Pharmacokinetics

Pharmacokinetics

• Pharmacokinetics is the quantitative study of drug movement in, through, and out of the body.
• It involves the study of absorption, distribution, metabolism, and excretion (ADME) of drugs.
• It essentially focuses on how the body affects the drug.

Pharmacokinetic (PK) Processes

Four pharmacokinetic properties determine the onset, intensity, and duration of a drug’s action:

1. Absorption
   • The process by which the drug enters the bloodstream from the site of administration.
   • Can be direct (e.g., intravenous) or indirect (e.g., oral, requiring crossing biological barriers).
2. Distribution
   • After absorption, the drug reversibly leaves the bloodstream and distributes into interstitial and intracellular fluids.
   • The extent and pattern of distribution depend on physicochemical properties of the drug and physiological factors (e.g., blood flow, tissue permeability).
3. Metabolism (Biotransformation)
   • The drug may be chemically altered by metabolic enzymes, primarily in the liver but also in other tissues.
   • Metabolism can activate, inactivate, or transform the drug to more water-soluble forms for elimination.
4. Elimination
   • The process by which the drug and its metabolites are removed from the body, mainly via urine, bile, or feces.
   • Elimination involves both excretion and metabolic conversion.

Pharmacokinetics and Pharmacological Action

• The pharmacological effects of a drug depend on its concentration at the target tissue.
• The time-course of the drug’s action typically mirrors the rise and fall of its concentration at the site of action.
• The concentration of a drug at any point in time after administration depends on two main processes:
  1. Translocation of drug molecules
     • Movement of the drug from the site of administration to different body parts (involving absorption, distribution, and excretion).
  2. Chemical transformation (Biotransformation)
     • Conversion of the drug to active or inactive forms, leading to the formation or disappearance of active drug molecules.