Focal Epilepsy ASMs
Focal epilepsy is the most common form of epilepsy in adults, and numerous antiseizure medications (ASMs) have been developed specifically or predominantly for this indication. The narrow-spectrum ASMs discussed here—carbamazepine, oxcarbazepine, eslicarbazepine, phenytoin, lacosamide, perampanel, gabapentin, and pregabalin—are effective against focal seizures but may exacerbate generalized seizure types (absence, myoclonic, atonic). They must therefore be avoided in patients with idiopathic generalized epilepsy (IGE) or unclassified epilepsy. Among these agents, the sodium channel blockers have a long history of proven efficacy, with carbamazepine serving as the historical reference standard. Newer agents such as lacosamide offer improved pharmacokinetic profiles, while perampanel provides a unique mechanism (AMPA receptor antagonism) not shared by any other ASM.
Bottom Line
- Carbamazepine/oxcarbazepine/eslicarbazepine: Three generations of dibenzazepine sodium channel blockers; carbamazepine is a potent enzyme inducer with autoinduction; oxcarbazepine causes more hyponatremia; eslicarbazepine offers once-daily dosing with fewer interactions; all may worsen generalized absence and myoclonic seizures; HLA-B*1502 screening mandatory before carbamazepine in Asian-descent patients
- Lacosamide: Enhances slow (not fast) sodium channel inactivation; favorable pharmacokinetic profile (no significant interactions); rapid titration possible; PR interval prolongation is the primary cardiac concern; available IV; effective in status epilepticus
- Phenytoin: Oldest sodium channel blocker still in use (since 1938); narrow therapeutic window with saturable nonlinear kinetics; potent enzyme inducer; chronic toxicity includes gingival hyperplasia, cerebellar atrophy, and decreased bone density; declining use in favor of newer agents
- Perampanel: Only selective AMPA receptor antagonist; effective for focal and GTC seizures; very long half-life (~105 hours); aggression and hostility (boxed warning) at higher doses; may be particularly effective in progressive myoclonic epilepsies
- Gabapentin/pregabalin: Bind the α2δ calcium channel subunit; narrow-spectrum focal seizure agents; gabapentin has dose-dependent saturable absorption; no drug interactions; also used for neuropathic pain and RLS
Carbamazepine
Mechanism and Pharmacokinetics
Carbamazepine blocks voltage-gated sodium channels in a voltage- and use-dependent fashion, reducing high-frequency neuronal firing. It has good oral bioavailability and ~75% protein binding. The primary metabolic pathway is hepatic via CYP3A4, producing the active metabolite carbamazepine-10,11-epoxide, which contributes to both efficacy and toxicity.
Carbamazepine Autoinduction
- Carbamazepine is unique among ASMs in that it induces its own metabolism (autoinduction)
- Over 2–4 weeks of therapy, clearance increases, half-life shortens, and serum concentration falls
- Practical implication: the dose that initially produces therapeutic levels will need upward adjustment; this is why gradual titration is essential
- Half-life after autoinduction: 12–17 hours (initially longer)
- Carbamazepine is also a potent inducer of CYP3A4 and other enzymes, reducing levels of many co-medications
Efficacy and Dosing
Carbamazepine is effective against focal seizures and generalized tonic-clonic seizures. It may exacerbate absence, myoclonic, and atonic seizures and should be avoided in IGE. Additional FDA indications include trigeminal neuralgia and bipolar disorder (acute mania).
- Starting dose: 100 mg twice daily or 200 mg at bedtime (extended-release)
- Titration: Increase by 200 mg every 3 days to target
- Target dose: 400–800 mg/d in 2 divided doses; higher doses may be needed
- Therapeutic range: 4–12 μg/mL
- Extended-release formulation (twice daily) provides steadier levels with evidence for improved tolerability and efficacy; 3-times-daily dosing needed for immediate-release formulations
Adverse Effects
Common dose-related effects include nausea, dizziness, sedation, tiredness, blurred vision, diplopia, and cognitive impairment. Hyponatremia may occur. Weight gain and decreased bone density are reported with long-term use. Mild leukopenia occurs in 10–20% of patients (usually benign); aplastic anemia is rare (~1 in 200,000).
