Individual ASM Profiles & PK
Individual ASM Profiles & Pharmacokinetics
What Do You Need to Know?
- Know each ASM by: mechanism, half-life, metabolism route, enzyme effect (inducer/inhibitor/none), key side effect, teratogenicity, and weight effect — the board tests all of these
- Phenytoin = zero-order kinetics: small dose changes cause disproportionately large level changes; always check free levels in low albumin, renal failure, or VPA co-administration
- Lamotrigine: mandatory 6–8 week titration (SJS/TEN risk); half-life doubles with VPA, halves with enzyme inducers; clearance increases 50–100% in pregnancy
- Cenobamate: most potent adjunctive ASM (21% seizure-free in drug-resistant focal epilepsy); mandatory slow titration (DRESS risk)
- Valproate: highest teratogenicity (MCM 10.3%); per 2024 AAN guidance, avoid in PWECP whenever possible (use only when benefits clearly outweigh risks and alternatives are inadequate); screen for POLG when clinical suspicion exists (children <2 yr, suspected mitochondrial disease, unexplained liver disease, developmental regression)
- No-interaction ASMs: LEV, GBP, PGB, LCM — ideal for polypharmacy, elderly, transplant, and oncology patients
- Pregnancy: LTG clearance increases 50–100% (monthly levels); postpartum dose taper over 2–3 weeks to avoid toxicity
🚩 Don’t Miss — Test-Day Priorities
- Phenytoin = zero-order (saturable) kinetics: 300 → 400 mg/d can push levels 13 → 30+ μg/mL; adjust by 30–60 mg increments and check FREE PHT in low albumin, renal failure, pregnancy, or VPA co-admin; IV must run via central line or with fosphenytoin to avoid PURPLE GLOVE syndrome; long-term → gingival hyperplasia, cerebellar atrophy, megaloblastic anemia, osteopenia
- Carbamazepine = autoinduction: levels fall over 2–4 wks → recheck 4 wk after any dose change; SIADH/hyponatremia; aplastic anemia (rare); worsens absence/myoclonus; HLA-B*15:02 mandatory screen in Asian descent for CBZ/OXC/PHT (SJS/TEN); HLA-A*31:01 in Europeans/Japanese
- Oxcarbazepine > CBZ for hyponatremia: more frequent and more severe, especially in elderly on diuretics; same HLA-B*15:02 risk; eslicarbazepine is same Na-channel class with fewer interactions
- Valproate = highest teratogen (MCM 10.3%): NTDs, cardiac, IQ ↓7–10 points (NEAD), autism risk — avoid in PWECP per 2024 AAN; fatal hepatotoxicity in children <2 yr with POLG/Alpers; pancreatitis, thrombocytopenia, hyperammonemia (esp + TPM — consider L-carnitine); PCOS, weight gain, tremor, alopecia; glucuronidation inhibitor → doubles LTG
- Lamotrigine SJS/TEN risk: mandatory 6–8 wk titration; halve LTG dose when on VPA, double on inducers; clearance ↑ 50–100% in pregnancy (monthly levels, target ≥65% preconception); taper postpartum over 2–3 wk to avoid toxicity; may worsen JME myoclonus in 5–10%
- Levetiracetam = SV2A, no interactions: renal dose; behavioral irritability/aggression/depression (10–15%, pyridoxine may help in peds); preferred in pregnancy, ICU, transplant, oncology; brivaracetam = higher-affinity SV2A replacement (not add-on) with fewer behavioral effects
- Topiramate red flags: cognitive slowing ("Dopa-max"), kidney stones, metabolic acidosis (check HCO3−), oligohidrosis + hyperthermia in children, acute angle-closure glaucoma in first month (emergent stop), weight loss, oral clefts ~1.4% (RR ~9–11×); zonisamide is similar profile + sulfa cross-reactivity caution
- Lacosamide = slow Na inactivation: PR prolongation → baseline ECG in elderly / structural heart disease; atrial arrhythmia risk; IV available; preferred in critically ill / minimal interactions
- Perampanel: only AMPA antagonist; t½ ~105 h (nightly dosing); boxed warning for aggression/hostility (~12–20% at 12 mg/d) and suicidality; DEA Schedule III
- Cenobamate — 21% seizure-free in drug-resistant focal (unprecedented), but DRESS risk requires mandatory slow REMS titration (~11 wk to 200 mg, ~17–20 wk to 400 mg); CYP2C19 inhibitor (raises PHT, PB, norclobazam) + CYP3A4/2B6 inducer (reduces OCP, bupropion, methadone, efavirenz)
