Alfentanil, sufentanil, and fentanyl are synthetic opioids that are metabolized by oxidative N-dealkylation in the liver. We have previously shown that cytochrome P-450 3A4 (CYP3A4) contributes significantly to human liver microsomal alfentanil oxidation. Since identification of specific drug-metabolizing enzymes allows prediction of the variables affecting drug metabolism, the purpose of the present study was to identify the P-450 enzymes responsible for sufentanil and fentanyl metabolism in human liver microsomes. Microsomal preparations fortified with a reduced nicotinamide-adenine dinucleotide phosphate-generating system were incubated with 0.25 microM 3H-fentanyl or 3H-sufentanil. Rates of N-dealkylated metabolite formation significantly correlated with nifedipine oxidation activity (a marker of CYP3A4 activity) for fentanyl and sufentanil (r = 0.93 and 0.87, n = 18, respectively), but not with the oxidation activity for ethoxyresorufin (CYP1A2), S-mephenytoin (CYP2C19), bufuralol (CYP2D6), or chlorzoxazone (CYP2E1). Gestodene and troleandomycin (chemical inhibitors of CYP3A4) and antibody to CYP3A4 inhibited N-dealkylation of fentanyl and sufentanil. Chemical inhibitors of CYP2C, 2E1, and 2D6 did not inhibit N-dealkylation of fentanyl and sufentanil. Recombinant CYP3A4 expressed in Escherichia coli showed N-dealkylation activity of fentanyl and sufentanil, while expressed CYP1A2, 2C10, and 2E1 enzymes did not. We conclude that CYP3A4 is responsible for fentanyl and sufentanil N-dealkylation in vitro.

Because there is considerable interindividual variation in both microsomal CYP3A4 activity and CYP3A4 substrate disposition, an established probe of in vivo CYP3A4 activity would represent an important advance in clinical practice. In a previous study, no correlation was found between the "Cerythromycin breath test and urinary dapsone recovery ratio. However these drugs were administered by different routes, with the orally administered dapsone being exposed to presystemic metabolism by the gut and renal metabolism before the measurement of the urinary ratio. To overcome the variable of route of administration, the aim of this study was to determine whether the elimination of two intravenously administered CYP3A4 substrates (alfentanil and erythromycin) correlate. We compared the metabolism of alfentanil to its CYP3A4-dependent metabolite, noralfentanil, with the erythromycin breath test in 14 young healthy white men. No significant correlation was found between alfentanil metabolism and the erythromycin breath test: alfentanil clearance versus erythromycin breath test, r = 0.45, p = 0.1; partial metabolic clearance to noralfentanil versus erythromycin breath test, r = 0.35, p = 0.23.Because these two CYP3A4 substrates were administered by the same (intravenous) route, we conclude that differences in the route of administration do not explain the lack of correlation between the erythromycin breath test and other probes of CYP3A4 metabolism.The cytochrome P450 super family of heme proteins plays a central role in the metabolism of both xenobiotics and endogenous compounds) Of the four major subfamilies involved in human metabolism, cytochrome P4503A (CYP3A) may be the most important because it is responsible for the metabolism of such a large number of substrates. The spectrum of

