This study aims at evaluating the effect of vortioxetine on cognitive dysfunction in major depressive disorder (MDD) patients with inadequate response to current antidepressant treatment.

Inclusion Criteria:

• The patient has MDD, diagnosed according to DSM-IV-TR™ recurrent MDE (classification 296.3x) as confirmed using the Mini International Neuropsychiatric Interview (MINI).
• The patient has depressive symptoms currently considered as non- or only partially responsive (inadequate response), to one adequate course of SSRI/SNRI antidepressant monotherapy and is candidate for a switch in the investigator's opinion.
• The patient wants to stop taking his/her current SSRI/SNRI treatment due to inadequate response confirmed by the Antidepressant Treatment Response Questionnaire (ATRQ), <50% response to current treatment).
• The patient must have been treated by SSRI/SNRI monotherapy (citalopram, paroxetine, sertraline, duloxetine, or venlafaxine) for at least 6 weeks at licensed doses prior to the Screening Visit.
• The patient has a PHQ-9 total score ≥14.
• The patient has a MADRS total score ≥ 22.
• The patient has had the current MDE for ≤1 year.
• The patient has a Perceived Deficits Questionnaire - Depression (PDQ-D) total score >25.
• The patient is a man or woman aged ≥18 and ≤65 years.

Exclusion Criteria:

• The patient has a score ≥70 on the DSST (Number of Correct Symbols) at the Baseline Visit.
• The patient has physical, cognitive, or language impairment of such severity as to adversely affect the validity of the data derived from the neuropsychological tests.
• The patient has any current psychiatric disorder or Axis I disorder (according to DSMIV-TR™ criteria) other than MDD, as assessed using MINI.
• The patient has a current or has had a diagnosis of dysthymic disorder within 3 months preceding the onset of current episode (DSM-IV-TR™ criteria).
• The patient has borderline, schizotypal, schizoid, paranoid, histrionic, antisocial personality disorders (axis II) as comorbid or primary diagnosis (DSM-IV-TR™ criteria).
• The patient has history of previous MDEs considered as treatment resistant defined as inadequate response (incomplete or no therapeutic response) to two prior courses of at least 6 weeks of conventional antidepressant drugs in adequate dosages or, the patient has treatment-resistant depression in the investigator's judgement.
• The patient suffers from personality disorders, mental retardation, pervasive development disorder, attention-deficit/hyperactivity disorder, organic mental disorders, or mental disorders due to a general medical condition (DSM-IV-TR™ criteria).
• The patient has a diagnosis of alcohol or other substance abuse or dependence (excluding nicotine or caffeine) (DSM-IV-TR™ criteria) that has not been in sustained full remission at least 2 years prior to the Screening Visit.
• The patient has a current diagnosis or history of manic or hypomanic episode, schizophrenia or any other psychotic disorder, including major depression with psychotic features (DSM-IV-TR™ criteria).

Other protocol defined inclusion and exclusion criteria may apply.