- Vigabatrin = first-line infantile spasms in TSC: irreversible GABA-T inhibitor; permanent visual field constriction → mandatory VFA q3 mo; reversible T2 BG/thalamic MRI changes; worsens absence/myoclonus
- Felbamate restricted — APLASTIC ANEMIA + HEPATIC FAILURE, last-resort for LGS; tiagabine can precipitate non-convulsive SE (avoid IGE); ethosuximide = T-type Ca, first-line CAE for pure absence only (no GTC coverage)
- Cardiac/REMS triad: Lacosamide → PR prolongation; Rufinamide → QT shortening (contraindicated in familial short QT); Fenfluramine → valvulopathy + PAH (mandatory echo baseline + q6 mo REMS)
🔍 Buzzwords & Pathognomonic FindingsAdverse effects / "must-not-miss" · Mechanism / kinetics · Drug interactions / pregnancy / monitoring
Adverse effects / "must-not-miss" reactions
- Purple glove syndrome → IV phenytoin extravasation
- Gingival hyperplasia + coarsened facies + cerebellar atrophy → phenytoin (chronic)
- SIADH / hyponatremia + autoinduction + aplastic anemia → carbamazepine
- Marked hyponatremia (worse than CBZ) → oxcarbazepine
- Fatal hepatotoxicity in child <2 yr → valproate in POLG/Alpers
- Pancreatitis + thrombocytopenia + hyperammonemia + alopecia → valproate
- SJS/TEN with rapid titration (esp on VPA) → lamotrigine
- Behavioral irritability / aggression / depression → levetiracetam
- Cognitive slowing ("Dopa-max") + word-finding difficulty + kidney stones + metabolic acidosis + oral clefts + acute angle-closure glaucoma + oligohidrosis → topiramate
- Kidney stones + oligohidrosis + sulfa cross-reactivity caution → zonisamide
- PR prolongation / atrial arrhythmia (ECG before starting in elderly) → lacosamide
- Boxed warning for aggression/hostility + suicidality → perampanel
- DRESS with rapid titration → cenobamate
- Permanent visual field constriction → mandatory VFA q3 mo; reversible T2 BG/thalamic MRI changes → vigabatrin
- Aplastic anemia + hepatic failure (restricted, last-resort LGS) → felbamate
- QT shortening (contraindicated in familial short QT) → rufinamide
- Valvulopathy + pulmonary arterial hypertension (REMS echo q6 mo) → fenfluramine
- Non-convulsive status when used in IGE → tiagabine
- SLE-like reaction → ethosuximide
- DRESS (fever + rash + eosinophilia + hepatitis/nephritis, 2–8 wk after start) → aromatic ASMs (PHT, CBZ, PB, LTG)
Mechanism / kinetics buzzwords
- Zero-order (saturable) kinetics → phenytoin
- Autoinduction of own CYP3A4 → carbamazepine
- SV2A binding → levetiracetam (and brivaracetam, ~20× higher affinity)
- Irreversible GABA-transaminase inhibitor → vigabatrin
- GAT-1 (GABA reuptake) inhibitor → tiagabine
- Enhances slow Na+ inactivation (unique) → lacosamide
- α2δ subunit of voltage-gated Ca2+ channel → gabapentin / pregabalin
- Selective AMPA receptor antagonist → perampanel
- T-type Ca2+ channel → ethosuximide (also ZNS partial)
- Carbonic anhydrase inhibition + multi-mechanism → topiramate, zonisamide
- Glucuronidation inhibitor → doubles LTG → valproate
- Dual Na+ channel + GABA-A positive allosteric modulator → cenobamate
- 1,5-benzodiazepine (less tolerance than clonazepam) → clobazam
- Saturable absorption (60% at 300 mg → ~27% at 1600 mg) → gabapentin
- 5-HT2C agonist + serotonin releaser → fenfluramine
Drug interactions / pregnancy / monitoring
- VPA doubles LTG — halve LTG dose; 25 mg QOD start with VPA on board → VPA + lamotrigine
- Enzyme inducers reduce OCP/DOAC/warfarin/many ASMs → use LNG-IUD or copper IUD → CBZ, PHT, phenobarbital, primidone
- OCPs lower LTG 40–60% — continuous OCP or LNG-IUD preferred → lamotrigine + estrogen
- LTG clearance ↑ 50–100% in pregnancy — monthly levels, target ≥65% preconception → lamotrigine in pregnancy
- Safest in pregnancy → lamotrigine + levetiracetam
- Folic acid ≥0.4 mg/d all PWECP; 4–5 mg/d on VPA or CBZ → preconception planning
