Ketamine

Schedule III dissociative anesthetic and the most consequential rapid-antidepressant breakthrough of the 21st century — a single 0.5 mg/kg IV infusion can lift treatment-resistant depression within hours, replicated in dozens of trials and FDA-approved as intranasal esketamine (Spravato, 2019). For Dylan: WATCH-LIST. He has no depression, no acute TBI, and the chronic-use risk profile (ketamine bladder, cognitive blunting in heavy users, fast tolerance, real abuse liability) makes daily/prophylactic use indefensible at age 20. The TBI-protection rationale is mechanistically real (NMDA-driven glutamate excitotoxicity is the central secondary-injury pathway in concussion) but the clinical evidence is mixed and the protection window is acute (immediate post-injury), not chronic. Hold for trigger: emergent TRD that fails SSRIs/SNRIs/bupropion → ketamine-assisted psychotherapy at a licensed clinic ($400-600/session, 6-session induction).

Ketamine is a phencyclidine-derived arylcyclohexylamine, originally synthesized by Calvin Stevens at Parke-Davis in 1962 (compound CI-581) as a shorter-acting, less-psychotomimetic alternative to PCP. First administered to a human prisoner volunteer in 1964 by Edward Domino and Guenter Corssen at the University of Michigan (Domino coined "dissociative anesthetic" after his wife Toni described the patients as "disconnected from their environment"). FDA-approved 1970 as Ketalar for anesthesia. Saw heavy use in the Vietnam War as the field anesthetic of choice for the wounded due to cardiovascular stability. The chiral compound exists as two enantiomers — S-(+)-ketamine (esketamine, ~3-4× more potent NMDA blocker) and R-(−)-ketamine (arketamine, slower-onset, longer-duration antidepressant in animal models, possibly via different mechanism). Racemic ketamine is the historical anesthetic preparation (50:50 mix). Esketamine was developed separately in Germany (Ketanest S, 1997). Spravato (intranasal esketamine) FDA approval March 5, 2019 for adults with TRD when used with another oral antidepressant; expanded indication 2020 for major depression with acute suicidal ideation.

Primary action — non-competitive, high-affinity, slow-off-rate NMDA receptor open-channel block (the opposite of memantine's kinetic profile):

The NMDA receptor is the brain's main calcium-permeable, voltage- and ligand-gated glutamate receptor — the central node of synaptic plasticity, learning, memory, and (when overactivated) glutamate excitotoxicity. Ketamine binds the dizocilpine (MK-801) site within the open channel, with Ki ~0.5 µM — similar nominal affinity to memantine, but with a critically slower off-rate. Where memantine pops out of the channel fast enough (millisecond timescale) to spare physiological synaptic transmission, ketamine stays bound much longer (seconds to minutes), producing a much deeper functional block at therapeutic concentrations.

This kinetic distinction is why ketamine produces dissociation, anesthesia, and (paradoxically) rapid antidepressant effects, while memantine is felt-effect-free at clinical doses. Both block the same receptor; only the kinetics differ.

The rapid-antidepressant mechanism — the "disinhibition hypothesis" (Krystal, Duman, others, 2000-2015):

The puzzle: ketamine is an NMDA antagonist, but its antidepressant effect requires enhanced glutamate signaling and BDNF release. Resolution: ketamine preferentially silences NMDA receptors on tonically-firing GABAergic interneurons (parvalbumin-positive PV interneurons in PFC) more than on pyramidal cells. PV interneurons normally provide tonic inhibition to pyramidal output. Silencing them removes the brake → cortical glutamate burst → AMPA receptor activation on pyramidal cells → BDNF release → TrkB activation → mTORC1 pathway → eEF2 dephosphorylation → translation of synaptic proteins → spinogenesis and synaptogenesis in PFC pyramidal neurons within hours.

This is mechanistically fast (hours) versus SSRIs (weeks), explaining the rapid antidepressant onset. It also explains why an AMPA antagonist (NBQX in animals) blocks the antidepressant effect — confirming the cascade is glutamate-burst → AMPA → downstream plasticity, not the NMDA antagonism per se.

Recent challenge (Williams 2018, Klein 2020): Naltrexone (μ-opioid antagonist) attenuated ketamine's antidepressant effect in a small Stanford crossover trial — suggesting μ-opioid receptor binding contributes. Ketamine has weak μ-opioid affinity (Ki ~30-50 µM, far above NMDA affinity), but at clinical concentrations the contribution may be non-trivial. The field is divided. Replications have been mixed. Practical implication: ketamine may work partly via opioid receptors, which has implications for both abuse liability and combination with opioid antagonists.

Secondary off-target actions:

• Sigma-1 receptor agonism (Ki ~0.1-1 µM) — a chaperone protein at the ER-mitochondria interface. Modulates calcium handling, dopamine/NMDA cross-talk, and BDNF. Likely contributes to neuroplasticity effects.

• HCN1 channel inhibition — hyperpolarization-activated cation channels in thalamic and cortical neurons. Likely contributes to anesthetic action and to dissociation (HCN1 knockouts are resistant to ketamine anesthesia).

• Monoamine reuptake inhibition (DAT, NET, SERT) — weak, micromolar affinity. Probably contributes to acute psychostimulant-like effects (tachycardia, BP elevation, mild euphoria).

• Muscarinic acetylcholine receptor antagonism — contributes to bronchodilation (clinically useful in asthma anesthesia) and to some autonomic effects.

• Voltage-gated sodium channel blockade at high concentrations — local anesthetic-like effect, partly explains analgesia.

• R-ketamine (arketamine) vs S-ketamine (esketamine): S-ketamine is ~3-4× more potent at NMDA, drives more dissociation per mg, and is the FDA-approved Spravato. R-ketamine in animal models produces longer-lasting antidepressant effects with less dissociation, and clinical trials of pure R-ketamine for depression are ongoing (Perception Neuroscience PCN-101 reached Phase 2 in 2022). For now, racemic ketamine and esketamine are the clinically-deployed forms.

Pharmacokinetics:

• IV: onset seconds, peak ~1 min, anesthetic duration 5-15 min, antidepressant effects detectable within hours.
• IM: onset 3-5 min, peak ~20 min, duration 30-60 min.
• Intranasal (Spravato): onset 5-10 min, peak ~40 min, full subjective effect 2 hr.
• Oral / sublingual lozenge: onset 15-30 min, peak ~1 hr, duration 2-4 hr. Oral bioavailability is poor (~16-20%) due to first-pass; sublingual lozenges held in the mouth ~10-15 min before swallowing achieve ~25-30% bioavailability with bypass of some first-pass.
• Half-life: 2-4 hours (parent ketamine); active metabolite norketamine has longer half-life (~5-12 hr) and contributes to some effects.
• Metabolism: CYP3A4 (major) and CYP2B6 (significant) → norketamine → hydroxynorketamine (HNK). The (2R,6R)-HNK metabolite is itself antidepressant in animal models and is part of the BDNF/AMPA-cascade story.