PubMed Health⌕ Search

Biomedical subjects

S D Hall

Publications and source records attributed to S D Hall.

At least 19 recordsLinked to original sources
PubMed2005

Diltiazem inhibits human intestinal cytochrome P450 3A (CYP3A) activity in vivo without altering the expression of intestinal mRNA or protein.

AIMS: To determine the effect of diltiazem on intestinal CYP3A activity and protein and mRNA expression in vivo in healthy subjects. METHODS: Intestinal biopsies were obtained from ten healthy controls and from ten healthy subjects after receiving diltiazem 120 mg bid for 7 days. Intestinal CYP3A activity, CYP3A4 protein and mRNA concentrations were quantified in both groups. Intestinal CYP3A activity was determined by incubation of small bowel homogenate with midazolam (25 microM) and NADPH for 5 min and the rate of formation of 1'-hydroxymidazolam was quantified. RESULTS: All subjects in the treatment group had detectable diltiazem concentration in the serum. While there was no significant difference in CYP3A4 protein and mRNA expression between the control and treatment groups, the formation of 1'-hydroxymidazolam (446 pmol min(-1) mg(-1) 6 (control) vs. 170 (CI 112, 228) pmol min(-1) mg(-1) 95% confidence interval (CI 269, 623) (diltiazem group)) was significantly reduced (P < 0.05). CONCLUSION: Diltiazem decreased small bowel CYP3A activity by 62% as a result of irreversible inhibition with no corresponding change in intestinal CYP3A4 mRNA or protein concentrations.

Adult↗
PubMed2002

Reversible and irreversible inhibition of CYP3A enzymes by tamoxifen and metabolites.

1. Preliminary studies have identified cytochrome P450 (CYP) 3A4 and CYP1B1 as the human CYPs inhibited by tamoxifen. To quantify the inhibitory potency of tamoxifen and its major metabolites, the metabolism of three substrates of CYP3A, midazolam, diltiazem and testosterone, and 7-ethoxyresorufin as a substrate of CYP1B1 were examined in catalytic assays carried out using human liver microsomes and cDNA-expression systems. 2. Tamoxifen, N-desmethyltamoxifen, 4-hydroxytamoxifen and 3-hydroxytamoxifen reversibly inhibited midazolam 1'-hydroxylation, diltiazem N-demethylation and testosterone 6beta-hydroxylation with K(i) ranging from 3 to 37 micro M in human liver microsomes. Tamoxifen, N-desmethyltamoxifen, 4-hydroxytamoxifen and 3-hydroxytamoxifen also reversibly inhibited the activity of cDNA-expressed CYP3A4, CYP3A5 and CYP1B1. 3. Tamoxifen and N-desmethyltamoxifen exhibited time-dependent inactivation of testosterone 6beta-hydroxylation by cDNA-expressed CYP3A4 (+ cytochrome b5) yielding k(inact) and K(i) of 0.04 min(-1) and 0.2 micro M for tamoxifen and 0.08 min(-1) and 2.6 micro M for N-desmethyltamoxifen. A metabolic intermediate complex (MIC) was also formed by tamoxifen and N-desmethyltamoxifen with CYP3A4 (+ cytochrome b5) and CYP3A4 but not with CYP3A5 or CYP3A7. Pre-incubation with 4-hydroxytamoxifen and 3-hydroxytamoxifen did not result in any CYP3A inactivation or detectable MIC formation. There was no detectable time-dependent inactivation or MIC formation with tamoxifen or metabolites with CYP1B1. 4. These data indicate that tamoxifen and its three major metabolites are effective inhibitors of CYP3A in vitro and that tamoxifen and N-desmethyltamoxifen are effective mechanism-based inhibitors. Thus, caution should be exercised when tamoxifen is coadministered with other CYP3A substrates.

Antineoplastic Agents, Hormonal↗
PubMed2001

In vitro exposure to highly cytopathic HIV-1 X4 strains increases expression of mucosa-associated integrins on CD4(+) T cells.

