Prenatal pharmacogenomics: a promising area for research.

Clinical applications of prenatal genetic screening currently focus on detection of aneuploidy and other genetic diseases in the developing fetus. Growing evidence suggests that the fetal genome may also be informative about fetal exposures through contributions to placental transport as well as placental and fetal metabolism. Possible clinical applications of prenatal pharmacogenomic screening include prospective optimization of medication selection and dosage, as well as retrospective assessment of whether a fetus was previously exposed to significant risk. Newly available noninvasive methods of prenatal genetic screening mean that relevant fetal genotypes could be made available to obstetricians for use in management of a current pregnancy. This promising area for research merits more attention than it has thus far received.The Pharmacogenomics Journal advance online publication, 10 May 2016; doi:10.1038/tpj.2016.33.

Interindividual variability of CYP2C19-catalyzed drug metabolism due to differences in gene diplotypes and cytochrome P450 oxidoreductase content.

Large interindividual variability has been observed in the metabolism of CYP2C19 substrates in vivo. The study aimed to evaluate sources of this variability in CYP2C19 activity, focusing on CYP2C19 diplotypes and the cytochrome P450 oxidoreductase (POR). CYP2C19 gene analysis was carried out on 347 human liver samples. CYP2C19 activity assayed using human liver microsomes confirmed a significant a priori predicted rank order for (S)-mephenytoin hydroxylase activity of CYP2C19*17/*17 > *1B/*17 > *1B/*1B > *2A/*17 > *1B/*2A > *2A/*2A diplotypes. In a multivariate analysis, the CYP2C19*2A allele and POR protein content were associated with CYP2C19 activity. Further analysis indicated a strong effect of the CYP2C19*2A, but not the *17, allele on both metabolic steps in the conversion of clopidogrel to its active metabolite. The present study demonstrates that interindividual variability in CYP2C19 activity is due to differences in both CYP2C19 protein content associated with gene diplotypes and the POR concentration.The Pharmacogenomics Journal advance online publication, 1 September 2015; doi:10.1038/tpj.2015.58.

Associations between diet and cardiometabolic risk among Yup'ik Alaska Native people using food frequency questionnaire dietary patterns.

BACKGROUND AND AIMS: In previous analyses, we identified three dietary patterns from food frequency questionnaire data among a sample of Yup'ik Alaska Native people living in Southwest Alaska: a "subsistence foods" dietary pattern and two market-based dietary patterns "processed foods" and "fruits and vegetables". In this analysis, we aimed to characterize the association between the dietary patterns and cardiometabolic (CM) risk factors (lipids, blood pressure, glucose, adiposity). METHODS AND RESULTS: We used multilevel linear regression to estimate the mean of each CM risk factor, comparing participants in the 4th to the 1st quartile of each dietary pattern (n = 637). Models were adjusted for age, sex, past smoking, current smoking, and physical activity. Mean log triglyceride levels were significantly higher among participants in the 4th compared to the 1st quartile of the processed foods dietary pattern (ß = 0.11). Mean HbA1c percent was significantly lower (ß = -0.08) and mean diastolic blood pressure (DBP) mm Hg was significantly higher (ß = 2.87) among participants in the 4th compared to the 1st quartile of the fruits and vegetables dietary pattern. Finally, mean log triglyceride levels and mean DBP mm Hg were significantly lower among participants in the 4th compared to the 1st quartile of the subsistence foods dietary pattern (ß = -0.10 and ß = -3.99 respectively). CONCLUSIONS: We found increased CM risk, as reflected by increased triglycerides, associated with eating a greater frequency of processed foods, and reduced CM risk, as reflected by lower triglycerides and DBP, associated with eating a greater frequency of subsistence foods.

Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for CYP3A5 Genotype and Tacrolimus Dosing.

Tacrolimus is the mainstay immunosuppressant drug used after solid organ and hematopoietic stem cell transplantation. Individuals who express CYP3A5 (extensive and intermediate metabolizers) generally have decreased dose-adjusted trough concentrations of tacrolimus as compared with those who are CYP3A5 nonexpressers (poor metabolizers), possibly delaying achievement of target blood concentrations. We summarize evidence from the published literature supporting this association and provide dosing recommendations for tacrolimus based on CYP3A5 genotype when known (updates at www.pharmgkb.org).

Genetic risk factors for major bleeding in patients treated with warfarin in a community setting.