- Therapeutic levels routinely useful → PHT, VPA, CBZ, PB, ESM (less rigid: LTG, LEV, LCM, ZNS)
- HLA-B*15:02 (Han Chinese, Thai, Malay, Filipino) → screen before CBZ/OXC/PHT
- HLA-A*31:01 (European, Japanese) → CBZ SJS/TEN, DRESS, maculopapular rash
- VFA every 3 months → vigabatrin (vision loss)
- Baseline ECG before starting in elderly / structural heart disease → lacosamide (PR prolongation)
- Baseline echo + q6 mo REMS → fenfluramine (valvulopathy / PAH)
- Stiripentol + CBD + cenobamate raise norclobazam (CYP2C19) → reduce clobazam 25–50% → "norclobazam-raising trio"
- Minimal interactions / no enzyme effect → LEV, LTG (mild), LCM, GBP, PGB, BRV
- Take with food (doubles bioavailability) → rufinamide
Master ASM Comparison Table
| Drug | Mechanism | Half-life | Metabolism | Key Interactions | Unique Side Effect | MCM Rate | Weight |
|---|---|---|---|---|---|---|---|
| Valproate | Multiple: Na+, T-Ca2+, GABA enhancement | 9–16 h | Hepatic (glucuronidation + β-oxidation) | Enzyme INHIBITOR; doubles LTG levels | Hepatotoxicity (POLG!), hyperammonemia (esp + TPM), pancreatitis, thrombocytopenia, tremor | 10.3% | Gain |
| Levetiracetam | SV2A binding | 6–8 h | Renal (no hepatic) | NO interactions | Irritability/behavioral (10–15%), psychiatric | 2.4–2.8% | Neutral |
| Brivaracetam | SV2A (20× higher affinity) | ~9 h | Hepatic CYP2C19 | Few; raises CBZ-epoxide, PHT ~20% | Fewer behavioral effects than LEV; NOT effective when added to LEV (same target) | Limited data | Neutral |
| Lamotrigine | Na+ channel (slow inactivation) | 25–33 h (alone); 60 h (+ VPA); 15 h (+ inducers) | Hepatic glucuronidation | Levels ↓ 40–60% by OCPs; doubled by VPA | SJS/TEN (mandatory 6–8 wk titration); mood stabilizer | 2.3–2.9% | Neutral |
| Carbamazepine | Na+ channel | 12–17 h (after autoinduction over 2–4 wks) | CYP3A4; potent INDUCER | Potent inducer; autoinduction | HLA-B*15:02 screen (Asian); rash cross-reactivity with OXC ~25%; diplopia, ataxia, hyponatremia, SIADH, aplastic anemia (rare) | 5.5% | Gain |
| Oxcarbazepine | Na+ channel (MHD active metabolite) | 9–11 h | Hepatic; NO autoinduction | Mild inducer (CYP3A4) | MORE hyponatremia than CBZ (esp elderly + diuretics); HLA-B*15:02 | ≤3% | Neutral |
| Eslicarbazepine | Na+ channel (pure S-enantiomer) | 13–20 h | Hepatic; once daily | Fewer interactions than CBZ/OXC | Lower hyponatremia than OXC; fewer cognitive effects | Limited data | Neutral |
| Phenytoin | Na+ channel | ZERO-ORDER (saturable; nonlinear) | Hepatic CYP2C9/2C19; potent INDUCER | Free fraction ↑ with low albumin, renal failure, VPA | HLA-B*15:02 risk (Asian descent — Han Chinese, Thai, Malay, Filipino): SJS/TEN — test before initiating; gingival hyperplasia, cerebellar atrophy, osteoporosis, peripheral neuropathy, coarsened facies, hirsutism | 6.4% | Neutral |
| Topiramate | Multiple: Na+, AMPA/kainate antagonist, GABA, carbonic anhydrase | ~21 h | 70% renal unchanged; 30% hepatic | Mild CYP3A4 inducer at ≥200 mg/d | Cognitive effects (primary limitation); kidney stones 4%; word-finding difficulty; oral clefts ~1.4% (RR ~9–11× background); acute angle-closure glaucoma (first month — emergent stop); metabolic acidosis (check serum HCO3−); oligohydrosis in children | ~3.9% | Loss |
| Zonisamide | T-type Ca2+ and Na+ | ~60 h (once daily) | ~65% hepatic | Minimal | Kidney stones; cognitive (less than TPM); oligohydrosis in children | ≤3% | Loss |
| Lacosamide | Enhances SLOW Na+ inactivation (unique) | ~13 h | 60% hepatic / 40% renal | Minimal | PR prolongation (ECG before starting); atrial arrhythmia risk in elderly / structural heart disease; DEA Schedule V; IV available for SE | Limited data | Neutral |
| Perampanel | ONLY selective AMPA antagonist | ~105 h | Extensive hepatic | Levels ↓ by inducers | Aggression/hostility boxed warning (~12–20% at 12 mg/d, dose-dependent); DEA Schedule III; effective in PME | Limited data | Neutral |
