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Oral administration

Following oral administration of a drug with two compartment characteristics, Cp is described by an equation with three exponential terms.

Diagram XIX-2 Scheme for Oral Two-Compartment Pharmacokinetic Model

The model is shown in Diagram XIX-2:

Differential equation

Cp = A.e-alpha.t + B.e-beta.t + C.e-ka.t

where A + B + C = 0

Integrated Equation

Figure XIX-13 Semi-Log Plot Showing Pronounced Distribution

Bioavailability calculations are the same as for the one compartment model, i.e., by comparison of AUC or U. These apply for any linear system. Also if , , and ka are sufficiently separated the method of residuals can be applied to determine the three values.

Figure XIX-14 Semi-Log Plot Without Distribution Phase Evident

The equation can also be used to calculate appropriate dosing regimens. For example if an average plasma concentration of 20 mg/L is required and V1 = 15 L, kel = 0.15 hr-1, F = 0.9 and a dosing interval of 12 hours is to be used then the required dose is:-

= 600 mg every 12 hours

An Excel spreadsheet to calculate Cp versus time after IV Bolus, IV Infusion (fast), IV Infusion (slow), and/or Oral is available here. This spreadsheet does include some macros for printing and changing Y-axis scaling.
This page was last modified: 12 February 2001

Copyright 2001 David W.A. Bourne

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