Example: An antihypertensive drug is completely metabolized by the liver. Its hepatic extraction ratio is 0.08 in a patient who has liver blood flow of 1.52 L/min. Calculate the hepatic clearance of this drug in this patient. The patient started taking an anticonvulsant which induces the liver microsomal enzymes. The intrinsic clearance after induction was found to be 0.264 L/min. Calculate the hepatic clearance of this drug if the anticonvulsant does not affect the hepatic blood flow.

Enzyme induction results in
increasing the CLint which
increases the hepatic
extraction ratio

E =

CLint

Q + CLint

The increase in the hepatic
extraction ratio increases the
hepatic clearance.

CLH = Q E

Enzyme induction resulted in a significant
increase in the intrinsic clearance and the
extraction ratio of this antihypertensive drug
(low extraction ratio drug). This caused an
increase in the drug hepatic clearance.

Answer:

a- CLH before induction:

CLH = 1.52 L/min x 0.08 = 0.122 L/min

b- CLH after induction:

E =

0.264 L/min

1.52 L/min + 0.264 L/min
= 0.148
CLH = 1.52 L/min x 0.148 = 0.225 L/min