The gut wall metabolism of ethinyloestradiol and its contribution to the pre-systemic metabolism of ethinyloestradiol in humans.

1 Five patients have been studied to determine the contribution of the gut wall to the pre-systemic metabolism of ethinyloestradiol. All patients had a catheter inserted into their hepatic portal vein as part of their surgical management. 2 After an oral dose of 50 micrograms (65 microCi) ethinyloestradiol, blood samples were taken from the hepatic portal vein and from a peripheral vein at intervals for 1 h. 3 In each patient the concentration of conjugated ethinyloestradiol in the portal vein was considerably higher than in the peripheral vein. 4 Although a number of assumptions have been made, calculations showed that the gut wall appeared to be twice as effective as the liver in conjugating ethinyloestradiol on the first pass. 5 In two patients there was no evidence of major uptake or metabolism of ethinyloestradiol in the lung.