| Cenobamate | Dual: Na+ channel + GABA-A PAM | 50–60 h | Extensive hepatic | CYP2C19 inhibitor; CYP3A4 inducer; CYP2B6 inducer (reduces bupropion, methadone, efavirenz) | DRESS risk → mandatory slow titration; 21% seizure-free in drug-resistant focal epilepsy | No data | Neutral |
| Clobazam | GABA-A (1,4-benzodiazepine) | 36–42 h | CYP3A4 → norclobazam (CYP2C19) | CYP2C19 polymorphism affects norclobazam levels; CYP2C19 poor metabolizers (~15–20% Asians, ~3% Caucasians) have 3–5× higher norclobazam → more sedation | Less sedation than clonazepam; FDA approved for LGS | Limited data | Neutral |
| Ethosuximide | T-type Ca2+ channel | 40–60 h | Hepatic CYP3A4 | Minimal | GI side effects; absence seizures ONLY; first-line CAE | Limited data | Neutral |
| Gabapentin | α2δ subunit Ca2+ channel | 5–7 h | 100% renal unchanged | NO interactions | SATURABLE absorption (60% at 300 mg → ~27% at 1600 mg per single dose); daily ceiling ~3600 mg/d (FDA max); benefit plateaus ~1800 mg/d; adjunctive only | Limited data | Gain |
| Pregabalin | α2δ subunit Ca2+ channel | ~6 h | 100% renal unchanged | NO interactions | Dose-independent bioavailability (unlike GBP); neuropathic pain; DEA Schedule V | Limited data | Gain |
Teratogenicity Ranking
ASM Teratogenicity — Major Congenital Malformation Rates
| Rank | ASM | MCM Rate | Key Malformations / Notes |
|---|---|---|---|
| 1 | Valproate | 10.3% | Neural tube defects, cardiac; avoid in PWECP whenever possible (2024 AAN: use only when benefits clearly outweigh risks and alternatives inadequate); neurodevelopmental: IQ ↓7–10 points (NEAD study), autism risk |
| 2 | Phenobarbital | 6.5% | Cardiac, orofacial |
| 3 | Phenytoin | 6.4% | Fetal hydantoin syndrome: midface hypoplasia, digit/nail hypoplasia |
| 4 | Carbamazepine | 5.5% | Neural tube defects, cardiac |
| 5 | Topiramate | ~3.9% | Oral clefts ~1.4% (RR ~9–11× vs background); autism risk; acute angle-closure glaucoma in first month (emergent stop); metabolic acidosis; oligohydrosis in children |
| 6 | Oxcarbazepine | ≤3% | Less data than CBZ; likely lower risk |
| 7 | Lamotrigine | 2.3–2.9% | Safest well-studied ASM in pregnancy; >7000 monotherapy exposures |
| 8 | Levetiracetam | 2.4–2.8% | Safe; >2100 monotherapy exposures; close to background rate (~2.5%) |
Board Pearl
VPA neurodevelopmental toxicity (NEAD study): In utero VPA exposure reduces IQ by 7–10 points at age 6 and increases autism spectrum disorder risk. This is dose-dependent and persists across all dose ranges. No other ASM shows comparable cognitive teratogenicity.
Pregnancy Pharmacokinetics
Clearance Changes in Pregnancy
| ASM | Clearance Change | Mechanism | Monitoring |
|---|---|---|---|
| Lamotrigine | ↑ 50–100% | Estrogen-driven ↑ glucuronidation | Monthly levels; target ≥65% of preconception level |
| Levetiracetam | ↑ 40–60% | Increased renal clearance + GFR | Monthly or quarterly levels |
| Oxcarbazepine | ↑ 30–50% | Increased glucuronidation of MHD | Quarterly levels |
| Phenytoin | Variable | Free fraction ↑ (lower albumin); total levels misleading | Free PHT levels only |
Key Pregnancy Management Points
- Preconception: Establish baseline ASM level during best seizure control — this becomes the pregnancy target
- During pregnancy: Increase ASM dose proactively as levels fall; do not wait for seizure breakthrough
- Postpartum: Taper dose increases over 2–3 weeks — estrogen drops rapidly, glucuronidation normalizes → ASM toxicity risk
- Folic acid: ≥0.4 mg/day all PWECP; 4–5 mg/day for VPA or CBZ (higher NTD risk)
Board Pearl
LTG in pregnancy: Estrogen drives glucuronidation, increasing LTG clearance by 50–100%. Levels must be checked monthly. If levels fall below 65% of the preconception baseline, increase the dose. Postpartum, LTG levels rebound within days — taper dose increases over 2–3 weeks to prevent toxicity (diplopia, ataxia).
