Increasing evidence has demonstrated that inflammatory cytokines play an important role in major depressive disorder (MDD) and are associated with treatment outcomes. Few studies have explored the trajectories of multiple inflammatory cytokines after repeated ketamine infusions in MDD. In this study, we conducted a secondary analysis to investigate the impact of ketamine on the modulation of the inflammatory pathway in depression and whether this pathway contributes to the antidepressant properties of ketamine. A total of 60 patients with depression received six ketamine infusions (0.5 mg/kg) during a 12-day period. The Montgomery-Asberg Scale (MADRS) was administered, and blood samples were collected at baseline and 24 h and 14 days after the sixth infusion (days 0, 13, and 26). Plasma levels of the 19 cytokines were measured using the Luminex assay. At baseline, inflammatory cytokines were associated with the severity of depression. The concentrations of pro-and anti-inflammatory factors, including granulocyte macrophage colony-stimulating factor (GM-CSF), fractalkine, interferon gamma (IFN-γ), interleukin (IL)-10, IL-12p70, IL-17A, IL-1β, IL-2, IL-4, IL-23, IL-5, IL-6, IL-7, and tumor necrosis factor alpha (TNF-α), were downregulated after repeated ketamine administration (all p < 0.05). In addition, alterations in the levels of IL-17A (r = −0.259, p = 0.046) and IL-6 (r = −0.262, p = 0.043) were correlated with symptom improvement. A lower level of interferon-inducible T cell alpha chemoattractant (ITAC) at baseline was predictive of ketamine treatment response on day 13 according to a stepwise linear regression analysis (β = −0.296, p = 0.040). Our results suggest that the inflammatory pathway may be involved in the antidepressant effects of ketamine, which may be conducive to future treatment strategy optimization.

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Background Treatment-resistant depression (TRD) and pain frequently coexist clinically. Ketamine has analgesic and antidepressant effects, but few studies have evaluated individual differences in antidepressant outcomes to repeated ketamine in TRD patients with comorbid pain. Our aims were to determine the difference in ketamine’s antidepressant effects in TRD patients with or without pain and then to examine whether inflammatory cytokines might contribute to ketamine’s effect. Methods Sixty-six patients with TRD received six infusions of ketamine. Plasma levels of 19 inflammatory cytokines were assessed at baseline and post-infusion (day 13 and day 26) using the Luminex assay. Plasma inflammatory cytokines of sixty healthy controls (HCs) were also examined. Results TRD patients with pain had a higher antidepressant response rate (χ2 = 4.062, P = 0.044) and remission rate (χ2 = 4.062, P = 0.044) than patients without pain. Before ketamine treatment, GM-CSF and IL-6 levels were higher in the pain group than in the non-pain and HC groups. In the pain group, levels of TNF-α and IL-6 at day 13 and GM-CSF, fractalkine, IFN-γ, IL-10, MIP-3α, IL-12P70, IL-17α, IL-1β, IL-2, IL-4, IL-23, IL-5, IL-6, IL-7, MIP-1β, and TNF-α at day 26 were lower than those at baseline; in the non-pain group, TNF-α levels at day 13 and day 26 were lower than those at baseline. In the pain group, the changes of IL-6 were associated with improvement in pain intensity (β = 0.333, P = 0.001) and depressive symptoms (β = 0.478, P = 0.005) at day 13. Path analysis showed the direct (β = 2.995, P = 0.028) and indirect (β = 0.867, P = 0.042) effects of changes of IL-6 on improvement in depressive symptoms both were statistically significant. Conclusion This study suggested that an elevated inflammatory response plays a critical role in individual differences in TRD patients with or without pain. Ketamine showed great antidepressant and analgesic effects in TRD patients with pain, which may be related to its effects on modulating inflammation. Trial registration ChiCTR, ChiCTR-OOC-17012239. Registered on 26 May 2017

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