Chemical composition and biological effects of kratom (Mitragyna speciosa): In vitro studies with implications for efficacy and drug interactions

The safety and efficacy of kratom (Mitragyna speciosa) for treatment of pain is highly controversial. Kratom produces more than 40 structurally related alkaloids, but most studies have focused on just two of these, mitragynine and 7-hydroxymitragynine. Here, we profiled 53 commercial kratom products using untargeted LC-MS metabolomics, revealing two distinct chemotypes that contain different levels of the alkaloid speciofoline. Both chemotypes were confirmed with DNA barcoding to be M. speciosa. To evaluate the biological relevance of variable speciofoline levels in kratom, we compared the opioid receptor binding activity of speciofoline, mitragynine, and 7-hydroxymitragynine. Mitragynine and 7-hydroxymitragynine function as partial agonists of the human µ-opioid receptor, while speciofoline does not exhibit measurable binding affinity at the µ-, δ- or ƙ-opioid receptors. Importantly, mitragynine and 7-hydroxymitragynine demonstrate functional selectivity for G-protein signaling, with no measurable recruitment of β-arrestin. Overall, the study demonstrates the unique binding and functional profiles of the kratom alkaloids, suggesting potential utility for managing pain, but further studies are needed to follow up on these in vitro findings. All three kratom alkaloids tested inhibited select cytochrome P450 enzymes, suggesting a potential risk for adverse interactions when kratom is co-consumed with drugs metabolized by these enzymes.

Comparison of a New Intranasal Naloxone Formulation to Intramuscular Naloxone: Results from Hypothesis-generating Small Clinical Studies

Easy‐to‐use naloxone formulations are needed to help address the opioid overdose epidemic. The pharmacokinetics of i.v., i.m., and a new i.n. naloxone formulation (2 mg) were compared in six healthy volunteers. Relative to i.m. naloxone, geometric mean (90% confidence interval [CI]) absolute bioavailability of i.n. naloxone was modestly lower (55%; 90% CI, 43–70% vs. 41%; 90% CI, 27–62%), whereas average (±SE) mean absorption time was substantially shorter (74 ± 8.8 vs. 6.7 ± 4.9 min). The opioid‐attenuating effects of i.n. naloxone were compared with i.m. naloxone (2 mg) after administration of oral alfentanil (4 mg) to a separate group of six healthy volunteers pretreated with 240 mL of water or grapefruit juice. The i.m. and i.n. naloxone attenuated miosis by similar extents after water (40 ± 15 vs. 41 ± 21 h*%) and grapefruit juice (49 ± 18 vs. 50 ± 22 h*%) pretreatment. Results merit further testing of this new naloxone formulation.

Quantitative prediction and clinical evaluation of an unexplored herb-drug interaction mechanism in healthy volunteers

Quantitative prediction of herb–drug interaction risk remains challenging. A quantitative framework to assess a potential interaction was used to evaluate a mechanism not previously tested in humans. The semipurified milk thistle product, silibinin, was selected as an exemplar herbal product inhibitor of raloxifene intestinal glucuronidation. Physiologically based pharmacokinetic (PBPK) model simulations of the silibinin–raloxifene interaction predicted up to 30% increases in raloxifene area under the curve (AUC_0‐inf) and maximal concentration (C_max). Model‐informed clinical evaluation of the silibinin–raloxifene interaction indicated minimal clinical interaction liability, with observed geometric mean raloxifene AUC_0‐inf and C_max ratios lying within the predefined no effect range (0.75–1.33). Further refinement of PBPK modeling and simulation approaches will enhance confidence in predictions and facilitate generalizability to additional herb–drug combinations. This quantitative framework can be used to develop guidances to evaluate potential herb–drug interactions prospectively, providing evidenced‐based information about the risk or safety of these interactions.

