IUPAC: 6-(2,3-dichlorophenyl)-1,2,4-triazine-3,5-diamine · C9H7Cl2N5 · MW 256.09 g/mol · CAS 84057-84-1 · ChEMBL CHEMBL741
Lamotrigine (Lamictal). A phenyltriazine anticonvulsant that became psychiatry's cleanest mood-floor agent — a use-dependent, state-dependent blocker of voltage-gated sodium (Nav) channels that preferentially stabilizes the inactivated state, damping high-frequency firing and reducing presynaptic glutamate release. FDA-approved for partial/generalized seizures and for the maintenance of bipolar I disorder, where it prevents the depressive pole without pushing patients into mania. Trade/context names: Lamictal, Lamictal XR/ODT, BW-430C.
Lamotrigine is not a receptor ligand and not a transporter substrate. It is a conditional plug for the voltage-gated sodium channel (Nav) — the same pore-forming α-subunit that carries the rising phase of every neuronal action potential. Its defining property is state- and use-dependence: it binds with far higher affinity to the channel's inactivated conformation than to the resting state, so it accumulates on channels that have just fired and are cycling through inactivation.
The consequence is an activity filter. Quietly firing neurons at physiological rates barely notice the drug; neurons firing in the high-frequency, sustained bursts that define a seizure focus — or the pathologically hyper-glutamatergic states implicated in bipolar illness — spend far more time inactivated, trap more lamotrigine, and are selectively silenced. Because Nav current drives the terminal depolarization that opens presynaptic Ca2+ channels, blocking it reduces glutamate release without shutting the network down. The cryo-EM structure of human Nav1.7 (PDB 8THH) resolves this at atomic scale: two lamotrigine molecules bound — one in the central cavity beneath the selectivity filter, one at the intracellular activation gate.
Lamotrigine occupies the central cavity of the Nav pore module, contacting the local-anesthetic receptor site on the DIV-S6 helix (the phenylalanine/tyrosine that also binds carbamazepine and phenytoin) and occluding Na+ flux. PDB 8THH places one LTG here.
Affinity is highest for the fast-inactivated channel. Binding shifts steady-state inactivation to more hyperpolarized voltages and slows recovery, so the pool of available channels shrinks precisely when firing is fastest — the molecular basis of use-dependence.
Yan and colleagues (2023) resolved a second lamotrigine at the intracellular activation gate ("BIG" site). This dual-pocket occupancy is a distinctive structural signature versus classic single-site local anesthetics and helps explain lamotrigine's slow, wedged unblock.
By clamping axonal and terminal Nav current, lamotrigine limits the terminal depolarization that gates Cav2.1/2.2 channels, cutting glutamate (and aspartate) release. This anti-glutamatergic action is the leading mechanistic account of both its antiseizure and mood-stabilizing effects.
It blocks CNS isoforms Nav1.1/1.2/1.6 and shows measurable action at peripheral/cardiac Nav1.5 and DRG Nav1.7/1.8 — the basis of both its broad-spectrum antiseizure profile and its dose-dependent cardiac Nav1.5 (Brugada-type) signal at overdose.
At higher concentrations lamotrigine weakly inhibits high-voltage-activated Ca2+ channels (N/P/Q-type) and HCN currents. These are minor relative to Nav block but may fine-tune the anti-glutamatergic effect. It is essentially inactive at sigma-1 and the classic neuroreceptors.
This mechanism is why lamotrigine behaves so differently from the GABAergic mood stabilizers (valproate, benzodiazepines) and from lithium. It does not sedate, it does not broadly potentiate inhibition, and it does not blunt affect — it trims the excitatory excursions that carry both seizures and, in the bipolar model, the glutamatergic dysregulation underlying depressive relapse.
Lamotrigine is almost completely absorbed (oral bioavailability ~98%), unaffected by food, and not appreciably first-pass metabolized. What makes its PK clinically treacherous is the exit route: it is cleared by hepatic glucuronidation via UGT1A4 (with UGT2B7), not the cytochrome P450 system. That single enzymatic bottleneck is exquisitely sensitive to co-medication, and its throughput can be halved or doubled by common psychiatric and antiepileptic drugs — which is exactly why lamotrigine dosing is protocol-driven rather than one-size-fits-all.
Metabolism cascade: the committed step is N2-glucuronidation to an inactive, water-soluble conjugate that is renally cleared. There is no active metabolite of consequence — the parent drug does all the work.
