Explain this PPT in easy and understandable language and focus on individual drugs and MCQ types questions for medicinal chemistry 1 exam
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📚 Antiepileptics — Medicinal Chemistry 1 Study Guide
🧠 PART 1: UNDERSTANDING THE BASICS
What is a Seizure/Convulsion?
A seizure is abnormal electrical activity in the brain. Types:
- Generalized — whole brain involved (e.g., tonic-clonic "grand mal")
- Partial/Focal — starts in one area
- Absence — brief loss of awareness (previously "petit mal"), no convulsions
- Unilateral, Erratic, Unclassified — less common categories
How Do Antiepileptic Drugs (AEDs) Work? (3 Mechanisms)
- Block voltage-gated ion channels (Na⁺, Ca²⁺, K⁺) — stops neurons from firing too rapidly
- Enhance GABA — GABA is the brain's "brake" neurotransmitter; more GABA = less seizure
- Reduce glutamate — glutamate is the brain's "accelerator"; less glutamate = less seizure
🏗️ PART 2: SAR (Structure-Activity Relationship) — General Rules
This is the most important MCQ topic in medicinal chemistry!
| Feature | Effect |
|---|---|
| Two substituents at C5 position (R and R', both hydrocarbons) | Essential for activity |
| Both R and R' = small alkyl groups | Active against absence seizures, NOT tonic-clonic |
| One R = aryl (benzene ring) | Active against tonic-clonic, NOT absence |
| Bulky aromatic group at C5 | Tonic-clonic + partial seizures activity |
| No bulky group at C5 | Absence seizure activity |
Memory tip: "Bulky = Big seizures (tonic-clonic); Small groups = Small seizures (absence)"
💊 PART 3: DRUG CLASSES — Detailed Notes
1. 🔴 BARBITURATES
| Drug | Key Points |
|---|---|
| Phenobarbital | Active against generalized tonic-clonic + partial seizures |
| Mephobarbital | N-demethylated → converted to phenobarbital (prodrug essentially) |
| Methabarbital | Used for absence seizures |
| Primidone | Metabolized (CYP2C9/19) → phenobarbital + PEMA (both active) |
Metabolism of Phenobarbital: p-hydroxylation → conjugation
2. 🟠 HYDANTOINS
Key structural feature: Like barbiturates but missing the 6-oxo group → weaker acids
| Drug | Key Points |
|---|---|
| Phenytoin | Na⁺ channel blocker; active against ALL seizures except absence |
| Fosphenytoin | Prodrug of phenytoin; better absorbed (IV/IM); phosphatase converts it to phenytoin |
| Ethotoin | Free H at C5 → very low potency |
| Mephenytoin | Intermediate potency |
Phenytoin Metabolism:
- p-hydroxylation of aromatic ring → glucuronide conjugation
- Poor water solubility → fosphenytoin (phosphate ester prodrug) was developed
Phenytoin Toxicity — VERY IMPORTANT:
- Metabolized to arene oxide (reactive intermediate)
- Arene oxide → deactivated by:
- Epoxide hydrolase → dihydrodiol (major urinary metabolite)
- Glutathione + glutathione transferase → GSH conjugate
- Arene oxide is responsible for:
- Hypersensitivity reactions (reacts with hepatic proteins)
- Teratogenicity (fetal hydantoin syndrome)
- Phenytoin is a potent liver enzyme inducer
3. 🟡 OXAZOLIDINEDIONES
Key structural feature: Replace N-H at position 1 of hydantoin with oxygen → oxazolidine-2,4-dione ring
| Drug | Key Points |
|---|---|
| Trimethadione | Absence seizures; metabolized → dimethadione (active) |
| Dimethadione | Ca²⁺ T-channel blocker; water soluble; excreted unchanged |
| Paramethadione | N-dealkylated → active metabolite (t½ = 14 days); excreted by kidney |
No bulky substituents at C5 = good for absence seizures
Toxicity: dermatological + hematological
4. 🟢 SUCCINIMIDES
Origin: Designed as a less toxic replacement for oxazolidinediones by replacing the "O" with CH₂
