JNTU B.Pharmacy III MEDICINAL CHEMISTRY-I Previous Question Paper (November/December-2020 - R17)

Phase I metabolism, also known as functionalization reactions, involves the introduction or unmasking of polar functional groups such as -OH, -NH₂, -SH, or -COOH on a drug molecule. These reactions generally increase the hydrophilicity of the drug, preparing it for conjugation in Phase II, or occasionally rendering it inactive or active (prodrug activation). The primary enzyme system responsible for these reactions is the Cytochrome P450 (CYP450) monooxygenase system located in the endoplasmic reticulum.

Key Phase I reactions include:

• Oxidation: The most common reaction. Examples include aromatic hydroxylation (e.g., conversion of phenytoin to p-hydroxyphenytoin), aliphatic hydroxylation (e.g., hydroxylation of the side chain of ibuprofen), and N-dealkylation (e.g., conversion of imipramine to desipramine).
• Reduction: Occurs less frequently than oxidation, typically involving azo or nitro groups. For example, the reduction of chloramphenicol to its corresponding amine metabolite by nitroreductases.
• Hydrolysis: Involves the cleavage of esters or amides by esterases or amidases. For example, the hydrolysis of aspirin (acetylsalicylic acid) into salicylic acid and acetic acid.

These reactions serve to modify the chemical structure of the drug, facilitating its excretion by increasing water solubility or providing a handle for subsequent Phase II conjugation reactions.

The biological activity of a drug molecule is fundamentally determined by its physicochemical properties, which govern its ability to reach the target site, interact with receptors, and undergo metabolic processes. These properties dictate the pharmacokinetic (ADME) and pharmacodynamic profiles of a drug.

1. Solubility and Partition Coefficient (Log P): Solubility is essential for drug absorption. The partition coefficient, defined as the ratio of drug concentration in octanol to water, measures lipophilicity. A balanced Log P is necessary; highly lipophilic drugs may be trapped in membranes, while highly hydrophilic drugs fail to penetrate lipid bilayers. The optimal balance ensures absorption and distribution.

2. Ionization (pKa): Most drugs are weak acids or bases. The pKa determines the degree of ionization at physiological pH, described by the Henderson-Hasselbalch equation:

Only the unionized form typically crosses biological membranes via passive diffusion, while the ionized form is often required for binding to the target receptor through ionic interactions.

3. Hydrogen Bonding: The ability to form hydrogen bonds influences both solubility and binding affinity. Excessive hydrogen bonding can hinder membrane permeability, whereas specific hydrogen bonds between the drug and receptor amino acid residues are critical for high-affinity binding and biological efficacy.

4. Molecular Weight and Size: Smaller molecules generally exhibit better membrane permeability. According to Lipinski’s Rule of Five, a molecular weight under 500 Da is preferred for oral bioavailability.

5. Steric Factors: The three-dimensional shape of a molecule determines its fit into a receptor binding site. Stereochemistry (enantiomers) can lead to drastic differences in biological activity, as one isomer may fit the receptor perfectly while the other may be inactive or toxic.

6. Electronic Effects: The distribution of electrons (inductive, resonance, and mesomeric effects) influences the chemical reactivity of the drug, its stability, and its ability to form covalent or non-covalent bonds with biological targets.

In summary, these properties are interdependent. Medicinal chemists optimize these parameters—often guided by the Rule of Five—to ensure that a lead compound possesses the ideal balance of potency, selectivity, and pharmacokinetic stability.