The interaction axis runs both ways. Enzyme inducers — carbamazepine, phenytoin, phenobarbital, rifampin, and estrogen-containing oral contraceptives — up-regulate UGT1A4 and can halve lamotrigine levels, so the maintenance dose must be pushed higher and pregnancy/pill changes demand re-titration. In the opposite direction, valproate is a potent UGT1A4 inhibitor that roughly doubles lamotrigine plasma concentrations and extends its half-life toward ~60–70 h. This is not a footnote: valproate co-therapy forces the lamotrigine starting dose and titration rate to be cut in half, because the same milligram schedule that is safe alone becomes a serious-rash hazard when valproate is on board (see Harm Reduction). Lamotrigine mildly auto-induces its own glucuronidation over the first weeks, another reason steady state — and dosing — shifts during titration.
A single molecular action — activity-gated Nav block that trims glutamate release — translates into a clinical profile that is almost the inverse of the antidepressants. Lamotrigine's signature is preventing the depressive pole of bipolar disorder while carrying one of the lowest rates of treatment-emergent mania in psychiatry. It raises the floor without lifting the ceiling into hypomania.
Pivotal maintenance trials (Calabrese, Bowden et al., 2003) established lamotrigine's FDA indication for delaying relapse in bipolar I. Its strength is prophylaxis against depressive episodes; its effect on preventing mania is weaker than lithium's. The practical picture: lithium and antipsychotics defend the manic ceiling, lamotrigine defends the depressive floor — which is why the two are so often paired.
Because it stabilizes excitability from the glutamatergic side rather than flooding monoamine synapses, lamotrigine does not provoke the mood elevation, cycle acceleration, or manic/mixed switching that unopposed antidepressants can trigger in bipolar patients. That safety at the manic pole is its central clinical advantage — a mood stabilizer that treats the down phase without destabilizing the up phase.
As an AED, lamotrigine covers focal-onset seizures, primary generalized tonic-clonic seizures, and the generalized seizures of Lennox-Gastaut syndrome. The same Nav-mediated suppression of high-frequency firing underlies all of these. It is generally weight-neutral and non-sedating, distinguishing it from valproate and the barbiturate/benzodiazepine AEDs.
The price of lamotrigine's tolerability is patience: the mandatory slow titration (weeks to reach a therapeutic dose) makes it useless for an acute manic or depressive crisis and demands adherence before any benefit appears. It is a maintenance and prevention drug, not a rescue drug — a fact that shapes where it fits in every bipolar treatment algorithm.
Common, usually benign effects — headache, dizziness, diplopia, nausea, mild insomnia — are dose- and titration-rate-dependent and often fade. The effect that dominates prescribing, however, is not on this list: it is the dermatologic risk that makes the titration schedule non-negotiable (below).
Evidence-based, non-moralistic. Lamotrigine is remarkably clean at steady state and low in overdose lethality alone — its defining danger is a hypersensitivity reaction driven almost entirely by how fast you go up. The titration schedule is not bureaucratic caution; it is the safety mechanism.
| Target | Affinity | Rel. | Mechanism |
|---|---|---|---|
|
Nav1.2
SCN2A · human (HEK, patch)
|
IC50 = 10 µM
pChEMBL 5.00
|
Use-dependent block | |
|
Nav site 2
[³H]BW202W92 displ. · rat forebrain
|
IC50 = 2.6 µM
pChEMBL 5.58
|
Channel blocker | |
|
Nav1.8
SCN10A · rat DRG / human
|
IC50 = 25 / 96 µM
|
Channel blocker | |
|
Nav1.5
SCN5A · human cardiac
|
IC50 = 62 µM
|
Cardiac (OD signal) | |
|
σ1R
Sigma-1 receptor · rat
|
IC50 > 10 µM
|
Negligible | |
|
hERG
KCNH2 · human (K⁺ channel)
|
IC50 ~229 µM
|
Very weak |
Lamotrigine is a near-rigid ligand — a flat dichlorophenyl ring hinged by a single rotatable bond onto a planar diaminotriazine. It surrenders almost no internal conformational entropy on binding, so the −TΔSconf penalty that dominates flexible orthosteric binders is small here. What the dual-pocket structure (PDB 8THH) exposes instead is a state-dependent entropy problem: the drug binds a pore that only presents its high-affinity site once the channel has inactivated. Association is therefore gated by the channel's own conformational cycle, and the dominant entropic cost is paid by the protein (locking DIV-S6 and the gate) and by desolvation of the water-filled central cavity, not by the ligand.
In FlexAID∆S terms, the Shannon-entropy collapse concentrates in solvent and side-chain microstates lining the cavity and gate, while the two bound LTG copies read as a cooperative, two-site occupancy that further lowers the configurational entropy of the blocked state — the structural correlate of "trapping" and slow unblock. A rigorous ΔG decomposition should weight ΔSsolvent and the receptor's inactivated-state conformational entropy far above ligand ΔSconf: use-dependence is an entropy schedule, in which affinity is bought only when the channel has already paid down its own conformational freedom.