The influence of warfarin pharmacogenomics on major bleeding risk has been little studied in long-term users and non-specialist care settings. We conducted a case-control study to evaluate associations between CYP2C9*2/*3, VKORC1(1173), and CYP4F2*3 variants and major bleeding among patients treated with warfarin in a community setting. We calculated major bleeding odds ratios, adjusting for race, duration of warfarin use, age, gender, and body mass index. In 265 cases and 305 controls with 3.4 and 3.7 mean years of warfarin use, respectively, CYP4F2*3 was associated with decreased major bleeding risk (odds ratio: 0.62; 95% confidence interval: 0.43-0.91). CYP2C9*2/*3 and VKORC1(1173) had null associations overall, but there was a nonsignificant increase in major bleeding risk in patients with duration <6 months (odds ratio: 1.30; 95% confidence interval: 0.60-2.83; odds ratio: 1.23; 95% confidence interval: 0.57-2.64, respectively). In summary, in the largest study of warfarin pharmacogenomics and major bleeding to date, we found a 38% lower risk in patients with CYP4F2*3, potentially reflecting interaction with warfarin and dietary vitamin K intake and warranting additional evaluation.

Measurement and compartmental modeling of the effect of CYP3A5 gene variation on systemic and intrarenal tacrolimus disposition.

We evaluated the hypothesis that cytochrome P450 3A5 (CYP3A5) expression can affect intrarenal tacrolimus accumulation. Tacrolimus was administered orally to 24 healthy volunteers who were selected on the basis of their CYP3A5 genotype. As compared with CYP3A5 nonexpressors, expressors had a 1.6-fold higher oral tacrolimus clearance and 2.0- to 2.7-fold higher metabolite/parent area under the curve (AUC) ratios for 31-desmethyl tacrolimus (31-DMT), 12-hydroxy tacrolimus, and 13-desmethyl tacrolimus (13-DMT). In addition, the apparent urinary tacrolimus clearance was 36% lower in CYP3A5 expressors as compared with nonexpressors. To explore the mechanism behind this observation, we developed a semiphysiological model of renal tacrolimus disposition and predicted that tacrolimus exposure in the renal epithelium of CYP3A5 expressors is 53% of that for CYP3A5 nonexpressors, when normalized to blood AUC. These data suggest that, at steady state, intrarenal accumulation of tacrolimus and its primary metabolites will depend on the CYP3A5 genotype of the liver and kidneys. This may contribute to interpatient differences in the risk of tacrolimus-induced nephrotoxicity.

Evidence of CYP3A allosterism in vivo: analysis of interaction between fluconazole and midazolam.

The allosteric effect of fluconazole (effector) on the formation of 1'-hydroxymidazolam (1'-OH-MDZ) and 4-hydroxymidazolam (4-OH-MDZ) from midazolam (MDZ), a substrate of CYP3A4/5--members of the cytochrome P450 superfamily of enzymes--was examined in healthy volunteers. Following pretreatment with fluconazole, the ratio of the areas under the curve (AUCs) for 4-OH-MDZ and MDZ (AUC(4-OH)/AUC(MDZ)) increased by 35-62%, whereas the ratio AUC(1'-OH)/AUC(MDZ) decreased by 5-37%; the ratio AUC(1'-OH)/AUC(4-OH) decreased by 46-58% after fluconazole administration and had no association with the CYP3A5 genotype. The in vitro formation of 1'-OH-MDZ was more susceptible to inhibition by fluconazole than that of 4-OH-MDZ. Fluconazole decreased the intrinsic formation-clearance ratio of 1'-OH-MDZ/4-OH-MDZ to an extent that was quantitatively comparable to in vivo observations. The elimination clearance of MDZ metabolites appeared unaffected by fluconazole. This study demonstrated that fluconazole alters formation of MDZ metabolites, both in vivo and in vitro, in a manner consistent with an allosteric interaction. The 1'-OH-MDZ/4-OH-MDZ ratio may serve as a biomarker of such interactions among MDZ, CYP3A4/5, and other putative effectors.

Evaluation of 6ß-hydroxycortisol, 6ß-hydroxycortisone, and a combination of the two as endogenous probes for inhibition of CYP3A4 in vivo.