Contraception Interactions
ASM Effects on Hormonal Contraception
| ASM Category | Effect on Hormonal Contraception | Clinical Recommendation |
|---|---|---|
| Potent enzyme inducers: CBZ, PHT, phenobarbital, primidone | Significantly reduce estrogen + progestin levels; contraceptive failure rate doubled | Non-hormonal (copper IUD) or LNG-IUD (levonorgestrel intrauterine device — local release, not affected by inducers) |
| Weak/moderate inducers: OXC, ESL, TPM >200 mg/d, cenobamate, PER 12 mg/d | May reduce efficacy, especially at higher doses | Consider LNG-IUD or copper IUD; higher-dose OCP if hormonal method preferred |
| No effect on contraception: LEV, LTG, VPA, LCM, GBP, PGB, ZNS, BRV | Do not reduce contraceptive efficacy | Standard contraceptive methods are appropriate |
The LTG – OCP Bidirectional Interaction
- Estrogen-containing OCPs lower LTG levels by 40–60% via induced glucuronidation
- During the pill-free week, LTG levels rebound → cyclical side effects
- Starting OCPs may require LTG dose increase; stopping OCPs may cause LTG toxicity
- LTG does NOT reduce OCP efficacy (the interaction is one-directional for contraceptive failure)
- Best practice: Continuous (non-cyclic) OCP use or LNG-IUD avoids the fluctuation
Board Pearl
Best contraception for women on enzyme-inducing ASMs = LNG-IUD. The levonorgestrel intrauterine device releases hormone locally, bypassing hepatic first-pass metabolism. It is not affected by enzyme inducers and provides >99% efficacy. Copper IUD is the non-hormonal alternative. Depot medroxyprogesterone (DMPA) may also remain effective, but data are limited.
High-Yield Individual Drug Pearls
Phenytoin — Zero-Order Kinetics
- Saturable metabolism: small dose change (e.g., 300 → 400 mg/d) = massive level increase; adjust by 30–60 mg increments only
- Free fraction ↑ with low albumin, renal failure, pregnancy, VPA — always check free PHT levels in these settings
- Paradoxical seizures at levels >30 μg/mL
- Chronic toxicity: gingival hyperplasia, cerebellar atrophy (irreversible), osteoporosis, peripheral neuropathy, coarsened facies, hirsutism
Carbamazepine — Autoinduction
- Autoinduction over 2–4 weeks: CBZ induces its own CYP3A4 → levels fall; recheck 4 weeks after any dose change
- HLA-B*15:02 mandatory in Asian-descent patients (Han Chinese, Thai, Malay, Filipino) for CBZ, OXC, AND PHT — SJS/TEN; test before initiating; ~25% cross-reactivity with OXC for rash
- HLA-A*31:01 (Caucasian and Japanese) — CBZ-associated SJS/TEN, DRESS, and maculopapular rash (broader phenotype than B*1502)
- Active metabolite CBZ-epoxide: VPA inhibits epoxide hydrolase → epoxide accumulates → neurotoxicity
Cenobamate — Mandatory Slow Titration
- Titration (REMS): 12.5 mg × 2 wk → 25 mg × 2 wk → 50 mg × 2 wk → 100 mg × 2 wk → 150 mg × 2 wk → 200 mg/d (target, ~11 weeks) → may increase by 50 mg q2wk to max 400 mg/d (~17–20 weeks total)
- DRESS risk eliminated by slow titration; CYP2C19 inhibitor (raises PHT, phenobarbital, norclobazam) + CYP3A4 inducer (reduces OCP efficacy) + CYP2B6 inducer (reduces bupropion, methadone, efavirenz)
- Phase 3: 21% seizure-free at 400 mg/d — unprecedented for adjunctive ASM therapy
Lacosamide, Perampanel, Gabapentinoids, Clobazam, Ethosuximide
- Lacosamide: enhances slow Na+ inactivation (complements fast-inactivation blockers); PR prolongation (ECG before starting); atrial arrhythmia risk in elderly / structural heart disease; minimal interactions; IV for SE
- Perampanel: only AMPA antagonist; t½ ~105 h; boxed warning aggression (~12–20% at 12 mg/d, dose-dependent); DEA Schedule III; useful in PME
- GBP: saturable absorption (60% at 300 mg → ~27% at 1600 mg per single dose); FDA daily max 3600 mg/d but most clinical benefit plateaus ~1800 mg/d. PGB: dose-independent bioavailability; both have zero interactions, 100% renal, adjunctive only
- Clobazam: 1,4-BZD (less sedation than clonazepam); CYP2C19 polymorphism affects norclobazam levels — CYP2C19 poor metabolizers (~15–20% Asians, ~3% Caucasians) have 3–5× higher norclobazam → more sedation; FDA for LGS
- Ethosuximide: T-type Ca2+; first-line CAE (Glauser et al.); absence seizures ONLY; GI side effects most common
Board Pearls & Clinical Pearls
Board Pearls
- PHT zero-order kinetics: 300 → 400 mg/d can push levels from 13 → 30+ μg/mL. Adjust by 30–60 mg only. Check FREE levels in hypoalbuminemia, renal failure, VPA co-administration
- VPA + TPM = hyperammonemia: Check ammonia in any VPA patient with unexplained confusion, especially if on TPM. Can occur with normal VPA levels
- VPA + LTG = double-edged sword: VPA doubles LTG levels. When adding LTG to existing VPA, use the explicit slow schedule: 25 mg QOD × 2 wk → 25 mg/d × 2 wk → 25–50 mg/d, target 100–200 mg/d (much slower than monotherapy titration). Best-documented synergistic ASM combination
- CBZ autoinduction: Levels fall over 2–4 weeks; recheck 4 weeks after any dose change
- LTG + OCP: OCPs lower LTG 40–60%. Starting OCPs → seizure risk; stopping → toxicity. Use continuous OCP or LNG-IUD
- GBP saturable absorption: 60% at 300 mg → ~27% at 1600 mg (per single dose); FDA daily max 3600 mg/d but clinical benefit plateaus ~1800 mg/d
- Cenobamate 21% seizure-free in drug-resistant focal epilepsy — unprecedented. Consider before surgery in non-ideal candidates
Clinical Pearl
BRV is a replacement for LEV, not an add-on. Both bind SV2A; combining them saturates the same target. Switch to BRV for intolerable behavioral effects. BRV has ~20× higher SV2A affinity and fewer behavioral side effects (3.2% vs. 10–15%).
