JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy III BIOPHARMACEUTICS AND PHARMACOKINETICS Previous Question Paper (July - 2021 - R17)

Protein binding significantly influences the therapeutic efficacy, safety, and duration of action of drugs. The drug-protein complex acts as a reservoir, maintaining the concentration of the free (unbound) drug in the systemic circulation. Only the free drug is pharmacologically active, capable of crossing cell membranes to reach the site of action, and available for metabolism or excretion.

Key clinical implications include:

1. Drug Distribution: High protein binding restricts the volume of distribution, keeping the drug largely confined to the vascular compartment.
2. Duration of Action: Bound drugs are protected from rapid metabolism and renal filtration, effectively prolonging the half-life and duration of action.
3. Drug-Drug Interactions: When two drugs compete for the same binding site on plasma proteins (e.g., albumin), the displacement of one drug by another can lead to a sudden increase in the free concentration of the displaced drug, potentially causing toxicity.
4. Disease States: Conditions such as hypoalbuminemia or renal failure alter protein levels, necessitating dosage adjustments to prevent adverse effects due to increased free drug fractions.

Hydrophilic drugs possess low lipid solubility, making passive diffusion through the lipid bilayer of the gastrointestinal (GIT) membrane inefficient. Their absorption primarily occurs through the following mechanisms:

1. Paracellular Transport: Small, water-soluble molecules (typically with a molecular weight less than 200 Da) pass through the aqueous-filled pores or tight junctions between adjacent epithelial cells. This process is passive and driven by concentration gradients.
2. Carrier-Mediated Transport: Many hydrophilic drugs mimic endogenous nutrients and are transported across the membrane via specialized membrane proteins. This includes facilitated diffusion (down a concentration gradient without energy) and active transport (against a concentration gradient using ATP). Examples include the absorption of certain vitamins and amino acid-like drugs via specific transporters like PEPT1.
3. Endocytosis: Large, highly hydrophilic molecules or macromolecules may be engulfed by the cell membrane through pinocytosis or receptor-mediated endocytosis, forming vesicles that transport the drug into or across the cell.

The rate of absorption for these drugs is often limited by the number of available carriers or the pore size of the tight junctions, rather than the lipid solubility of the molecule.