Ketamine monotherapy versus adjunctive ketamine in adults with treatment-resistant depression: Results from the Canadian Rapid Treatment Centre of Excellence.

A proportion of individuals with major depressive disorder (MDD) do not receive adequate therapeutic benefit from conventional monoaminergic antidepressant drugs, leading to treatment-resistant depression (TRD). Ketamine has been shown to provide rapid and significant efficacy in treating patients with TRD. The majority of published studies have investigated the adjunctive efficacy of ketamine with one or more monoaminergic antidepressants. There remains a clinical need to ascertain the relative effectiveness of ketamine monotherapy versus adjunctive ketamine treatment in adults with TRD. In this retrospective study, we investigate multidimensional, self-reported outcomes (i.e., antidepressant, anti-suicidality, antianxiety, and anti-functional impairment) of 220 patients to compare monotherapy (n = 39) and adjunctive (n = 181) ketamine treatment for TRD at a community-based clinic. Both groups had clinically and statistically significant antidepressant effects (p < 0.05). Individuals receiving ketamine monotherapy exhibited a significantly greater reduction on the suicidal ideation (SI) item of the Quick Inventory for Depressive Symptomatology-Self Report 16-Item (QIDS-SR) than the adjunctive group, with a small effect size [F (1, 265) = 4.73; p = 0.03*; partial η = 0.02], and a significantly higher proportion of partial responders at post-infusion 4 (p = 0.034*). No other between-group differences were significant. Limitations include the small sample, single-centred, open-label, non-randomized, uncontrolled, retrospective nature of this study and indication bias. Our real-world evidence suggests that ketamine may be effective as monotherapy or adjunct to monoamine-based treatments. A priority research and clinical vista is to identify subsets of individuals with TRD who are most likely to have a desired therapeutic outcome with monotherapy versus adjunctive ketamine treatment.