HLA-B*1502 Screening
- The HLA-B*1502 allele is strongly predictive of carbamazepine-induced Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) in individuals of Asian descent (Han Chinese, Thai, Malaysian, Filipino, Indonesian, South Indian)
- FDA-mandated genetic testing of HLA-B polymorphisms before initiating carbamazepine in at-risk populations
- Other rare idiosyncratic reactions: hypersensitivity syndrome (fever, rash, organ involvement), lupus-like syndrome, hepatotoxicity
- Cross-reactivity with oxcarbazepine for rash: ~25%
- Abrupt withdrawal may cause severe rebound seizures—always taper gradually
Oxcarbazepine
Key Differences from Carbamazepine
Oxcarbazepine is a structural analogue of carbamazepine but with important metabolic differences. It is rapidly converted to the active metabolite S-licarbazepine (monohydroxy derivative, MHD), responsible for most antiseizure activity (80%). Unlike carbamazepine, it does not undergo autoinduction and is not affected by CYP3A4 inhibitors (erythromycin, fluoxetine, grapefruit juice).
- Enzyme effects: Weak CYP3A4 inducer (reduces oral contraceptive efficacy at doses >900 mg/d); weak CYP2C19 inhibitor (may raise phenytoin levels at high doses)
- Half-life: MHD half-life 8–10 hours; extended-release preparation allows once-daily dosing
- Starting dose: 150–300 mg twice daily; titrate by 300 mg/wk
- Target dose: 600–1200 mg/d; maximum 2400 mg/d
- Therapeutic range: 15–35 μg/mL (MHD)
Oxcarbazepine and Hyponatremia
- Oxcarbazepine is more likely to cause hyponatremia than carbamazepine
- Symptomatic hyponatremia is more common in older adults and those taking diuretics
- Sodium levels should be monitored in at-risk patients, particularly in the first 3 months
- Cross-reactivity with carbamazepine for rash: ~25%
- Abrupt withdrawal can precipitate generalized tonic-clonic seizures—always taper
Eslicarbazepine Acetate
Eslicarbazepine acetate is the third-generation dibenzazepine, a prodrug rapidly converted to S-licarbazepine (the active enantiomer of the MHD of oxcarbazepine). It blocks sodium channels and may preferentially enhance slow inactivation, unlike carbamazepine.
| Feature | Carbamazepine | Oxcarbazepine | Eslicarbazepine Acetate |
|---|---|---|---|
| Active metabolite | Carbamazepine-10,11-epoxide (active, causes side effects) | S-licarbazepine (80%) + R-licarbazepine (contributes to side effects) | S-licarbazepine only (purer active enantiomer) |
| Autoinduction | Yes (over 2–4 weeks) | No | No |
| Enzyme induction | Potent (CYP3A4, CYP2C9, UGT) | Weak (CYP3A4 at high doses) | Weak (CYP3A4) |
| Half-life | 12–17 hours (post-autoinduction) | 8–10 hours (MHD) | 13–20 hours (plasma); 20–24 hours (CSF) |
| Dosing frequency | Twice or three times daily | Twice daily (once daily ER) | Once daily |
| Hyponatremia risk | Moderate | Higher | Lower (Na ≤125 mEq/L in up to 1.5%) |
| Cognitive effects | Documented on neuropsychological testing | Similar to carbamazepine | Less pronounced than carbamazepine |
| Bone density | Decreased with long-term use | Less data | Prospective study: no adverse effect |
| IV formulation | Yes (approved 2016) | No | No |
| Cost | Low (generic) | Moderate (generic available) | Higher (brand) |
- Dosing: Start 400 mg once daily → increase to 800 mg/d after 1 week → increase to 1200 mg/d if needed; most patients do not require titration beyond 800 mg/d
- Conversion from carbamazepine: 300 mg eslicarbazepine for every 200 mg carbamazepine (when CBZ dose ≤800 mg); slower conversion for higher CBZ doses
- Should be avoided in IGE; best not combined with classic sodium channel blockers (though the combination with lamotrigine is less problematic than with carbamazepine)
Phenytoin
Mechanism and Pharmacokinetics
Phenytoin (in use since 1938) binds the active state of the sodium channel to prolong fast inactivation, reducing high-frequency firing. Available as oral and parenteral formulations. The prodrug fosphenytoin is preferred for parenteral use due to fewer local reactions and reliable IM absorption.