To determine whether infection with HIV-1 strains of different tropisms would influence expression of the mucosa-associated integrins alpha 4 beta 7 and alpha E beta 7 or the lymph node homing receptor L-selectin on peripheral T lymphocytes, cells were infected with the CXCR4-tropic (X4)/syncytium-inducing (SI) HIV-1(IIIB) strain or with X4/SI or CCR5-tropic (R5)/non-SI (NSI) primary human isolates. Flow cytometric analyses of CD4(+) T cells from cultures infected with HIV-1(IIIB) and one X4/SI primary HIV-1 isolate revealed a significant increase in surface expression of alpha 4 beta 7 and alpha E beta 7 12 days after infection. L-selectin expression was not significantly affected on CD4(+) T cells. However, infection with another X4/SI and two R5/NSI primary HIV-1 isolates did not significantly alter homing receptor expression on CD4(+) T cells. Since a higher degree of CD4 cytopathicity occurred in those cultures having increased integrin expression, these data suggest that significantly altered mucosal homing receptor expression on CD4(+) T cells may result as a "bystander" effect after infection with some cytopathic isolates of HIV-1.

CD4-Positive T-Lymphocytes↗
PubMed2001

Sequence diversity in CYP3A promoters and characterization of the genetic basis of polymorphic CYP3A5 expression.

Variation in the CYP3A enzymes, which act in drug metabolism, influences circulating steroid levels and responses to half of all oxidatively metabolized drugs. CYP3A activity is the sum activity of the family of CYP3A genes, including CYP3A5, which is polymorphically expressed at high levels in a minority of Americans of European descent and Europeans (hereafter collectively referred to as 'Caucasians'). Only people with at least one CYP3A5*1 allele express large amounts of CYP3A5. Our findings show that single-nucleotide polymorphisms (SNPs) in CYP3A5*3 and CYP3A5*6 that cause alternative splicing and protein truncation result in the absence of CYP3A5 from tissues of some people. CYP3A5 was more frequently expressed in livers of African Americans (60%) than in those of Caucasians (33%). Because CYP3A5 represents at least 50% of the total hepatic CYP3A content in people polymorphically expressing CYP3A5, CYP3A5 may be the most important genetic contributor to interindividual and interracial differences in CYP3A-dependent drug clearance and in responses to many medicines.

Alleles↗
PubMed2001

Hepatic and intestinal cytochrome P450 3A activity in cirrhosis: effects of transjugular intrahepatic portosystemic shunts.

Transjugular intrahepatic portosystemic shunt (TIPS) is performed to treat some complications of cirrhosis. This study investigated the effects of cirrhosis and TIPS on intestinal and hepatic cytochrome P450 3A (CYP3A) activity. Nine volunteers were cirrhotic patients with TIPS, 9 were cirrhotic controls (matched for sex, age, etiology, and Child-Pugh class), and 9 were sex- and age-matched healthy volunteers. Simultaneous doses of midazolam were given intravenously (0.05 mg/kg) and orally (3 mg of [15N3]midazolam). Peripheral and portal venous blood samples were assayed for midazolam and [15N3]midazolam. The systemic clearance of midazolam was significantly greater (P <.05) in healthy volunteers (0.42 +/- 0.10 L x h(-1) x kg(-1)) compared with cirrhotic controls (0.20 +/- 0.05) and with cirrhotic patients with TIPS (0.21 +/- 0.09). Hepatic availability followed the same trend. The bioavailability of midazolam was significantly higher (P <.05) in cirrhotic patients with TIPS (0.76 +/- 0.20) compared with cirrhotic controls (0.27 +/- 0.14) and with healthy volunteers (0.30 +/- 0.10). The intestinal availability was significantly greater (P <.05) in cirrhotic patients with TIPS (0.83 +/- 0.17) compared with cirrhotic controls (0.32 +/- 0.16) and with healthy volunteers (0.42+/-0.15). As expected, hepatic CYP3A activity was reduced in cirrhosis. However, in cirrhotic patients with TIPS, there was a marked loss in first-pass metabolism of midazolam as a result of diminished intestinal CYP3A activity.

Administration, Oral↗
PubMed2001

The effect of rifampin administration on the disposition of fexofenadine.

