JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy III Year BIOPHARMACEUTICS AND PHARMACOKINETICS Previous Question Paper (August - 2022 - R17)

Drug absorption is the process by which a drug proceeds from the site of administration to the site of measurement (usually the systemic circulation) within the body. It involves the movement of drug molecules across biological membranes.

The primary mechanisms of drug absorption across the gastrointestinal tract (GIT) are:

• Passive Diffusion: The most common mechanism where drug molecules move from a region of higher concentration to lower concentration. It is a non-saturable, non-energy-dependent process governed by Fick's first law of diffusion.
• Carrier-Mediated Transport: Involves specialized membrane proteins. Facilitated diffusion follows the concentration gradient without energy, while Active transport moves drugs against the concentration gradient using ATP and is saturable.
• Vesicular Transport (Endocytosis): A process where the cell membrane engulfs large particles or macromolecules to form vesicles, allowing them to enter the cell. This includes pinocytosis (fluid) and phagocytosis (particulate).
• Ion-Pair Transport: Occurs when highly ionized drugs form neutral complexes with endogenous ions (like mucin), allowing the neutral complex to diffuse through the lipoidal membrane.
• Pore Transport (Convective Transport): Small, water-soluble molecules pass through aqueous-filled channels or pores in the membrane, driven by hydrostatic or osmotic pressure.

Pre-systemic metabolism, often termed the first-pass effect, occurs when a drug is metabolized by enzymes in the gut wall or liver before reaching the systemic circulation. This process significantly reduces the bioavailability of the drug. If a drug is highly susceptible to hepatic or intestinal metabolism, only a fraction of the administered dose enters the blood, necessitating higher oral doses or alternative routes of administration to achieve therapeutic plasma concentrations.