Phenytoin Nonlinear Kinetics
- Phenytoin has saturable (zero-order) metabolism: at a certain serum concentration (usually within the therapeutic range), the half-life starts increasing and small dose increases produce disproportionately large concentration rises
- Example: at 400 mg/d, the serum concentration may be ~13 μg/mL; increasing to 500 mg/d may push the concentration beyond 30 μg/mL with a high risk of toxicity
- When adjusting dose near the therapeutic range: use small increments of 30–60 mg only
- Paradoxical increase in seizures has been documented at concentrations >30 μg/mL
- Protein binding: ~90%; free fraction increases in hepatic/renal failure, hypoalbuminemia, pregnancy, age >50, and with concurrent valproate—check free phenytoin levels in these situations
Dosing and Monitoring
- Starting dose: 200–400 mg/d (can be initiated as a bedtime dose)
- Therapeutic range: Total 10–20 μg/mL; free 1–2 μg/mL
- Oral loading: 18 mg/kg divided into 3 doses given 2–3 hours apart
- IV administration: ECG and blood pressure monitoring required; maximum rate 50 mg/min for phenytoin, 150 mg/min for fosphenytoin
- IM phenytoin: Contraindicated (erratic absorption, sterile abscess); use fosphenytoin IM instead
Adverse Effects
Phenytoin is a potent enzyme inducer (CYP3A4, CYP2C9) with numerous drug interactions. Dose-related toxicity includes ataxia, nystagmus, dysarthria, diplopia, and cognitive impairment. Idiosyncratic reactions include allergic rash (~6%), rarely SJS/TEN, and hypersensitivity syndrome. Chronic toxicity includes gingival hyperplasia, acne, hirsutism, cerebellar atrophy, decreased bone density, anemia, and peripheral neuropathy.
Lacosamide
Mechanism and Pharmacokinetics
Lacosamide blocks sodium channels by enhancing slow inactivation—a distinct mechanism from traditional sodium channel blockers that enhance fast inactivation. This difference may explain its efficacy when combined with fast-inactivation sodium channel blockers and its generally better tolerability.
- Bioavailability: Excellent oral bioavailability
- Protein binding: Not clinically significant
- Metabolism: ~60% hepatic (inactive metabolites), ~40% unchanged renal elimination
- Half-life: ~13 hours
- Interactions: None or minimal—a significant advantage for polypharmacy patients
Efficacy and Dosing
Lacosamide is effective against focal seizures and generalized tonic-clonic seizures (Class I evidence for both). It is unlikely to exacerbate generalized seizures in most patients. FDA-approved for monotherapy and adjunctive therapy in patients ≥4 years. The parenteral formulation is effective against nonconvulsive seizures in critically ill patients, and multiple reports support efficacy in status epilepticus.
- Starting dose: 100 mg/d (once at bedtime or in 2 divided doses) for 1 week, then 100 mg twice daily
- Titration: Increase by 100 mg q1–2wk as needed
- Target dose: 200 mg/d; maximum 600 mg/d
- When used as adjunctive therapy, greater efficacy and better tolerability if combined with a non-sodium channel blocker
Lacosamide and Cardiac Conduction
- Lacosamide may produce dose-dependent PR interval prolongation
- Clinically significant in patients with known cardiac conduction abnormalities (first-degree AV block, second-degree AV block)
- Risk increases when combined with other PR-prolonging drugs (beta-blockers, calcium channel blockers, digoxin)
- An ECG before initiation is recommended in patients with cardiac history or risk factors
- Lacosamide is a controlled substance (DEA Schedule V)
Perampanel
Mechanism and Pharmacokinetics
Perampanel is the only selective noncompetitive AMPA glutamate receptor antagonist among ASMs, providing a unique mechanism of action. It has excellent oral bioavailability, 95% protein binding, extensive hepatic metabolism, and a remarkably long half-life of ~105 hours, allowing once-daily dosing.
Efficacy and Dosing
Perampanel is effective for focal seizures (monotherapy and adjunctive) and generalized tonic-clonic seizures (adjunctive). Anecdotal evidence supports efficacy against myoclonic and absence seizures. Case reports and series suggest particular effectiveness in progressive myoclonic epilepsies, which are usually therapy-resistant. Several reports document successful use in refractory and super-refractory status epilepticus.
- Starting dose: 2 mg/d for 1–3 weeks, then 4 mg/d
- Titration: Increase by 2 mg every 3 weeks as needed
- Target dose: 4 mg/d in monotherapy; up to 8 mg/d; maximum 12 mg/d with enzyme inducers
- Long half-life advantages: Allows abrupt discontinuation without taper (unlike most ASMs); reduces the impact of missed doses; associated with reduced health care utilization in two studies
- At 12 mg/d (not 8 mg/d), accelerates metabolism of levonorgestrel, reducing oral contraceptive efficacy
Perampanel: Aggression and Behavioral Effects (Boxed Warning)
- Aggression, hostility, and irritability are significant dose-dependent adverse effects, with an estimated incidence of ~20% at 12 mg/d
- This led to an FDA boxed warning regarding serious psychiatric and behavioral reactions
- Behavioral changes are more common in patients with intellectual disability
- Other common adverse effects: dizziness, somnolence, headache, fatigue, ataxia, blurred vision
- Perampanel is a controlled substance (DEA Schedule III)
- Enzyme inducers reduce perampanel efficacy; higher doses required but behavioral side effects increase proportionally
Gabapentin and Pregabalin
Mechanism
Both bind to the α2δ subunit of voltage-gated calcium channels, reducing calcium influx and neurotransmitter release under hyperexcitable conditions. They are narrow-spectrum agents effective only against focal seizures and may exacerbate generalized myoclonic and absence seizures.