OBJECTIVE: Our objective was to assess the effect of rifampin (INN, rifampicin) on the pharmacokinetics of fexofenadine and to assess the influence of advanced age and sex. METHODS: Twelve young volunteers (6 men and 6 women; age range, 22 to 35 years) and twelve elderly volunteers (6 men and 6 women; age range, 65 to 76 years) received a 60-mg oral dose of fexofenadine before and after treatment with 600 mg of oral rifampin for 6 days. Blood and urine were collected for 48 hours and assayed for fexofenadine, azacyclonol, and rifampin by HPLC with either fluorescence or mass spectrometry detection. RESULTS: All of the groups had a significant increase (P <.05) in the oral clearance of fexofenadine after rifampin treatment: young men, 2955 +/- 1516 versus 5524 +/- 3410 mL/min; young women, 2632 +/- 996 versus 7091 +/- 5,379 mL/min; elderly men, 1760 +/- 850 versus 4608 +/- 1159 mL/min; and elderly women, 2210 +/- 554 versus 4845 +/- 1600 mL/min. The peak serum concentration of fexofenadine was also significantly reduced (P <.05) by rifampin treatment: young men, 77 +/- 31 versus 52 +/- 17 ng/mL; young women, 72 +/- 19 versus 36 +/- 14 ng/mL; elderly men, 106 +/- 42 versus 52 +/- 14 ng/mL; elderly women, 76 +/- 23 versus 46 +/- 19 ng/mL. Half-life (150 to 230 minutes), time to maximum concentration (130 to 205 minutes), renal clearance (95 to 153 mL/min), and fraction unbound (2.9% to 3.7%) of fexofenadine showed no significant difference between control and treatment. The amount of azacyclonol, a CYP3A4 mediated metabolite of fexofenadine, eliminated renally increased on average 2-fold after rifampin dosing; however, this pathway accounted for less than 0.5% of the dose. No effect of age or sex on fexofenadine disposition or serum trough rifampin concentration (0.2 microg/mL to 1.8 microg/mL) was observed before or after rifampin treatment. CONCLUSION: This study showed that rifampin effectively increased fexofenadine oral clearance and that this effect was independent of age and sex. We conclude that the cause of the increased oral clearance of fexofenadine is a reduced bioavailability caused by induction of intestinal P-glycoprotein.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2001

The human pregnane X receptor: genomic structure and identification and functional characterization of natural allelic variants.

The pregnane X receptor (PXR)/steroid and xenobiotic receptor (SXR) transcriptionally activates cytochrome P4503A4 (CYP3A4) when ligand activated by endobiotics and xenobiotics. We cloned the human PXR gene and analysed the sequence in DNAs of individuals whose CYP3A phenotype was known. The PXR gene spans 35 kb, contains nine exons, and mapped to chromosome 13q11-13. Thirty-eight single nucleotide polymorphisms (SNPs) were identified including six SNPs in the coding region. Three of the coding SNPs are non-synonymous creating new PXR alleles [PXR*2, P27S (79C to T); PXR*3, G36R (106G to A); and PXR*4, R122Q (4321G to A)]. The frequency of PXR*2 was 0.20 in African Americans and was never found in Caucasians. Hepatic expression of CYP3A4 protein was not significantly different between African Americans homozygous for PXR*1 compared to those with one PXR*2 allele. PXR*4 was a rare variant found in only one Caucasian person. Homology modelling suggested that R122Q, (PXR*4) is a direct DNA contact site variation in the third alpha-helix in the DNA binding domain. Compared with PXR*1, and variants PXR*2 and PXR*3, only the variant PXR*4 protein had significantly decreased affinity for the PXR binding sequence in electromobility shift assays and attenuated ligand activation of the CYP3A4 reporter plasmids in transient transfection assays. However, the person heterozygous for PXR*4 is normal for CYP3A4 metabolism phenotype. The relevance of each of the 38 PXR SNPs identified in DNA of individuals whose CYP3A basal and rifampin-inducible CYP3A4 expression was determined in vivo and/or in vitro was demonstrated by univariate statistical analysis. Because ligand activation of PXR and upregulation of a system of drug detoxification genes are major determinants of drug interactions, it will now be useful to extend this work to determine the association of these common PXR SNPs to human variation in induction of other drug detoxification gene targets.

Alleles↗
PubMed2001

In vivo effect of clarithromycin on multiple cytochrome P450s.

