This is a double-blind (patients and study personnel do not know the identity of the administered treatments), randomized (the drug is assigned by chance), placebo-controlled (placebo is a substance that appears identical to the treatment and has no active ingredients), parallel arm study (each group of patients will be treated at the same time). The study will consist of a screening phase of up to 4 weeks, a 4-week double-blind treatment phase (Day 1 to Day 29), and a 3-week post treatment (follow up) phase. In the double-blind phase, patients will receive over 4 weeks either intravenous (IV) infusions of placebo (2 or 3 times weekly) or IV infusions of ketamine (2 or 3 times weekly). The total study duration for each patient will be a maximum of 13 weeks.
