JNTU B.Pharmacy II MEDICINAL CHEMISTRY-I Previous Question Paper (July - 2021 - R17)

The biological activity of a drug molecule is fundamentally determined by its physicochemical properties, which govern its ability to reach the target site, bind to receptors, and be metabolized or excreted. Key properties include:

1. Solubility and Partition Coefficient (Log P): Drugs must possess a balance between hydrophilicity (for transport in aqueous body fluids) and lipophilicity (for crossing lipid-rich biological membranes). The partition coefficient, defined as the ratio of drug concentration in octanol to water, is a critical predictor of membrane permeability. Excessive lipophilicity may lead to poor solubility and sequestration in adipose tissue, while excessive hydrophilicity prevents membrane penetration.

2. Ionization (pKa): Most drugs are weak acids or bases and exist in equilibrium between ionized and unionized forms. According to the pH-partition hypothesis, only the unionized form can readily cross biological membranes via passive diffusion, while the ionized form is often necessary for ionic bonding with target receptors. The pKa of a drug determines the degree of ionization at a specific physiological pH.

3. Hydrogen Bonding: Hydrogen bonds are essential for the specificity of drug-receptor interactions. They influence both the solubility of the drug and its binding affinity. The number and orientation of hydrogen bond donors and acceptors determine how well a drug fits into the binding pocket of a protein or enzyme.

4. Steric Factors: The spatial arrangement of atoms, including molecular shape, size, and stereochemistry, dictates how a drug fits into a receptor. Enantiomers often exhibit different biological activities because the receptor site is chiral. Steric bulk can also influence the rate of metabolic degradation by blocking access to functional groups.

5. Electronic Effects: The distribution of electrons within a molecule, influenced by inductive and resonance effects, determines the reactivity and binding strength of the drug. Electron-withdrawing or electron-donating groups can alter the pKa of functional groups and the strength of electrostatic interactions with the target.

6. Surface Activity: Compounds that alter the surface tension of biological membranes can affect cell permeability and integrity. This is particularly relevant for surfactants or drugs that interact directly with the lipid bilayer.

Together, these properties constitute the basis of structure-activity relationships (SAR), allowing medicinal chemists to optimize lead compounds for improved potency, bioavailability, and reduced toxicity.

Drug metabolism, primarily occurring in the liver, is influenced by a variety of physiological, chemical, and environmental factors that determine the rate and extent of drug biotransformation.

The key factors affecting drug metabolism include:

• Genetic Factors: Genetic polymorphism significantly impacts enzyme activity. Variations in genes encoding cytochrome P450 (CYP) enzymes can lead to phenotypes such as poor, intermediate, extensive, or ultra-rapid metabolizers, causing inter-individual differences in drug response.
• Physiological Factors: Age is a critical factor; neonates have immature hepatic enzyme systems, while the elderly often exhibit reduced liver blood flow and enzyme activity. Gender can also influence metabolism due to hormonal differences affecting specific enzyme isoforms.
• Nutritional Status: Malnutrition or deficiencies in proteins, vitamins, and minerals can impair the synthesis of drug-metabolizing enzymes and cofactors, thereby decreasing the rate of metabolism.
• Drug-Drug Interactions: Enzyme induction and inhibition are major concerns. Enzyme inducers (e.g., rifampicin, phenobarbital) increase the synthesis of CYP enzymes, leading to faster metabolism and decreased drug efficacy. Conversely, enzyme inhibitors (e.g., ketoconazole, grapefruit juice) decrease enzyme activity, potentially leading to drug toxicity due to accumulation.
• Disease States: Hepatic diseases such as cirrhosis, hepatitis, or liver cancer directly damage hepatocytes, reducing the metabolic capacity. Cardiac failure can reduce hepatic blood flow, which limits the metabolism of drugs with high hepatic extraction ratios.
• Environmental Factors: Exposure to environmental pollutants, pesticides, or cigarette smoke can induce hepatic enzymes. For example, polycyclic aromatic hydrocarbons in cigarette smoke induce CYP1A2, increasing the metabolism of drugs like theophylline.
• Route of Administration: Drugs administered orally undergo the first-pass effect in the liver and gut wall before reaching systemic circulation, which significantly alters their metabolic profile compared to parenteral administration.