Modulation of ER Stress and Inflammation by S-Ketamine, R-Ketamine, and Their Metabolites in Human Microglial Cells: Insights into Novel Targets for Depression Therapy.

Ketamine and its metabolites—including R-ketamine, S-ketamine, and the hydroxynorketamines (2S,6S-HNK and 2R,6R-HNK)—directly reduce markers of endoplasmic reticulum (ER) stress and inflammation in human microglial cells. In cells treated with tunicamycin to induce ER stress, all compounds lowered expression and protein levels of CHOP and GRP78, two key components of the unfolded protein response (UPR). In microglia stimulated with lipopolysaccharide (LPS), the compounds decreased levels of the inflammatory cytokine IL-6 and, to a lesser extent, IL-8. These findings point to a glia-targeted mechanism for modulating ER stress and neuroinflammation, supporting further in vivo research to develop antidepressants with fewer psychoactive side effects than current treatments.

Study at a glance

Abstract

Despite affecting millions worldwide, major depressive disorder (MDD) remains a therapeutic challenge, with approximately one-third of patients failing to respond to standard treatments. The need for innovative, molecularly driven therapies has turned attention to ketamine and its enantiomers. While S-ketamine is clinically approved for treatment-resistant depression (TRD), it has various psychoactive side effects and potential for abuse. Hence, it is necessary to identify alternative compounds, such as R-ketamine, and their metabolites (e.g., 2S,6S-hydroxynorketamine and 2R,6R-hydroxynorketamine, collectively referred to as HNKs). Emerging evidence suggests that the pathophysiology of MDD involves two processes regulated by the unfolded protein response (UPR): endoplasmic reticulum (ER) stress and neuroinflammation. As such, they represent promising therapeutic targets. The study provides the first direct comparison of ketamine enantiomers and their metabolites in modulating ER stress and inflammatory signaling in human microglial cells (HMC3), which play key roles in neuroimmune communication. Both S-ketamine and R-ketamine, along with their metabolites, significantly reduced both the expression and protein levels of CHOP and GRP78-two critical UPR components-under tunicamycin-induced ER stress conditions. Additionally, the compounds significantly decreased IL-6 levels and, to a lesser extent, IL-8 levels in lipopolysaccharide (LPS)-stimulated microglia, indicating anti-inflammatory potential. Taken together, these findings highlight a novel glia-targeted mechanism by which ketamine and its metabolites modulate ER stress and neuroinflammation. CHOP and GRP78 appear to be stress-responsive molecular markers within the UPR pathway. These results justify further in vivo validation and support the development of antidepressants with fewer psychoactive effects.