In Vitro to In Vivo Scalars for Drug Clearance in Nonalcoholic Fatty Liver and Steatohepatitis

In vitro-in vivo extrapolation (IVIVE) allows prediction of clinical outcomes across populations from in vitro data using specific scalars tailored to the biologic characteristics of each population. This study experimentally determined scalars for patients with varying degrees of nonalcoholic fatty liver disease (NAFLD), ranging from fatty liver to nonalcoholic steatohepatitis (NASH) and cirrhosis. Microsomal, S9, and cytosol fractions were extracted from 36 histologically normal and 66 NAFLD livers (27 nonalcoholic fatty liver [NAFL], 13 NASH, and 26 NASH with cirrhosis). Corrected microsomal protein per gram liver (MPPGL) progressively decreased with disease severity (26.8, 27.4, and 24.3 mg/g in NAFL, NASH, and NASH/cirrhosis, respectively, compared with 35.6 mg/g in normal livers; ANOVA, P < 0.001). Homogenate, S9, and cytosolic protein showed a consistent trend of decline in NASH/cirrhosis relative to normal control (post-hoc t test, P < 0.05). No differences across the groups were observed in homogenate, S9, cytosolic, and microsomal protein content in matched kidney samples. MPPGL-based scalars that combine protein content with liver size revealed that the reduction in MPPGL in NAFL and NASH was compensated by the reported increase in liver size (relative scalar ratios of 0.96 and 0.99, respectively), which was not the case with NASH/cirrhosis (ratio of 0.63), compared with healthy control. Physiologically based pharmacokinetics-informed global sensitivity analysis of the relative contribution of IVIVE scalars (hepatic CYP3A4 abundance, MPPGL, and liver size) to variability in exposure (area under the curve) to three CYP3A substrates (alprazolam, midazolam, and ibrutinib) revealed enzyme abundance as the most significant parameter, followed by MPPGL, whereas liver volume was the least impactful factor. SIGNIFICANCE STATEMENT: Nonalcoholic fatty liver disease-specific scalars necessary for extrapolation from in vitro systems to liver tissue are lacking. These are required in clearance prediction and dose selection in nonalcoholic fatty liver and steatohepatitis populations. Previously reported disease-driven changes have focused on cirrhosis, with no data on the initial stages of liver disease. The authors obtained experimental values for microsomal, cytosolic, and S9 fractions and assessed the relative impact of microsomal scalars on predicted exposure to substrate drugs using physiologically based pharmacokinetics.

Introducing the Dynamic Well-Stirred Model for Predicting Hepatic Clearance and Extraction Ratio

The well-stirred model (WSM) incorporating the fraction of unbound drug (fu) to account for the effect of plasma binding on intrinsic clearance has been widely used for predicting hepatic clearance under the assumption that drug protein binding reaches equilibrium instantaneously. Our theoretical analysis reveals that the effect of protein binding on intrinsic clearance is better accounted for with the dynamic free fraction (fD), a measure of drug protein binding affinity, which leads to a putative dynamic well-stirred model (dWSM) without the instantaneous equilibrium assumption. Using recombinant CYP3A4 as the in vitro clearance system, we demonstrate that the binding effect of albumin on the intrinsic clearance of both highly bound midazolam and highly free verapamil is fully corrected by their corresponding fD values, respectively. On the other hand, fu only corrects the binding effect of albumin on the intrinsic clearance of verapamil, and yields severe over-correction of the intrinsic clearance of midazolam. The results suggest that the traditional WSM is suitable for highly free drugs like verapamil but not necessarily for highly bound drugs such as midazolam due to the violation of the instantaneous equilibrium assumption or under-estimating the true free drug concentration. In comparison, the dWSM incorporating fD holds true as long as drug elimination follows steady-state kinetics, and hence, it is more broadly applicable to drugs with different protein binding characteristics. Here we demonstrate with 36 diverse drugs, that the dWSM significantly improves the accuracy of predicting human hepatic clearance and liver extraction ratio from in vitro microsomal clearance data, highlighting the importance of drug plasma protein binding kinetics in addressing the under-prediction of hepatic clearance by the WSM.

High-throughput assay to simultaneously evaluate activation of CYP3A and the direct and time-dependent inhibition of CYP3A, CYP2C9, and CYP2D6 using liquid chromatography-tandem mass spectrometry

In the early stages of drug discovery, adequate evaluation of the potential drug-drug interactions (DDIs) of drug candidates is important. Several CYP3A activators are known to lead to underestimation of DDIs. These compounds affect midazolam 1'-hydroxylation but not midazolam 4-hydroxylation.We used both metabolic reactions of midazolam to evaluate the activation and inhibition of CYP3A activators simultaneously. For our CYP inhibition assay using cocktail probe substrates, simultaneous liquid chromatography-tandem mass spectrometry monitoring of 1'-hydroxymidazolam and 4-hydroxymidazolam for CYP3A was established in addition to monitoring of 4-hydroxydiclofenac and 1'-hydroxybufuralol for CYP2C9 and CYP2D6.The results of our cocktail inhibition assay were well correlated with those of a single inhibition assay, as were the estimated inhibition parameters for typical CYP3A inhibitors. In our assay, a proprietary compound that activated midazolam 1'-hydroxylation and tended to inhibit 4-hydroxylation was evaluated along with known CYP3A activators. All compounds were well characterised by comparison of the results of midazolam 1'- and 4-hydroxylation.In conclusion, our CYP cocktail inhibition assay can detect CYP3A activation and assess the direct and time-dependent inhibition potentials for CYP3A, CYP2C9, and CYP2D6. This method is expected to be very efficient in the early stages of drug discovery.

Comparison of 1Beta- and 5Beta-hydroxylation of Deoxycholate and Glycodeoxycholate as In Vitro Index Reactions for Cytochrome P450 3A Activities

Cytochrome P450 3A (CYP3A) participates in the metabolism of more than 30% of clinical drugs. The vast intra- and inter-individual variations in CYP3A activity pose great challenges to drug development and personalized medicine. It has been disclosed that human CYP3A4 and CYP3A7 are exclusively responsible for the tertiary oxidations of deoxycholic acid (DCA) and glycodeoxycholic acid (GDCA) regioselectivity at C-1β and C-5β This work aimed to compare the 1β- and 5β-hydroxylation of DCA and GDCA as potential in vitro CYP3A index reactions in both human liver microsomes and recombinant P450 enzymes. The results demonstrated that the metabolic activity of DCA 1β- and 5β-hydroxylation was 5-10 times higher than that of GDCA, suggesting that 1β-hydroxyglycodeoxycholic acid and 5β-hydroxyglycodeoxycholic acid may originate from DCA oxidation followed by conjugation in humans. Metabolic phenotyping data revealed that DCA 1β-hydroxylation, DCA 5β-hydroxylation, and GDCA 5β-hydroxylation were predominantly catalyzed by CYP3A4 (>80%), while GDCA 1β-hydroxylation had approximately equal contributions from CYP3A4 (41%) and 3A7 (58%). Robust Pearson correlation was established for the intrinsic clearance of DCA 1β- and 5β-hydroxylation with midazolam (MDZ) 1'- and 4-hydroxylation in fourteen single donor microsomes. Although DCA 5β-hydroxylation exhibited a stronger correlation with MDZ oxidation, DCA 1β-hydroxylation exhibited higher reactivity than DCA 5β-hydroxylation. It is therefore suggested that DCA 1β- and 5β-hydroxylations may serve as alternatives to T 6β-hydroxylation as in vitro CYP3A index reactions. SIGNIFICANCE STATEMENT: The oxidation of DCA and GDCA is primarily catalyzed by CYP3A4 and CYP3A7. This work compared the 1β- and 5β-hydroxylation of DCA and GDCA as in vitro index reactions to assess CYP3A activities. It was disclosed that the metabolic activity of DCA 1β- and 5β-hydroxylation was 5-10 times higher than that of GDCA. Although DCA 1β-hydroxylation exhibited higher metabolic activity than DCA 5β-hydroxylation, DCA 5β-hydroxylation demonstrated stronger correlation with MDZ oxidation than DCA 1β-hydroxylation in individual liver microsomes.

Cannabidiol Inhibits In Vitro Human Liver Microsomal Metabolism of Remdesivir

Introduction: The year 2020 began with the world being flounced with a wave of novel coronavirus (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) disease, named COVID-19. Based on promising pre-clinical and clinical data, remdesivir (RDV) was the first drug to receive FDA approval and so far, it is the most common therapy for treatment of SARS-CoV-2/MERS-CoV. However, following intravenous administration, RDV metabolizes majorly by human liver carboxylesterase 1 (CES1) and marginally by the CYP3A4 enzyme in merely less than an hour. Its resultant active metabolite is a hydrophilic nucleoside with very limited accumulation within lung tissues. Therefore, there is a need to investigate strategies to overcome such premature metabolism issues and improve the antiviral efficacy of RDV at the target site. Objective: Considering the major CES1-mediated metabolism of RDV on systemic administration, we intend to explore the remarkable CES1 plus CYP3A4 inhibitory activity of cannabidiol (CBD) against in vitro microsomal metabolism of RDV to indicate its therapeutic potential as an adjuvant to RDV in the treatment and management of COVID-19. Methods: We investigated the in vitro human liver microsomal metabolism of RDV in the presence of two potential CES1 inhibitors-CBD and nelfinavir, and two standard CYP3A4 inhibitors-ritonavir (RITO) and cyclosporin A. The microsomal metabolism assay was further validated by using a well-characterized CYP3A4-selective substrate, midazolam (MDZ), in the presence of CBD and RITO. Results: Our findings depicted that RDV was rapidly and completely metabolized by human liver microsomes within 60 min. Coincubation with CBD substantially reduced microsomal metabolism of RDV and prolonged its in vitro half-life from 8.93 to 31.07 min. CBD showed significantly higher inhibition of RDV compared with known CES1 and CYP3A4 inhibitors. Inhibition of MDZ metabolism by CBD and RITO further validated the assay. Conclusions: The current study strongly suggests that CBD significantly inhibits human liver microsomal metabolism of RDV and extends its in vitro half-life. Thus, concomitant administration of CBD with RDV intravenous injection could be a promising strategy to prevent premature metabolism in COVID-19 patients.

