JNTU B.Pharmacy II MEDICINAL CHEMISTRY-I Previous Question Paper (March-2022 - R17)

Physicochemical properties play a critical role in the interaction between a drug molecule and its biological target. These properties dictate how a drug is transported, distributed, and bound to its receptor.

Hydrogen bonding is a dipole-dipole interaction occurring between a hydrogen atom covalently bonded to an electronegative atom (like O, N, or F) and another electronegative atom with a lone pair. In drug action, hydrogen bonds are essential for the specificity of drug-receptor binding. They help stabilize the drug-receptor complex and influence the drug's solubility and membrane permeability.

Partition coefficient (log P) is the ratio of the concentration of a drug in an organic phase (usually n-octanol) to its concentration in an aqueous phase (buffer at pH 7.4). It measures the lipophilicity of a drug. A drug must have an optimal balance of lipophilicity to cross lipid-rich biological membranes (e.g., the blood-brain barrier) while remaining sufficiently water-soluble to be transported in the blood.

Ionization refers to the dissociation of a drug into charged ions, which depends on the drug's pKa and the pH of the physiological environment, as described by the Henderson-Hasselbalch equation:

Only the unionized form of a drug typically crosses biological membranes via passive diffusion, whereas the ionized form is often required for strong ionic interactions with amino acid residues at the receptor binding site.

Chelation involves the formation of a stable, ring-like complex between a metal ion and a ligand (the drug) containing two or more donor atoms. Chelation can significantly alter drug action: it may be used therapeutically to remove toxic heavy metals from the body (e.g., EDTA for lead poisoning), or it may be an undesirable side effect where a drug binds to essential metal ions (like Ca2+, Mg2+, or Fe2+), potentially leading to reduced bioavailability or toxicity.

Drug metabolism, or biotransformation, is the process by which the body chemically modifies drugs to facilitate their excretion. This process is categorized into two main phases, which together convert lipophilic drugs into more polar, water-soluble metabolites.

Phase I reactions involve the introduction or unmasking of polar functional groups (such as -OH, -NH2, -SH, or -COOH) on the drug molecule. These reactions are primarily mediated by the cytochrome P450 (CYP450) monooxygenase system located in the endoplasmic reticulum of hepatocytes. Common Phase I reactions include oxidation, reduction, and hydrolysis. For example, the oxidation of diazepam by CYP3A4 leads to the formation of N-desmethyldiazepam, which increases the molecule's polarity and prepares it for further conjugation.

Phase II reactions, also known as conjugation reactions, involve the covalent attachment of an endogenous polar molecule (such as glucuronic acid, sulfate, glutathione, or amino acids) to the drug or its Phase I metabolite. These reactions are catalyzed by transferase enzymes. The resulting conjugate is typically highly water-soluble, pharmacologically inactive, and readily excreted via urine or bile. A classic example is the glucuronidation of morphine by the enzyme UDP-glucuronosyltransferase (UGT), which attaches glucuronic acid to the phenolic hydroxyl group of morphine, rendering it inactive and facilitating its renal elimination.

The importance of these phases lies in their sequential nature: Phase I reactions provide the necessary functional handles for Phase II enzymes to act, while Phase II reactions ensure the final product is sufficiently hydrophilic to prevent reabsorption in the renal tubules. This metabolic pathway is critical for terminating the pharmacological activity of most drugs and preventing the accumulation of potentially toxic lipophilic substances in the body.