An endogenous probe for CYP3A activity would be useful for early identification of in vivo cytochrome P450 (CYP) 3A4 inhibitors. The aim of this study was to determine whether formation clearance (CL(f)) of the sum of 6ß-hydroxycortisol and 6ß-hydroxycortisone is a useful probe of CYP3A4 inhibition in vivo. In human liver microsomes (HLMs), the formation of 6ß-hydroxycortisol and 6ß-hydroxycortisone was catalyzed by CYP3A4, and itraconazole inhibited these reactions with half maximal inhibitory concentration (IC(50))(,u) values of 3.1 nmol/l and 3.4 nmol/l, respectively. The in vivo IC(50,u) value of itraconazole for the combined CL(f) of 6ß-hydroxycortisone and 6ß-hydroxycortisol was 1.6 nmol/l. The greater inhibitory potency in vivo is probably due to circulating inhibitory itraconazole metabolites. The maximum in vivo inhibition was 59%, suggesting that f(m,CYP3A4) for cortisol and cortisone 6ß-hydroxylation is ~60%. Given the significant decrease in CL(f) of 6ß-hydroxycortisone and 6ß-hydroxycortisol after 200-mg and 400-mg single doses of itraconazole, this endogenous probe can be used to detect moderate and potent CYP3A4 inhibition in vivo.

Ethical issues in developing pharmacogenetic research partnerships with American Indigenous communities.

Pharmacogenetic research offers the potential to improve the safety and efficacy of drug prescribing. Assuring that the benefits of this research reach indigenous and other medically underserved people is an important justice concern. First, however, a legacy of mistrust, derived from traditional research practices that disempower communities, must be overcome. Linking pharmacogenetic research to collaborative, power-sharing research partnerships provides a valuable opportunity to develop new and positive precedents for genetic research in indigenous communities.

Accurate prediction of dose-dependent CYP3A4 inhibition by itraconazole and its metabolites from in vitro inhibition data.

Inhibitory drug metabolites may contribute to drug-drug interactions (DDIs). The aim of this study was to determine the importance of inhibitory metabolites of itraconazole (ITZ) in in vivo cytochrome P450 (CYP) 3A4 inhibition. The pharmacokinetics of ITZ and midazolam (MDZ) were determined in six healthy volunteers in four sessions after administration of MDZ with and without oral ITZ. After doses of 50, 200, and 400 mg of ITZ, the clearance of orally administered MDZ decreased by 27, 74, and 83%, respectively. The in vivo half maximal inhibitory concentration (IC(50)) for ITZ ranged from 5 to 132 nmol/l in the six subjects. The metabolites of ITZ were estimated to account for ~50% of the total CYP3A4 inhibition, with the relative contribution increasing with time after ITZ dosing. Of the total of 18 interactions observed, 15 (84%) could be predicted within a twofold error margin, with improved accuracy observed when ITZ metabolites were included in the predictions. This study shows that the metabolites of ITZ contribute to CYP3A4 inhibition and need to be accounted for in quantitative rationalization of ITZ-mediated DDIs.

Reduced duodenal cytochrome P450 3A protein expression and catalytic activity in patients with cirrhosis.

The small intestine and liver express high levels of cytochrome P450 3A (CYP3A), an enzyme subfamily that contributes significantly to drug metabolism. In patients with cirrhosis, reduced metabolism of drugs is typically attributed to decreased liver function, but it is unclear whether drug metabolism in the intestine is also compromised. In this study, we compared CYP3A protein expression and in vitro midazolam hydroxylation in duodenal mucosal biopsies from subjects with normal liver function (controls; n = 20) and subjects with various levels of severity of cirrhosis (n = 23). In samples from subjects with cirrhosis, duodenal CYP3A expression and total midazolam hydroxylation were lower by 47 and 34%, respectively, as compared with samples from controls. Greater decreases in CYP3A expression were seen in subjects with more severe cirrhosis. Therefore, patients with advanced cirrhosis may have greater drug exposure following oral dosing as a result of both impaired liver function and decreased intestinal CYP3A expression and activity.

Assessment of the impact of renal impairment on systemic exposure of new molecular entities: evaluation of recent new drug applications.

