ABSTRACT
An understanding of the pharmacodynamics and pharmacokinetics of a drug is essential for its safe use. A therapeutic effect of a drug and its appropriate dosage regimens are based upon the relationships of time and drug concentration at the active site (target organ or biophase) and the resultant drug response. For reasons of feasibility, correlation of drug effects and drug concentration in blood or urine are most often monitored for systemically acting agents. The three factors that are generally assessed are: (a) the area under the serum concentration versus time curve, (b) the peak serum concentration, and (c) the time to peak serum concentration. Analytical difficulties sometimes preclude the measurement of drug or metabolite levels in the body fluids, in which case drug effects are assessed by observing an appropriate pharmacologic response (1,2). Pharmacokinetics are the current mainstay parameters of bioequivalence assessments because blood and urine assays are available for nearly all drugs, and because pharmacokinetic methods are generally well understood. The key assumption is that equivalent pharmacokinetic parameters indicate equivalent therapeutics-or, more precisely, that observed differences in pharmacokinetic parameters between formulations are predictive of differences in clinical performance of the two formulations. However, little attempt to verify this key assumption is made (3-5).