Impact of Heterotropic Allosteric Modulation on the Time-Dependent Inhibition of Cytochrome P450 3A4

The current study was designed to investigate the influence of allosteric effectors on the metabolism of the prototypical cytochrome P450 (CYP) 3A4 substrate midazolam (MDZ), and on the determination in vitro time-dependent inhibition (TDI) of CYP3A4 using human liver microsomes (HLM). As the concentration of midazolam increased to 250 µM in HLMs, homotropic cooperativity resulted in a decrease in the 1'-hydroxymidazolam to 4-hydroxymidazolam ratio to a maximum of 1.1. The presence of varying concentrations of testosterone, progesterone (PGS), or carbamazepine (CBZ) in HLMs with MDZ could recapitulate the effect of homotropic cooperativity such that the formation rates of the 1'hydroxymidazolam and 4-hydroxymidazolam were equal even at low concentrations of MDZ. The presence of PGS (10 or 100 µM) and CBZ (100 or 1000 µM) in in vitro TDI determination of four known CYP3A4 time-dependent inactivators (clarithromycin, troleandomycin, mibefradil, raloxifene) simultaneously decreased potency and inactivation rate constant, resulting in fold changes in inactivation efficiency on average of 1.6-fold and 13-fold for the low and high concentrations of allosteric modulator tested, respectively. The formation of a metabolic-intermediate complex (MIC) for clarithromycin and troleandomycin decreased in the presence of the allosteric modulators in a concentration-dependent manner, reaching a new steady state formation that could not be overcome with increased incubation time. Maximum reduction of the MIC formed by clarithromycin was up to ∼91%, while troleandomycin MIC decreased up to ∼31%. These findings suggest that the absence of endogenous allosteric modulators may contribute to the poor translation of HLM-based drug-drug interaction predictions. SIGNIFICANCE STATEMENT: The reported overprediction of in vitro human liver microsome time-dependent inhibition of CYP3A4 and observed drug interactions in vivo remains an issue in drug development. We provide characterization of allosteric modulators on the CYP3A4 metabolism of the prototypical substrate midazolam, demonstrating the ability of the modulators to recapitulate the homotropic cooperativity of midazolam. Furthermore, we demonstrate that allosteric heterotropic cooperativity of CYP3A4 can impact the time-dependent inhibition kinetics of known mechanisms-based inhibitors, providing a potential mechanism to explain the overprediction.

The Activity of Members of the UDP-Glucuronosyltransferase Subfamilies UGT1A and UGT2B is Impaired in Patients with Liver Cirrhosis

BACKGROUND AND OBJECTIVE

The impact of liver cirrhosis on the activity of UDP-glucuronosyltransferases (UGTs) is currently not well characterized. We investigated the glucuronidation capacity and glucuronide accumulation in patients with liver cirrhosis.

METHODS

We administered the Basel phenotyping cocktail (caffeine, efavirenz, flurbiprofen, omeprazole, metoprolol, midazolam) to patients with liver cirrhosis (n = 16 Child A, n = 15 Child B, n = 5 Child C) and n = 12 control subjects and obtained pharmacokinetic profiles of substrates and primary metabolites and their glucuronides.

RESULTS

Caffeine and its metabolite paraxanthine were only slightly glucuronidated. The metabolic ratio (AUCglucuronide/AUCparent, MR) was not affected for caffeine but decreased by 60% for paraxanthine glucuronide formation in Child C patients. Efavirenz was not glucuronidated whereas 8-hydroxyefavirenz was efficiently glucuronidated. The MR of 8-hydroxyefavirenz-glucuronide formation increased three-fold in Child C patients and was negatively correlated with the glomerular filtration rate. Flurbiprofen and omeprazole were not glucuronidated. 4-Hydroxyflurbiprofen and 5-hydroxyomeprazole were both glucuronidated but the corresponding MRs for glucuronide formation were not affected by liver cirrhosis. Metoprolol, but not α-hydroxymetoprolol, was glucuronidated, and the MR for metoprolol-glucuronide formation dropped by 60% in Child C patients. Both midazolam and its metabolite 1'-hydroxymidazolam underwent glucuronidation, and the corresponding MRs for glucuronide formation dropped by approximately 80% in Child C patients. No relevant glucuronide accumulation occurred in patients with liver cirrhosis.

CONCLUSIONS

Detailed analysis revealed that liver cirrhosis may affect the activity of UGTs of the UGT1A and UGT2B subfamilies according to liver function. Clinically significant glucuronide accumulation did not occur in the population investigated.

CLINICAL TRIAL REGISTRATION

NCT03337945.

Midazolam as a Probe for Heterotropic Drug-Drug Interactions Mediated by CYP3A4

Human cytochrome P450 CYP3A4 is involved in the processing of more than 35% of current pharmaceuticals and therefore is responsible for multiple drug-drug interactions (DDI). In order to develop a method for the detection and prediction of the possible involvement of new drug candidates in CYP3A4-mediated DDI, we evaluated the application of midazolam (MDZ) as a probe substrate. MDZ is hydroxylated by CYP3A4 in two positions: 1-hydroxy MDZ formed at lower substrate concentrations, and up to 35% of 4-hydroxy MDZ at high concentrations. The ratio of the formation rates of these two products (the site of metabolism ratio, SOM) was used as a measure of allosteric heterotropic interactions caused by effector molecules using CYP3A4 incorporated in lipid nanodiscs. The extent of the changes in the SOM in the presence of effectors is determined by chemical structure and is concentration-dependent. MD simulations of CYP3A4 in the lipid bilayer suggest that experimental results can be explained by the movement of the F-F' loop and concomitant changes in the shape and volume of the substrate-binding pocket. As a result of PGS binding at the allosteric site, several residues directly contacting MDZ move away from the substrate molecule, enabling the repositioning of the latter for minor product formation.

Alterations of Cytochrome P450-Mediated Drug Metabolism during Liver Repair and Regeneration after Acetaminophen-Induced Liver Injury in Mice

Acetaminophen (APAP)-induced liver injury (AILI) is the leading cause of acute liver failure in the United States, but its impact on metabolism, therapeutic efficacy, and adverse drug reactions (ADRs) of co- and/or subsequent administered drugs are not fully investigated. The current work explored this field with a focus on the AILI-mediated alterations of cytochrome P450-mediated drug metabolism. Various levels of liver injury were induced in mice by treatment with APAP at 0, 200, 400, and 600 mg/kg. Severity of liver damage was determined at 24, 48, 72, and 96 hours by plasma levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), microRNA miR122, and tissue staining. The expression and activities of CYP3A11, 1A2, 2B10, 2C29, and 2E1 were measured. Sedation efficacy and ADRs of midazolam, a CYP3A substrate, were monitored after APAP treatment. ALT, AST, and miR122 increased at 24 hours after APAP treatment with all APAP doses, whereas only groups treated with 200 and 400 mg/kg recovered back to normal levels at 72 and 96 hours. The expression and activity of the cytochromes P450 significantly decreased at 24 hours with all APAP doses but only recovered back to normal at 72 and 96 hours with 200 and 400, but not 600, mg/kg of APAP. The alterations of cytochrome P450 activities resulted in altered sedation efficacy and ADRs of midazolam, which were corrected by dose justification of midazolam. Overall, this work illustrated a low cytochrome P450 expression window after AILI, which can decrease drug metabolism and negatively impact drug efficacy and ADRs. SIGNIFICANCE STATEMENT: The data generated in the mouse model demonstrated that expression and activities of cytochrome P450 enzymes and correlated drug efficacy and ADRs are altered during the time course of liver repair and regeneration after liver is injured by treatment with APAP. Dose justifications based on predicted changes of cytochrome P450 activities can achieve desired therapeutic efficacy and avoid ADRs. The generated data provide fundamental knowledge for translational research to drug treatment for patients during liver recovery and regeneration who have experienced AILI.

UPLC-MS/MS analysis of the Michaelis-Menten kinetics of CYP3A-mediated midazolam 1'- and 4-hydroxylation in rat brain microsomes

Midazolam (MDZ) is a short-acting benzodiazepine with rapid onset of action, which is metabolized by CYP3A isoenzymes to two hydroxylated metabolites, 1'-hydroxymidazolam and 4-hydroxymidazolam. The drug is also commonly used as a marker of CYP3A activity in the liver microsomes. However, the kinetics of CYP3A-mediated hydroxylation of MDZ in the brain, which contains much lower CYP content than the liver, have not been reported. In this study, UPLC-MS/MS and metabolic incubation methods were developed and validated for simultaneous measurement of low concentrations of both hydroxylated metabolites of MDZ in brain microsomes. Different concentrations of MDZ (1-500 µM) were incubated with rat brain microsomes (6.25 µg) and NADPH over a period of 10 min. After precipitation of the microsomal proteins with acetonitrile, which contained individual isotope-labeled internal standards for each metabolite, the analytes were separated on a C₁₈ UPLC column and detected by a tandem mass spectrometer. Accurate quantitation of MDZ metabolism in the brain microsomes presented several challenges unique to this tissue, which were resolved. The optimized method showed validation results in accordance with the FDA acceptance criteria, with a linearity ranging from 1 to 100 nM and a lower limit of quantitation of 0.4 pg on the column for each of the two metabolites. The method was successfully used to determine the Michaelis-Menten (MM) kinetics of MDZ 1'- and 4-hydroxylase activities in rat brain microsomes (n = 5) for the first time. The 4-hydroxylated metabolite had 2.4 fold higher maximum velocity (p < 0.01) and 1.9 fold higher (p < 0.05) MM constant values than the 1'-hydroxylated metabolite. However, intrinsic clearance values of the two metabolites were similar. The optimized analytical and metabolic incubation methods reported here may be used to study the effects of various pathophysiological and pharmacological factors on the CYP3A-mediated metabolism of MDZ in the brain.

