DRUG KINETICS-PART-3

MOVEMENTS OF THE DRUG BY THE BODY

First Order Reaction

In this kind of order of reaction,the rate of the reaction or the changes in concentration(C) is  proportional to the initial concentration(C0) of the drug with respect to time.

Mathematically this can be written as,

                    C is directly proportional to C0

                                       or

                               dC/dt = -kC

The drug concentration changes with respect to time(dC/dt) equals to the product of the rate constant(-k)

and the remaining drug concentration(C).

In otherwords the rate of reaction is directly proportional to concentration of the reactants.This can be expressed as C=C0e-kt in which C is the concentration of the reactants,C0 is the initial concentration,k is the first order rate constant in units of reciprocal of time,and t is time.

                           
                              C=C0e-kt

                         Int C = -kt+Int C0

                         log C =-kt/2.303+log C0

On putting the above equation on a graph we can see a negative descending linear slope which equals to -kt/2.303.It shows the time increases exponentially as the initial concentration(C0) of the reactants decreases.

The half life (t1/2) of the reaction can be defined as the time required for the concentration of a drug(C0) of a drug to one half of its initial value(C1/2).

The half life of a drug which is moved by the body on a first order reaction basis can be expressed as follows.

                     t1/2    =  0.693/k

According to the above expression the half life of the first order reaction is a constant.

The half life of the drug is very important which says that by how much speed a drug is eliminated from the body.The higher value of half life the slower the drug is eliminated.