Two-compartment catenary model for extravascular administration

This model is representative for the conditions described in the previous section, except for the mode of administration which can be oral, rectal or parenteral by means of injection into muscle, fat, under the skin, etc. (Fig. 39.7). In addition to the central plasma compartment, the model involves an absorption compartment to which the drug is rapidly delivered. This may be to the gut in the case of tablets, syrups and suppositories or into adipose, muscle or skin tissues in the case of injections. The transport from the absorption site to the central compartment is assumed to be one-way and governed by the transfer constant (Fig. 39.7a). The linear differential model for this problem can be defined in the following way  

The analytic solution of the system of differential equations in eq. (39.14) can be written as follows  

The plasma function can be rewritten in terms of concentrations C rather than 

When j,p k, the time course of the plasma concentration Cp is dominated by the rate of elimination which is the slower of the two. This is desirable when a drug must be delivered as rapidly as possible to the plasma compartment, for example in the relief of acute pain. At sufficiently large times following administration, the 

If fcpg, the time course of Cp is dominated by the slower rate of absorption. This is desirable when a drug is to be delivered over a prolonged period of time, for example in the relief of chronic pain. When a sufficiently large period of time has elapsed, the transient effect of elimination has decayed and the solution for the plasma compartment in eq. (39.16) becomes approximately  

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Two-compartment catenary model for extravascular administration with incomplete absorption... 

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