The in vivo effects of oral clarithromycin administration on the in vivo activity of cytochrome P450 1A2, 2C9, and 2D6 were determined. The cytochrome P450 probes caffeine (CYP1A2), tolbutamide (CYP2C9), and dextromethorphan (CYP2D6) were administered as an oral cocktail prior to and 7 days after oral clarithromycin (500 mg twice daily) administration to 12 healthy male subjects. Blood and urine samples were collected and assayed for each of the compounds and their metabolites using high-performance liquid chromatography. The CYP1A2 indices, oral caffeine clearance (6.2 +/- 3.3 l/h before and 5.7 +/- 4.2 l/h after, p > 0.05) and the 6-h paraxanthine to caffeine serum concentration ratio (0.49 +/- 0.3 before and 0.44 +/- 0.3 after, p > 0.05), were unchanged following clarithromycin dosing. Neither the tolbutamide oral clearance (0.77 +/- 0.28 l/h before and 0.72 +/-0.24 l/h after, p > 0.05) nor the tolbutamide urinary metabolic ratio (779 +/- 294 before and 681 +/- 416 after, p > 0.05) indices of CYP2C9 were altered by clarithromycin administration. In the case of CYP2D6, the dextromethorphan to dextrorphan urinary ratio was not significantly different before (0.021 +/- 0.04) and after (0.024 +/- 0.06) clarithromycin dosing. In conclusion, clarithromycin does not appear to alter the in vivo catalytic activity of CYP1A2, CYP2C9, and CYP2D6 in healthy individuals as assessed by caffeine, tolbutamide, and dextromethorphan, respectively.

Adult↗
PubMed2001

The effects of St John's wort (Hypericum perforatum) on human cytochrome P450 activity.

BACKGROUND: St John's wort (Hypericum perforatum) is a popular over-the-counter dietary supplement and herbal remedy that has been implicated in drug interactions with substrates of several cytochrome P450 (CYP) isozymes. The effect of St John's wort on CYP activity in vivo was examined with a probe drug cocktail. METHODS: Twelve healthy subjects (5 female, 7 male) completed this 3-period, open-label, fixed schedule study. Tolbutamide (CYP2C9), caffeine (CYP1A2), dextromethorphan (CYP2D6), oral midazolam (intestinal wall and hepatic CYP3A), and intravenous midazolam (hepatic CYP3A) were administered before, with short-term St John's wort dosing (900 mg), and after 2 weeks of intake (300 mg 3 times a day) to determine CYP activities. RESULTS: Short-term administration of St John's wort had no effect on CYP activities. Long-term St John's wort administration caused a significant (P <.05) increase in oral clearance of midazolam from 121.8 +/- 70.7 to 254.5 +/- 127.8 and a corresponding significant decline in oral bioavailability from 0.28 +/- 0.15 to 0.17 +/- 0.06. In contrast to the >50% decrease in the area under the plasma concentration-time curve (AUC) when midazolam was administered orally, long-term St John's wort administration caused a 20% decrease in AUC when midazolam was given intravenously. There was no change in CYP1A2, CYP2C9, or CYP2D6 activities as a result of St John's wort administration. CONCLUSION: Long-term St John's wort administration resulted in a significant and selective induction of CYP3A activity in the intestinal wall. St John's wort did not alter the CYP2C9, CYP1A2, or CYP2D6 activities. Reduced therapeutic efficacy of drugs metabolized by CYP3A should be anticipated during long-term administration of St John's wort.

Administration, Oral↗
PubMed2000

Stereoselective sulfoxidation of sulindac sulfide by flavin-containing monooxygenases. Comparison of human liver and kidney microsomes and mammalian enzymes.