Clinical Pearl
Postpartum ASM toxicity is underrecognized. LTG clearance increases 50–100% during pregnancy; after delivery, estrogen drops and glucuronidation normalizes within days. Taper dose increases over 2–3 weeks or LTG levels spike (diplopia, ataxia). Same principle for LEV and OXC.
Clinical Pearl
Screen for POLG when clinical suspicion exists. VPA in POLG carriers (Alpers syndrome) causes fatal hepatotoxicity. POLG testing is not universally required before every VPA prescription, but should be obtained when there are red flags: children <2 years, suspected mitochondrial disease, unexplained liver disease, developmental regression, myopathy, ophthalmoplegia, or family history. Strong indication in pediatric refractory epilepsy of unknown cause.
Dosing Quick-Reference (Start / Maintenance / Max)
| ASM | Starting Dose | Maintenance / Target | Max Daily Dose |
|---|---|---|---|
| Levetiracetam (LEV) | 500 mg BID | 1000–3000 mg/d (divided BID) | 3000 mg/d |
| Valproate (VPA) | 250 mg BID | 750–2000 mg/d | 60 mg/kg/d |
| Lamotrigine (LTG) — monotherapy | 25 mg/d × 2 wk → 50 mg/d × 2 wk → 100 mg/d × 1 wk | 100–400 mg/d (divided BID) | 500 mg/d (with inducers may need higher) |
| Carbamazepine (CBZ) | 200 mg BID | 600–1200 mg/d | ~1600 mg/d |
| Oxcarbazepine (OXC) | 300 mg BID | 900–2400 mg/d | 2400 mg/d |
| Phenytoin (PHT) | Load 20 mg/kg IV/PO | 4–6 mg/kg/d (300–400 mg/d typical, divided) | Titrate to level; monitor |
| Topiramate (TPM) | 25 mg/d | 200–400 mg/d (increase weekly) | 400–1600 mg/d (indication-dependent) |
| Zonisamide (ZNS) | 100 mg/d | 200–400 mg/d | 600 mg/d |
| Lacosamide (LCM) | 50 mg BID | 200–400 mg/d | 400 mg/d |
| Perampanel (PER) | 2 mg qHS | 8–12 mg/d (qHS dosing) | 12 mg/d |
| Brivaracetam (BRV) | 50 mg BID | 100–200 mg/d | 200 mg/d |
| Gabapentin (GBP) | 300 mg TID | 900–1800 mg/d (benefit plateaus >1800) | 3600 mg/d (FDA max; benefit plateau ~1800) |
| Pregabalin (PGB) | 75 mg BID | 150–600 mg/d (divided BID–TID) | 600 mg/d |
| Cenobamate (CNB) | 12.5 mg/d (slow REMS titration) | 200 mg/d target (~11 wks) | 400 mg/d (~17–20 wks) |
Board Pearl
LTG titration with VPA: Do NOT use the monotherapy schedule. With VPA on board, start at 25 mg every other day × 2 wk → 25 mg/d × 2 wk → 25–50 mg/d, target 100–200 mg/d. Going faster invites SJS/TEN.
