JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD Pharm. D III Year MEDICINAL CHEMISTRY Previous Question Paper (June/July - 2022 - R08)

Quantitative Structure-Activity Relationship (QSAR) parameters are essential for predicting biological activity based on chemical structure.

Hansch Analysis: This is a mathematical approach that correlates biological activity with physicochemical properties, specifically lipophilicity (log P), electronic effects (Hammett constant), and steric factors (Taft constant). It allows for the development of regression equations to optimize lead compounds by identifying which parameter most influences potency.

Hammett Constant (σ): This parameter quantifies the electronic effect of substituents on an aromatic ring. It is derived from the ionization constants of substituted benzoic acids. A positive σ value indicates an electron-withdrawing group, while a negative value indicates an electron-donating group, helping to predict how electronic changes affect drug-receptor binding.

Taft Constant (Es): This parameter specifically measures the steric bulk of a substituent. Unlike the Hammett constant, it is derived from the rates of acid-catalyzed hydrolysis of aliphatic esters. It is crucial for understanding how the size of a substituent hinders or facilitates the approach of a drug molecule to its target binding site.

Prodrug design is a strategic approach to enhance the aqueous solubility of poorly soluble drugs, thereby improving their bioavailability and formulation potential.

• Phosphate Esters: Attaching a phosphate group to a hydroxyl or amine group of a drug significantly increases its polarity and water solubility. These are typically cleaved by endogenous alkaline phosphatases in the blood to release the active drug.
• Hemiesters of Dicarboxylic Acids: Converting a hydroxyl-containing drug into a succinate or glutarate ester introduces a free carboxylic acid group. This allows for the formation of water-soluble salts (e.g., sodium or potassium salts) at physiological pH.
• Amino Acid Esters: Conjugating amino acids to a drug molecule provides both a polar amine and a carboxylic acid moiety. These derivatives are highly soluble and are rapidly hydrolyzed by non-specific peptidases.
• N-Mannich Bases: For drugs with amide or imide groups, forming N-Mannich bases with formaldehyde and a secondary amine can increase solubility by introducing a basic center that forms soluble salts.