Pharmacokinetics of Oral Administration

Student Objectives for this Chapter

• To draw the scheme and differential equations for a one compartment pharmacokinetic model with first order absorption
• To recognize and use the integrated equations for this pharmacokinetic model
• To define and use the parameters ka and F
• To understand the influence of ka and F values on plasma concentration versus time curves

So far we have considered the pharmacokinetics of intravenously administered drugs, either as a bolus or by infusion. If we know kel and V for a particular patient we can calculate appropriate doses or dosing rates (infusion rates) to produce the necessary therapeutic concentrations.

In the previous Chapter we considered a number of routes of drug administration. Most of the routes of administration were extravascular; for example IM, SC, and most importantly oral. With this type of drug administration the drug isn't placed in the central compartment but must be absorbed through at least one membrane. This has a considerable effect on drug pharmacokinetics and may cause a reduction in the actual amount of drug which is absorbed.

Most commonly the absorption process follows first order kinetics. Even though many oral dosage forms are solids, which must dissolve before being absorbed, the overall absorption process can often be considered to be a single first order process. At least that's the assumption we will use for now.

Scheme or diagram

Schematically this model can be represented as:-

Figure 17.1.1 Representing Oral Administration, One Compartment Pharmacokinetic Model

Where Xg is the amount of drug to be absorbed, Xp is the amount of drug in the body, and ka is the first order absorption rate constant.

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Copyright 2001-3 David W. A. Bourne (david@boomer.org)