|Introduction to Veterinary Pharmacokinetics:||Home||Administration||Distribution||Elimination||Modeling||Models||Glossary|
Attempt to describe the actual events which control drug absorption, distribution, and elimination
Attempt to accurately predict the time course of drug concentrations in one (usually blood or plasma) or two (urine as well) body fluids. Predictions are generally made for tissues which can be sampled from intact patients.
"Pharmacokinetics" on package inserts represent these kinds of model.
All pharmacokinetics are biochemical or physiological processes. For the most part, these processes obey Michaelis-Menten kinetics. At low amounts of substrate, capacity (for aborption, elimination, etc.) greatly exceeds concentration. Changing the concentration changes the rate (pharmacokinetics depend on one thing so are first order). At very high amounts of substrate, capacity reaches a maximum. Changing the concentration has no effect on rate (pharmacokinetics no longer depend on concentration - zero order). In between, the pharmacokinetics change as the dose changes (pharmacokinetics are mixed order or dose dependent).
First Order (The pharmacokinetics of MOST drugs is first order at therapeutic doses.)
Zero order (special dose forms, high concentrations of some drugs)
Dose Dependent Behavior
For most drugs (99%), it is logical to assume that the relationship between the dose we give and the concentration(s) that the dose produces in the body are linear. (We double the dose, the concentrations double, the effects double). The PHARMACOKINETICS of these drugs are said to be dose-independent.
Occassionally, drug dosing behave differently:
Michaelis-Menten saturation curve showing how the rate of a process changes with the concentration of substrate.
This curve is labeled to show three pharmacokinetic states (dependencies). First order at low doses (typical for drug therapy), zero order at high doses (ethanol behaves this way at "effective" doses), and mixed order in between.