Initially, observational studies suggested a possible effect of Lacosamide on depressive and anxious symptoms in individuals with epilepsy, and later, an open-label study demonstrated the efficacy of lacosamide in improving depressive and manic symptoms in individuals with bipolar disorder (BD). The primary objective of this study is to evaluate the efficacy of combining lacosamide as an augmentation treatment to first- or second-line medication treatments in moderate to severe major depressive episodes of treatment-resistant BD I and II (failure of at least two adequate treatments during the current episode). The main hypothesis of the study is that lacosamide produces a greater reduction in depression scores compared to a placebo treatment and that both groups will exhibit similar rates of side effects and adverse events. We will conduct a double-blind, randomized, parallel-group pilot study, comparing the enhancement of the treatment that patients had been using with lacosamide and placebo, over a duration of 12 weeks. Forty subjects aged between 18 and 65 years with a diagnosis of BD (I or II) in a moderate or severe major depressive episode, despite the use of first- or second-line treatments, will be selected. The primary outcome will be the assessment of lacosamide efficacy through the difference in scores on the Hamilton Depression Rating Scale (HAMD-17) from the initial visit to the end of week 12 of intervention between the lacosamide and placebo groups.

Inclusion Criteria:

1. Diagnosis of bipolar disorder (type I or II) confirmed by structured clinical interview;
2. acute major depressive episode of moderate or severe intensity;
3. no response to at least two adequate pharmacological interventions to treat the current episode.

Exclusion Criteria:

1. Current diagnosis of schizophrenia, dementia, intellectual disability, organic mental disorder (by clinical assessment). Comorbidity with other psychiatric disorders (personality disorders, anxiety disorders, substance use disorders, eating disorders, and attention deficit disorder) will be permitted provided the primary diagnosis is bipolar disorder;
2. acute suicidal ideation (defined by HAMD-17 item 3 ≥ 3 points or by clinical assessment);
3. current depressive episode with psychotic features (by clinical assessment);
4. suspected or confirmed pregnancy;
5. severe or unstable clinical illnesses;
6. previous history of non-response to an adequate course of at least 8 sessions of electroconvulsive therapy;
7. previous history of non-response to an adequate course of ketamine treatment.