DRUG KINETICS-PART-5

MOVEMENTS OF THE DRUG BY THE BODY

Movements after Single Oral Dose

Any drug which has been given by oral formulations such as tablet,syrup,elixir,capsules,or pills is rapidly absorbed by the intestine in a first order kinetics that means the higher the drug concentration the higher the absorption.Elimination also follows in a first order kinetics.

When the values are plotted on a graph by taking the plasma concentration on the y-axis and the time in the x-axis it shows the elevation of the absorption line to a peak plasma level at the given time followed by the declining line shows the rapid elimination of the drug in the post absorption period.

The following equation express the the drug movement by the body in a first order absorption and elimination principle.

                   Cp  = [FD0kA/VD(kA-k)](e to the power -kAt minus e to the power -kt)

where D0 - is the drug given by oral route.

           VD -is the volume of distribution,

           k-is the first order elimination constant. 

           kA-is the first order absorption rate constant and      F- is the fraction of the drug's bioavailability.

From the above equation it can be observed that any change in any of the following factors F,D0,VD,kA,and k will affect the plasma concentration Cp.

The time for the maximum or peak absorption can be expressed by the following equation,

               Tmax   =  2.3 log (kA-k)/kA-k

It is observed from the above equation that Tmax is affected by the rate constants only and not by any other factors.

The maximum plasma concentration Cmax can be calculated as follows,

                Cmax=[FD0kA/VD(kA-k)](e to the power of -kAtmax minus e to the power of -ktmax)

Lag Time

Lag time is well defined as a delay in systemic absorption due to delayed stomach emptying or other factors.