The stereoselective sulfoxidation of the pharmacologically active metabolite of sulindac, sulindac sulfide, was characterized in human liver, kidney, and cDNA-expressed enzymes. Kinetic parameter estimates (pH = 7.4) for sulindac sulfoxide formation in human liver microsomes (N = 4) for R- and S-sulindac sulfoxide were V(max) = 1.5 +/- 0.50 nmol/min/mg, K(m) = 15 +/- 5.1 microM; and V(max) = 1.1 +/- 0.36 nmol/min/mg, K(m) = 16 +/- 6.1 microM, respectively. Kidney microsomes (N = 3) produced parameter estimates (pH = 7.4) of V(max) = 0.9 +/- 0.29 nmol/min/mg, K(m) = 15 +/- 2.9 microM; V(max) = 0.5 +/- 0.21 nmol/min/mg, K(m) = 22 +/- 1.9 microM for R- and S-sulindac sulfoxide, respectively. In human liver and flavin-containing monooxygenase 3 (FMO3) the V(max) for R-sulindac sulfoxide increased 60-70% at pH = 8.5, but for S-sulindac sulfoxide was unchanged. In fourteen liver microsomal preparations, significant correlations occurred between R-sulindac sulfoxide formation and either immunoquantified FMO or nicotine N-oxidation (r = 0.88 and 0.83; P < 0.01). The R- and S-sulindac sulfoxide formation rate also correlated significantly (r = 0.85 and 0.75; P < 0.01) with immunoquantified FMO in thirteen kidney microsomal samples. Mild heat deactivation of microsomes reduced activity by 30-60%, and a loss in stereoselectivity was observed. Methimazole was a potent and nonstereoselective inhibitor of sulfoxidation in liver and kidney microsomes. n-Octylamine and membrane solubilization with lubrol were potent and selective inhibitors of S-sulindac sulfoxide formation. cDNA-expressed CYPs failed to appreciably sulfoxidate sulindac sulfide, and CYP inhibitors were ineffective in suppressing catalytic activity. Purified mini-pig liver FMO1, rabbit lung FMO2, and human cDNA-expressed FMO3 efficiently oxidized sulindac sulfide with a high degree of stereoselectivity towards the R-isomer, but FMO5 lacked catalytic activity. The biotransformation of the sulfide to the sulfoxide is catalyzed predominately by FMOs and may prove to be useful in characterizing FMO activity.

Animals↗
PubMed2000

Productive human immunodeficiency virus-1 infection of epithelial cell lines of salivary gland origin.

To ascertain whether epithelial cells of oral cavity origin may be infected with human immunodeficiency virus (HIV-1), a study to determine susceptibility to infection of salivary gland epithelial cell lines (HSY and HSG) was undertaken. Because of the potential for oral-genital transmission, an endometrial cell line, HEC-1, was also studied. Epithelial cell monolayers were infected with cell-free HTLVIIIB or a primary HIV-1 isolate. Several lines of evidence indicated that inoculation of these cell lines with HIV-1 led to productive infection: 1) p24 antigen was present in supernatants, with levels peaking on days 3-4; 2) provirus was found in cells by polymerase chain reaction; 3) virions present in supernatants were infectious as confirmed by coculture with the T-lymphoblastoid line CEM-NKr. Following a period of virus production, HIV-1 entered a latency phase over 10 weeks. All epithelial cell lines were positive for galactosylceramide (GalC) and CXCR4. HSY was weakly positive for surface CD4, and also expressed mRNA for CD4 and CCR5, as did HEC-1. Blocking studies indicated that anti-GalC, but not anti-CD4, significantly reduced productive infection, and that regulated on activation normal T cell expressed and secreted (RANTES) but not stromal cell-derived factor (SDF-1) could partially block infection of the M-tropic primary isolate. These results suggest that epithelial cells in the oral cavity and the genital tract might be targets of HIV-1 and potentially serve as a mediator of systemic infection.

Antibodies, Monoclonal↗
PubMed2000

Intravenous diltiazem and CYP3A-mediated metabolism.

AIMS: To study whether intravenous diltiazem, a calcium channel blocker commonly prescribed for hypertension and stable angina, is an inhibitor of the CYP3A enzymes by using oral lovastatin, an HMG Co-A reductase inhibitor, as a substrate. METHODS: Ten healthy volunteers were studied in a randomized two-way crossover design. The two arms were 1) administration of a 20 mg dosage of lovastatin orally and 2) administration of a 20 mg dosage of lovastatin orally 1 h after an intravenous loading dosage and constant infusion of diltiazem. Blood samples were collected up to 25 h in order to quantify lovastatin and diltiazem concentrations in the separated serum. Lovastatin and diltiazem concentrations were quantified by GC-MS and h.p.l.c., respectively. RESULTS: Intravenous diltiazem did not significantly affect the oral AUC, Cmax, t(1/2), or tmax of lovastatin. CONCLUSIONS: These data suggest that the interaction of lovastatin with diltiazem does not occur systemically and is primarily a first-pass effect. Thus, drug interactions with diltiazem may become evident when a patient is moved from intravenous to oral dosing.

