Interleukin 6 as a Treatment Target for Depression: A Proof-of-Concept Randomized Clinical Trial.

Importance: Interleukin 6 (IL-6), a keystone inflammatory cytokine, is a credible mechanistic candidate for causing depression. However, randomized clinical trials testing its treatment potential remain scarce. Objective: To identify likely treatment-sensitive outcomes and effect size for systemic IL-6 inhibition in patients with difficult-to-treat depression. Design, Setting, and Participants: This 4-week, proof-of-concept, double-blind, parallel-arm, placebo-controlled randomized clinical trial recruited adults with moderate-to-severe International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) depression, poor antidepressant response, low-grade systemic inflammation (high-sensitivity C-reactive protein [hs-CRP] level ≥0.3 mg/dL on 2 tests), and depression somatic symptoms (Beck Depression Inventory II somatic symptoms score ≥7) from primary and secondary care and self-referral from 2018 to 2022. Participants were randomized into minimization balanced groups on depression severity and sex. Assessments were conducted at baseline and 7, 14, and 28 days after infusion. Data were analyzed from 2023 to 2025. Intervention: Single intravenous infusion of IL-6R antagonist tocilizumab (8 mg/kg, maximum 800 mg/patient) or normal saline. Main Outcomes and Measures: The primary outcome was depression somatic symptoms at 14 days after infusion. The secondary outcome was total depression severity. Exploratory outcomes included fatigue, anxiety, anhedonia, quality of life, and cognition. Outcomes were assessed using validated scales and interpreted against clinically meaningful thresholds. Results: A total of 30 participants (mean [SD] age, 41.1 [12.3] years; 24 [80.0%] female) were randomized, including 14 in the tocilizumab group and 16 in the placebo group. Of these, 29 participants received the assigned infusion and completed follow-up. As expected for a small proof-of-concept study, no results reached statistical significance, including little improvement in depression somatic symptoms at day 14 (adjusted mean difference: -0.12; 95% CI, -2.51 to 2.28). However, a pattern of greater stepwise improvement over time was observed with tocilizumab in somatic symptoms, depression severity, fatigue, psychological symptoms, state anxiety, and quality of life, with the largest effects observed at the final follow-up (day 28). Tocilizumab may also improve more individual depressive symptoms. Treatment effects were within ranges considered clinically meaningful for depression severity, fatigue, anxiety, and quality of life. At the final follow-up, remission (7 participants [53.9%] vs 5 participants [31.3%]; number needed to treat [NNT] = 5) and response (6 participants [46.2%] vs 3 participants [18.8%]; NNT = 4) rates favored tocilizumab compared with placebo. Baseline hs-CRP level, but not IL-6 level, tracked depression improvement, suggesting hs-CRP may better predict immunotherapy response in depression than drug-specific biomarkers. Tocilizumab was well tolerated, with no serious adverse events or withdrawals. Conclusions and Relevance: These findings highlight treatment-sensitive outcomes, effect sizes, and patient selection methods for testing systemic IL-6 inhibition in patients with difficult-to-treat depression, and call for a large-scale efficacy trial of anti-IL-6 treatment in depression. Trial Registration: isrctn.org Identifier: ISRCTN16942542.