Therapeutic Drug Monitoring (TDM) — Target Levels
| ASM | Therapeutic Range | Notes |
|---|---|---|
| Phenytoin (PHT) | 10–20 μg/mL total; 1–2 μg/mL free | Check FREE levels in hypoalbuminemia, renal failure, pregnancy, VPA co-admin |
| Valproate (VPA) | 50–100 μg/mL | Trough; check ammonia if encephalopathic |
| Carbamazepine (CBZ) | 4–12 μg/mL | Recheck 4 wk after dose change (autoinduction) |
| Phenobarbital (PB) | 15–40 μg/mL | Long t½ (~80–100 h); steady state ~3 wk |
| Ethosuximide (ESM) | 40–100 μg/mL | Absence epilepsy |
| Lamotrigine (LTG) | 3–14 μg/mL (less rigid) | Useful for pregnancy monitoring (target ≥65% preconception) |
| Levetiracetam (LEV) | 12–46 μg/mL (less commonly monitored) | Useful in pregnancy, renal impairment |
| Lacosamide (LCM) | ~5–10 μg/mL (informal target) | Not routinely required |
Renal Dose Adjustments (CrCl-based)
| ASM | Renal Excretion | CrCl-Based Adjustment |
|---|---|---|
| Levetiracetam (LEV) | ~66% renal unchanged | CrCl 50–80: 500–1000 mg BID; 30–50: 250–750 mg BID; <30: 250–500 mg BID; HD: 500–1000 mg q24h + 250–500 mg post-dialysis supplement |
| Brivaracetam (BRV) | Hepatic primary; renal metabolites | Minimal adjustment in renal impairment (advantage vs LEV); not recommended in ESRD/HD |
| Lacosamide (LCM) | 40% renal unchanged | CrCl ≤30 or HD: max 300 mg/d; supplement up to 50% post-HD |
| Gabapentin (GBP) | 100% renal unchanged | CrCl 30–59: 400–1400 mg/d; 15–29: 200–700 mg/d; <15: 100–300 mg/d; HD: 125–350 mg post-dialysis |
| Pregabalin (PGB) | 100% renal unchanged | CrCl 30–60: 50% reduction; 15–30: 75% reduction; <15: ~90% reduction; supplement post-HD |
| Topiramate (TPM) | ~70% renal unchanged | CrCl <70: halve the dose; supplement post-HD |
| Zonisamide (ZNS) | ~35% renal unchanged | Slower titration in CrCl <50; avoid CrCl <20 |
| Vigabatrin (VGB) | ~80% renal unchanged | CrCl <50 requires adjustment: 50–80: reduce 25%; 30–50: reduce 50%; 10–30: reduce 75% |
Clinical Pearl
Renal/dialysis ASM picks: BRV has minimal renal adjustment (vs LEV which needs dose reduction and post-HD supplement). LTG and VPA do not need renal dose adjustment but VPA is highly protein-bound — free fraction rises in uremia. LCM, GBP, PGB are dialyzable — supplement after HD.
Seizure-Aggravating ASMs (Avoid in IGE / JME / Absence)
High-Yield Safety Box
- AVOID in generalized epilepsies (IGE, JME, absence):
- Carbamazepine (CBZ) — worsens absence and myoclonus (Na-channel mechanism)
- Oxcarbazepine (OXC) — same mechanism as CBZ
- Phenytoin (PHT) — can worsen absence and myoclonus
- Gabapentin (GBP) — worsens absence and myoclonus
- Pregabalin (PGB) — same as GBP
- Tiagabine (TGB) — can precipitate non-convulsive SE in IGE (GABAergic)
- Vigabatrin (VGB) — worsens absence and myoclonus (GABAergic)
- Use with caution: Lamotrigine (LTG) — can worsen myoclonus in ~5–10% of JME patients; remains effective in IGE GTCs and absence overall
- Preferred for IGE/JME/absence: VPA (men, non-PWECP), LEV, LTG (with myoclonus caveat), TPM, ZNS, PER, clobazam; ESM for pure absence
Newer ASMs — Stiripentol, Fenfluramine, Cannabidiol, Rufinamide
Stiripentol (Diacomit)
- Mechanism: GABA-A allosteric modulator; potent CYP inhibitor (CYP2C19, CYP3A4)
- FDA-approved: Dravet syndrome (2018) — adjunctive with VPA + clobazam (mandatory combination)
- Key interaction: Markedly raises norclobazam levels (via CYP2C19 inhibition) → sedation, ataxia — reduce clobazam dose preemptively
- Side effects: Somnolence, decreased appetite, weight loss, ataxia, tremor; neutropenia (monitor CBC)
- Dosing: 50 mg/kg/d divided BID–TID (max 3000 mg/d)
Fenfluramine (Fintepla)
- Mechanism: Serotonin releaser + 5-HT2C receptor agonist; sigma-1 receptor activity
- FDA-approved: Dravet syndrome (2020), Lennox-Gastaut syndrome (2022)
- REMS — mandatory cardiac monitoring: Baseline echocardiogram + q6 months during therapy and 3–6 months after discontinuation to monitor for valvular heart disease and pulmonary arterial hypertension (PAH) (legacy concern from fen-phen era)
