The Method of Inspection

Often, it is unnecessary to calculate an exact value for an absorption rate constant. For example, when several oral tablets containing the same drug substance are all found to be completely absorbed, it may be sufficient to merely determine if the absorption rates are similar. In another instance, it would be possible to choose between an elixir and a sustained-release tablet without assigning accurate numbers to the absorption rate constant for the two dosage forms.

In these instances, the time of the peak in the plasma concentration versus time curve provides a convenient measure of the absorption rate. For example, if three tablets of the same drug are found to be completely absorbed and all give plasma peaks at one hour, it could be concluded that all three tablets are absorbed at essentially the same rate. (In fact, if all tablets are completely absorbed and all peak at the same time, it would be expected that all three plasma concentration versus time curves would be identical, within experimental error.)

The time of the peak can also be used to roughly estimate the absorption rate constant. If it is assumed that ka is at least 5 x kel, then it can be assumed that absorption is at least 95% complete at the peak time; that is, the peak time represents approximately five absorption half-lives (Table 1). The absorption half-life can then be calculated by dividing the time of the peak by 5, and the absorption rate constant can be calculated by dividing the absorption half-life by 0.693.

Example. Inspection of Figure 8 gives a peak time of about 2.5 hours. The absorption half-life can be estimated to be 0.5 hour and the absorption rate constant to be 1.4/hr.

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