Adult↗
PubMed2000

In vivo effects of interleukin-10 on human cytochrome P450 activity.

BACKGROUND: Injection of lipopolysaccharide into human volunteers leads to an increase in serum interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha and a significant decrease in cytochrome P450 (CYP)-mediated drug metabolism. The in vivo effects of the noninflammatory cytokine interleukin-10 (IL-10) on CYP-mediated drug metabolism was examined. METHODS: IL-10 (8 microg/kg) and placebo were administered for 6 days to 12 healthy volunteers in a double-blind crossover study. Tolbutamide (CYP2C9), caffeine (CYP1A2), dextromethorphan (CYP2D6 and CYP3A), and midazolam (CYP3A) were administered on days 4 and 5 to determine individual CYP activities. RESULTS: Few clinically apparent side effects were observed after administration of IL-10; however, blood chemistries reflected an acute-phase response. A significant drop in serum albumin (mean percentage change +/- SD between groups; 4.7% +/- 6.0%, P < or = .02), a significant increase in serum ferritin (736% +/- 717%, P < or = .001), and a significant reduction in platelet count (49% +/- 12%, P < or = .0001) was observed after administration of IL-10. IL-10 significantly (P < or = .02) decreased CYP3A activity 12% +/- 17%, as reflected by midazolam clearance. CYP2C9 activity was significantly (P < or = .005) increased by 38% +/- 35%, as reflected by the tolbutamide urinary metabolic ratio and oral clearance. However, administration of IL-10 resulted in a 40% increase in the fraction unbound of tolbutamide. Therefore no difference in the unbound clearance of tolbutamide was observed between placebo (23.3 +/- 9.7 L/h) or IL-10 (23.5 +/- 11.4 L/h) administration. No significant changes in either CYP1A2 or CYP2D6 activities were observed between placebo and treatment arms of the study. CONCLUSION: IL-10 administration resulted in an acute-phase response. Administration of IL-10 did not alter CYP1A2, CYP2C9, and CYP2D6 activities. CYP3A-mediated biotransformation was reduced by administration of IL-10.

Adult↗
PubMed2000

The interaction of diltiazem with simvastatin.

BACKGROUND: Simvastatin is an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase that is used as a cholesterol-lowering agent and is metabolized by cytochrome P450 3A (CYP3A) enzymes. Diltiazem is a substrate and an inhibitor of CYP3A enzymes and is commonly coadministered with cholesterol-lowering agents such as simvastatin. The objective of this study was to quantify the effect of diltiazem on the pharmacokinetics of simvastatin. METHOD: A fixed-order study was conducted in 10 healthy volunteers with a 2-week washout period between the phases. In one arm of the study, a single 20-mg dose of simvastatin was administered orally; the second arm entailed administration of a single 20-mg dose of simvastatin orally after 2 weeks of treatment with 120 mg diltiazem twice a day. RESULTS: Diltiazem significantly increased the mean peak serum concentration of simvastatin by 3.6-fold (P < .05) and simvastatin acid by 3.7-fold (P < .05). Diltiazem also significantly increased the area under the serum concentration-time curve of simvastatin 5-fold (P < .05) and the elimination half-life 2.3-fold (P < .05). There was no change in the time to peak concentration for simvastatin and simvastatin acid. CONCLUSION: Diltiazem coadministration resulted in a significant interaction with simvastatin, probably by inhibiting CYP3A-mediated metabolism. Concomitant use of diltiazem or other potent inhibitors of CYP3A with simvastatin should be avoided, or close clinical monitoring should be used.

Adult↗
PubMed2000

The effect of hormone replacement therapy on CYP3A activity.

