JNTU B.Pharmacy III MEDICINAL CHEMISTRY-II Previous Question Paper (March-2021 - R17)

H1-antagonists are classified based on their chemical structure into the following groups:

• Ethanolamine ethers: Diphenhydramine, Doxylamine, Clemastine.
• Ethylenediamines: Mepyramine, Tripelennamine.
• Alkylamines: Chlorpheniramine, Pheniramine.
• Piperazines: Cyclizine, Meclizine, Cetirizine.
• Tricyclics (Phenothiazines): Promethazine, Trimeprazine.
• Piperidines: Cyproheptadine, Fexofenadine.

The synthesis of Diphenhydramine hydrochloride involves the condensation of benzhydryl bromide with 2-dimethylaminoethanol in the presence of a base (such as sodium carbonate) or via the reaction of benzhydrol with 2-dimethylaminoethyl chloride in the presence of an acid catalyst. The resulting base is then treated with hydrochloric acid to form the stable hydrochloride salt.

Mechanism of Action: H1-antagonists act as inverse agonists rather than simple competitive antagonists. They bind to the H1-receptor in its inactive conformation, thereby stabilizing the inactive state and shifting the equilibrium away from the active state. This prevents histamine from binding to the receptor and inhibits the downstream signaling pathways, such as the activation of phospholipase C and the subsequent increase in intracellular calcium, which are responsible for allergic symptoms like vasodilation, capillary permeability, and smooth muscle contraction.

Gastric proton pump inhibitors (PPIs) are a class of drugs that reduce gastric acid production by irreversibly inhibiting the H+/K+-ATPase enzyme system in the secretory surface of gastric parietal cells. While cimetidine is a H2-receptor antagonist rather than a PPI, it is foundational in the treatment of gastric acid-related disorders.

Classification of Proton Pump Inhibitors:

• Substituted Benzimidazoles: Omeprazole, Esomeprazole, Lansoprazole, Pantoprazole, Rabeprazole.
• Imidazopyridine derivatives: Tenatoprazole.

Mechanism of Action:

PPIs are prodrugs that require activation in the acidic environment of the parietal cell canaliculi. Once absorbed, they accumulate in the acidic secretory canaliculi where they undergo acid-catalyzed conversion into active sulfenamide derivatives. These active species form stable covalent disulfide bonds with the cysteine residues of the H+/K+-ATPase enzyme, effectively blocking the final step of gastric acid secretion.

Synthesis of Cimetidine:

The synthesis of cimetidine typically involves the following steps:

1. Formation of the imidazole ring: 4-methyl-5-hydroxymethylimidazole is prepared by the reaction of 1,3-dihydroxyacetone with formaldehyde and ammonia.
2. Thioether linkage: The hydroxymethyl group is converted to a chloromethyl intermediate, which is then reacted with cysteamine hydrochloride to form the thioether linkage.
3. Guanidine formation: The terminal amino group of the cysteamine chain is reacted with N-cyano-N'-methyl-S-methylisothiourea. This step introduces the cyanoguanidine moiety, which is essential for the H2-receptor antagonist activity of cimetidine.