Pharmacokinetics of Oral Administration
Objective:
=> 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:-
Diagram VIII-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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