BACKGROUND: The effect of menopause and hormone replacement therapy on hepatic and intestinal wall CYP3A activity is poorly defined. This study was therefore designed to determine the effect of menopause and estrogen replacement therapy on hepatic and intestinal CYP3A activity with a specific CYP3A substrate, midazolam. METHODS: Twelve young women (27 +/- 5 years), 10 elderly women receiving estrogen replacement therapy (71 +/- 6 years), and 14 elderly women not receiving estrogen replacement therapy (71 +/- 5 years) received simultaneous intravenous (0.05 mg/kg over 30 minutes) and oral (3 to 4 mg of a stable isotope, 15N3-midazolam) doses of midazolam. Serum and urine samples were assayed for midazolam, 15N3-midazolam, and metabolites by use of gas chromatography-mass spectrometry. RESULTS: No significant (P > .05) differences were observed in systemic clearance and oral clearance between the three groups. Likewise, no differences were observed in oral, hepatic, or intestinal availability. A significant correlation was observed between oral and intestinal availability and not hepatic availability. CONCLUSION: Neither menopause nor menopause with estrogen replacement therapy altered intestinal or hepatic CYP3A activity relative to that in a control group of young women.

Administration, Oral↗
PubMed2000

Tissue distribution and interindividual variation in human UDP-glucuronosyltransferase activity: relationship between UGT1A1 promoter genotype and variability in a liver bank.

The variability in a liver bank and tissue distribution of three probe UDP-glucuronosyltransferase (UGT) activities were determined as a means to predict interindividual differences in expression and the contribution of extrahepatic metabolism to presystemic and systemic clearance. Formation rates of acetaminophen-O-glucuronide (APAPG), morphine-3-glucuronide (M3G), and oestradiol-3-glucuronide (E3G) as probes for UGT1A6, 2B7, and 1A1, respectively, were determined in human kidney, liver, and lung microsomes, and in microsomes from intestinal mucosa corresponding to duodenum, jejunum and ileum. While formation of E3G and APAPG were detectable in human kidney microsomes, M3G formation rates from kidney microsomes approached the levels seen in liver, indicating significant expression of UGT2B7. Interestingly, rates of E3G formation in human intestine exceeded the hepatic rates by several fold, while APAPG and M3G formation rates were low. The intestinal apparent Km value for E3G formation was essentially identical to that seen in liver, consistent with intestinal UGT1A1 expression. No UGT activities were observed in lung. Variability in APAPG and M3G activity across a bank of 20 human livers was modest (< or = 7-fold), compared to E3G formation, which varied approximately 30-fold. The E3G formation rates were found to segregate by UGT1A1 promoter genotype, with wild-type (TA)6 rates significantly greater than homozygous mutant (TA)7 individuals. Kinetic analyses were performed to demonstrate that the promoter mutation altered apparent Vmax without significantly affecting apparent Km. These results suggest that glucuronidation, and specifically UGT1A1 activity, can profoundly contribute to intestinal first pass metabolism and interindividual variability due to the expression of common allelic variants.

Acetaminophen↗
PubMed2000

An in vitro model for predicting in vivo inhibition of cytochrome P450 3A4 by metabolic intermediate complex formation.

An in vitro model is proposed to account for the clinically observed inhibition of cytochrome P450 (CYP) 3A that results from administration of clarithromycin, fluoxetine, or diltiazem. Rates for loss of CYP3A4 enzymatic activity resulting from metabolic intermediate complex formation and the concentration dependencies thereof were determined in vitro for clarithromycin, fluoxetine, and N-desmethyl diltiazem, which is the primary metabolite of diltiazem. Using the in vitro concentration-dependent rates for loss of activity, in vivo rates of CYP3A4 inactivation were predicted for these compounds at a clinically relevant unbound plasma concentration of 0.1 microM. Based on the predicted rates combined with published rates for in vivo CYP3A degradation, our model predicts that fluoxetine, clarithromycin, and the primary metabolite of diltiazem reduce the steady-state concentration of liver CYP3A4 to approximately 72, 39, or 21% of initial levels, respectively. These reductions correspond to 1.4-, 2.6-, or 4.7-fold increases, respectively, in the area under the plasma concentration-time curve of a coadministered drug that is eliminated exclusively by hepatic CYP3A4 metabolism. These predicted results are in good agreement with reported clinical data. The major implication of this work is that fluoxetine, clarithromycin, and the primary metabolite of diltiazem, at clinically relevant concentrations, inactivate CYP3A4 enzymatic activity at rates sufficient to affect in vivo concentrations of CYP3A4 and thereby affect the clearance of compounds eliminated by this pathway. We speculate that mechanisms involving substrate-mediated mechanistic inactivation of CYPs play a major role in many clinically observed drug-drug interactions.

Anti-Bacterial Agents↗