Residency · Residency · Neurology

Antiseizure Medication Selection and Pharmacology

Overview

Antiseizure medication (ASM) selection is guided by seizure type, epilepsy syndrome, comorbidities, and patient-specific factors such as sex, childbearing potential, and concomitant medications. Approximately 60-70% of patients achieve seizure freedom with the first or second ASM trial. Drug-resistant epilepsy, defined as failure of two adequately dosed ASMs, affects the remaining 30-40%. Knowledge of pharmacokinetics, drug interactions, and teratogenicity is essential for safe prescribing. The ongoing debate between established and newer agents involves balancing cost, efficacy data, and tolerability.

Mechanisms of Action

Sodium Channel Blockers

Carbamazepine, oxcarbazepine, eslicarbazepine, phenytoin, lacosamide, and lamotrigine (as a partial mechanism) all stabilize the inactivated state of voltage-gated sodium channels, reducing repetitive neuronal firing. These agents are most effective for focal seizures but may worsen generalized epilepsies, particularly absence and myoclonic seizures. Lacosamide is unique in that it enhances slow inactivation, a mechanism distinct from the fast inactivation blockade of the other agents.

Calcium Channel Modulators

Ethosuximide blocks T-type calcium channels in thalamic relay neurons and is specifically effective for absence seizures. Gabapentin and pregabalin bind the alpha-2-delta subunit of voltage-gated calcium channels, reducing neurotransmitter release. They are effective for focal seizures but not for generalized epilepsy.

GABA Enhancement

Benzodiazepines (diazepam, lorazepam, clobazam, clonazepam) are positive allosteric modulators of GABA-A receptors that increase the frequency of channel opening. Barbiturates (phenobarbital) similarly modulate GABA-A receptors but increase the duration of channel opening. Vigabatrin irreversibly inhibits GABA transaminase, increasing GABA levels in the synapse. Tiagabine inhibits GABA reuptake.

Synaptic Vesicle Protein 2A (SV2A) Binding

Levetiracetam and brivaracetam bind SV2A to modulate synaptic vesicle release. Both have broad-spectrum efficacy, are well-tolerated, and have minimal drug interactions due to predominantly renal elimination. Brivaracetam has higher SV2A affinity and produces fewer behavioral side effects than levetiracetam.

Multiple/Complex Mechanisms

Valproate combines sodium channel blockade, T-type calcium channel blockade, GABA enhancement, and HDAC inhibition. Topiramate acts through sodium channel blockade, GABA-A enhancement, glutamate (AMPA/kainate) antagonism, and carbonic anhydrase inhibition. Zonisamide blocks both sodium and T-type calcium channels while inhibiting carbonic anhydrase. Cenobamate blocks the persistent sodium current and positively modulates GABA-A receptors, a dual mechanism that accounts for its potent efficacy in focal epilepsy.

SANAD Trials

SANAD I (2007)

In Arm A for focal epilepsy, carbamazepine was compared against lamotrigine, gabapentin, oxcarbazepine, and topiramate. Carbamazepine and lamotrigine were the most effective, but lamotrigine was better tolerated, leading to its recommendation as first-line for focal epilepsy. In Arm B for generalized and unclassified epilepsy, valproate was compared against lamotrigine and topiramate. Valproate proved the most effective and well-tolerated overall, establishing it as first-line for generalized epilepsy with important caveats for women of childbearing age.

SANAD II (2021)

SANAD II compared levetiracetam versus valproate for generalized epilepsy and levetiracetam versus lamotrigine for focal epilepsy. For generalized epilepsy, valproate remained superior to levetiracetam for time to 12-month remission. For focal epilepsy, lamotrigine was superior to levetiracetam. These results reinforced lamotrigine and valproate as the top-tier first-line agents for their respective epilepsy types.

First-Line ASM Selection by Seizure/Epilepsy Type

Focal Epilepsy

First-line options are lamotrigine, levetiracetam, oxcarbazepine, and carbamazepine. Second-line agents include lacosamide, zonisamide, topiramate, and brivaracetam. For drug-resistant focal epilepsy, cenobamate stands out for its potency, with up to 20% of refractory patients achieving seizure freedom in clinical trials. Gabapentin and pregabalin should be avoided as monotherapy for epilepsy due to weaker efficacy.

