The calculation of Clint values andfm
The fm can be calculated by comparing thein vitro clearance in the presence of an inhibitor to that in its
absence. To determine the Clint of a drug in the absence
of ketoconazole, its concentration at each time point can be measured
using the HMPL-MS method. The initial rate of drug disappearance can
then be calculated by using linear regression of the concentration
versus time data, which yields the slope value, k. This value can be
used to determine the in vitro Clint by the
following formula:
Clint (mL min-1mg-1) =\(\frac{k(\min^{-1})}{\text{Concentration\ of\ liver\ microsomes}(\text{mg\ mL}-1\ )}\)
To measure the Clint in the presence of ketoconazole,
the inhibitor is added to the microsomal incubation, and the same
procedure is repeated. The presence of the inhibitor reduces the
metabolism reaction, resulting in a lower Clint value.
The fm can be calculated using the formula:
fm =\(\frac{CLintwithout\ inhibitorCLintwith\ inhibitor}{\text{CLintwithout\ inhibitor}}\)