Effects of alcohol-induced increase in CYP2E1 content in human liver microsomes on the activity and cooperativity of CYP3A4

We investigate the effect of the alcohol-induced increase in the content of CYP2E1 in human liver microsomes (HLM) on the function of CYP3A4. Membrane incorporation of the purified CYP2E1 into HLM considerably increases the rate of metabolism of 7-benzyloxyquinoline (BQ) and attenuates the homotropic cooperativity observed with this CYP3A4-specific substrate. It also eliminates the activating effect of α-naphthoflavone (ANF) seen in some HLM samples. To probe the physiological relevance of these effects, we compared three pooled preparations of HLM from normal donors (HLM-N) with a pooled preparation from ten heavy alcohol consumers (HLM-A). The composition of the P450 pool in all samples was characterized by the mass-spectrometric determination of 11 cytochrome P450 species. The fractional content of CYP2E1 in HLM-A was from 2.0 to 3.4 times higher than in HLM-N. In contrast, the content of CYP3A4 in HLM-A was the lowest among all samples. Despite that, HLM-A exhibited a much higher metabolism rate and a lower homotropic cooperativity with BQ, similar to CYP2E1-enriched HLM-N. To substantiate the involvement of interactions between CYP2E1 and CYP3A4 in these effects, we probed hetero-association of these proteins in CYP3A4-containing Supersomes™ with a technique employing CYP2E1 labeled with BODIPY-618 maleimide. These experiments evinced the interactions between the two enzymes and revealed an inhibitory effect of ANF on their association. Our results demonstrate that the functional properties of CYP3A4 are fundamentally dependent on the composition of the cytochrome P450 ensemble and suggest a possible impact of chronic alcohol exposure on the pharmacokinetics of drugs metabolized by CYP3A4.

The Effect of GS-548351 on the Pharmacokinetics of Midazolam Following Multiple Doses of ANS-6637 in Healthy Adults

ANS-6637, a pro-drug of GS-548351, is a selective, reversible inhibitor of aldehyde dehydrogenase isoform 2 under development as an anticraving agent for the treatment of substance use disorders. In vitro testing indicates that GS-548351 is an inhibitor and inducer of cytochrome P450 family 3, subfamily A (CYP3A). In this phase 1 single-center, open-label, fixed-sequence drug-drug interaction study we assessed the impact of steady-state GS-548351 on single-dose pharmacokinetics of midazolam, an index substrate for CYP3A. Twelve healthy volunteers received 600 mg of ANS-6637 by mouth daily from study days 3 to 8 and a single 5-mg oral dose of midazolam on days 1 and 8. Pharmacokinetic samples were collected over 24 hours on days 1 and 8, then analyzed using liquid chromatography-tandem mass spectrometry. The prespecified no-effect range for the 90% confidence interval (CI) of the geometric mean ratio (GMR) of midazolam coadministered with ANS-6637 (day 8) compared with midazolam alone (day 1) was 0.7-1.43. There was an increase in midazolam AUC0-∞ (GMR [90%CI]) that was within the no-effect range (1.26 [1.12-1.425]) and an increase in midazolam Cmax that was outside the range (1.22 [1.03-1.45]). The AUC0-∞ (1.08 [0.91-1.27]) and Cmax (0.95 [0.75-1.2]) of 1-hydroxymidazolam, the primary metabolite of midazolam, were also within the no-effect range. A single grade 3 adverse event (alanine aminotransferase elevation) was identified and resolved following discontinuation of the study drug. Overall, multidose ANS-6637 was well tolerated and did not alter the PK of midazolam beyond a small increase in AUC0-∞ that is unlikely to be clinically significant.

Distribution of clomipramine, citalopram, midazolam, and metabolites in skeletal tissue after chronic dosing in rats

In recent years, the use of skeletal tissue as an alternative matrix in forensic toxicology has received new interest. In cases where extreme decomposition has taken place, analysis of skeletal tissue is often the only option left. In this article, a fully validated method is presented and the distribution of clomipramine, citalopram, midazolam, and metabolites after chronically administration is examined within skeletal tissue. Rats were chronically dosed with respectively clomipramine, citalopram, or midazolam. Extracts were quantitatively analyzed using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS). Clomipramine, citalopram, and metabolites, respectively desmethylclomipramine and desmethylcitalopram are shown to be detectable in all bone types sampled. Midazolam and its metabolite α-OH-midazolam could not be detected. The absence of midazolam in extracts gives an indication that drugs with pKa values under physiological pH are badly or not incorporated in bone tissue. Bone and post-mortem blood concentrations were compared. A range of different bone types was compared and showed that the concentration is strongly dependent on the bone type. In concordance with previous publications, the humerus shows the highest drug levels. Skeletal tissue concentrations found ranged from 1.1 to 587.8 ng/g. Comparison of the same bone type between the different rats showed high variances. However, the drugs-metabolite ratio proved to have lower variances (<20%). Moreover, the drugs-metabolite ratio in the sampled bones is in close concordance to the ratios seen in blood within a rat. From this, we can assume that the drugs-metabolite ratio in skeletal tissue may prove to be more useful than absolute found concentration.

Cytochrome P450-dependent drug oxidation activities in commercially available hepatocytes derived from human induced pluripotent stem cells cultured for 3 weeks

Hepatocyte-like cells differentiated from human induced pluripotent stem (iPS) cells are of great interest for applications in pharmacological research. For drug metabolism testing, commercially available hepatocytes derived from human iPS cells are generally recommended to be used 1 week after seeding on plates. In this study, however, after 3-4 weeks of culture according to the manufacturer's instructions, human cytochrome P450 (P450) 2C9- and 2C19-dependent diclofenac 4'-hydroxylation and omeprazole 5-hydroxylation activities of the iPS-derived hepatocytes had significantly increased above the activities at 1 week and had reached levels similar to those in HepaRG cells, a human hepatocyte-like cell line. This increase in activities was associated with increasing P450 2C9 and 2C19 mRNA levels. Human P450 3A4-dependent midazolam 1'/4-hydroxylation activities in the iPS-derived hepatocytes were also enhanced after 3 weeks of culture, but the levels were low compared with those of HepaRG cells. These results indicate that the induction of mRNA of typical P450s in human iPS-derived hepatocyte-like cells occurred after 3 weeks of normal culture conditions. However, the induction levels varied considerably depending on the pregnane X receptor pathway and/or the P450 isoform. Our findings that the hepatic functions of human iPS-derived hepatocytes were enhanced by 3 weeks of simple culture could facilitate the use of these cells for drug metabolism and toxicity testing.

Effect of Benifuuki Tea on Cytochrome P450-mediated Metabolic Activity in Rats

BACKGROUND/AIM

Benifuuki tea has recently been used as an alternative therapy for pollinosis, and it may be consumed with pharmaceutical drugs. This study aimed to examine cytochrome P450 (CYP)-mediated food-drug interactions with Benifuuki tea in rats.

MATERIALS AND METHODS

The inhibitory effects of Benifuuki tea and (-)-epigallocatechin-3-O-(3-O-methyl) gallate (EGCG3"Me) on CYP activities were evaluated in vitro. Midazolam pharmacokinetics was investigated after two treatments with Benifuuki tea. In an ex vivo study, CYP activities were determined after 1-week-treatment with the tea.

RESULTS

Benifuuki tea and EGCG3"Me inhibited CYP2D and CYP3A activities in a concentration-dependent manner in vitro. However, MDZ metabolism did not change by Benifuuki treatment in vivo and ex vivo. In contrast, CYP2D activity was decreased ex vivo.

CONCLUSION

Normal intake of Benifuuki tea is not likely to cause food-drug interactions by CYP3A inhibition or induction. In contrast, Benifuuki tea consumption may lead to food-drug interactions through the inhibition of CYP2D.

Reinforcing effectiveness of midazolam, ethanol, and sucrose: behavioral economic comparison of a mixture relative to its component solutions

Benzodiazepines (BZs) are relatively safe when administered alone. However, these drugs can produce severe side effects when coadministered with ethanol. Despite these adverse consequences, rates of concurrent BZ and ethanol misuse are increasing, and it is unclear whether this behavior is maintained by an enhanced reinforcing effect of the mixture. To address this issue, the current study compared the reinforcing effectiveness of sucrose solutions mixed with midazolam, ethanol, or both. Eight male rats were trained to orally self-administer solutions of either sucrose (S), sucrose+midazolam (SM), sucrose+ethanol (SE), or sucrose+midazolam+ethanol (SME). The response requirement was increased between sessions until the number of reinforcers earned was zero and the relationship between response requirement and reinforcers earned was analyzed using the exponential model of demand. Although baseline intake was similar across drug conditions, consumption of SM was least affected by increases in cost, indicating that it possessed the highest reinforcing effectiveness (i.e. least elastic). The reinforcing effectiveness of S, SE, and SME did not differ significantly. The finding that the reinforcing effectiveness of the SME was less than that of SM does not support the supposition that BZ and ethanol coadministration is maintained by a higher reinforcing effectiveness of the mixture.

