Sustained pharmacodynamic effects of S‐ketamine on cortical excitability and resting‐state brain activity: A randomized, placebo‐controlled trial

Major depressive disorder remains highly prevalent and disabling, while conventional monoaminergic antidepressants are limited by slow onset, incomplete efficacy and tolerability problems. The introduction positions S-ketamine as a rapid-acting antidepressant with clinical promise, but notes that its mechanisms are still not fully understood and that the field lacks pharmacodynamic biomarkers that can capture both beneficial and adverse central nervous system effects in humans. The study was designed to examine whether Transcranial Magnetic Stimulation (TMS) and resting-state pharmaco-electroencephalography (pEEG) could detect acute, delayed and sustained central nervous system effects of S-ketamine, with particular interest in changes that might relate to its delayed antidepressant action. The authors state that this was the first study to investigate delayed effects beyond 24 h post-dose on cortical excitability and spontaneous brain activity in healthy volunteers, using S-ketamine as a tool compound for biomarker development.

The researchers conducted a randomised, double-blind, double-dummy, placebo-controlled, four-way crossover study in 16 healthy volunteers aged 18-45 years. Participants were screened medically, and those with an increased risk of TMS-related complications or significant neuropsychiatric illness were excluded. The design compared one therapeutic intravenous dose of S-ketamine with two oral doses and placebo, allowing the investigators to contrast parent-drug and metabolite exposure. The intravenous condition used 0.40 mg/kg given over 40 min. The two oral doses were 0.45 mg/kg and 0.20 mg/kg. Oral administration began 15 min after the start of the intravenous infusion so that the expected time to maximum concentration would be aligned as closely as possible across routes. The dosing strategy was based on simulated S-ketamine and S-norketamine concentrations from population pharmacokinetic models, and the routes were chosen to maximise pharmacokinetic differences between parent compound and metabolites. pEEG was recorded in alternating eyes closed and eyes open periods according to IPEG guidance, and TMS was applied using standard guidelines. The motor hotspot of the dominant abductor digiti minimi muscle was stimulated, resting motor threshold was determined at each session, and both single-pulse and paired-pulse TMS protocols were used alongside simultaneous TMS-EMG and TMS-EEG recording. pEEG outcomes included total spectral power by frequency band, electrode and eye state. TMS-EMG outcomes included motor-evoked potential amplitude and paired-pulse measures of short and long intracortical inhibition, including long-interval intracortical inhibition at 50, 100 and 300 ms. The sample size calculation was based on motor-evoked potential amplitude; 16 participants were estimated to provide 80% power to detect a mean difference of 350 microvolts at a two-sided 0.05 significance level. Analyses used mixed-effects analysis of covariance models with treatment, time, period and treatment-related terms. pEEG data were log-transformed and back-transformed so results could be expressed as percentage change, and cluster-based permutation testing was used for TMS-EEG. Post-hoc concentration-effect analyses examined relationships between pharmacokinetic measures and pharmacodynamic responses.

For TMS-EMG, the intravenous dose produced both acute and sustained effects. At 4.5 h post-dose, 0.40 mg/kg intravenous S-ketamine significantly reduced motor-evoked potential amplitude compared with placebo (estimated difference -430.17 microvolts, 95% CI -806.74 to -53.61; p=0.03). At 7 days, the same dose significantly attenuated long-interval intracortical inhibition at 50 ms (estimated difference 57.83%, 95% CI 14.24 to 101.43; p=0.01). No other TMS-EMG endpoints were affected, and neither oral dose showed clear acute, delayed or sustained effects. For TMS-EEG, all treatments showed significant acute effects at 4.5 h. Delayed effects at 24 h were observed only with 0.40 mg/kg intravenous S-ketamine and 0.45 mg/kg oral S-ketamine, while no treatment showed sustained TMS-EEG effects at 7 days. The acute pattern involved changes in early TMS-evoked potential components, specifically an increased N45 component and decreased P60 component in the contralateral centro-parietal region. The delayed pattern involved an increased P180 component in the contralateral parieto-occipital region. The authors note two consistent cluster patterns, one acute and one delayed, and report that these did not clearly differ by route of administration. In the pEEG data, acute reductions in delta, alpha and beta power were seen up to 6 h post-dose, whereas delayed increases in delta power emerged at 24 h and persisted to 7 days. These delayed effects were seen with both intravenous and high-dose oral S-ketamine, and were most evident in eyes-open recordings over parieto-occipital regions. Post-hoc analyses found negative linear concentration-effect relationships for delta power with both S-ketamine and S-norketamine during the acute phase, whereas long-interval intracortical inhibition at 7 days showed a positive linear concentration-effect relationship with S-ketamine. The authors interpret these pharmacokinetic-pharmacodynamic findings as suggesting that the sustained effect was more closely related to the parent compound than to metabolites.

The authors interpret the findings as evidence that S-ketamine produces delayed and sustained neurophysiological effects in healthy volunteers that are distinct from its acute effects. They argue that the acute changes are consistent with the known early pharmacology of S-ketamine, while the later increase in cortical excitability and spontaneous brain activity may reflect downstream processes beyond initial NMDAR antagonism. In particular, the 7-day reduction in intracortical inhibition is presented as potentially relevant to longer-term neuroplastic mechanisms, and the delayed changes in TMS-EEG and pEEG are discussed as possible biomarkers of sustained antidepressant efficacy. Relative to earlier research, the authors note that prior studies have mostly focused on peak or acute ketamine effects, often with racemic ketamine or different analytical approaches. They emphasise that their measurements were taken after peak exposure, and therefore may capture a later phase of the pharmacodynamic profile that differs from previously reported acute findings. They also point out that late TMS-evoked potential components and delta power changes were located in parieto-occipital regions, raising the possibility of posterior network involvement, although they describe this interpretation as speculative. Several limitations are acknowledged. The study did not sample at peak S-ketamine concentrations, so it cannot replicate earlier acute-phase findings. The crossover design limited follow-up to 7 days, and the low subanaesthetic doses, while therapeutically relevant, may not generalise beyond the intended clinical range. The researchers also note that one TMS-EMG measure, intracortical facilitation, was omitted because of time constraints. Because repeated testing over multiple days can introduce variability, and no correction for multiple comparisons was applied, the risk of type I error is higher, especially at later time points. They therefore present the work as hypothesis-generating and call for larger, better-powered replication studies with denser post-dose sampling. In terms of implications, the authors suggest that TMS and pEEG may be useful biomarker candidates for screening rapid-acting antidepressant compounds and for understanding the temporal evolution of S-ketamine's pharmacodynamic effects. They state that the delayed findings align temporally with the onset of S-ketamine's antidepressant action at around 24 h post-dose, but they do not claim direct clinical validation.

The authors conclude that the study provides evidence suggestive of delayed pharmacodynamic effects of S-ketamine on cortical excitability and resting-state brain activity in healthy volunteers, extending beyond 24 h post-dose and persisting for up to 7 days. They present TMS- and pEEG-derived measures as promising candidate biomarkers for future validation in rapid-acting antidepressant research.