(35) Movement of Drug Molecule from Biological Barriers | Transport Across Cell Membrane | English

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Pharmacokinetic- “What does BODY 1 do to the DRUG 2” we have simple interchanged the BODY and DRUG to make a definition. And as per the definition, our body responds to any drug by giving ADME effects.
1. A Stands for - Absorption
2. D stands for - Distribution
3. M stands for - Metabolism
4. E stands for- Elimination

Transport of drug from the small intestine to the systemic circulation takes place by many methods like-

1. Passive Diffusion- Does not need energy for the movement till equilibrium achieved. Drugs diffuse across a cell membrane from a region of high concentration (GI fluids) to one of low concentration (blood). The diffusion rate is directly proportional to the gradient but also depends on the molecule’s lipid solubility, size, degree of ionization, and the area of the absorptive surface. Because the cell membrane is lipoid, lipid-soluble drugs diffuse most rapidly. Small molecules tend to penetrate membranes more rapidly than larger ones.

2. Active Transport- Need energy. Active transport is selective, requires energy expenditure, and may involve transport against a concentration gradient. Active transport seems to be limited to drugs structurally similar to endogenous substances like- ions, vitamins, sugars, amino acids. These drugs are usually absorbed from specific sites in the small intestine.

3. Endocytosis and Exocytosis- Process of engulfing either Pinocytosis or Phagocytosis. In Pinocytosis, fluid or particles are engulfed by a cell. The cell membrane invigilates, encloses the fluid or particles, then fuses again, forming a vesicle that later detaches and moves to the cell interior. Energy expenditure is required. Pinocytosis probably plays a small role in drug transport, except for protein drugs.

4. Carrier mediated transport – Need some molecule which will carry and drop the substance. Certain molecules with low lipid solubility penetrate membranes more rapidly than expected. One theory is facilitated passive diffusion: A carrier molecule in the membrane combines reversibly with the substrate molecule outside the cell membrane, and the carrier-substrate complex diffuses rapidly across the membrane, releasing the substrate at the interior surface. In such cases, the membrane transports only substrates with a relatively specific molecular configuration, and the availability of carriers limits the process. The process does not require energy expenditure, and transport against a concentration gradient cannot occur.

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