A highly sensitive method for the simultaneous UHPLC-MS/MS analysis of clonidine, morphine, midazolam and their metabolites in blood plasma using HFIP as the eluent additive

In intensive care units, the precise administration of sedatives and analgesics is crucial in order to avoid under- or over sedation and for appropriate pain control. Both can be harmful to the patient, causing side effects or pain and suffering. This is especially important in the case of pediatric patients, and dose-response relationships require studies using pharmacokinetic-pharmacodynamic modeling. The aim of this work was to develop and validate a rapid ultra-high performance liquid chromatographic-tandem mass spectrometric method for the analysis of three common sedative and analgesic agents: morphine, clonidine and midazolam, and their metabolites (morphine-3-glucuronide, morphine-6-glucuronide and 1'-hydroxymidazolam) in blood plasma at trace level concentrations. Low concentrations and low sampling volumes may be expected in pediatric patients; we report the lowest limit of quantification for all analytes as 0.05ng/mL using only 100μL of blood plasma. The analytes were separated chromatographically using the C18 column with the weak ion-pairing additive 1,1,1,3,3,3-hexafluoro-2-propanol and methanol. The method was fully validated and a matrix matched calibration range of 0.05-250ng/mL was attained for all analytes In addition, between-day accuracy for all analytes remained within 93-108%, and precision remained within 1.5-9.6% for all analytes at all concentration levels over the calibration range.

The usage of a three-compartment model to investigate the metabolic differences between hepatic reductase null and wild-type mice

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The Cytochrome P450 (CYP) system is involved in 90% of the human body's interactions with xenobiotics and due to this, it has become an area of avid research including the creation of transgenic mice. This paper proposes a three-compartment model which is used to explain the drug metabolism in the Hepatic Reductase Null (HRN) mouse developed by the University of Dundee (Henderson, C. J., Otto, D. M. E., Carrie, D., Magnuson, M. A., McLaren, A. W., Rosewell, I. and Wolf, C. R. (2003) Inactivation of the hepatic cytochrome p450 system by conditional deletion of hepatic cytochrome p450 reductase. J. Biol. Chem. , 13480-13486). The model is compared with a two-compartment model using experimental data from studies using wild-type and HRN mice. This comparison allowed for metabolic differences between the two types of mice to be isolated. The three sets of drug data (Gefitinib, Midazolam and Thalidomide) showed that the transgenic mouse has a decreased rate of metabolism.

Evaluation of the effects of hydrophilic organic solvents on CYP3A-mediated drug-drug interaction in vitro

This study evaluated the effects of the commonly used hydrophilic organic solvents, acetonitrile, methanol, ethanol, 1-propanol, dimethyl sulfoxide (DMSO), N,N-dimethylformamide, polyethylene glycol and propylene glycol, on CYP3A in pooled human liver microsomes, using testosterone and midazolam as substrates. Furthermore, we examined the modulation effect of organic solvents on CYP3A inhibition by ketoconazole. Testosterone 6b-hydroxylation activity was potently inhibited in the presence of DMSO and 1-propanol in a concentration-dependent manner. Midazolam 1'-hydroxylation activity, however, was weakly inhibited only by 1% of DMSO, the highest concentration used in this study. Moreover, the potency of ketoconazole to inhibit CYP3A activities was variable, depending on the organic solvent used as a dissolving solvent for ketoconazole. Our data indicate that each organic solvent had an effect on CYP3A4 activity, evaluated by both substrates with different magnitudes. Furthermore, it was shown that the effects of organic solvents on CYP3A activity are substrate-dependent. The present study also shows that methanol had little effect on either substrate.

A High-Throughput Method for Enzyme Kinetic Studies

A simple and flexible setup for conducting drug metabolism studies is described in this report. A heating block was designed for the Multimek liquid handler platform for incubation of multiple samples at 37 °C in a 96-well format. This setup enables the rapid performance of drug metabolism experiments on a large number of samples. In this report, the authors present the validation of the system by 1) showing reproducible and consistent determination of the in vitro half-life of midazolam in every well across the entire plate and 2) determination of metabolic parameter values of midazolam, testosterone, diclofenac, warfarin, and dextromethorphan and inhibition parameter values of quinidine and ketoconazole, all comparable to literature values. In addition, the authors demonstrate the application of the setup to determining the metabolic stability of a set of proprietary compounds, the inhibition of activity of cytochrome P450 (CYP) enzymes, and the conduct of a single combination experiment that can simultaneously determine the metabolic stability and CYP inhibition activity. Overall, the system represents a simple, high-throughput and useful tool for drug metabolism screening in drug discovery. ( Journal of Biomolecular Screening 2003:544-554)

Evaluating a physiologically based pharmacokinetic model for predicting the pharmacokinetics of midazolam in Chinese after oral administration

AIM

To evaluate the SimCYP simulator ethnicity-specific population model for predicting the pharmacokinetics of midazolam, a typical CYP3A4/5 substrate, in Chinese after oral administration.

METHODS

The physiologically based pharmacokinetic (PBPK) model for midazolam was developed using a SimCYP population-based simulator incorporating Chinese population demographic, physiological and enzyme data. A clinical trial was conducted in 40 Chinese subjects (the half was females) receiving a single oral dose of 15 mg midazolam. The subjects were separated into 4 groups based on age (20-50, 51-65, 66-75, and above 76 years), and the pharmacokinetics profiles of each age- and gender-group were determined, and the results were used to verify the PBPK model.

RESULTS

Following oral administration, the simulated profiles of midazolam plasma concentrations over time in virtual Chinese were in good agreement with the observed profiles, as were AUC and Cmax. Moreover, for subjects of varying ages (20-80 years), the ratios of predicted to observed clearances were between 0.86 and 1.12.

CONCLUSION

The SimCYP PBPK model accurately predicted the pharmacokinetics of midazolam in Chinese from youth to old age. This study may provide novel insight into the prediction of CYP3A4/5-mediated pharmacokinetics in the Chinese population relative to Caucasians and other ethnic groups, which can support the rational design of bridging clinical trials.

The Pharmacokinetics of the CYP3A Substrate Midazolam in Morbidly Obese Patients Before and One Year After Bariatric Surgery

PURPOSE

Bariatric surgery is nowadays commonly applied as treatment for morbid obesity (BMI > 40 kg/m(2)). As information about the effects of this procedure on a drug's pharmacokinetics is limited, we aimed to evaluate the pharmacokinetics of CYP3A probe substrate midazolam after oral and intravenous administration in a cohort of morbidly obese patients that was studied before and 1 year post bariatric surgery.

METHODS

Twenty morbidly obese patients (aged 26-58 years) undergoing bariatric surgery participated in the study of which 18 patients returned 1 year after surgery. At both occasions, patients received 7.5 mg oral and 5 mg intravenous midazolam separated by 160 ± 48 min. Per patient and occasion, a mean of 22 blood samples were collected. Midazolam concentrations were analyzed using population pharmacokinetic modeling.

RESULTS

One year after bariatric surgery, systemic clearance of midazolam was higher [0.65 (7%) versus 0.39 (11%) L/min, mean ± RSE (P < 0.01), respectively] and mean oral transit time (MTT) was faster [23 (20%) versus 51 (15%) minutes (P < 0.01)], while oral bioavailability was unchanged (0.54 (9%)). Central and peripheral volumes of distribution were overall lower (P < 0.05).

CONCLUSIONS

In this cohort study in morbidly obese patients, systemic clearance was 1.7 times higher 1 year after bariatric surgery, which may potentially result from an increase in hepatic CYP3A activity per unit of liver weight. Although MTT was found to be faster, oral bioavailability remained unchanged, which considering the increased systemic clearance implies an increase in the fraction escaping intestinal first pass metabolism.

[Study of change in activity of hepatic drug metabolism enzymes in rat model of chronic unpredictable mild stress]

This study aimed to explore the impact of depression caused by chronic unpredictable mild stress (CUMS) on in vivo activity of six kinds of CYP450 isoforms in rats. According to 'Katz' method, the model of CUMS was established. Tolbutamide, chlorzoxazone, theophylline, midazolam, omeprazole and dextromethorphan were chosen as probe substrates of CYP2C6, CYP2E1, CYP1A2, CYP3A2, CYP2D1 and CYP2D2 of rats. Plasma concentration of six kinds of CYP450 in control group and model group were determined by LC-MS/MS and computed pharmacokinetic parameters. Consequently, metabolism of theophylline and chlorzoxazone accelerated significantly (P < 0.01), but tolbutamide, dextromethorphan, omeprazole and midazolam had no significant difference. The present study proved that depression caused by CUMS had strong induction to CYP1A2 and medium induction to CYP2E1.

Application of a novel regulatable Cre recombinase system to define the role of liver and gut metabolism in drug oral bioavailability

The relative contribution of hepatic compared with intestinal oxidative metabolism is a crucial factor in drug oral bioavailability and therapeutic efficacy. Oxidative metabolism is mediated by the cytochrome P450 mono-oxygenase system to which cytochrome P450 reductase (POR) is the essential electron donor. In order to study the relative importance of these pathways in drug disposition, we have generated a novel mouse line where Cre recombinase is driven off the endogenous Cyp1a1 gene promoter; this line was then crossed on to a floxed POR mouse. A 40 mg/kg dose of the Cyp1a1 inducer 3-methylcholanthrene (3MC) eliminated POR expression in both liver and small intestine, whereas treatment at 4 mg/kg led to a more targeted deletion in the liver. Using this approach, we have studied the pharmacokinetics of three probe drugs--paroxetine, midazolam, nelfinavir--and show that intestinal metabolism is a determinant of oral bioavailability for the two latter compounds. The Endogenous Reductase Locus (ERL) mouse represents a significant advance on previous POR deletion models as it allows direct comparison of hepatic and intestinal effects on drug and xenobiotic clearance using lower doses of a single Cre inducing agent, and in addition minimizes any cytotoxic effects, which may compromise interpretation of the experimental data.

