Superiority of stable isotope techniques in the assessment of the bioavailability of drugs undergoing extensive first pass elimination. Studies of the relative bioavailability of verapamil tablets.

Although the absorption of verapamil is almost complete after oral administration, its bioavailability is low due to extensive hepatic first-pass metabolism. Besides large interindividual differences in first-pass metabolism, pronounced day-to-day intraindividual variations in first-pass metabolism are observed, leading to erroneous results in relative bioavailability studies. Stable isotope techniques, which permit simultaneous administration of a solution and a tablet, can successfully be used to overcome these difficulties. The method has the advantage that two experiments can be carried out in a single test. Furthermore, the number of subjects required in bioavailability studies can be greatly reduced. Using this technique the bioavailability of verapamil tablets (Isoptin 80) relative to a stable labelled solution of verapamil was found to be 108.1%, with a 95% confidence interval between 89.1 and 127.1%.