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Omeprazole is a potent inhibitor of H+-ATPase in the gastric parietal cell [1], and is widely used for the treatment of peptic ulcer disease and reflux oesophagitis [2]. Omeprazole has recently been shown to be metabolized by the cytochrome P450 enzymes 3A4 (CYP3A4) and 2C19 (CYP2C19) [3]. Because of the effects of cimetidine on drug metabolism [4], there has been considerable interest in the effects of omeprazole on drug metabolism. Omeprazole has been shown to inhibit the metabolism of a number of drugs including phenytoin, warfarin, and diazepam [5][6][7]. All of these drugs are substrates of CYP2C19 thus suggesting that inhibition of CYP2C19 is produced by omeprazole. A much larger number of drugs are metabolized by CYP3A4 which is the major cytochrome P450 present in human liver. Thus the effects of omeprazole on CYP3A4 activity has considerable importance. Although no single probe has developed universal acceptance as a measure of CYP3A4 activity in vivo, the erythromycin breath test has been the most widely used in vivo index of CYP3A4 activity and we have previously shown that it is inhibited by the CYP3A4 inhibitor ketoconazole [16]. This test involves the intravenous administration of 14C labelled methylerythromycin and the collection of 14Co2 exhaled in breath [8].The purpose of the present study was to determine whether omeprazole, in addition to being a substrate of CYP3A4 also inhibits its activity as measured by the erythromycin breath test.After obtaining the approval of the Vanderbilt Committee for the Protection of Human Subjects, 12 healthy male volunteers aged 20-30 years were recruited into the study and gave written informed consent. The study was a placebo controlled, singleblind crossover design in two parts, each part of similar design. First each volunteer took one placebo capsule (identical to omeprazole 20 mg) twice daily for 7 days. After an overnight fast, the erythromycin breath test was performed on the morning of day seven and then the subjects crossed over to omeprazole. The tests were separated by at least 1 week. For the period of the study the subjects abstained from drinking alcohol and grapefruit juice. The erythromycin breath test was performed as previously The excretion of 14Co2 in the first hour accounted for 3.6 ± 0.6 (s.d.)% of the administered dose following placebo and was similar (3.6 ± 1.1%) after 20 mg omeprazole twice a day for 7 days ( Figure 1).The effects of omeprazole on drug metabolism have been studied in the past and these studies have demonstrated that omeprazole inhibits the metabolism of substrates of CYP2C19. The focus on CYP2C19 substrates preceded the recognition that omeprazole is metabolized not only by CYP2C19, as thought previously, but also by CYP3A4 [10]. Thus it was important to determine whether omeprazole also inhibited the metabolism of CYP3A4 substrates in vivo. CYP3A4 constitutes the dominant portion of cytochrome P450 in human liver and is responsible for the metabolism of a very large range of drugs and endogenous substrates and for th...

The isoproterenol-mediated increase in NE release is inhibited by halothane anesthesia, indicating that halothane inhibits prejunctional beta 2-adrenergic receptor regulation of NE release.

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indicative of renal and hepatic function. Alcohol, caffeine Effects of ketoconazole on the erythromycin and grapefruit-derived products were not permitted for breath test and the dapsone recovery ratio 24 h prior to and during the study period, and all of the Cytochrome P4503A (CYP3A)-a term used to indicate subjects were non-smokers. Each subject received placebo the collective metabolic activity of CYP3A4 and CYP3A5 followed by 300 mg ketoconazole daily, each for 7 days. in adult humans-is the most abundant of the hepatic After administration of the final dose in each study arm, CYP isoforms [1]. Because of this and its broad substrate the ERMBT was performed in the fasted state following specificity, CYP3A is involved to a varying extent in the rapid intravenous injection of 3 mCi [ 14 C-N-methyl]-metabolism of a large number of drugs [1]. Moreover, its erythromycin (DuPont New England Nuclear, Boston, localization in both the intestinal epithelium and liver MA), collection of expired air over the subsequent makes it important in the often marked oral first-pass 60 min, and measurement of 14 CO 2 as previously metabolism of many of its substrates [2]. Additionally, described [7]. One hour later, 100 mg dapsone ( Jacobus CYP3A's unimodal frequency distribution exhibits conPharmaceutical Co., Inc., Princeton, NJ) was orally siderable (5-to 10-fold) variability in activity that can be administered and urine collected for the subsequent 8 h, readily altered by inducers and inhibitors [1]. Accordingly, using ascorbic acid (1 g ) as preservative. Dapsone and its a means by which CYP3A activity and its modulation in N-hydroxy metabolite were determined by an h.p.l.c.-individuals could be readily assessed in vivo would be of based procedure and the DPRR (amount of dapsone value. The erythromycin breath test (ERMBT), based on hydroxylamine excreted/amount of dapsone and its the CYP3A-mediated N-demethylation of radiolabeled hydroxylamine excreted) estimated as described previously erythromycin, has been developed and used in several [14]. Between treatment differences in the indices were clinical studies for this purpose [3][4][5][6][7][8][9]. However, this test analyzed by a paired Student's t-test with P=0.05 being has several limitations, not the least of which is that it only taken as the minimal level of significance. measures hepatic metabolism [4, 5, 10, 11] and then only Seven days of ketoconazole administration reduced the that mediated by CYP3A4 [12]. Accordingly, intestinal ERMBT value markedly by 76% (2.23±0.5 vs 0.56±0.2% metabolism following oral administration and any