Altered morphine glucuronide and bile acid disposition in patients with nonalcoholic steatohepatitis

The functional impact of altered drug transport protein expression on the systemic pharmacokinetics of morphine, hepatically derived morphine glucuronide (morphine-3- and morphine-6-glucuronide), and fasting bile acids was evaluated in patients with biopsy-confirmed nonalcoholic steatohepatitis (NASH) compared to healthy subjects. The maximum concentration (Cmax ) and area under the concentration-time curve (AUC0-last ) of morphine glucuronide in serum were increased in NASH patients (343 vs. 225 nM and 58.8 vs. 37.2 µM*min, respectively; P ≤ 0.005); morphine pharmacokinetics did not differ between groups. Linear regression analyses detected an association of NASH severity with increased morphine glucuronide Cmax and AUC0-last (P < 0.001). Fasting serum glycocholate, taurocholate, and total bile acid concentrations were associated with NASH severity (P < 0.006). Increased hepatic basolateral efflux of morphine glucuronide and bile acids is consistent with altered hepatic transport protein expression in patients with NASH and may partially explain differences in efficacy and/or toxicity of some highly transported anionic drugs/metabolites in this patient population.

Induction of CYP3A4 by 1 alpha,25-dihydroxyvitamin D3 is human cell line-specific and is unlikely to involve pregnane X receptor

Under certain culture conditions, exposure of the human colon adenocarcinoma cell line Caco-2 to 1,25-(OH)(2)-D(3) induces expression of CYP3A4 to levels comparable to that in human small intestinal epithelium. To determine whether 1,25-(OH)(2)-D(3) could be used to restore CYP3A expression in other culture models, we examined several cell lines derived from malignancies of human tissues known to express CYP3A enzymes: Hep G2 (liver), LS180 (colon), HPAC (pancreas), Hs746T (stomach). Primary cultures of human hepatocytes from two donors were also examined. 1,25-(OH)(2)-D(3) increased CYP3A catalytic activity in LS180 (15-fold), HPAC (6-fold), and hepatocytes (2- to 3-fold); this was accompanied by induction of CYP3A4 mRNA and CYP3A immunoreactive protein. However, 1,25-(OH)(2)-D(3) had no effect on CYP3A expression in Hs746T or Hep G2. Known ligands for pregnane X receptor (PXR) (rifampin, dexamethasone, and dexamethasone t-butyl acetate) markedly induced CYP3A4 expression in human hepatocytes. In contrast, these ligands had little or no effect on CYP3A4 expression in Caco-2 cells, even at concentrations 1 to 2 orders of magnitude greater than effective concentrations of 1,25-(OH)(2)-D(3) or two other vitamin D receptor (VDR) ligands (25-OH-D(3) and 1-OH-D(3)). The retinoic acid receptor ligand all-trans-retinoic acid augmented the 1,25-(OH)(2)-D(3)-mediated induction of CYP3A4 catalytic activity up to 2-fold in Caco-2 cells, while having no demonstrable effect on levels of CYP3A4 mRNA or protein. The retinoid X receptor ligand 9-cis-retinoic acid appeared to slightly reduce CYP3A4 catalytic activity. We conclude that 1,25-(OH)(2)-D(3) can be used to increase CYP3A4 expression in some, but not all, human cell lines derived from tissues known to express CYP3A enzymes. The mechanisms involved in this induction are unlikely to involve PXR and may involve VDR.

The role of hepatic and extrahepatic UDP-glucuronosyltransferases in human drug metabolism

At present, the methods and enzymology of the UDP-glucuronosyltransferases (UGTs) lag behind that of the cytochromes P450 (CYPs). About 15 human UGTs have been identified, and knowledge about their regulation, substrate selectivity, and tissue distribution has progressed recently. Alamethicin has been characterized as a treatment to remove the latency of microsomal glucuronidations. Most UGT isoforms appear to have a distinct hepatic and/or extrahepatic expression, resulting in significant expression in kidney, intestine, and steroid target tissues. The gastrointestinal tract possesses a complex expression pattern largely containing members of the UGT1A subfamily. Thus, these forms are poised to participate in the first pass metabolism of oral drugs. The authors and others have identified a significant expression of UGT1A1 in human small intestine, an enzyme possessing considerable allelic variability and a polymorphic expression pattern in intestine. Intestinal glucuronidation therefore plays a major role not only in first pass metabolism, but also in the degree of interindividual variation in overall oral bioavailability. Due to issues such as significant genetic variability and tissue localization in first-pass organs, clearance due to UGT1A1 should be minimized for new drugs.

