Unit 1: Intro to Medicinal Chemistry & Drug Metabolism

Semester 4

BP402T

Intro to Medicinal Chemistry & Drug Metabolism

Medicinal Chemistry intersectionally combines organic chemistry, pharmacology, and biology to design, discover, and develop new pharmaceutical agents. This unit explores how the physicochemical properties of a drug molecule (like its solubility, ionization state, and 3D shape) dictate how it interacts with the human body. Furthermore, it extensively covers Drug Metabolism—the biochemical modifications the body uses to detoxify and excrete foreign chemicals (xenobiotics) via Phase I and Phase II pathways.

Syllabus & Topics

1History and development of Medicinal Chemistry.
2Physicochemical properties in relation to biological action: Ionization, Solubility, Partition Coefficient.
3Hydrogen bonding, Protein binding, and Chelation.
4Bioisosterism, Optical and Geometrical isomerism.
5Drug metabolism: Principles and pathways.
6Phase I reactions (Oxidation, Reduction, Hydrolysis).
7Phase II reactions (Glucuronic acid, Sulfate, Amino acid, Glutathione conjugations).
8Factors affecting drug metabolism (Age, Species, Route, Enzyme induction/inhibition).

Learning Objectives

Understand how physicochemical properties govern drug absorption and receptor binding.

Explain the principles and applications of bioisosterism in drug design.

Detail the enzymatic pathways of Phase I and Phase II drug metabolism.

Predict the metabolic fate of common functional groups in drug molecules.

Frequently Asked Questions (FAQs)

Q1. What is the Concept of Bioisosterism?

Bioisosterism refers to the replacement of one chemical group in a molecule with another group that has similar physical or chemical properties and produces broadly similar biological effects. It is widely used in drug design to reduce toxicity, improve bioavailability, enhance potency, or modify the metabolic profile of a lead compound.

Q2. How Does the Partition Coefficient (P) Affect Drug Action?

The partition coefficient (P) indicates a drug’s lipophilicity (fat solubility) relative to its hydrophilicity (water solubility). For a drug to exert its effect, it must cross lipid cell membranes (requiring sufficient lipophilicity, high log P) while also remaining adequately soluble in aqueous blood for transport. Therefore, an optimal balance between lipid and water solubility is essential for effective drug action.

Q3. What is the Difference Between Phase I and Phase II Metabolism?

Phase I (functionalization) reactions introduce or expose polar functional groups such as –OH, –NH₂, or –SH through oxidation, reduction, or hydrolysis, commonly mediated by the Cytochrome P450 enzyme system. Phase II (conjugation) reactions involve attachment of a highly polar molecule such as glucuronic acid, sulfate, or glutathione to the drug or its Phase I metabolite, increasing water solubility and facilitating excretion.

Q4. Which Organ is Primarily Responsible for Drug Metabolism?

The Liver is the primary site of drug metabolism. It contains high concentrations of metabolic enzymes, especially the cytochrome P450 system, which convert lipophilic xenobiotics into more hydrophilic metabolites for elimination.