P11 (S100A10) as a potential predictor of ketamine response in patients with SSRI-resistant depression

•Peripheral p11 levels were analysed in relation to ketamine treatment.•P11 levels were reduced in CD4+ and CD8+ T cells, though not compared to placebo.•Baseline p11 levels in cytotoxic T cells correlated with antidepressant response.•P11 levels might be useful as predictor for ketamine treatment response. Ketamine can act as antidepressant in patients with major depressive disorder (MDD) who are treatment-resistant. P11 has been implicated in ketamine's mechanism of action and proposed as biomarker for treatment response to other antidepressants. This study explores the effect of ketamine on peripheral p11 and the potential role for p11 as response marker for ketamine treatment. Thirty Selective Serotonin Reuptake Inhibitor resistant MDD patients were randomized to either 0.5 mg/kg ketamine or placebo intravenous treatment. Using multicolor Flow Cytometry, peripheral p11 levels were measured before and 1-2 days after treatment. P11 levels were decreased within the ketamine group in both cytotoxic T cell and T helper cells populations, although this did not significantly differ from changes seen in the placebo group. Baseline p11 levels in cytotoxic T cells were significantly correlated with antidepressant response to ketamine treatment. This study was part of a larger study examining the effect of ketamine on the serotonin system in MDD patients, therefore the number of study subjects was limited to that of the primary study. High baseline p11 levels in cytotoxic T cells were associated with a stronger reduction of depressive symptoms in MDD patients after ketamine treatment. Future studies should confirm if peripheral p11 levels could be used as a predictor of ketamine treatment response.