Seville orange juice-felodipine interaction: comparison with dilute grapefruit juice and involvement of furocoumarins

OBJECTIVE

Our objective was to determine whether Seville orange juice produces a grapefruit juice-like interaction with felodipine and whether bergamottin, 6',7'-dihydroxybergamottin, or other furocoumarins are involved.

METHODS

In a randomized three-way crossover design, 10 volunteers received a felodipine 10-mg extended-release tablet with 240 mL of Seville orange juice, dilute grapefruit juice (that contained equivalent total molar concentrations of bergamottin plus 6',7'-dihydroxybergamottin), or common orange juice (negative control). The pharmacokinetics of felodipine and its dehydrofelodipine metabolite were determined. Juice concentrations of furocoumarins were measured. CYP3A4 inhibitory activity of newly identified furocoumarins was assessed.

RESULTS

The felodipine area under the plasma concentration-time curve was increased by 76% and 93% after Seville orange juice and grapefruit juice ingestion, respectively, compared with common orange juice. The effects of Seville orange juice and grapefruit juice were similar in that the felodipine maximum concentration was augmented while the terminal elimination half-life was unchanged and the dehydrofelodipine area under the plasma concentration time-curve was increased, but the dehydrofelodipine-felodipine area under the plasma concentration-time curve ratio was reduced. Bergamottin and 6',7'-dihydroxybergamottin concentrations were 5 and 36 micromol/L, respectively, in Seville orange juice and were 16 and 23 micromol/L, respectively, in dilute grapefruit juice. A newly identified furocoumarin, bergapten, was detected only in Seville orange juice (31 micromol/L), and it was found to be a mechanism-based inhibitor of recombinant CYP3A4. Relative to the control, 6',7'-dihydroxybergamottin (10 micromol/L) inhibited CYP3A4 activity in cultured intestinal epithelial cells by 93%, whereas bergapten (10 micromol/L) inhibited the activity by only 34%.

CONCLUSIONS

Seville orange juice and grapefruit juice interact with felodipine by a common mechanism, which is probably inactivation of intestinal CYP3A4. Bergamottin and 6',7'-dihydroxybergamottin may be "marker substances" in foods for this interaction. The lack of interaction between Seville orange juice and cyclosporine (INN, ciclosporin) suggests that grapefruit juice may also inhibit intestinal P-glycoprotein, whereas Seville orange juice may selectively "knock out" intestinal CYP3A4.

Can oral midazolam predict oral cyclosporine disposition?

Effective cyclosporine therapy is confounded by large interindividual differences in oral bioavailability and a narrow therapeutic window. Because cytochrome P450 (CYP) 3A-mediated first-pass metabolism contributes to this unpredictable bioavailability, an in vivo oral CYP3A phenotyping probe could be a valuable tool in optimizing cyclosporine therapy. Based on similarities in the metabolic kinetics of cyclosporine and midazolam by the liver and intestinal mucosa, we evaluated whether midazolam oral clearance would predict cyclosporine oral clearance when the two drugs were administered to 20 medically stable kidney transplant recipients. Despite earlier findings in liver transplant recipients who displayed a strong correlation between the systemic clearances of midazolam and cyclosporine, there was a weak correlation between their oral clearances in the current group of subjects (r(s)=0.50, P=0.03). Differing extents of intestinal first-pass metabolic extraction between the two drugs, inhibition of midazolam metabolism by cyclosporine at the level of the intestine, and/or P-glycoprotein-mediated intestinal efflux of cyclosporine (but not midazolam) may account for this poor correlation. We conclude that although oral midazolam is unlikely to be clinically useful as a probe for cyclosporine disposition, its utility in the prediction of other orally administered CYP3A substrates cannot be out ruled.

Immunochemical identification of UGT isoforms in human small bowel and in caco-2 cell monolayers

Previous work had suggested the presence of significant levels of UDP-glucuronosyltransferase 1A1 (UGT1A1) catalytic activity in human small intestinal microsomes, with undetectable to low UGT1A6 and 2B7 activities. To confirm the presence of UGT1A1 isoform in human small bowel, to explore the possible absence of UGT1A6 and 2B7 in the organ, and to examine induced Caco-2 cells as a potential model for human intestinal metabolism, Western blot analysis was performed using specific antibodies to the relevant UGT isoforms. Significant expression of UGT1A1 protein was observed in all samples of human small intestinal microsomes, while UGT1A6 expression was undetectable to faint and UGT2B7 immunoreactivity was faint to detectable. Caco-2 cells treated with typical enzyme-inducing agents resulted in low UGT2B7 expression but failed to mimic the UGT1A1 levels found in human small bowel. Further work needs to be performed to develop a comprehensive in vitro model for human small intestinal first-pass metabolism.

