Models in pharmacokinetics
Linear vs Nonlinear drug elimination
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If rate of elimination proportional to plasma concentration
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linear or first order (constant fraction of the drug eliminated per unit time)
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occurs for most normal kinetic processes.
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If rate of elimination is constant irrespective of concentration ---> nonlinear or zero order reaction (constant amount of the drug eliminated per unit time)
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Occurs in toxicity, saturable mechanisms of elimination,
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At very high dose, reaction changes from first order to zero order
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Conc. increases disproportionately with increase in dose
Compartmental models
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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
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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
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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.
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Elimination is only from central compartment
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Most drugs follow two compartment open models with first order processes of absorption and elimination
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Last modified: Wednesday, 25 April 2012, 6:59 AM