The effect of complementary and alternative medicines on CYP3A4-mediated metabolism of three different substrates: 7-benzyloxy-4-trifluoromethyl-coumarin, midazolam and docetaxel

OBJECTIVE

Concomitant use of complementary and alternative medicine (CAM) and anticancer drugs can affect the pharmacokinetics of anticancer drugs by inhibiting the metabolizing enzyme cytochrome P450 3A4 (CYP3A4) (EC 1.14.13.157). Several in vitro studies determined whether CAM can inhibit CYP3A4, but these studies revealed contradictory results. A plausible explanation for these conflicting results is the use only of a single model CYP3A4 substrate in each study. Therefore, the objective was to determine the potential of selected CAM (β-carotene, Echinacea, garlic, Ginkgo biloba, ginseng, grape seed extract, green tea extract, milk thistle, saw palmetto, valerian, vitamin B6, B12 and C) to inhibit CYP3A4-mediated metabolism of different substrates: 7-benzyloxy-4-trifluoromethyl-coumarin (BFC), midazolam and docetaxel. The effect of CAM on CYP3A4-mediated metabolism of an anticancer drug has never been determined before in vitro, which makes this study unique. The oncolytic CYP3A4 substrate docetaxel was used to establish the predictive value of the model substrates for pharmacokinetic interactions between CAM and anticancer drugs in vitro, and to more closely predict these interactions in vivo.

METHODS

The inhibition of CYP3A4-mediated metabolism of 7-benzyloxy-4-trifluoromethyl-coumarin (BFC) by CAM was assessed in Supersomes, using the fluorometric CYP3A4 inhibition assay. In human liver microsomes (HLM) the inhibition of CYP3A4-mediated metabolism of midazolam and docetaxel was determined, using liquid-chromatography coupled to tandem mass spectrometry (LC-MS/MS).

KEY FINDINGS

The results confirmed grape seed and green tea as potent inhibitors and milk thistle as moderate inhibitor of CYP3A4-mediated metabolism of BFC, midazolam and docetaxel.

CONCLUSION

Clinical studies are required to determine the clinical relevance of the determined CYP3A4 inhibition by grape seed, green tea and milk thistle.

Midazolam induces cellular apoptosis in human cancer cells and inhibits tumor growth in xenograft mice

Midazolam is a widely used anesthetic of the benzodiazepine class that has shown cytotoxicity and apoptosisinducing activity in neuronal cells and lymphocytes. This study aims to evaluate the effect of midazolam on growth of K562 human leukemia cells and HT29 colon cancer cells. The in vivo effect of midazolam was investigated in BALB/c-nu mice bearing K562 and HT29 cells human tumor xenografts. The results show that midazolam decreased the viability of K562 and HT29 cells by inducing apoptosis and S phase cell-cycle arrest in a concentration-dependent manner. Midazolam activated caspase-9, capspase-3 and PARP indicating induction of the mitochondrial intrinsic pathway of apoptosis. Midazolam lowered mitochondrial membrane potential and increased apoptotic DNA fragmentation. Midazolam showed reactive oxygen species (ROS) scavenging activity through inhibition of NADPH oxidase 2 (Nox2) enzyme activity in K562 cells. Midazolam caused inhibition of pERK1/2 signaling which led to inhibition of the anti-apoptotic proteins Bcl-XL and XIAP and phosphorylation activation of the pro-apoptotic protein Bid. Midazolam inhibited growth of HT29 tumors in xenograft mice. Collectively our results demonstrate that midazolam caused growth inhibition of cancer cells via activation of the mitochondrial intrinsic pathway of apoptosis and inhibited HT29 tumor growth in xenograft mice. The mechanism underlying these effects of midazolam might be suppression of ROS production leading to modulation of apoptosis and growth regulatory proteins. These findings present possible clinical implications of midazolam as an anesthetic to relieve pain during in vivo anticancer drug delivery and to enhance anticancer efficacy through its ROS-scavenging and pro-apoptotic properties.

Pomegranate Juice Does Not Impair Clearance of Oral or Intravenous Midazolam, a Probe for Cytochrome P450-3A Activity: Comparison With Grapefruit Juice

The effect of pomegranate juice (PJ) or grapefruit juice (GFJ) on CYP3A activity was studied in vitro and in healthy human volunteers. In human liver microsomes, the mean 50% inhibitory concentrations (IC(50)) for PJ and GFJ versus CYP3A (triazolam alpha-hydroxylation) were 0.61% and 0.55%, (v/v) respectively, without preincubation of inhibitor with microsomes. After preincubation, the IC(50) for PJ increased to 0.97% (P < .05), whereas the IC(50) for GFJ decreased to 0.41% (P < .05), suggesting mechanism-based inhibition by GFJ but not PJ. Pretreatment of volunteer subjects (n = 13) with PJ (8 oz) did not alter the elimination half-life, volume of distribution, or clearance of intravenous midazolam (2 mg). Administration of PJ also did not affect C(max), total area under the curve (AUC), or clearance of oral midazolam (6 mg). However, GFJ (8 oz) increased midazolam C(max) and AUC by a factor of 1.3 and 1.5, respectively, and reduced oral clearance to 72% of control values. Thus, PJ does not alter clearance of intravenous or oral midazolam, whereas GFJ impairs clearance and elevates plasma levels of oral midazolam.

In vitro inhibition of cytochrome P450 3A4 by Aronia melanocarpa constituents

Extracts, subfractions, isolated anthocyanins and procyanidins, and two phenolic acids from aronia [Aronia melanocarpa] were investigated for their CYP3A4 inhibitory effects, using midazolam as the probe substrate and recombinant insect cell microsomes expressing CYP3A4 as the enzyme source. Procyanidin B5 was a considerably stronger CYP3A4 inhibitor in vitro than the isomeric procyanidin B2 and comparable to bergamottin, a known CYP3A4 inhibitor from grapefruit juice. The inhibitory activity of proanthocyanidin-containing fractions was correlated to the degree of polymerization. Among the anthocyanins, cyanidin 3-arabinoside showed stronger CYP3A4 inhibition than cyanidin 3-galactoside and cyanidin 3-glucoside. Thus, the ability to inhibit CYP3A4 in vitro seems to be influenced by the sugar unit linked to the anthocyanidin.

Isolation and identification of intestinal CYP3A inhibitors from cranberry (Vaccinium macrocarpon) using human intestinal microsomes

Cranberry juice is used routinely, especially among women and the elderly, to prevent and treat urinary tract infections. These individuals are likely to be taking medications concomitantly with cranberry juice, leading to concern about potential drug-dietary substance interactions, particularly in the intestine, which, along with the liver, is rich in expression of the prominent drug metabolizing enzyme, cytochrome P450 3A (CYP3A). Using a systematic in vitro-in vivo approach, a cranberry juice product was identified recently that elicited a pharmacokinetic interaction with the CYP3A probe substrate midazolam in 16 healthy volunteers. Relative to water, cranberry juice inhibited intestinal first-pass midazolam metabolism. In vitro studies were initiated to identify potential enteric CYP3A inhibitors from cranberry via a bioactivity-directed fractionation approach involving dried whole cranberry [Vaccinium macrocarpon Ait. (Ericaceae)], midazolam, and human intestinal microsomes (HIM). Three triterpenes (maslinic acid, corosolic acid, and ursolic acid) were isolated. The inhibitory potency (IC(50)) of maslinic acid, corosolic acid, and ursolic acid was 7.4, 8.8, and < 10 µM, respectively, using HIM as the enzyme source and 2.8, 4.3, and < 10 µM, respectively, using recombinant CYP3A4 as the enzyme source. These in vitro inhibitory potencies, which are within the range of those reported for two CYP3A inhibitory components in grapefruit juice, suggest that these triterpenes may have contributed to the midazolam-cranberry juice interaction observed in the clinical study.

In-vivo kinetics of the interaction between midazolam and erythromycin in rats, taking account of metabolic intermediate complex formation

To predict, quantitatively, the extent of drug interaction during repeated administration of a metabolic inhibitor, we analysed the effects of erythromycin treatment under several regimens on the area under the concentration curve (AUC) of midazolam in rats. Midazolam was administered into the portal vein 12 h after erythromycin treatment for 1, 2 or 3 days, or 12, 24, 36, 48, 72 and 96 h after erythromycin treatment for 4 days, and the plasma-concentration profiles of midazolam were analysed to assess the AUC. Moreover, the contents of total cytochrome P450 and inactive metabolic intermediate (MI) complex were simultaneously quantitated. While the AUC value of midazolam was not affected by the administration of erythromycin for 1 day, repeated administration of erythromycin evoked an increase in AUC ratio (AUC in erythromycin-treated rats/AUC in vehicle-treated rats), which reached a maximum value of 1.99 at 12 h after 4 days' treatment with erythromycin. The total content of cytochrome P450 in liver microsomes was unaffected by erythromycin treatment. Although the MI complex was undetectable after 1 day's treatment with erythromycin, its content increased with duration of erythromycin treatment, and the complex disappeared after the end of erythromycin treatment with a half-life of 12.3 h. In conclusion, the interaction between erythromycin and midazolam could be well predicted when the formation of MI complex in the liver was taken into account.