Comparison of CYP2D6 content and metoprolol oxidation between microsomes isolated from human livers and small intestines

PURPOSE

To assess the role of intestinal CYP2D6 in oral first-pass drug clearance by comparing the enzyme content and catalytic activity of a prototype CYP2D6 substrate, metoprolol, between microsomes prepared from human intestinal mucosa and from human livers.

METHODS

Microsomes were prepared from a panel of 31 human livers and 19 human intestinal jejunal mucosa. Microsomes were also obtained from the jejunum, duodenum and ileum of four other human intestines to assess regional distribution of intestinal CYP2D6. CYP2D6 content (pmole/mg microsomal protein) was determined by Western blot. CYP2D6 activity was measured by alpha-hydroxylation and O-demethylation of metoprolol.

RESULTS

Kinetic studies with microsomes from select livers (n = 6) and jejunal mucosa (n = 5) yielded K(M) estimates of 26 +/- 9 microM and 44 +/- 17 microM, respectively. The mean Vmax (per mg protein) for total formation of alpha-OH-M and ODM was 14-fold higher for the liver microsomes compared to the jejunal microsomes. Comparisons across intestinal regions showed that CYP2D6 protein content and catalytic activity were in the order ofjejunum > duodenum > ileum. Excluding the poor metabolizer genotype donors, CYP2D6 content varied 13- and 100-fold across the panels of human livers (n = 31) and jejunal mucosa (n = 19), respectively. Metoprolol alpha-hydroxylation activity and CYP2D6 content were highly correlated in the liver microsomes (r = 0.84, p < 0.001) and jejunal microsomes (r = 0.75, p < 0.05). Using the well-stirred model, the mean microsomal intrinsic clearance (i.e., Vmax/K(M)) for the livers and jejunum were scaled to predict their respective in vivo organ intrinsic clearance and first-pass extraction ratio. Hepatic and intestinal first-pass extractions of metoprolol were predicted to be 48% and 0.85%, respectively.

CONCLUSIONS

A much lower abundance and activity of CYP2D6 are present in human intestinal mucosa than in human liver. Intestinal mucosal metabolism contributes minimally to the first-pass effect of orally administered CYP2D6 substrates, unless they have exceptionally high microsomal intrinsic clearances and/or long residence time in the intestinal epithelium.

Effects of a chargrilled meat diet on expression of CYP3A, CYP1A, and P-glycoprotein levels in healthy volunteers

BACKGROUND & AIMS

Carcinogenic heterocyclic amines and polycyclic aromatic hydrocarbons present in chargrilled meat are substrates for inducible CYP1A and CYP3A enzymes and for P-glycoprotein. We examined whether consumption of a chargrilled meat diet results in induction of these proteins.

METHODS

Ten healthy adults were fed a diet enriched with chargrilled meat for 7 days. Duodenal biopsy specimens were obtained on days 1, 5, and 12 and analyzed for CYP1A, CYP3A, and P-glycoprotein messenger RNA (mRNA) and protein. On days 5 and 12, hepatic CYP3A4 and CYP1A2 activities were measured and colon biopsies were performed. The levels of polycyclic aromatic hydrocarbon DNA adducts in peripheral blood mononuclear cells were measured on days 1, 4, 11, and 26.

RESULTS

There was no detectable induction of CYP3A4, CYP3A5, or P-glycoprotein mRNAs or protein in small intestine or colon and no induction of hepatic CYP3A4 enzyme activity. In contrast, the chargrilled meat diet resulted in unequivocal induction of CYP1A enzymes in the liver and small intestine of each subject. There was an inverse correlation between the level of peripheral polycyclic aromatic hydrocarbon DNA adducts measured on day 11 and both liver CYP1A2 activity (P = 0.027) and enterocyte CYP1A1 protein concentration (P = 0.046).

