The study will enroll approximately 90 patients. Participants who meet the study entry criteria will be assessed for response to ketamine by administering 2 intravenous (IV) infusions of ketamine (0.5 mg/kg given over 40 minutes; maximum dose 40 mg/day) on Day -5 (±1) and Day -1 (±1). To determine response to ketamine, the Day -5 predose Montgomery Åsberg Depression Rating Scale (MADRS) total score (before the first ketamine infusion) will be compared with the MADRS total score predose on Day 1 (24 hours after the second ketamine infusion on Day -1). Two cohorts (ketamine responders and ketamine nonresponders) of participants will be enrolled in parallel into the study.