## 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