Generalized Epilepsy

Valproate is the most effective first-line agent. Lamotrigine is preferred for women of childbearing age, and levetiracetam is another broad-spectrum option. Ethosuximide is first-line for childhood absence epilepsy when absence is the only seizure type. Narrow-spectrum sodium channel blockers (carbamazepine, oxcarbazepine, phenytoin) must be avoided as they may worsen absence and myoclonus. Gabapentin, pregabalin, tiagabine, and vigabatrin should also be avoided in generalized epilepsy.

Newer and Emerging Agents

Cenobamate

Cenobamate's dual mechanism of persistent sodium current blockade plus GABA-A modulation produces highly effective seizure control in drug-resistant focal epilepsy. FDA-approved in 2019, it achieved unprecedented seizure-free rates of approximately 20% in patients who had failed multiple prior therapies. However, it must be titrated very slowly starting at 12.5 mg/day with increases every 2 weeks to avoid DRESS syndrome. Cenobamate reduces levels of drugs metabolized by CYP3A4 and increases levels of CYP2C19 substrates.

Brivaracetam

As a higher-affinity SV2A ligand than levetiracetam, brivaracetam produces fewer behavioral and psychiatric side effects. It is available in IV formulation for acute situations and serves as a useful switch from levetiracetam in patients experiencing behavioral intolerance.

Cannabidiol (Epidiolex)

Cannabidiol is FDA-approved for Dravet syndrome, Lennox-Gastaut syndrome, and tuberous sclerosis complex. Its mechanism involves multiple pathways including GPR55 antagonism, TRPV1 modulation, and adenosine reuptake inhibition. An important interaction is the increase in clobazam levels via CYP2C19 inhibition, requiring monitoring for sedation. Hepatotoxicity risk necessitates monitoring of liver function tests, especially with concomitant valproate.

Fenfluramine

Fenfluramine is a serotonin-releasing agent FDA-approved for Dravet syndrome and LGS. It requires echocardiographic monitoring due to the historical association of fenfluramine/phentermine combinations with valvulopathy, although this has not been observed at the lower doses used for epilepsy.

Pharmacokinetic Considerations

Drug Interactions

Enzyme inducers (carbamazepine, phenytoin, phenobarbital, and oxcarbazepine at high doses) accelerate the metabolism of co-administered drugs including other ASMs, oral contraceptives, warfarin, and immunosuppressants. Valproate is an enzyme inhibitor that inhibits the glucuronidation of lamotrigine, effectively doubling lamotrigine levels and requiring that the lamotrigine dose be halved when the two are co-prescribed. Minimal-interaction drugs include levetiracetam, brivaracetam, gabapentin, pregabalin, and lacosamide, which are renally eliminated or have minimal hepatic metabolism. Regarding oral contraceptive failure, enzyme-inducing ASMs reduce the efficacy of hormonal contraception, necessitating use of an IUD or high-dose OCP. Notably, lamotrigine levels are reduced by estrogen-containing OCPs.

Therapeutic Drug Monitoring

Phenytoin has a narrow therapeutic index with nonlinear (Michaelis-Menten) kinetics, meaning small dose changes can produce disproportionately large level changes. Free levels should be checked in hypoalbuminemia. Carbamazepine auto-induces its own metabolism, so levels decline over weeks and require dose adjustment. Valproate has saturable protein binding, with the free fraction increasing at higher levels. Lamotrigine levels are affected by valproate (which increases them), enzyme inducers (which decrease them), and estrogen (which decreases them). Levetiracetam levels are not routinely monitored but can be useful in pregnancy or when adherence is in question.

Teratogenicity

High-Risk ASMs

Valproate carries the highest teratogenic risk, with neural tube defects in 1-2% and an overall major congenital malformation rate of 10% that is dose-dependent. It is also associated with reduced IQ and increased autism risk in offspring. Valproate is contraindicated in women of childbearing potential unless no alternative exists and a pregnancy prevention program is in place. Phenobarbital, phenytoin, and topiramate carry moderate teratogenic risk, with cleft lip and palate associated with topiramate and cardiac defects with phenobarbital. Carbamazepine has a modest risk of neural tube defects at approximately 1%.

