Dose-dependent pharmacokinetics and cancer chemotherapy.

Dose-dependent pharmacokinetics have been reported more frequently for anticancer drugs than for other drugs, probably because anticancer drugs are studied over a wide range of doses during early evaluation and because of the increasing use of anticancer drugs at very high doses. Dose-dependent pharmacokinetics are reflected most commonly as an increase in the biological half-life of a drug and a greater than proportional increase in plasma concentration of the drug and in area under the drug concentration-time curve with increase in dose. Occasionally the rate of drug removal increases with increasing dose. These nonlinear changes in drug concentrations with dose may lead to increases in toxicity out of proportion to increases in dose. Appreciation of the possibility of dose-dependent pharmacokinetics is important in the clinical pharmacologic evaluation of new drugs, and may be essential for the design of effective therapeutic regimens.