| Drug | Substituent at C3 | Activity | Metabolism |
|---|---|---|---|
| Ethosuximide | No bulky group (ethyl only) | Absence only | Oxidation of ethyl → hydroxyethyl (inactive) |
| Methsuximide | Bulky group | Absence + some partial seizures | N-dealkylation → active metabolite → p-hydroxylation |
| Phensuximide | Bulky (phenyl) | Absence + some tonic-clonic | N-dealkylation → active metabolite; t½ 5–12 hr |
5. 🔵 UREA DERIVATIVES (Urea & Monoacylureas)
Carbamazepine
- Two phenyl groups on the urea nitrogen → antigeneralized tonic-clonic activity pattern
- Mechanism: Na⁺ channel blocker
- Used in: tonic-clonic + partial seizures
- Metabolism: Epoxidation at the (Z) cis-stilbene double bond → 10,11-epoxide → 10,11-trans-diol
- The epoxide is responsible for idiosyncratic reactions (aplastic anemia)
- Oxcarbazepine was developed to avoid epoxide formation
- Carbamazepine and oxcarbazepine = iminostilbene derivatives
Oxcarbazepine
- Reduced → monohydroxy compound (major active metabolite)
- No epoxidation → less toxic than carbamazepine
- Used for partial seizures
- Mechanism: Na⁺ channel block
Phenacemide
- Most active aromatic acetylurea
- Any substitution on nitrogen → does NOT increase activity
- Aromatic substitution → activity decreases, hypnotic activity increases
- Serious toxicity limits use
6. 🟣 BENZODIAZEPINES
| Drug | Key Points |
|---|---|
| Clonazepam | Absence + myoclonic seizures; tolerance develops quickly |
Metabolism: Hydroxylation → glucuronidation; nitro group reduction → acetylation
7. ⚪ MISCELLANEOUS
Valproic Acid
- Used for: typical + atypical absence + tonic-clonic seizures
- Mechanisms:
- Na⁺ channel blocker
- Blocks T-type Ca²⁺ channels
- Metabolism: Carboxylic acid conjugation + oxidation of hydrocarbon chain
- Hepatotoxic metabolites: 2,4-diene-VPA and 4-epoxy-VPA
- Teratogenic; inhibitor of cytochrome P450
Gabapentin
- Structurally: 3-substituted GABA
- May activate L-glutamic acid decarboxylase (GAD) → increases GABA
- Has a binding site on Ca²⁺ channels
- Competes for biosynthesis of L-glutamic acid
- Broad-spectrum anticonvulsant (multiple mechanisms)
Felbamate
- Structurally similar to the anxiolytic meprobamate
- Mechanisms: Na⁺ channel blocker + NMDA receptor antagonist
- Used for: refractory tonic-clonic + complex partial seizures
- Serious side effects: aplastic anemia, hepatic failure
- Metabolism: CYP3A4 + CYP2E1 → 2-hydroxyfelbamate + p-hydroxyfelbamate → glucuronides
🧩 PART 4: MCQ-STYLE QUESTIONS
Q1. Which substituents at C5 of barbiturate ring make a drug active against ABSENCE seizures?
- A) Both aryl groups
- B) Both lower alkyl groups ✅
- C) One aryl, one alkyl
- D) No substituents
Q2. Fosphenytoin is a prodrug of phenytoin. How is it converted to phenytoin?
- A) CYP3A4 hydroxylation
- B) N-demethylation
- C) Phosphatase-catalyzed hydrolysis of phosphate ester ✅
- D) Glucuronide hydrolysis
Q3. The reactive intermediate responsible for hypersensitivity reactions and teratogenicity of phenytoin is:
- A) Dihydrodiol
- B) Catechol
- C) Arene oxide ✅
- D) p-Hydroxy phenytoin
Q4. Primidone is metabolized to which two active anticonvulsant metabolites?
- A) Phenytoin + PEMA
- B) Phenobarbital + PEMA ✅
- C) Carbamazepine + phenobarbital
- D) Dimethadione + phenobarbital
Q5. Succinimides were developed as a less toxic alternative to which class?
- A) Barbiturates
- B) Hydantoins
- C) Oxazolidinediones ✅
- D) Benzodiazepines
Q6. Oxcarbazepine was developed to avoid which toxic intermediate formed by carbamazepine?
- A) Dihydrodiol
- B) 10,11-epoxide ✅
- C) Arene oxide
- D) N-oxide
Q7. Gabapentin is structurally related to which neurotransmitter?