The US Food and Drug Administration (FDA) is currently developing a guidance for industry to replace a previous guidance, "Pharmacokinetics in Patients With Impaired Renal Function--Study Design, Data Analysis, and Impact on Dosing and Labeling" (renal guidance) issued in May 1998. The impact of the 1998 renal guidance was assessed following a survey of 94 new drug applications (NDAs) for small-molecule new molecular entities (NMEs) approved over the past 5 years (2003-2007). The survey results indicate that 57% of these NDAs included renal impairment study data, that 44% of those with renal data included evaluation in patients on hemodialysis, and that 41% of those with renal data resulted in recommendation of dose adjustment in renal impairment. In addition, the survey results provided evidence that renal impairment can affect the pharmacokinetics of drugs that are predominantly eliminated by nonrenal processes such as metabolism and/or active transport. The latter finding supports our updated recommendation to evaluate pharmacokinetic/pharmacodynamic alterations in renal impairment for those drugs that are mainly eliminated by nonrenal processes, in addition to those that are mainly excreted unchanged by the kidney.

Effects of pregnancy on CYP3A and P-glycoprotein activities as measured by disposition of midazolam and digoxin: a University of Washington specialized center of research study.

The objectives of the study were to evaluate the effects of pregnancy on CYP3A and P-glycoprotein (P-gp) activities, as measured by disposition of midazolam and digoxin, respectively. Thirteen women received digoxin (0.25 mg p.o.) and midazolam (2 mg p.o.) in random order, separated by 1-2 weeks at 28-32 weeks gestation, and the same order was repeated at 6-10 weeks postpartum. Plasma and urine concentrations were determined by liquid chromatography-mass spectrometry and analyzed by noncompartmental methods. Midazolam CL/F(unbound) (593 +/- 237 l/min vs. 345 +/- 103 l/min; P = 0.007), digoxin CL(Renal, unbound) (272 +/- 45 ml/min vs. 183 +/- 37 ml/min; P < 0.002) and digoxin CL(secretion,) (unbound) (109 +/- 34 ml/min vs. 58 +/- 22 ml/min; P < 0.002) were higher during pregnancy than postpartum. These data are consistent with increased hepatic and/or intestinal CYP3A and renal P-gp activities during pregnancy.

Contribution of itraconazole metabolites to inhibition of CYP3A4 in vivo.

Itraconazole (ITZ) is metabolized in vitro to three inhibitory metabolites: hydroxy-itraconazole (OH-ITZ), keto-itraconazole (keto-ITZ), and N-desalkyl-itraconazole (ND-ITZ). The goal of this study was to determine the contribution of these metabolites to drug-drug interactions caused by ITZ. Six healthy volunteers received 100 mg ITZ orally for 7 days, and pharmacokinetic analysis was conducted at days 1 and 7 of the study. The extent of CYP3A4 inhibition by ITZ and its metabolites was predicted using this data. ITZ, OH-ITZ, keto-ITZ, and ND-ITZ were detected in plasma samples of all volunteers. A 3.9-fold decrease in the hepatic intrinsic clearance of a CYP3A4 substrate was predicted using the average unbound steady-state concentrations (C(ss,ave,u)) and liver microsomal inhibition constants for ITZ, OH-ITZ, keto-ITZ, and ND-ITZ. Accounting for circulating metabolites of ITZ significantly improved the in vitro to in vivo extrapolation of CYP3A4 inhibition compared to a consideration of ITZ exposure alone.

Influence of CYP3A5 genotype on the pharmacokinetics and pharmacodynamics of the cytochrome P4503A probes alfentanil and midazolam.

The hepatic and first-pass cytochrome P4503A (CYP3A) probe alfentanil (ALF) is also metabolized in vitro by CYP3A5. Human hepatic microsomal ALF metabolism is higher in livers with at least one CYP3A5*1 allele and higher CYP3A5 protein content, compared with CYP3A5*3 homozygotes with little CYP3A5. The influence of CYP3A5 genotype on ALF pharmacokinetics and pharmacodynamics was studied, and compared to midazolam (MDZ), another CYP3A probe. Healthy volunteers (58 men, 41 women) were genotyped for CYP3A5 *1, *3, *6, and *7 alleles. They received intravenous MDZ then ALF, and oral MDZ and ALF the next day. Plasma MDZ and ALF concentrations were determined by mass spectrometry. Dark-adapted pupil diameters were determined coincident with blood sampling. In CYP3A5(*)3/(*)3 (n=62), (*)1/(*)3 (n=28), and (*)1/(*)1 (n=8) genotypes, systemic clearances of ALF were 4.6+/-1.8, 4.8+/-1.7, and 3.9+/-1.7 ml/kg/min and those of MDZ were 7.8+/-2.3, 7.7+/-2.3, and 6.0+/-1.4 ml/kg/min, respectively (not significant), and apparent oral clearances were 11.8+/-7.2, 13.3+/-6.1, and 12.6+/-8.2 ml/kg/min for ALF and 35.2+/-19.0, 36.4+/-15.7, and 29.4+/-9.3 ml/kg/min for MDZ (not significant). Clearances were not different between African Americans (n=25) and Whites (n=68), or between CYP3A5 genotypes within African Americans. ALF pharmacodynamics was not different between CYP3A5 genotypes. There was consistent concordance between ALF and MDZ, in clearances and extraction ratios. Thus, in a relatively large cohort of healthy subjects with constitutive CYP3A activity, CYP3A5 genotype had no effect on the systemic or apparent oral clearances, or pharmacodynamics, of the CYP3A probes ALF and MDZ, despite affecting their hepatic microsomal metabolism.