CONCLUSIONS

Ingestion of chargrilled meat results in induction of CYP1A enzymes but not CYP3A4 or P-glycoprotein. This observation, combined with the correlation between adduct levels and CYP1A expression, supports an adaptive role for CYP1A but not CYP3A4 or P-glycoprotein.

Cytochrome P-450 1A1 expression in human small bowel: interindividual variation and inhibition by ketoconazole

Human cytochrome P-450 1A1 (CYP1A1) is located primarily in extrahepatic tissues. To begin the characterization of this enzyme in the small intestine, we screened a bank of 18 human small intestinal microsomal preparations for CYP1A1 catalytic [(7-ethoxyresorufin O-deethylase (EROD)] activity and protein content. Although EROD activity was below detectable limits in 12 of the preparations, 6 exhibited measurable activity (1.4-123.5 pmol/min/mg), some exceeding that for 2 human liver microsomal preparations (11.0 and 26.4 pmol/min/mg). This variation was not due to variable quality of the preparations because each sample displayed readily detectable CYP3A4 catalytic activity and immunoreactive protein. We inadvertently found that intestinal EROD activity was inhibitable by ketoconazole at a concentration commonly believed to selectively inhibit CYP3A4. The possibility that CYP3A4 metabolizes 7-ethoxyresorufin was excluded because there was no correlation between intestinal CYP3A4 catalytic and EROD activity, and cDNA-expressed human CYP3A4 exhibited no EROD activity. Moreover, CYP1A1 immunoreactive protein was most abundant in the three intestinal preparations with the highest EROD activities, and the mean apparent Ki of ketoconazole observed for these three preparations (40 nM) was essentially identical with that for cDNA-expressed human CYP1A1 (37 nM). In summary, there is large interindividual variation in CYP1A1 expression in human small bowel, and ketoconazole is not a selective CYP3A4 inhibitor in in vitro metabolism studies involving intestinal tissue obtained from some individuals. These observations raise the possibility that in vivo drug interactions involving ketoconazole could result from CYP1A1 inhibition in the intestine in some individuals.

Molecular and physical mechanisms of first-pass extraction

This is a report of a symposium held at the March 1997 meeting of the American Society for Pharmacology and Therapeutics in San Diego. Our understanding of the events that control first-pass drug elimination in humans has increased tremendously by two sequential discoveries. First, cytochrome P-450s 3A4 and 5 are expressed at high concentrations in both hepatocytes and upper intestinal enterocytes, and therefore limit the systemic availability of many drugs. Second, P-glycoprotein is expressed at the lumenal surface of the intestinal epithelium and therefore also acts to oppose the absorption of unchanged drug. The following discussion brings together our current understandings of these interrelated phenomena to aid a more complete picture of how they may contribute both qualitatively and quantitatively to first-pass elimination.

Characterization of interintestinal and intraintestinal variations in human CYP3A-dependent metabolism

Cytochrome P450 3A (CYP3A) metabolizes a diverse array of clinically important drugs. For some of these (e.g., cyclosporine, verapamil, midazolam), CYP3A in the intestinal mucosa contributes to their extensive and variable first-pass extraction. To further characterize this phenomenon, we measured CYP3A content and catalytic activity toward the probe substrate midazolam in mucosa isolated from duodenal, jejunal and ileal sections of 20 human donor intestines. For comparison, the same measurements were performed for 20 human donor livers, eight of which were obtained from the same donors as eight of the intestines. Excellent correlations existed between homogenate and microsomal CYP3A content for the three intestinal regions. Median microsomal CYP3A content was greatest in the duodenum and lowest in the ileum (31 vs. 17 pmol/mg of protein). With respect to midazolam 1'-hydroxylation kinetics, the median Km for each intestinal region was similar to the median hepatic Km, approximately 4 microM. In contrast, the median Vmax decreased from liver to duodenum to jejunum to ileum (850 vs. 644 vs. 426 vs. 68 pmol/min/mg). Intrinsic clearance (Vmax/Km) followed a similar trend for the intestinal regions; median duodenal intrinsic clearance was comparable to hepatic intrinsic clearance (157 and 200 microl/min/mg, respectively). Vmax correlated with CYP3A content for all tissues except the ileum. Duodenal and jejunal Vmax and CYP3A content varied by >30-fold among donors. Microsomes prepared from every other 1-foot section of six intestines were also analyzed for CYP3A as well as for two coenzymes. In general, CYP3A activity, CYP3A content and CYP reductase activity rose slightly from duodenum to middle jejunum and then declined to distal jejunum and ileum. Cytochrome b5 content and cytochrome b5 reductase activity varied little throughout the intestinal tract. Regional intrinsic midazolam 1'-hydroxylation clearance was greatest for the jejunum, followed by the duodenum and ileum (144, 50 and 19 ml/min, respectively). Collectively, these results demonstrate that the upper small intestine serves as the major site for intestinal CYP3A-mediated first-pass metabolism and provides a rationale for interindividual differences in oral bioavailability for some CYP3A substrates.