- A) Glutamate
- B) Dopamine
- C) GABA (it is a 3-substituted GABA) ✅
- D) Glycine
Q8. Which antiepileptic is structurally similar to the anxiolytic drug meprobamate?
- A) Valproic acid
- B) Clonazepam
- C) Gabapentin
- D) Felbamate ✅
Q9. Dimethadione (active metabolite of trimethadione) works by:
- A) Na⁺ channel block
- B) GABA enhancement
- C) T-type Ca²⁺ channel block ✅
- D) NMDA antagonism
Q10. The hepatotoxicity of valproic acid is associated with which metabolites?
- A) 2,4-diene-VPA only
- B) 4-epoxy-VPA only
- C) Both 2,4-diene-VPA and 4-epoxy-VPA ✅
- D) Hydroxy-VPA glucuronide
Q11. Ethotoin has very low potency because:
- A) It has two aryl groups at C5
- B) It has no C5 substituents
- C) It has a free hydrogen at C5 ✅
- D) It lacks the hydantoin ring
Q12. Which antiepileptic inhibits CYP450 AND blocks both Na⁺ and T-type Ca²⁺ channels?
- A) Phenytoin
- B) Carbamazepine
- C) Valproic acid ✅
- D) Gabapentin
Q13. Tolerance develops quickly with which antiepileptic used for absence seizures?
- A) Ethosuximide
- B) Valproic acid
- C) Clonazepam ✅
- D) Trimethadione
Q14. Which structural change distinguishes oxazolidinediones from hydantoins?
- A) Addition of a carbonyl group
- B) Replacement of N-H at position 1 with oxygen ✅
- C) Addition of an aryl group at C5
- D) Removal of the ring nitrogen
Q15. Mephobarbital is thought to owe most of its activity to:
- A) Itself (active drug)
- B) PEMA
- C) Phenobarbital (via N-demethylation) ✅
- D) Dimethadione
📋 QUICK REFERENCE TABLE
| Drug | Class | Seizure Type | Mechanism | Key Metabolism |
|---|---|---|---|---|
| Phenobarbital | Barbiturate | Tonic-clonic, partial | GABA↑ | p-hydroxylation |
| Primidone | Barbiturate pro-drug | Tonic-clonic, partial | → Phenobarbital+PEMA | CYP2C9/19 |
| Methabarbital | Barbiturate | Absence | GABA↑ | — |
| Phenytoin | Hydantoin | All except absence | Na⁺ block | Arene oxide → p-OH |
| Fosphenytoin | Hydantoin prodrug | All except absence | → Phenytoin | Phosphatase |
| Trimethadione | Oxazolidinedione | Absence | → Dimethadione | N-demethylation |
| Dimethadione | (active metabolite) | Absence | T-Ca²⁺ block | Excreted unchanged |
| Ethosuximide | Succinimide | Absence only | T-Ca²⁺ block | Oxidation (inactive) |
| Carbamazepine | Urea derivative | Tonic-clonic, partial | Na⁺ block | Epoxide→diol (toxic) |
| Oxcarbazepine | Urea derivative | Partial | Na⁺ block | → Monohydroxy |
| Valproic acid | Miscellaneous | Absence + tonic-clonic | Na⁺ + T-Ca²⁺ block | Conjugation+oxidation |
| Gabapentin | Miscellaneous | Broad spectrum | Ca²⁺ channel + GABA↑ | — |
| Felbamate | Miscellaneous | Tonic-clonic, partial | Na⁺ block + NMDA block | CYP3A4/2E1 |
| Clonazepam | Benzodiazepine | Absence, myoclonic | GABA↑ | Hydroxylation+acetylation |
⚡ HIGHEST YIELD EXAM POINTS
- Bulky aryl at C5 = tonic-clonic; small alkyl at C5 = absence
- Phenytoin's arene oxide causes hypersensitivity AND teratogenicity
- Fosphenytoin = phosphate ester prodrug → converted by phosphatase
- Primidone → phenobarbital + PEMA (both active)
- Carbamazepine epoxide → aplastic anemia; oxcarbazepine avoids this
- Succinimides = less toxic version of oxazolidinediones (O→CH₂)
- Ethosuximide = no bulky group → absence only
- Valproic acid inhibits CYP450 (unlike phenytoin/carbamazepine which induce it)
- Gabapentin = 3-substituted GABA analog
- Felbamate structurally similar to meprobamate (anxiolytic)
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