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 =\(\frac{CLintwithout\ inhibitorCLintwith\ inhibitor}{\text{CLintwithout\ inhibitor}}\)