Concordance between trifluoroacetic acid and hepatic protein trifluoroacetylation after disulfiram inhibition of halothane metabolism in rats.

BACKGROUND: Cytochrome P4502E1(CYP2E1)-mediated oxidation of halothane to a reactive intermediate (trifluoroacyl chloride) that covalently binds to hepatic proteins forming trifluoroacetylated neoantigens is believed to be the initiating event in a complex immunologic cascade culminating in antibody formation and severe hepatic necrosis ('halothane hepatitis') in susceptible patients. Trifluoroacyl chloride may also hydrolyze to the stable metabolite trifluoroacetic acid (TFA). CYP2E1 inactivation by disulfiram or its primary metabolite, diethyldithiocarbamate, inhibits human halothane oxidation to TFA in vitro and in vivo. Nevertheless, disulfiram effects on hepatic protein trifluoroacetylation by halothane in vivo are unknown. This investigation tested the hypotheses that disulfiram prevents halothane-dependent protein trifluoroacetylation in vivo, and that TFA represents a biomarker for hepatic protein trifluoroacetylation. METHODS: Rats were pretreated with isoniazid (CYP2E1 induction), isoniazid followed by disulfiram (CYP2E1 inhibition), or nothing (controls), then anesthetized with halothane or nothing (controls). Plasma and urine TFA were quantified by ion HPLC; hepatic microsomal TFA-proteins were analyzed by Western blot. RESULTS: CYP2E1 induction increased both TFA and TFA-protein formation compared with uninduced halothane-treated rats. Disulfiram, even after CYP2E1 induction, nearly abolished both TFA and TFA-protein formation. Pretreatments similarly affected both TFA and TFA-protein formation across all groups. CONCLUSIONS: Disulfiram inhibition of CYP2E1-mediated halothane oxidation prevents hepatic protein trifluoroacetylation. Based on the concordance between TFA and TFA-protein formation, TFA appears to be a valid biomarker for TFA-protein formation. Disulfiram inhibition of human halothane oxidation in vivo, previously assessed by diminished TFA formation, probably also confers inhibition of hepatic TFA-protein formation.

Topiramate and phenytoin pharmacokinetics during repetitive monotherapy and combination therapy to epileptic patients.

PURPOSE: To evaluate the potential pharmacokinetic interactions between topiramate (TPM) and phenytoin (PHT) in patients with epilepsy by studying their pharmacokinetics (PK) after monotherapy and concomitant TPM/PHT treatment. METHODS: Twelve patients with epilepsy stabilized on PHT monotherapy were enrolled in this study, with 10 and seven patients completing the phases with 400 and 800 mg TPM daily doses, respectively. TPM was added at escalating doses, and after stabilization at the highest tolerated TPM dose, PHT doses were tapered. Serial blood and urine samples were collected for PK analysis during the monotherapy phase or the lowest PHT dose after taper and the concomitant TPM/PHT phase. Potential metabolic interaction between PHT and TPM also was studied in vitro in human liver microsomal preparations. RESULTS: In nine of the 12 patients, PHT plasma concentrations remained stable, with a mean (+/-SD) area under the curve (AUC) ratio (combination therapy/monotherapy) of 1.13 +/- 0.17 (range, 0.89-1.23). Three patients had AUC ratios of 1.25, 1.39, and 1.55, respectively, and with the addition of TPM (800, 400, and 400 mg daily, respectively), their peak PHT plasma concentrations increased from 15 to 21 mg/L, 28 to 36 mg/L, and 27 to 41 mg/L, respectively. Human liver microsomal studies with S-mephenytoin showed that TPM partially inhibited CYP2C19 at very high concentrations of 300 microM (11% inhibition) and 900 microM (29% inhibition). Such high plasma concentrations would correspond to doses in humans that are 5 to 15 times higher than the recommended dose (200-400 mg). TPM clearance was approximately twofold higher during concomitant TPM/PHT therapy CONCLUSIONS: This study provides evidence that the addition of TPM to PHT generally does not cause clinically significant PK interaction. PHT induces the metabolism of TPM, causing increased TPM clearance, which may require TPM dose adjustments when PHT therapy is added or is discontinued. TPM may affect PHT concentrations in a few patients because of inhibition by TPM of the CYP2C19-mediated minor metabolic pathway of PHT.

