DRUG KINETICS-NON COMPARTMENT METHODS

NON COMPARTMENT PHARMACOKINETICS

These methods involves comparison of the area under curves (AUC) in a graph plotted by taking the plasma concentration on the y-axis against the time t on the x-axis.The pharmacokinetic parameters for absorption,distribution and elimination are estimated with non compartment methods.

1.Mean Residence Time (MRT):-

The MRT is the mean average time for the drug molecules to reside inside the body.The synonyms are Mean Transit Time or Mean Sojourn Time.

The MRT depends on the route of administration with the assumption that the elimination of the drug molecules are drived from a central compartment.

The MRT is calculated by dividing the total transit or resident time for all drug molecules by the total number of drug molecules.

MRT For IV Bolus Injection

This can be calculated as follows.

                MRTIV =AUMC/AUC0-infinity

where AUMC is the area under the first moment verses time curve from t=0 to t=infinity,and AUC0-infinity is the plasma drug concentration versus time curve from t=0 to t=infinity,which is otherwise known as zero moment curve.

The MRTIV is related to the elimination half life by the following expression,

                     MRTIV =1/k

During MRTIV 62.3% intravenous bolus dose is eliminated.

The MRT for a drug given by a noninstantaneous input is longer than the MRTIV

Mean Absorption Time (MAT)

MAT is described as the difference between MRT and MRTIV after an extra vascular route is used.

                     MAT = MRTPO -MRTIV

when first order absorption occurs 

                     MAT = 1/ka

Steady State Volume Of Distribution

The steady state volume of distribution is the amount of drug in the body at steady state and the average steady state drug concentration.

After an I.V.bolus injection VSS is calculated as follows,

                      VSS = DoseIV (AUMC)/(AUC)squired