Pharmacokinetics: dose-dependent changes.

Stable-isotope tracer methods are described for answering the following questions about a drug: Does the drug exhibit dose-dependent changes in pharmacokinetic properties? What are the drug's Michaelis constant (Km) and maximum velocity (Vmax) for enzymatic biotransformation; Are the dose-dependent pharmacokinetic changes great enough to be clinically important? and Which routes of the drug's biotransformation are responsible for the drug's dose-dependent pharmacokinetic properties? Illustrative data are provided from tracer studies performed with a drug with dose-dependent pharmacokinetic properties, phenytoin, and a drug that does not exhibit dose-dependent pharmacokinetic properties, phenobarbital. The advantages and disadvantages of the described stable-isotope methods are discussed.