- Side effects: Decreased appetite, weight loss, somnolence, fatigue; rare valvulopathy/PAH
- Dosing: Start 0.1 mg/kg BID; target 0.2–0.35 mg/kg BID (max 26 mg/d without stiripentol; 17 mg/d with stiripentol due to interaction)
- Interaction: Stiripentol/clobazam co-administration requires dose reduction
Cannabidiol (Epidiolex)
- Mechanism: Multiple, not fully characterized; modulates intracellular calcium, adenosine signaling; not a CB1/CB2 agonist at therapeutic doses
- FDA-approved: Dravet syndrome, Lennox-Gastaut syndrome, tuberous sclerosis complex (TSC)
- Key interactions (CYP3A4 and CYP2C19 inhibitor): Raises norclobazam markedly → sedation (often need to reduce clobazam dose by 25–50%); raises warfarin INR
- Hepatotoxicity: Transaminitis, especially when co-administered with VPA — baseline + monthly LFTs × 6 months, then periodic
- Side effects: Somnolence, decreased appetite, diarrhea, transaminitis
- Dosing: Start 2.5 mg/kg BID; target 5–10 mg/kg BID (max 20 mg/kg/d)
Rufinamide (Banzel)
- Mechanism: Sodium channel modulator (prolongs inactive state)
- FDA-approved: Lennox-Gastaut syndrome (drop attacks — tonic and atonic seizures)
- CRITICAL: QT SHORTENING — CONTRAINDICATED in familial short QT syndrome; baseline ECG advised
- Food enhances absorption — take with food (bioavailability nearly doubles)
- Interactions: VPA raises rufinamide levels (~70%); rufinamide is a weak CYP3A4 inducer (may reduce OCP, triazolam)
- Side effects: Somnolence, headache, dizziness, fatigue, nausea/vomiting; rare DRESS
- Dosing: Start 10 mg/kg/d (peds) or 400–800 mg/d (adults) divided BID; target 45 mg/kg/d (peds, max 3200 mg/d) or 3200 mg/d (adults)
Board Pearl
The "norclobazam-raising trio": Stiripentol, cannabidiol, and cenobamate all inhibit CYP2C19 and markedly raise norclobazam levels → sedation, ataxia. When adding any of these to a patient on clobazam, anticipate the interaction and reduce clobazam by 25–50%. Especially relevant in Dravet and LGS patients who are typically on clobazam already.
Board Pearl
Cardiac surveillance ASMs:
• Lacosamide → PR prolongation + atrial arrhythmia (ECG before starting in elderly / structural heart disease)
• Rufinamide → QT shortening (contraindicated in familial short QT)
• Fenfluramine → valvulopathy and pulmonary arterial hypertension (mandatory echo baseline + q6 months REMS)
References
- Abou-Khalil B. Update on antiseizure medications 2025. Continuum (Minneap Minn) 2025;31(1):123–165.
- Tomson T, Battino D, Bonizzoni E, et al. Dose-dependent teratogenicity of valproate in mono- and polytherapy. Neurology 2015;85(10):866–872.
- Cohen JM, Alvestad S, Cesta CE, et al. Comparative safety of antiseizure medication monotherapy for major malformations. Ann Neurol 2023;93:551–562.
- Meador KJ, Baker GA, Browning N, et al. Fetal antiepileptic drug exposure and cognitive outcomes at age 6 years (NEAD study). Lancet Neurol 2013;12(3):244–252.
- Krauss GL, Klein P, Brandt C, et al. Safety and efficacy of adjunctive cenobamate (YKP3089) in patients with uncontrolled focal seizures. Lancet Neurol 2020;19(1):38–48.
- Marson AG, Al-Kharusi AM, Alwaidh M, et al. The SANAD study of effectiveness of carbamazepine, gabapentin, lamotrigine, oxcarbazepine, or topiramate for treatment of partial epilepsy. Lancet 2007;369(9566):1000–1015.
- Glauser TA, Cnaan A, Shinnar S, et al. Ethosuximide, valproic acid, and lamotrigine in childhood absence epilepsy. N Engl J Med 2010;362(9):790–799.
- Tomson T, Battino D, Bromley R. Management of epilepsy in pregnancy: a report from the International League Against Epilepsy Task Force on Women and Pregnancy. Epileptic Disord 2019;21(6):497–517.
- Patsalos PN, Spencer EP, Berry DJ. Therapeutic drug monitoring of antiepileptic drugs in epilepsy: a 2018 update. Ther Drug Monit 2018;40(5):526–548.
- Sabers A, Ohman I, Christensen J, Tomson T. Oral contraceptives reduce lamotrigine plasma levels. Neurology 2003;61(4):570–571.
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