nervous system (including but not limited to any kind of
seizures, stroke or any psychiatric disorders other than schizophrenia)

10. Any subject who on the Mini-international neuropsychiatric Interview (M.I.N.I.) has a
categorical diagnosis of another current major psychiatric disorder.

11. History of malignancy within the last 5 years, except for basal cell carcinoma

12. Planned elective surgery requiring general anaesthesia, or hospitalisation for more
than 1 day during the study period

13. Significant history of drug dependence or abuse (including alcohol, as defined in
DSM-5-substance use disorder or in the opinion of the investigator) within the last
two years prior to informed consent, or a positive urine drug screen for cocaine,
opioid, PCP, amphetamine, heroin, or marijuana at screening

14. Patient needs to take long-acting hypnotics and anxiolytics (i.e. Diazepam)

15. Patients taking medications that are known to be strong or moderate CYP3A4 inhibitors

16. Participation in another trial with an investigational drug or procedure within 30
days or 6 half-lives (whichever is longer) or participation in another trial with any
cognitive-enhancing therapy or procedure within 90 days prior to screening

17. Previous participation in any BI 409306 study

18. Not fluent in the language of the batteries/questionnaires which will be used in the
country