Fentanyl metabolism by human hepatic and intestinal cytochrome P450 3A4: implications for interindividual variability in disposition, efficacy, and drug interactions

The synthetic opioid fentanyl undergoes extensive metabolism in humans. Systemic elimination occurs primarily by hepatic metabolism. When administered as a lozenge for oral transmucosal absorption, swallowed fentanyl is subject to first pass metabolism in the liver and possibly small intestine. Little is known, however, about the identity and formation of human fentanyl metabolites. This investigation identified routes of human liver microsomal fentanyl metabolism and their relative importance, tested the hypothesis that fentanyl is metabolized by human duodenal microsomes, and identified the predominantly responsible cytochrome P450 isoforms. A GC/MS assay using deuterated internal standards was developed for fentanyl metabolites. Piperidine N-dealkylation to norfentanyl was the predominant pathway of liver microsomal metabolism. Amide hydrolysis to despropionylfentanyl and alkyl hydroxylation to hydroxyfentanyl were comparatively minor pathways. Hydroxynorfentanyl was identified as a minor, secondary metabolite arising from N-dealkylation of hydroxyfentanyl. Liver microsomal norfentanyl formation was significantly inhibited by the mechanism-based P450 3A4 inhibitor troleandomycin and the P450 3A4 substrate and competitive inhibitor midazolam, and was significantly correlated with P450 3A4 protein content and catalytic activity. Of six expressed human P450 isoforms (P450s 1A2, 2B6, 2C9, 2D6, 2E1, and 3A4), only P450 3A4 exhibited significant fentanyl dealkylation to norfentanyl. These results indicate the predominant role of P450 3A4 in the primary route of hepatic fentanyl metabolism. Human duodenal microsomes also catalyzed fentanyl metabolism to norfentanyl; the average rate was approximately half that of hepatic metabolism. Rates of duodenal norfentanyl formation were diminished by troleandomycin and midazolam, and were significantly correlated with P450 3A4 activity, suggesting a prominent role for P450 3A4. These results demonstrate that human intestinal as well as liver microsomes catalyze fentanyl metabolism, and N-dealkylation by P450 3A4 is the predominant route in both organs. The fraction of fentanyl lozenge that is swallowed likely undergoes significant intestinal, as well as hepatic, first-pass metabolism. Intestinal and hepatic first-pass metabolism, as well as systemic metabolism, may be subject to individual variability in P450 3A4 expression and to drug interactions involving P450 3A4.

Expression of enzymatically active CYP3A4 by Caco-2 cells grown on extracellular matrix-coated permeable supports in the presence of 1alpha,25-dihydroxyvitamin D3

The human colon carcinoma cell line, Caco-2, is widely used as a model for oral absorption of xenobiotics. The usefulness of Caco-2 cells has been limited, however, because they do not express appreciable quantities of CYP3A4, the principle cytochrome P450 present in human small bowel epithelial cells. We report that treatment of Caco-2 cells with 1 alpha,25-dihydroxyvitamin D3, beginning at confluence, results in a dose- and duration-dependent increase in CYP3A4 mRNA and protein, with little apparent effect on the expression of CYP3A5 or CYP3A7. This treatment also results in increases in NADPH cytochrome P450 reductase and P-glycoprotein (the MDR1 gene product) but has no detectable effect on expression of CYP1A1, CYP2D6, cytochrome b5, liver or intestinal fatty acid binding proteins, or villin. Maximal expression of CYP3A4 requires an extracellular matrix on a permeable support and the presence of serum. In the treated cells, the intrinsic formation clearance of 1'-hydroxymidazolam (a reaction characteristically catalyzed by CYP3A enzymes) was estimated to be somewhat lower than that of human jejunal mucosa (1.14 and 3.67 ml/min/g of cells, respectively). The 1'-OH-midazolam/4-OH-midazolam product ratio produced by the cells (approximately 5.3) is comparable to, but somewhat lower than, that observed in human jejunal microsomes (7.4-15.4), which may reflect the presence of CYP3A7 in the Caco-2 cells. 25-Hydroxyvitamin D3 is less efficacious but reproduces the effects of the dihydroxy compound, whereas unhydroxylated vitamin D is without appreciable effect. These observations, together with the time course of response, suggest that the vitamin D receptor may be involved in CYP3A4 regulation. The culture model we describe should prove useful in defining the role of CYP3A4 in limiting the oral bioavailability of many xenobiotics.

