The use of pharmacokinetics in drug design

The time taken to achieve a plateau concentration may be reduced by using a larger than usual initial dose. This loading dose, as it is known, gives a relatively high initial plasma concentration, which acts as an elevated starting point for the succeeding normal doses. This reduces the time taken to reach the plateau concentration, that is, the therapeutic window, and so can be particularly useful in cases of serious illness. 

Pharmacokinetic data is used to differentiate between active substances with good and poor pharmacokinetic characteristics. For example, substances with poor absorption, high first pass metabolism and an unsuitable half-life (too long or too short) will normally be discarded in favour of substances with more appropriate pharmacokinetic properties. 

Pharmacokinetics is used in all the development stages of a drug from preclinical to Phase IV trials (see section 11.3). Legislation normally demands 

Megestrol acetate (A) is an oral contraceptive. What pharmacokinetic parameters should be determined for other potential oral contraceptives in order to compare their actions with compound A Give reasons for your choice. 

Time (hours) Plasma concentration (fig cm-3) Time (hours) Plasma concentration fug cm-3) 

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