Pharmacokinetics

Pharmacokinetics deals with the absorption, distribution, metabolism and excretion drugs in the
body.
A. Biotransport of drug: It is translocation of a solute from one side of the biological barrier to
the other.

1. Structure of biological membrane: The outer surface of the cell covered by a very thin
   structure known as plasma membrane. It is composed of lipid and protein molecules. The

membrane proteins have many functions like (a) contributing structure to the membrane (b)
acting as enzyme (c) acting as carrier for transport of substances (d) acting as receptors.
The plasma membrane is a semipermeable membrane allowing certain chemical
substances to pass freely e.g. it allows water, glucose, etc. but it won’t allow sucrose until it
is converted into glucose and fructose.

2. Passage of drug across membrane.
   i) Simple diffusion
   (a) Passive transfer
   ii) Filtration
   i) Facilitated diffusion
   (b) Specialized transport ii) Active transport
   iii) Endocytosis.
   (a) i) Simple diffusion: Movement of a solute through a biological barrier from the phase of
   higher concentration to phase of lower concentration. No need of energy e.g. highly
   lipid soluble drugs.
   ii) Filtration: Is the process by which water soluble drug of relatively low molecular
   weight crosses the plasma membrane through pores as a result of hydrodynamic
   pressure gradient across the membrane e.g. urea and ethylene glycol.
   (b) i) Facilitated diffusion: It meansthe passage of drug across the biological membrane
3. along the concentration gradient by the protein carrier mediated system also called as
4. carrier mediated diffusion. It depends on number of carrier e.g. tetracycline, pyrimidine.
5. ii) Active transport: The process by which drugs pass across the biological membrane
6. most often against their concentration gradient with the help of carriers along with the
7. expenditure of energy e.g. alpha methyl dopa, levodopa, 5-fluoro-uracil, 5 bromouracil.
8. iii) Endocytosis: It is the process by which the large molecules are engulfed by the cell
9. membrane and releases them intracellularly e.g. protein, toxins (botulinum, diphtheria)
10. Differences amongst different transport systems
11. Characteristics Simple diffusion Facilitated Active transport
12. Incidence Commonest Less common Least common
13. Process Slow Quick Very Quick
14. Movement Along concentration
15. gradient
16. Along concentration
17. gradient
18. Against concentration
19. gradient
20. Carrier Not needed Needed Needed
21. Energy Not required Not required Required
22. B. Drug absorption: Absorption is the process by which the drug enters in to the systemic
23. circulation from the site of administration through biological barrier. In case of intravenous or
24. intra-arterial administration the drug bypasses absorption processes and it enters into the
25. circulation directly.