Cloning, expression, and characterization of CYP3A43 cDNA in cynomolgus macaque (Macaca fascicularis)

Cynomolgus macaques are frequently used in drug metabolism studies due to their evolutionary closeness to humans. Despite their importance, genes encoding drug-metabolizing enzymes have not been fully identified in this species. In this study, the cDNA orthologous to human cytochrome P450 3A43 (CYP3A43) was isolated. Deduced amino acids of this cDNA had a high sequence identity ( approximately 95%) to human CYP3A43 cDNA and contained characteristic motifs for CYP3A proteins, heme-binding region and substrate recognition sites. Among 10 tissues analyzed, cynomolgus CYP3A43 was expressed in liver, adrenal gland, and lung, with the highest expression seen in liver. Cynomolgus CYP3A43 protein heterologously expressed in Escherichia coli exhibited metabolic activity toward midazolam 1'-hydroxylation. These results indicated that cynomolgus CYP3A43 was expressed in liver and encoded a functional drug-metabolizing enzyme, and could contribute to overall drug metabolism in cynomolgus macaque liver if expressed as a protein.

Comparison of pharmacokinetics of cyclosporine A in cadaveric and living-related renal transplant recipients and in an experimental rat model of renal failure

To elucidate the differences in the cyclosporine A (CyA) PK between cadaveric and living-related renal transplantation (CRT and LRT, respectively) recipients, a retrospective cohort study of clinical PK was conducted. Data from 80 patients who received LRT (n=75) and CRT (n=5) over 4 years were included. The incidence of acute rejection in CRT recipients was over 5 times higher than that in LRT recipients. On day 14 after transplantation, the area under the blood concentration versus time curve (AUC) per dose up to 4 h in CRT recipients was 65.3 % that of LRT recipients, however, there was no difference in the blood trough levels. Unlike LRT, renal failure derived from long ischemia time was observed in CRT recipients, and it is speculated that renal failure affects the PK of CyA. Moreover, we performed intravenous. (i.v.) and intraduodenal (i.d.) PK studies of CyA using renal failure model rats prepared by renal ischemia-reperfusion (RIR rats). There were no differences in PK profiles after i.v. administration of CyA between RIR and control rats; however, AUC up to infinity (1.81+/-0.18 microg.h/ml) in RIR rats after i.d. administration was significantly lower than in control rats (5.01+/-1.78 microg.h/ml). In addition, the absorption of CyA and midazolam, an ideal probe for CYP3A, from the intestinal loop in RIR rats was significantly less (69.8% and 42.8 %, respectively) than in control rats. These results suggest that the contribution of intestinal metabolism by CYP3A to decreasing CyA absorption in RIR rats is significant, namely, there is a possibility that the reason for poor absorption of CyA in CRT recipients is increasing intestinal CYP3A activity is maybe renal injury derived from long renal ischemia. The results of this study provide a useful information for therapeutic drug monitoring of CyA in CRT recipients.

Relative roles of CYP2C19 and CYP3A4/5 in midazolam 1′-hydroxylation

1. During the characterization of recombinant CYP2C19, it was observed that this enzyme metabolized midazolam, which is generally regarded as CYP3A4/5 substrate, and we therefore decided to pursue this observation further. 2. CYP2C19 showed a Michaelis-Menten pattern for midazolam 1'-hydroxylation and was inhibited by (+)-N-3-benzylnirvanol and S-mephenytoin, which are a standard potent inhibitor and a substrate of CYP2C19, respectively. 3. The inhibitory potency by CYP3A4/5 inhibitor on the midazolam 1'-hydroxylation in human liver microsomes (HLM) was correlated with the CYP3A4/5 specific catalytic activity, but such correlation was not observed in CYP2C19 enzyme. The in vitro intrinsic clearance value for midazolam 1'-hydroxylation was not changed by the addition of (+)-N-3-benzylnirvanol in four individual HLM preparations. 4. These results indicated that although CYP2C19 is capable of catalyzing midazolam 1'-hydroxylation, CYP3A4/5 play a more important role.

Model for the drug–drug interaction responsible for CYP3A enzyme inhibition. II: establishment and evaluation of dexamethasone-pretreated female rats

1. Cytochrome P450 (CYP) 3A catalysis of testosterone 6beta-hydroxylation in female rat liver microsomes was significantly induced, then reached a plateau level after pretreatment with 80 mg kg(-1) day(-1) dexamethasone (DEX) for 3 days. 2. Midazolam was mainly metabolized by CYP3A in DEX-treated female rat liver microsomes from an immuno-inhibition study, and the apparent K(m) was 1.8 microM, similar to that in human microsomes. 3. Ketoconazole and erythromycin, typical CYP3A inhibitors, demonstrated extensive inhibition of midazolam metabolism in DEX-treated female rat liver microsomes, and the apparent K(i) values were 0.088 and 91.2 microM, respectively. The values were similar to those in humans, suggesting that DEX-treated female rat liver microsomes have properties similar to those of humans. 4. After oral administration of midazolam, the plasma midazolam concentration in DEX-treated female rats significantly decreased compared with control female rats. The area under the plasma concentration curve (AUC) and elimination half-life were one-11th and one-20th of those of control female rats, respectively. 5. Using DEX-treated female rats, the effect of CYP3A inhibitors on midazolam pharmacokinetics was evaluated. The AUC and maximum concentration in plasma (C(max)) increased when ketoconazole was co-administered with midazolam. 6. It was shown that the drug-drug interaction that occurs in vitro is also observed in vivo after oral administration of midazolam. In conclusion, the DEX-treated female rat could be a useful model for evaluating drug-drug interactions based on CYP3A enzyme inhibition.

The effect of multiple-dose, oral rifaximin on the pharmacokinetics of intravenous and oral midazolam in healthy volunteers

STUDY OBJECTIVE

To evaluate the potential of rifaximin, an oral nonabsorbed (< 0.4%) structural analog of rifampin, to induce human hepatic and/or intestinal cytochrome P450 (CYP) 3A enzymes, with use of a known CYP3A probe, midazolam.

DESIGN

Prospective, randomized, open-label, two-period, crossover study.

SETTING

Clinical research center.

SUBJECTS

Twenty-seven healthy adult volunteers.

INTERVENTION

During the first treatment period, subjects received a single dose of either intravenous midazolam 2 mg over 30 minutes or oral midazolam 6 mg on day 0. From days 3-10, they received rifaximin 200 mg every 8 hours. On days 6 (after the 9th dose of rifaximin) and 10 (after the 21st dose of rifaximin), subjects received a concomitant single dose of intravenous or oral midazolam. After a 15-day washout period, subjects were crossed over to the other formulation of midazolam, and the treatment schedule was repeated, with the second treatment period starting on day 26 and single-dose administration of midazolam on days 26, 32, and 36. Serial plasma samples were collected for pharmacokinetic analyses.

MEASUREMENTS AND MAIN RESULTS

The pharmacokinetic parameters of single-dose intravenous or oral midazolam were determined alone and after coadministration of rifaximin for 3 and 7 days. Rifaximin coadministration did not alter the measured pharmacokinetic parameters for midazolam or its major metabolite, 1'-hydroxymidazolam. The 90% confidence intervals for the maximum concentration and area under the concentration-time curve from time zero extrapolated to infinity (bioavailability) were all within 80-125% for intravenous and oral midazolam. Therefore, no drug interaction was observed between rifaximin and midazolam. Coadministration of midazolam and rifaximin was well tolerated.

CONCLUSION

Overall, 3-7 days of rifaximin 200 mg 3 times/day did not alter single-dose midazolam pharmacokinetics. Rifaximin also does not appear to induce intestinal or hepatic CYP3A activity.

Differential metabolism of gefitinib and erlotinib by human cytochrome P450 enzymes

PURPOSE

To examine the enzyme kinetics of gefitinib and erlotinib metabolism by individual cytochrome P450 (CYP) enzymes, and to compare their effects on CYP3A activity, with the aim to better understand mechanisms underlying pharmacokinetic variability and clinical effects.

EXPERIMENTAL DESIGN

Enzyme kinetics were examined by incubating gefitinib or erlotinib (1.5-50 micromol/L) with recombinant human CYP3A4, CYP3A5, CYP2D6, CYP1A1, CYP1A2, and CYP1B1 (10-160 pmol/mL). Their effects on CYP3A activity were examined by comparing midazolam metabolism in the presence and absence of gefitinib or erlotinib in human liver and intestinal microsomes. Parent compounds and metabolites were monitored by high-performance liquid chromatography with a photodiode detector or tandem mass spectrometer.

RESULTS

Both drugs were metabolized primarily by CYP3A4, CYP3A5, and CYP1A1, with respective maximum clearance (Cl(max)) values for metabolism of 0.41, 0.39, and 0.57 mL/min/nmol for gefitinib and 0.24, 0.21, 0.31 mL/min/nmol for erlotinib. CYP2D6 was involved in gefitinib metabolism (Cl(max), 0.63 mL/min/nmol) to a large extent, whereas CYP1A2 was considerably involved in erlotinib metabolism (Cl(max), 0.15 mL/min/nmol). Both drugs stimulated CYP3A-mediated midazolam disappearance and 1-hydroxymidazolam formation in liver and intestinal microsomes.