| Feature | Gabapentin | Pregabalin |
|---|---|---|
| Oral bioavailability | Low and variable; decreases with increasing dose (saturable active transport: 60% at 300 mg → 29% at 1600 mg TID) | Very good; dose-independent |
| Protein binding | Negligible | None |
| Metabolism | None (excreted unchanged in urine) | None (excreted unchanged in urine) |
| Half-life | 5–7 hours | ~6 hours |
| Drug interactions | None (except antacids may impair absorption) | None |
| Dosing | Start 300–400 mg/d; increase by 300–400 mg/d; target 1200 mg/d in 3 divided doses; max 4800 mg/d | Start 75–150 mg/d; increase by 75–150 mg/wk; target 300 mg/d; max 600 mg/d |
| Epilepsy indication | Adjunctive for focal seizures; less effective than lamotrigine as monotherapy (large RCT) | Adjunctive for focal seizures (FDA); inferior to lamotrigine as first-line |
| Other indications | Postherpetic neuralgia (FDA); gabapentin enacarbil ER for RLS | Diabetic neuropathic pain, postherpetic neuralgia, fibromyalgia, spinal cord injury pain (FDA) |
| Key side effects | Drowsiness, dizziness, ataxia, weight gain, peripheral edema; cognitive slowing in elderly; emotional lability in children; myoclonus | Dizziness, somnolence, weight gain, peripheral edema; myoclonus at higher doses |
| Controlled substance | In some US states | Yes (DEA Schedule V) |
When to Choose Gabapentinoids for Epilepsy
- Best role is as adjunctive therapy for focal epilepsy, not as first-line monotherapy
- Particularly useful when the patient has comorbid neuropathic pain, headache, restless legs syndrome, or insomnia
- Major advantage: absence of drug interactions makes them safe in elderly patients on polypharmacy
- Major disadvantage: narrow spectrum (worsen generalized seizures), weight gain, and peripheral edema, particularly in the elderly
- Gabapentin's saturable absorption is a clinical limitation: increasing doses above 1200–1800 mg/d may not proportionally increase efficacy
- Neither agent should be used as first-line therapy based on comparative trial data showing inferiority to lamotrigine
Choosing Among Focal Epilepsy ASMs
| ASM | Best Suited For | Key Limitations |
|---|---|---|
| Carbamazepine (ER) | Focal epilepsy (historical gold standard); trigeminal neuralgia; bipolar disorder; cost-sensitive settings | Potent enzyme inducer; autoinduction; HLA-B*1502 testing required; may worsen IGE; abrupt withdrawal risk |
| Oxcarbazepine | First-line focal epilepsy; approved monotherapy; better tolerability than IR carbamazepine | Hyponatremia (especially elderly on diuretics); weak enzyme induction; abrupt withdrawal risk |
| Eslicarbazepine | Once-daily focal epilepsy monotherapy or adjunctive; carbamazepine replacement with fewer interactions | Higher cost; should avoid combining with other sodium channel blockers; limited data in generalized epilepsy |
| Phenytoin | Acute seizure management (IV/oral loading); resource-limited settings (low cost) | Nonlinear kinetics; narrow therapeutic window; enzyme inducer; chronic cosmetic and systemic toxicity |
| Lacosamide | First-line focal epilepsy; rapid titration; minimal interactions; IV for status epilepticus | PR prolongation (avoid with cardiac conduction disease); controlled substance; best combined with non-sodium channel blocker |
| Perampanel | Refractory focal/GTC epilepsy; progressive myoclonic epilepsy; adherence concerns (long half-life) | Behavioral effects (aggression boxed warning); controlled substance; enzyme inducers reduce efficacy |
| Gabapentin | Adjunctive focal epilepsy with comorbid pain; elderly (no interactions) | Dose-dependent absorption limit; not monotherapy; weight gain; narrow spectrum |
| Pregabalin | Adjunctive focal epilepsy with neuropathic pain or fibromyalgia | Not first-line; inferior to lamotrigine; weight gain; narrow spectrum; controlled substance |
References
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