The time course of a drug in the body is frequently represented as a concentration-time profile in which the concentrations of a drug in the body are measured analytically and the results plotted in semilogarithmic form against time. A representative profile of a drug given intravenously is presented in Figure 5.1. Drug concentrations are measured in samples typically taken from the brachial vein, since this vein is readily accessible, since sampling results in minimal patient discomfort and since obtained values reflect the concentrations of drug in the bloodstream. Concentrations in the blood may not be identical to concentrations at the site of action, such as a receptor, but one hopes they serve as a surrogate that correlates in a proportional manner.
Figure 5.1 shows that for a drug given intravenously, maximum concentrations are achieved almost instantaneously, since absorption across membranes is not required, though distributive processes may also occur (not depicted for the sake of simplicity).The concentrations of drug in the blood decline over time according to the elimination rate of that particular drug. More commonly, drug is given via extravascular routes (e.g., orally), so absorption and distribution must occur, and therefore it will take some time before maximum concentrations are achieved.
The blood concentration-time profile for a theoretical drug given extravascularly (e.g., orally) is shown in Figure 5.2. Some pharmacokinetic parameters, such as Cmax, Tmax, area under the curve, and half-life, can be estimated by visual inspection or computation from a con
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