CONCLUSIONS

Gefitinib is more susceptible to CYP3A-mediated metabolism than erlotinib, which may contribute to the higher apparent oral clearance observed for gefitinib. Metabolism by hepatic and extrahepatic CYP1A may represent a determinant of pharmacokinetic variability and response for both drugs. The differential metabolizing enzyme profiles suggest that there may be differences in drug-drug interaction potential and that stimulation of CYP3A4 may likely play a role in drug interactions for erlotinib and gefitinib.

Decreased intestinal CYP3A and P-glycoprotein activities in rats with adjuvant arthritis

Adjuvant-induced arthritis (AA) rats have been used as an animal model for rheumatoid arthritis. Several studies have shown that the pharmacokinetics of a number of drugs are altered in AA rats. We investigated the effects of AA on the barrier functions of the intestine using a rat model. Intestinal CYP3A activities (midazolam 1'-hydroxylation and 7-benzyloxy-4-(trifluoromethyl)-coumarin 7-hydroxylation) in AA rats were significantly decreased compared with those in normal rats, with marked decrease observed in the upper segment of intestine. Intestinal P-glycoprotein (P-gp) activity at upper segment was also significantly decreased in AA rats to 60% of that in normal rats, and the other segments (middle and lower) of intestine also exhibited tendencies toward decrease in P-gp activity. This decrease was supported by the finding that levels of mdr1a mRNA and P-gp protein were decreased in AA rats. No significant differences were observed in intestinal paracellular and transcellular permeability between AA and normal rats. These results suggest that intestinal CYP3A and P-gp activities are decreased in AA rats, and that the pharmacokinetics and bioavailabilities of drugs whose membrane permeation is limited by intestinal CYP3A and/or P-gp may be altered in rheumatic diseases.

Characterization of cytochrome P450-mediated drug metabolism in cats

In this study we examined activities of cytochrome P450 (CYP)1A, 2C, 2D and 3A using hepatic microsomes from five male and five female cats. CYP1A, 2C, 2D and 3A activities were referred by ethoxyresorufin O-deethylation (EROD), tolbutamide hydroxylation (TBH), bufuralol 1'-hydroxylation (BLH) and midazolam 1'- and 4-hydroxylation respectively. The anti-rat CYP1A2 and CYP3A2 serum significantly inhibited EROD and midazolam 1'- and 4-hydroxylation, suggesting that EROD and midazolam 1'- and 4-hydroxylation were catalysed by CYP1A and 3A in cats respectively. Quinidine inhibited BLH in cats microsomes at quite low concentrations, suggesting that BLH was catalysed by CYP2D in cats. Tolbutamide hydroxylation activities were negligible in hepatic microsomes from both male and female cats, suggesting CYP2C activities of cats are extremely low. This suggests that CYP2C substrates should be carefully administered to cats. Although there is no sexual difference in CYP1A activities, there are differences in CYP2D and 3A activities of cats. CYP2D activities were higher (3-fold), but CYP3A activities were lower (one-fifth) in female cats. These results might suggest that CYP2D and 3A substrates should be prescribed for male and female cats using different dosage regimen.

Effects of hypothermia on drug disposition, metabolism, and response: A focus of hypothermia-mediated alterations on the cytochrome P450 enzyme system

OBJECTIVES

Therapeutic hypothermia has been shown to decrease neurologic damage in patients experiencing out-of-hospital cardiac arrest. In addition to being treated with hypothermia, critically ill patients are treated with an extensive pharmacotherapeutic regimen. The effects of hypothermia on drug disposition increase the probability for unanticipated toxicity, which could limit its putative benefit. This review examines the effects of therapeutic hypothermia on the disposition, metabolism, and response of drugs commonly used in the intensive care unit, with a focus on the cytochrome P450 enzyme system.

DATA SOURCES AND STUDY SELECTION

A MEDLINE/PubMed search from 1965 to June 2006 was conducted using the search terms hypothermia, drug metabolism, P450, critical care, cardiac arrest, traumatic brain injury, and pharmacokinetics.

DATA EXTRACTION AND SYNTHESIS

Twenty-one studies were included in this review. The effects of therapeutic hypothermia on drug disposition include both the effects during cooling and the effects after rewarming on drug metabolism and response. The studies cited in this review demonstrate that the addition of mild to moderate hypothermia decreases the systemic clearance of cytochrome P450 metabolized drugs between approximately 7% and 22% per degree Celsius below 37degreesC during cooling. The addition of hypothermia decreases the potency and efficacy of certain drugs.

CONCLUSIONS

This review provides evidence that the therapeutic index of drugs is narrowed during hypothermia. The magnitude of these alterations indicates that intensivists must be aware of these alterations in order to maximize the therapeutic efficacy of this modality. In addition to increased clinical attention, future research efforts are essential to delineate precise dosing guidelines and mechanisms of the effect of hypothermia on drug disposition and response.

The CYP3A4*18 allele, the most frequent coding variant in asian populations, does not significantly affect the midazolam disposition in heterozygous individuals

The objective of this study was to identify CYP3A4 variants in Koreans and to characterize their functional consequences in vitro and in vivo. Four single nucleotide polymorphisms were identified in 50 Koreans by direct DNA sequencing. In an additional genotyping using 248 subjects, CYP3A4(*)18 was confirmed as the most frequent coding variant in Koreans at 1.7%, and its frequency was similar to that of Asians, suggesting that CYP3A4(*)18 would be the highest coding variant in Asians. The recombinant CYP3A4.18 protein prepared in baculovirus expression system showed 67.4% lower Vmax and 1.8-fold higher K(m) for midazolam 1'-hydroxylation compared with the wild type. The mean values of Cmax and area under the concentration curve (AUC) in the CYP3A4(*)1/(*)18 and CYP3A5(*)1/(*)3 subjects (n = 8) were 63% and 32% higher than in CYP3A4(*)1/(*)1 and CYP3A5(*)1/(*)3 carriers (n = 8), respectively. Although the in vitro assay exhibited a significant reduction of the enzyme activity for midazolam, the in vivo differences associated with the CYP3A4(*)1/(*)18 tend to be low (P < 0.07 in Cmax and P < 0.09 in AUC). In summary, the heterozygous CYP3A4(*)1/(*)18 does not appear to cause a significant change of midazolam disposition in vivo; however, the clinical relevance of CYP3A4(*)18/(*)18 remains to be evaluated.

Use of Isolated Hepatocyte Preparations for Cytochrome P450 Inhibition Studies: Comparison with Microsomes forKiDetermination

Predicting drug-drug interactions requires an assessment of the drug concentration available to the enzyme active site, both in vivo, and within an in vitro incubation. These predictions are confounded when the inhibitor accumulates within the liver, either as a result of active transport processes or intracellular binding (including lysosomal trapping). In theory, hepatocytes should provide a more accurate estimation of inhibitory potency compared with microsomes for those compounds that undergo hepatic accumulation. However, they are not routinely used for Ki determination and there is limited comparative information available. Therefore, the aims of this study were to compare Ki values determined in rat microsomes and freshly isolated hepatocytes using six cytochrome P450 inhibitors (miconazole, fluconazole, ketoconazole, quinine, fluoxetine, and fluvoxamine) with a range of uptake properties (cell-to-medium concentration ratios 4.2-6000). Inhibition studies were performed using four probe substrates for CYP2C, CYP2D, and CYP3A enzymes (tolbutamide and phenytoin, dextromethorphan and midazolam, respectively). Comparison of unbound Ki values (range 0.05-30 microM) showed good agreement between microsomes and hepatocytes for inhibition of 18 pathways of metabolism. In addition to this, there was no relationship between the cell-to-medium concentration ratios (covering over 3 orders of magnitude) and the microsomal to hepatocyte Ki ratio of these inhibitors. These data suggest that the hepatic accumulation of these inhibitors results from intracellular binding rather than the involvement of uptake transporters and indicate that microsomes and hepatocytes appear to be equivalent for determining the inhibitory potency of the six inhibitors investigated in the present study.

Transporter-mediated drug interactions: clinical implications and in vitro assessment

Although they are less frequently compared with the reported cases of CYP-mediated drug interactions, clinically significant transporter-mediated drug interactions, which are mainly based on efflux transporter or P-glycoprotein data, have been reported. Unlike the CYP-mediated drug interactions that can be readily defined by inhibition or induction of CYP enzymes, the evidence for the so-called transporter-mediated drug interactions is often less conclusive. The difficulty in defining transporter-mediated drug interactions is due mainly to the interplay between transporters and drug-metabolizing enzymes in drug disposition, and the lack of specific and potent inhibitors for each transporter and enzyme. An important lesson learned from animal studies is that transporter inhibition has a much greater impact on the tissue distribution of drugs than on the systemic exposure of drugs measured in plasma. The potential risk of transporter-mediated drug interactions might be underestimated if only plasma concentrations are monitored.

Effect of ion suppression on judgment of enzyme inhibition and avoidance of error by utilizing a stable isotope-labeled probe substrate: example of CYP3A4 inhibition with [13C4,15N] labeled midazolam as a substrate

An advantage of using LC-MS(/MS) for in vitro CYP inhibition screening is that it does not require extensive sample preparation and chromatographic separation. Attention must be paid, however, to ion suppression effects on analytes caused by the test compound as well as endogenous compounds. In this study, we have shown the ion suppression of 1'-hydroxymidazolam (analyte) and dextrorphan (IS) by erythromycin, as an example, which may cause over- or underestimation of CYP3A4 inhibition. To avoid this kind of effect, we proposed to use a stable isotope-labeled substrate and determine labeled metabolites by using unlabeled authentic compounds of each metabolite. We showed that CYP3A4 activity was determined with high accuracy and precision by using stable isotope-labeled midazolam even in the presence of an ion suppressor at high concentrations in the samples. This method is useful not only for the CYP inhibition screening but also for testing drug candidates to predict changes in metabolite formation by the possible co-administered drugs.

