Ketamine for Relapse Prevention in Recurrent Depressive Disorder

Participants will be recruited at admission to St Patrick's University Hospital for treatment of the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV)-diagnosed recurrent unipolar depression and followed-up weekly to assess recovery according to standard criteria. Blood samples for epigenetic studies will be taken at baseline. Treatment-as-usual will continue throughout the entire trial. Participants who meet standardised response criteria will then be invited to be randomised to course of four two-weekly ketamine or midazolam (active comparator) infusions. Block randomisation will be independently performed. Physical, psychotomimetic and cognitive outcomes will be monitored before, during and after infusions. Blood samples will be taken at four time-points in the first infusion session and before the final infusion for neuroplasticity biomarker studies.Trial Interventions: participants will receive four two-weekly infusions of either ketamine at 0.05mg/kg or midazolam at 0.045mg/kg. All infusions will be administered by a consultant anaesthetist. Repeated infusions of ketamine have been shown to be safe and well-tolerated by patients with mental illness. Minor haemodynamic changes and psychotomimetic side-effects can occur and will be assessed regularly during infusions and for 200 minutes afterwards. Participants will be followed up over six months to assess for relapse according to standardised criteria. This is the highest-risk period for relapse and investigators hypothesize that ketamine will provide additional neurotrophic support (assessed by the laboratory biomarker project) which will result in lower relapse rates when compared to midazolam.