Peripheral Biomarkers for Ketamine Response: Clinical Status and Research Gaps

Current Evidence: No Reliable Peripheral Biomarkers for Ketamine Response

There is currently no reliable peripheral biomarker that predicts or monitors response to ketamine or esketamine in treatment-resistant depression (TRD), according to a comprehensive scoping review published on September 23, 2026 (OpenAlex W7214124547). The review, conducted according to PRISMA-ScR guidelines, synthesized 46 studies—including 17 randomized controlled trials (RCTs)—and found substantial inconsistency across results. Despite the growing clinical use of ketamine and esketamine for TRD, the field lacks validated blood-based or other peripheral biomarkers to guide patient selection or monitor efficacy.

Most studies to date have focused on brain-derived neurotrophic factor (BDNF), inflammatory cytokines such as interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-α), interleukin-10 (IL-10), and interleukin-8 (IL-8), as well as metabolites from the kynurenine pathway. However, none of these markers have demonstrated reproducible associations with clinical response across multiple studies. The review highlights that heterogeneity in study design, analytical methods, and definitions of clinical response have contributed to the lack of consensus.

Mechanistic Context: Why Biomarkers Matter in Ketamine Therapy

Peripheral biomarkers—such as blood proteins, cytokines, or metabolites—are sought as objective indicators to predict or track response to antidepressant therapies, including ketamine and its S-enantiomer, esketamine. The rationale is that such markers could help identify which patients are most likely to benefit, monitor safety, and optimize dosing strategies. Ketamine's rapid antidepressant effects are believed to involve neuroplasticity, glutamatergic modulation, and anti-inflammatory pathways, making BDNF and inflammatory cytokines logical candidates for investigation.

However, the review found that even well-studied markers like BDNF and IL-6 show inconsistent changes after ketamine administration, and their correlation with clinical improvement is weak or absent. This inconsistency may reflect the complex, multifactorial mechanisms of ketamine's action, as well as methodological differences between studies. Notably, the review points out that esketamine-specific biomarker data remain especially sparse, despite its regulatory approval and growing clinical use.

Research and Policy Implications: The Need for Standardization and Multimarker Approaches

The lack of validated biomarkers for ketamine and esketamine response has direct implications for clinical trial design, regulatory guidance, and clinical practice. Without reliable predictors, patient selection for ketamine therapy remains largely empirical, and safety monitoring relies on clinical observation rather than objective measures. The review emphasizes the need for future studies to adopt standardized protocols, larger and more diverse patient populations, and prospective designs that can robustly assess biomarker utility.

• Standardization of definitions for clinical response and remission is critical to enable meaningful cross-study comparisons.
• Harmonized analytical methods for biomarker measurement are necessary to reduce variability and improve reproducibility.
• Multimarker panels that integrate neuroplastic, inflammatory, and metabolic pathways may offer greater predictive power than single markers.
• Inclusion of esketamine-treated populations in biomarker research is essential, given its distinct pharmacology and regulatory status.

For policymakers and research funders, the review provides a clear mandate: support large-scale, prospective, and multimodal studies that can move the field beyond fragmented and inconclusive findings.

Risks, Unknowns, and a Critical Insight: The Pitfalls of Over-Reliance on Peripheral Markers

Peripheral biomarkers may not fully capture the central nervous system (CNS) changes underlying ketamine's antidepressant effects, raising the risk that even well-designed studies could miss key mechanistic signals. The review notes that over-reliance on peripheral markers—without integrating neuroimaging, cerebrospinal fluid (CSF) studies, or digital phenotyping—could limit progress. A concrete example: several candidate biomarkers showed promising associations in small, single-site studies, but failed to replicate in larger, multi-center trials, suggesting site- or assay-specific artifacts.

Additionally, the heterogeneity of TRD itself—encompassing diverse biological and psychosocial factors—means that a "one-size-fits-all" biomarker is unlikely. This underscores the importance of stratified medicine approaches and the integration of clinical, biological, and digital data streams.

Looking Forward: Toward Precision Psychiatry in Ketamine Treatment

Progress in identifying reliable biomarkers for ketamine and esketamine response will require coordinated, multidisciplinary efforts and a willingness to move beyond reductionist models. The future of precision psychiatry in this space likely lies in combining peripheral biomarker panels with neuroimaging, genetics, and real-world clinical data. As regulatory agencies and payers increasingly demand evidence of personalized benefit and safety, the field must prioritize robust, replicable science over early adoption of unproven markers.

Until such evidence emerges, clinicians and researchers should approach biomarker findings with caution, relying on established clinical criteria for patient selection and monitoring. The next generation of trials—if designed with these lessons in mind—may finally unlock the potential of biomarkers to improve outcomes in TRD and beyond.

How we research: This article was written and reviewed by Dr. Alex Greene, MD, PhD, board-certified psychiatrist and clinical trialist, on 2026-09-25. All primary claims are supported by direct reference to the cited OpenAlex review and regulatory documents where relevant.