Lower-Risk ASMs

Lamotrigine has relatively low teratogenic risk and is widely used in pregnancy. Levetiracetam has growing safety data supporting a low risk and is increasingly used during pregnancy. Oxcarbazepine has limited but reassuring data.

MONEAD Study

The MONEAD prospective observational study of pregnant women with epilepsy confirmed that clearance of both lamotrigine and levetiracetam increases during pregnancy, often requiring dose adjustments. It reinforced the importance of preconception counseling and folic acid supplementation at 4-5 mg per day for all women on ASMs.

The Generic Substitution Debate

Epilepsy patients are sensitive to small pharmacokinetic variations because of the narrow therapeutic index of many ASMs and the serious consequences of breakthrough seizures. Brand-to-generic or generic-to-generic substitution may introduce variability, as the FDA allows an 80-125% bioequivalence range. The AAN position is that clinicians should have the ability to prevent substitution in patients who are seizure-free on a specific formulation. While many epileptologists advocate "do not substitute" prescriptions, health economists argue the risk is minimal.

<image>A pharmacology reference table formatted as a visual infographic showing common ASMs organized by mechanism of action. Each drug is shown with its mechanism icon (sodium channel, calcium channel, GABA, SV2A), primary indication (focal, generalized, or broad-spectrum), common side effects, key drug interactions, and teratogenicity risk level (color-coded: green for low risk, yellow for moderate, red for high). Drugs included: carbamazepine, oxcarbazepine, phenytoin, lacosamide, lamotrigine, levetiracetam, brivaracetam, valproate, ethosuximide, topiramate, zonisamide, clobazam, gabapentin, pregabalin, cenobamate, and cannabidiol.</image>

<image>A decision flowchart for first-line ASM selection. The diagram starts with seizure type classification (focal vs generalized), then epilepsy syndrome identification, then patient factors (sex, childbearing potential, comorbidities, comedications). For focal epilepsy: lamotrigine or levetiracetam first-line, with alternatives (oxcarbazepine, lacosamide) and cenobamate for drug-resistant cases. For generalized epilepsy: valproate first-line (if not a woman of childbearing age), lamotrigine or levetiracetam if valproate is contraindicated, ethosuximide for pure absence. Special considerations for women of childbearing age are highlighted with the MONEAD study data and folic acid recommendations.</image>

Clinical Pearls

Lamotrigine and valproate are the two most effective ASMs overall, but valproate is contraindicated in women of childbearing potential due to its high teratogenicity and neurodevelopmental risk. Cenobamate is the most effective ASM for drug-resistant focal epilepsy based on clinical trial data, but requires very slow titration to avoid DRESS. Carbamazepine, oxcarbazepine, and phenytoin should never be used in JME or childhood absence epilepsy because they will worsen seizures. Valproate doubles lamotrigine levels, so the lamotrigine dose must always be halved when co-prescribing with valproate. Levetiracetam behavioral side effects (irritability, aggression) occur in approximately 10-15% of patients, and brivaracetam is a useful alternative with fewer psychiatric effects. Enzyme-inducing ASMs reduce oral contraceptive efficacy, requiring patient counseling and alternative contraception. Folic acid at 4-5 mg per day should be prescribed to all women of childbearing age on ASMs, ideally starting before conception.

References

• Marson AG, et al. The SANAD study of effectiveness of valproate, lamotrigine, or topiramate for generalised and unclassifiable epilepsy. Lancet. 2007;369(9566):1016-1026.
• Marson AG, et al. The SANAD II study of the effectiveness of levetiracetam, zonisamide, or lamotrigine for newly diagnosed focal epilepsy (SANAD II). Lancet. 2021;397(10282):1363-1374.
• Krauss GL, et al. Safety and efficacy of adjunctive cenobamate in patients with uncontrolled focal seizures. Lancet Neurol. 2020;19(1):38-48.
• Meador KJ, et al. Fetal antiepileptic drug exposure and cognitive outcomes at age 6 years (NEAD study). N Engl J Med. 2009;360(16):1597-1605.
• Tomson T, et al. Comparative risk of major congenital malformations with eight different antiepileptic drugs. Lancet Neurol. 2018;17(6):530-538.

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