PharmSim

Formulas for multiple extravascular bolus administration

During multiple extravascular bolus administration, the plasma concentration of a drug at a certain moment in time depends on:

• the administered dose

• the volume of distribution

• the clearance

• the oral availibility

• the absorbtion rate constant

• the dosing interval

The oral availibility, dose and volue of distribution together determine the maximal amount of drug that could reach the plasma. In addition the absorbtion rate and the elimination rate determine how much of the drug enteres and leaves the plasma per unit of time. Both are expressed as a power of e. For each dose administered before the point of time that is considered, this process should be evaluated. The formulas, the plasma concentration (Cp) at a certain time is given by:

where

The drug action over time depends on the plasma concentration, and, in addition on the maximal effect of the drug (Emax), which is 100% in this applet, and the EC50. If the plasma concentration equals the EC50, the drug action should be 50% by definition. Furthermore, when the plasma concentration is approaches 0, the action should also approach zero, and when the plasma concentration equals a number of times the EC50, the action equals 1. In a formula, this looks as follows:

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