Models in pharmacokinetics

MODELS IN PHARMOKINETICS

Linear vs Nonlinear drug elimination

  • If rate of elimination proportional to plasma concentration
  • linear or first order (constant fraction of the drug eliminated per unit time)
  • occurs for most normal kinetic processes.
  • If rate of elimination is constant irrespective of concentration ---> nonlinear or zero order reaction (constant amount of the drug eliminated per unit time)
  • Occurs in toxicity, saturable mechanisms of elimination,
  • At very high dose, reaction changes from first order to zero order
  • Conc. increases disproportionately with increase in dose

Compartmental models

  • A mathematical description of processes based on the number of phases. One compartment or two-compartment or multi-compartment a concept of imaginary division of the body into number of compartments.  The compartments do not correlate to any physical or anatomical compartments
  • A single compartment model consists of only one elimination phase. A two compartment model consists of a rapid distribution phase and a slow elimination phase 
  • Usually, I compartment indicates blood and highly perfused tissues and II compartment indicates peripheral tissues. In rare cases a third compartment corresponding to a deeper tissue component.
  • Elimination is only from central compartment
  • Most drugs follow two compartment open models with first order processes of absorption and elimination
Last modified: Wednesday, 25 April 2012, 6:59 AM