First-pass metabolism of midazolam by the human intestine

The in vivo intestinal metabolism of the CYP3A probe midazolam to its principal metabolite, 1'-hydroxymidazolam, was investigated during surgery in 10 liver transplant recipients. After removal of the diseased liver, five subjects received 2 mg midazolam intraduodenally, and the other five received 1 mg midazolam intravenously. Simultaneous arterial and hepatic portal venous blood samples were collected during the anhepatic phase; collection of arterial samples continued after reperfusion of the donor liver. Midazolam, 1'-hydroxymidazolam, and 1'-hydroxymidazolam glucuronide were measured in plasma. A mass balance approach that considered the net change in midazolam (intravenously) or midazolam and 1'-hydroxymidazolam (intraduodenally) concentrations across the splanchnic vascular bed during the anhepatic phase was used to quantitate the intestinal extraction of midazolam after each route of administration. For the intraduodenal group, the mean fraction of the absorbed midazolam dose that was metabolized on transit through the intestinal mucosa was 0.43 +/- 0.18. For the intravenous group, the mean fraction of midazolam extracted from arterial blood and metabolized during each passage through the splanchnic vascular bed was 0.08 +/- 0.11. Although there was significant intersubject variability, the mean intravenous and intraduodenal extraction fractions were statistically different (p = 0.009). Collectively, these results show that the small intestine contributes significantly to the first-pass oxidative metabolism of midazolam catalyzed by mucosal CYP3A4 and suggest that significant first-pass metabolism may be a general phenomenon for all high-turnover CYP3A4 substrates.

Oral first-pass elimination of midazolam involves both gastrointestinal and hepatic CYP3A-mediated metabolism

OBJECTIVE

To determine in humans the relative roles of intestinal and hepatic metabolism in the oral first-pass elimination of a CYP3A substrate using midazolam as a model compound.

METHODS

Midazolam was administered intravenously (1 mg) or orally (2 mg) to 20 healthy young subjects (10 men and 10 women) in a random fashion, and the disposition of the drug and its 1'-hydroxy metabolite were determined. In separate in vitro studies, the CYP3A-mediated formation of 1'-hydroxymidazolam by human hepatic and intestinal microsomes was investigated.

RESULTS

No gender-related differences were noted in either the systemic (370 +/- 114 ml/min [mean +/- SD]) or oral (1413 +/- 807 ml/min) clearance values of midazolam. Despite complete oral absorption, measured oral bioavailability was on average about 50% less than that predicted on the assumption that only the liver contributed to first-pass metabolism. Pharmacokinetic estimation of the intestinal component indicated an extraction ratio (0.43 +/- 0.24) that was similar to that of the liver (0.44 +/- 0.14). 1'-Hydroxymidazolam was extensively but variably formed in vitro by both hepatic and intestinal microsomes and, although the intrinsic clearance (V(max)/Km) was higher in the liver preparations (540 +/- 747 versus 135 +/- 92 microliters/min/mg protein), this difference was not statistically significant.

CONCLUSIONS

These results show that the small intestine can be a major site for presystemic, CYP3A-mediated metabolism after oral administration. Moreover, it appears that this represents a true first-pass effect. In addition, intestinal and hepatic metabolism may be important factors in interindividual variability in disposition after oral administration of midazolam and similar CYP3A substrates. Finally, intestinal localization of CYP3A may be significant in metabolism-based drug-drug interactions.