BACKGROUND AND AIMS: In humans gut wall metabolism can be quantitatively as important as hepatic drug metabolism in limiting the systemic exposure to drugs after oral administration. However, it has been proposed that the role of gut wall metabolism might be overemphasized, because high luminal drug concentrations would lead to a saturation of gut wall metabolism. Therefore we investigated the impact of concentration and rate of intraluminal drug delivery on absorption (Fabs) and gastrointestinal extraction (EGI) of a luminally administered cytochrome P450 (CYP) 3A4 substrate (verapamil) using a multilumen perfusion catheter in combination with a stable isotope technique. METHODS: Two 20-cm-long, adjacent jejunal segments were isolated with the multilumen perfusion catheter in 7 subjects. In this study 80 mg of unlabeled verapamil (d0-verapamil15 min) was infused into one segment over a 15-minute period, 80 mg of 3-fold deuterated verapamil (d3-verapamil240 min) was administered over a 240-minute period into the other segment, and simultaneously, 5 mg of 7-fold deuterated verapamil (d7-verapamil) was injected intravenously over a 15-minute period. RESULTS: The rate of intraluminal drug delivery had only a modest effect on bioavailability of the verapamil isotopes (after correction for Fabs) (F/Fabsd3-verapamil240 min versus d0-verapamil15 min, 0.24 0.10 versus 0.20 0.09; P