Phase 2a TS-161 Trial Halted: No Antidepressant Effect in TRD

Phase 2a TS-161 Trial in Treatment-Resistant Depression Halted for Futility

The phase 2a clinical trial of TS-161, a group II metabotropic glutamate receptor (mGlu2/3) antagonist, was stopped early after interim analysis found no significant antidepressant effect in adults with treatment-resistant depression (TRD). According to the trial report (OpenAlex W7211915078), the study enrolled 11 unmedicated participants in a randomized, double-blind, placebo-controlled, crossover design. After three weeks of daily oral TS-161 (50–100 mg) or placebo, no significant difference was observed in Montgomery-Åsberg Depression Rating Scale (MADRS) scores at the primary endpoint (day 21, P = 0.91). Only one participant responded to TS-161 by day 7, and no remissions were recorded. The trial was halted due to futility and low recruitment, underscoring the persistent challenge of finding effective rapid-acting antidepressants for TRD.

Mechanism of Action: Glutamatergic Modulation and Biomarker Engagement

TS-161 is an orthosteric mGlu2/3 antagonist designed to increase glutamate release and enhance monoaminergic signaling, paralleling some of the mechanisms attributed to ketamine’s rapid antidepressant effects. Preclinical models and phase 1 safety data previously suggested promise for this approach. In this phase 2a trial, TS-161 demonstrated central nervous system (CNS) engagement through several proxy biomarkers. Magnetic resonance spectroscopy (MRS) indicated a qualitative trend toward increased glutamate metabolite values, and magnetoencephalography (MEG) showed elevated gamma power in lateral cortical regions (p FDR <0.01). Peripheral brain-derived neurotrophic factor (BDNF) levels also trended upward (P = 0.055). However, these neurobiological changes did not translate into measurable clinical benefit in this small, early-phase population.

Policy and Research Implications: Translational Gaps and Trial Design Challenges

The lack of clinical efficacy for TS-161, despite evidence of CNS target engagement, highlights the translational gap between preclinical glutamatergic modulation and meaningful antidepressant outcomes in humans. This result echoes prior failures of other non-ketamine glutamatergic modulators, such as NR2B-selective NMDA antagonists, to deliver robust clinical effects in larger trials. For policymakers, regulators, and research funders, these findings reinforce the need for rigorous, adequately powered studies and the importance of patient-centric endpoints. The trial’s early halt for low recruitment also illustrates a practical barrier: recruiting unmedicated TRD patients for crossover designs is difficult, especially when the agent lacks established efficacy or psychoactive effects that might motivate participation. Future studies may need to reconsider inclusion criteria, recruitment strategies, and the balance between biomarker endpoints and clinical outcomes.

Risks, Limitations, and Unknowns

The main risks highlighted by the TS-161 trial are not safety-related—adverse events were mild and consistent with earlier phase 1 data—but rather relate to the feasibility and interpretability of early-phase CNS drug trials. The small sample size (n=11) and single-site design limit the generalizability of the findings. The absence of a robust clinical signal, despite biomarker changes, raises questions about the validity of these proxies as predictors of antidepressant efficacy. Moreover, the trial’s early termination for recruitment challenges is a concrete example of how logistical issues can confound scientific interpretation and slow progress in the field. The non-obvious implication here is that, for non-classical rapid-acting antidepressants, patient and clinician expectations—shaped by the dramatic effects of ketamine—may hinder recruitment and trial completion for agents with more subtle or uncertain effects.

Outlook: Implications for Future Rapid-Acting Antidepressant Development

While the TS-161 trial does not support further development of this specific mGlu2/3 antagonist for TRD at present, the study provides valuable lessons for the design and execution of future trials in this space. Researchers and sponsors should carefully weigh the feasibility of recruiting unmedicated TRD populations, the choice of crossover versus parallel designs, and the selection of biomarkers that have proven predictive value for clinical outcomes. For investors and institutions, the halted TS-161 program serves as a cautionary tale about the high attrition rate and translational uncertainty in the search for next-generation, rapid-acting antidepressants beyond ketamine and classical psychedelics. The field is likely to see continued exploration of glutamatergic targets, but with greater scrutiny of early-phase signals and a renewed focus on patient-centered endpoints and trial logistics.

How we research: This article was written and reviewed by Dr. Alex M. Greene, PhD (neuroscience, clinical trials analyst). Reviewed on 2026-09-08. Primary source: OpenAlex W7211915078.