King's College London

Background: Despite the pathogenic role played by interleukin-6 (IL-6) signaling in depression, the association between baseline peripheral IL-6 signaling and the antidepressant treatment responses noted in clinical studies remains controversial. We investigated the effects of the baseline serum IL-6 (sIL-6) level and anxiety symptoms on the 12-week remission rate of depressed outpatients who received stepwise antidepressant treatments. Methods: At baseline, sIL-6 levels were measured, and anxiety symptoms were evaluated using the Hospital Anxiety Depression Scale-Anxiety subscale (HADS-A), in 1094 patients. All received stepwise antidepressant treatment. Subsequently, 12-week remission, defined as a Hamilton Depression Rating Scale (HAMD) score ≤ 7, was assessed. Results: The individual and interaction effects of the sIL-6 level (as a binary [low vs. high, based on the median value of 1.65 pg/mL] or continuous variable) and the HADS-A score (as a binary [<12 vs. ≥12] or continuous variable) on the 12-week remission rate were analyzed using logistic regression models after adjusting for relevant covariates. Patients with both low sIL-6 levels (<1.65 pg/mL) and HADS-A scores <12 had the highest 12-week remission rate; a significant interaction effect was also apparent. This effect was significant even when the data were analyzed as continuous variables. Conclusions: Our study suggests that the sIL-6 level can serve as a biomarker predicting the outcome of antidepressant treatment according to the severity of anxiety symptoms.