Prediction of bioavailability for drugs with a high first-pass effect using oral clearance data.

For drugs with a high hepatic clearance, bioavailability is low due to the so-called "first pass effect". Prediction of the bioavailability for these drugs has been only loosely tested. It is proposed that by plotting the reciprocal of bioavailability versus the oral clearance, a straight line with intercept of unity and slope of reciprocal of hepatic blood flow should ensue. For lignocaine and verapamil, this relationship was found to be strong and gave good predictability, whereas for propranolol this relationship was weak and gave poor predictability. The proposed method may be of value in determining whether the low bioavailability of a drug is due to hepatic first pass metabolism.