In-vivo phenotyping for CYP3A by a single-point determination of midazolam plasma concentration.

We investigated whether a single plasma midazolam concentration could serve as an accurate predictor of total midazolam clearance, an established in-vivo probe measure of cytochrome P450 3A (CYP3A) activity. In a retrospective analysis of data from 224 healthy volunteers, non-compartmental pharmacokinetic parameters were estimated from plasma concentration-time curves following intravenous (IV) and/or oral administration. Based on statistical moment theory, the concentration at the mean residence time (MRT) should be the best predictor of the total area under the curve (AUC). Following IV or oral midazolam administration, the average MRT was found to be approximately 3.5 h, suggesting that the optimal single sampling time to predict AUC was between 3 and 4 h. Since a 4-h data point was common to all studies incorporated into this analysis, we selected this time point for further investigation. The concentrations of midazolam measured 4 h after an IV or oral dose explained 80 and 91% of the constitutive interindividual variability in midazolam AUC, respectively. The 4-h midazolam measurement was also an excellent predictor of drug-drug interactions involving CYP3A induction and inhibition. Compared with baseline values, the direction and magnitude of change in midazolam AUC and the 4-h concentration were completely concordant for all study subjects. We conclude that a single 4-h midazolam concentration following IV or oral administration represents an accurate marker of CYP3A phenotype under constitutive and modified states. Moreover, the single-point approach offers an efficient means to phenotype and identify individuals with important genetic polymorphisms that affect CYP3A activity.

Transcriptional control of intestinal cytochrome P-4503A by 1alpha,25-dihydroxy vitamin D3.

It was previously shown that CYP3A4 is induced in the human intestinal Caco-2 cell model by treatment with 1alpha,25-dihydroxy vitamin D3 (1,25-D3). We demonstrate the vitamin D analog, 19-nor-1alpha,25-dihydroxy vitamin D2, is also an effective inducer of CYP3A4 in Caco-2 cells, but with half the potency of 1,25-D3. We report that treatment of LS180 cells, a human intestinal cell line, with 1 to 10 nM 1,25-D3 dose dependently increased CYP3A4 protein and CYP3A4 mRNA expression. CYP3A4- and CYP3A23-promoter-Luciferase reporter constructs transiently transfected into LS180 cells were transcriptionally activated in a dose-dependent manner by 1,25-D3, whereas mutation of the nuclear hormone receptor binding motif (ER6) in the CYP3A4 promoter abrogated 1,25-D3 activation of CYP3A4. Although the CYP3A4 ER6 promoter element has been shown to bind the pregnane X receptor (PXR), this receptor does not mediate 1,25-D3 induction of CYP3A4 because a) PXR is not expressed in Caco-2 cells; b) PXR mRNA expression is not induced by 1,25-D3 treatment of LS180 cells; and c) the ligand binding domain of human PXR was not activated by 1,25-D3. 1,25-D3 uses the vitamin D receptor to induce CYP3A4 because a) the vitamin D receptor (VDR)-retinoid X receptor (RXR) heterodimer binds specifically to the CYP3A4 ER6; b) selective mutation of the CYP3A4 ER6 disrupted the binding of VDR-RXR; and c) reporter constructs containing only three copies of the CYP3A4 ER6 linked to a TK-CAT reporter were activated by 1,25-D3 only in cells cotransfected with a human VDR expression plasmid. These data support the hypothesis that 1,25-D3 and VDR induce expression of intestinal CYP3A by binding of the activated VDR-RXR heterodimer to the CYP3A PXR response element and promoting gene transcription.