Medicinal Chemistry: Anticonvulsants, Antipsychotics & SAR

Anticonvulsant Drugs

Anticonvulsants are agents used to prevent or control seizures by reducing abnormal, excessive neuronal activity in the brain.

Classification

• Hydantoins: Phenytoin
• Succinimides: Ethosuximide
• Iminostilbenes: Carbamazepine
• Barbiturates: Phenobarbital
• Others: Valproic acid, Diazepam

General SAR

• Hydrophobic aromatic ring is essential.
• Electron-withdrawing groups increase activity by improving membrane penetration.
• Imide or amide groups are critical for binding.
• Optimal lipophilicity is required; excessive polarity leads to inactivity, while excessive lipophilicity causes toxicity.
• Planarity increases CNS activity by facilitating receptor binding.

General Mechanism of Action

Anticonvulsants act by blocking Na⁺ channels, enhancing GABA activity, or blocking Ca²⁺ channels, resulting in reduced neuronal firing.

Key Drugs

• Phenytoin: Blocks voltage-gated Na⁺ channels. Used for epilepsy and status epilepticus. Synthesis: Benzil + Urea.
• Ethosuximide: Blocks T-type Ca²⁺ channels in the thalamus. Used for absence seizures. Synthesis: Succinic acid derivative + ammonia.
• Carbamazepine: Blocks voltage-gated Na⁺ channels. Used for partial/generalized seizures, trigeminal neuralgia, and bipolar disorder.

Antipsychotic Drugs

Antipsychotics treat psychosis (schizophrenia, mania) by reducing hallucinations, delusions, and agitation.

Classification

• Typical (First Generation): Phenothiazines (Chlorpromazine), Butyrophenones (Haloperidol), Thioxanthenes (Chlorprothixene).
• Atypical (Second Generation): Clozapine, Risperidone, Olanzapine.

SAR of Phenothiazines

• Tricyclic phenothiazine nucleus is essential.
• Substitution at position 2 (e.g., Cl, CF3) increases activity.
• A 3-carbon side chain at N-10 is optimal.
• Tertiary amines in the side chain provide higher activity.

Chlorpromazine Hydrochloride

Mechanism: Blocks D2 dopamine receptors, as well as α1, H1, and muscarinic receptors. Uses: Schizophrenia, mania, anxiety, and anti-emetic.

Benzodiazepines and Sedative-Hypnotics

SAR of Benzodiazepines

• 1,4-Benzodiazepine nucleus is essential.
• Electron-withdrawing groups at C-7 increase potency.
• Substitution at the C-5 phenyl ring enhances activity.
• Carbonyl group at C-2 is required for binding.

Key Drugs

• Diazepam: Enhances GABA-A action, increasing Cl⁻ influx. Used for anxiety, insomnia, and muscle spasms.
• Barbital: Enhances GABA-A receptor activity. Used for sedation and insomnia.

Cholinergic and Anticholinergic Agents

Dicyclomine Hydrochloride

Mechanism: Antimuscarinic; blocks M1 and M3 receptors to inhibit GI smooth muscle contraction. Uses: IBS and intestinal spasms.

Ipratropium Bromide

Mechanism: Competitive muscarinic antagonist; blocks M3 receptors in bronchial smooth muscle. Uses: COPD and asthma.

Neostigmine

Mechanism: Reversible acetylcholinesterase (AChE) inhibitor. Uses: Myasthenia gravis and post-operative ileus.

Adrenergic Agents

Beta Blockers

SAR: Aryloxypropanolamine nucleus is essential. The β-hydroxyl group and secondary amine are required for receptor binding. Propranolol: Non-selective β-blocker used for hypertension and angina. Tolazoline: α-adrenergic antagonist used for peripheral vasospasm.

Sympathomimetics

SAR: Phenylethanolamine nucleus is essential. Bulky N-substitution increases β-selectivity. Salbutamol: Selective β2 agonist for asthma. Phenylephrine: Selective α1 agonist for nasal congestion.

Physicochemical Parameters and Metabolism

Key Principles

• Bioisosterism: Replacing atoms/groups with similar ones to maintain activity.
• Stereoisomerism: 3D arrangement affects receptor binding (e.g., Thalidomide).
• Partition Coefficient: Determines lipid solubility and absorption.
• Ionization: Unionized forms cross membranes more easily.

Drug Metabolism

Phase I: Functionalization (Oxidation, Reduction, Hydrolysis) via CYP450 enzymes. Phase II: Conjugation (Glucuronidation, Sulfation, Acetylation) to increase water solubility for excretion.