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%); MUST avoid in PWECP per 2024 AAN guidelines; screen for POLG before starting
- 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
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*1502 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*1502 | ≤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 | 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 | ~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); 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 (~20% at 12 mg/d); 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 | 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 | 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 → 29% at 1600 mg TID); 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; MUST avoid in PWECP (2024 AAN); 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 (specific association); autism risk |
| 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*1502 mandatory in Asian-descent patients (SJS/TEN); ~25% cross-reactivity with OXC for rash
- Active metabolite CBZ-epoxide: VPA inhibits epoxide hydrolase → epoxide accumulates → neurotoxicity
Cenobamate — Mandatory Slow Titration
- Titration (REMS): 12.5 mg × 2 wks → 25 mg × 2 wks → 50 mg × 2 wks → 100 mg × 2 wks → +50 mg q2wk to max 400 mg/d (~10–14 weeks)
- DRESS risk eliminated by slow titration; CYP2C19 inhibitor (raises PHT, phenobarbital, norclobazam) + CYP3A4 inducer (reduces OCP efficacy)
- 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); minimal interactions; IV for SE
- Perampanel: only AMPA antagonist; t½ ~105 h; boxed warning aggression (~20% at 12 mg/d); DEA Schedule III; useful in PME
- GBP: saturable absorption (60% at 300 mg → 29% at 1600 mg); practical ceiling ~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; 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 — halve LTG dose/titration. But this is the 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 → 29% at 1600 mg; practical ceiling ~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 before VPA. VPA in POLG carriers causes fatal hepatotoxicity. Screen in children <2 years and in suspected mitochondrial disease (unexplained liver disease, developmental regression, myopathy, ophthalmoplegia).
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.