Simultaneous analysis of cytochrome P450 probes—dextromethorphan, flurbiprofen and midazolam and their major metabolites by HPLC-mass-spectrometry/fluorescence after single-step extraction from plasma

Cytochrome P450 enzymes catalyze oxidative metabolism of most pharmaceutical compounds. Consequently dextromethorphan, flurbiprofen, midazolam and other compounds are commonly used as probe substrates to evaluate cytochrome P450 function in humans. A "cocktail" approach employing simultaneous administration of two or more of the probe substrates has been used by various investigators in recent years. An analytical strategy to simultaneously extract and analyze dextromethorphan, flurbiprofen and midazolam and their major metabolites (dextrorphan, 4'-hydroxy-flurbiprofen and 1'-hydroxy-midazolam) by HPLC-MS/fluorescence was developed and is described here. The three probe substrates and their major metabolites were extracted simultaneously by means of a solid-phase (Bond Elut Certify cartridges) extraction procedure from 200 microl of pig plasma. The extraction efficiency was more than 79.5% for each of the six analytes. The extracted compounds were chromatographically separated on a Luna C8(II) column (50 mm Lx3 mm ID) in a single run of 20 min and analyzed by either fluorescence (flurbiprofen and 4'-hydroxy-flurbiprofen) or selective ion monitoring (dextromethorphan, dextrorphan, midazolam and 1'-hydroxy-midazolam) with positive electrospray ionization. The limit of quantification was 2.5 ng/ml for midazolam and 5 ng/ml for the other five analytes. The assay was precise and accurate (error: -9.1 to 12.1) with total CVs of 13.9% or better for each of the 6 analytes. This method was used to analyze concentrations of the three probes and their metabolites in plasma after intravenous administration to a healthy pig.

[Hepatic CYP3A activity in association with development of osteonecrosis of the femoral head]

Osteonecrosis of the femoral head (ONFH) is one of the major side effects of corticosteroid therapy. Since corticosteroids are metabolized by hepatic cytochrome P450 3A (CYP3A), a low endogenous activity of this enzyme may exert excessive or toxic effect to bone and contribute to development of ONFH. To test this hypothesis, we measured hepatic CYP3A activity in 130 patients (26 with steroid-induced ONFH, 29 alcohol-related ONFH, and 75 normal controls without corticosteroid therapy and ONFH) by measuring the clearance of midazolam (MDZ) , a substrate specific to CYP3A, and found that MDZ clearance in steroid-induced ONFH patients was significantly retarded when compared with that in controls or alcohol-related ONFH patients. Multivariate analysis revealed that only MDZ clearance was significantly related with steroid-induced ONFH and the patients with low MDZ clearance due to low hepatic CYP3A activity provided 9-fold greater risk for steroid-induced ONFH. Therefore, low hepatic CYP3A activity would be significant risk factor to the onset of steroid-induced ONFH.

Asymmetric Intestinal First-Pass Metabolism Causes Minimal Oral Bioavailability of Midazolam in Cynomolgus Monkey

Oral bioavailability of some drugs is substantially lower in cynomolgus monkeys than in various other species, including humans. In the present study, midazolam was used as a model drug to investigate the reason for the lower bioavailability in these monkeys. The bioavailability of midazolam after oral administration was minimal in monkeys and rats, being only 2.1 and 1.1%, respectively. In monkeys, this low bioavailability could not be explained simply in terms of a hepatic first-pass effect. To examine the roles of intestinal metabolism and transport, we evaluated apical-to-basal and basal-to-apical transport of midazolam, and the formation of metabolites in small intestinal tissues using an Ussing-type chamber. The values of mucosal extraction ratio were estimated to be 0.97, 0.93, and 0.89 during apical-to-basal transport in the upper, middle, and lower small intestine of monkeys, respectively, whereas the corresponding values for rats were close to zero, indicating that extensive metabolism of midazolam occurs, particularly in the upper region of the small intestine in monkeys, but not rats. Interestingly, formation of the metabolites was much greater during transport in the apical-to-basal direction than in the basal-to-apical direction, and this could be well explained by a mathematical model based on the assumption that extensive metabolism is associated with the uptake process of midazolam from the apical cell surface. Thus, we conclude that an asymmetric distribution of metabolic activity in the small intestine, leading to extensive metabolism during uptake from the apical cell surface, accounts for the minimal oral bioavailability of midazolam in cynomolgus monkeys.

Comparison of cytochrome P450 inhibition assays for drug discovery using human liver microsomes with LC–MS, rhCYP450 isozymes with fluorescence, and double cocktail with LC–MS

The disparity of IC(50)s from CYP450 inhibition assays used to assess drug-drug interaction potential was investigated, in order to have evidence for selecting a reliable in vitro CYP450 inhibition assay to support drug discovery. Three assays were studied: individual rhCYP isozymes and corresponding coumarin derivative-probe substrates with fluorescent detection, human liver microsomes (HLM) and cocktail drug-probe substrates with LC-MS detection, and double cocktail rhCYP isozymes mix and drug-probe mix with LC-MS detection. Data comparisons showed that the rhCYP-fluorescent assay and the cocktail assay with HLM-LC-MS had weak correlation. Detection method and probe substrates were shown to not be the major cause of the disparity in IC(50)s. However, the enzyme source and composition (HLM versus, rhCYP) caused disparity in IC(50)s. Specifically, the high concentrations of CYP isozymes often used with HLM-based assays produced high probe substrate conversion and test compound metabolism, which should both contribute to artificially higher IC(50)s. Non-specific binding of substrate to higher concentration proteins and lipids in the HLM-based assays should also contribute to higher IC(50)s. The modified double cocktail assay was found to overcome limitations of the other two assays. It uses an rhCYP isozymes mix, drug-probe substrate mix, low protein concentration, and LC-MS detection. The double cocktail assay is sensitive, selective, and high throughout for use in drug discovery to provide an early alert to potential toxicity with regard to drug-drug interaction, prioritize chemical series, and guide structural modification to circumvent CYP450 inhibition.

Risk Assessment for Drug-Drug Interaction Caused by Metabolism-Based Inhibition of CYP3A Using Automated in Vitro Assay Systems and Its Application in the Early Drug Discovery Process

The CYP3A family is a major drug metabolism enzyme in humans. Metabolism-based inhibition of CYP3A might cause clinically significant drug-drug interactions (DDIs). To assess the risk of DDIs caused by metabolism-based inhibition (MBI) of CYP3A, we established an automated single time- and concentration-dependent inhibition assay. To create a diagram to assess DDI risk of compounds in the early discovery stage, we classified 171 marketed drugs by the possibility of the occurrence of in vivo DDI caused by MBI from the relationship between the inactivation activity determined in the MBI screening, the therapeutic blood or plasma concentration, and the in vivo DDI information. This analysis revealed that the DDI risk depends on both the MBI potential and the blood concentration of a compound, and provided the criteria of the DDI risk. In the assay, three compounds (midazolam, nifedipine, and testosterone) were compared as CYP3A probe substrates. The results show that the evaluation for MBI does not depend on the probe substrates used in the assay. In addition, we established an automated assay to distinguish quasi-irreversible and irreversible binding to CYP3A in which the quasi-irreversible inhibitors such as diltiazem, verapamil, and nicardipine were dissociated from CYP3A by the addition of potassium ferricyanide, whereas the irreversible inhibitors such as clozapine, delavirdine, and mibefradil were not. It provides useful information related to chemical structures likely to cause MBI. By using these MBI assays supported by an extensive database of marketed compounds, a systematic MBI evaluation paradigm was established and has been incorporated into our drug discovery process.

Reduction of Site-Specific CYP3A-Mediated Metabolism for Dual Angiotensin and Endothelin Receptor Antagonists in Various in Vitro Systems and in Cynomolgus Monkeys

2-{Butyryl-[2'-(4,5-dimethyl-isoxazol-3-ylsulfamoyl)-biphenyl-4-ylmethyl]-amino}-N-isopropyl-3-methyl-butyramide (BMS-1) is a potent dual acting angiotensin-1 and endothelin-A receptor antagonist. The compound was subject to rapid metabolic clearance in monkey and human liver microsomes and exhibited low systemic exposure and marked interanimal variability in cynomolgus monkeys after p.o. administration. The variability pattern was identical to that of midazolam given p.o. in the same monkeys, as measured by area under the curve and Cmax values, suggesting that CYP3A-mediated metabolism might play a role in the rapid clearance and observed interanimal variability. Subsequent in vitro metabolism studies using human liver microsomes and cDNA-expressed human cytochrome P450 (P450) enzymes revealed that BMS-1 was a CYP3A4 substrate and was not metabolized by other human P450 enzymes. Mass spectral and NMR analyses of key metabolites led to the identification of the dimethyl isoxazole group as a major metabolic soft spot for BMS-1. Replacement of the 4-methyl group on the isoxazole ring with halogens not only improved overall metabolic stability but also decreased CYP3A-mediated hydroxylation of the isoxazole 5-methyl group. As exemplified by 2-{butyryl-[2'-(4-fluoro-5-methyl-isoxazol-3-ylsulfamoyl)-biphenyl-4-ylmethyl]-amino}-N-isopropyl-3-methyl-butyramide (BMS-3), a fluorinated analog of BMS-1, the structural modification resulted in an increase in the systemic exposure relative to previous analogs and a dramatic reduction in interanimal variability in the monkeys after p.o. administration. In addition, BMS-3 could be metabolized by both CYP2C9 and CYP3A4, thus avoiding the reliance on a single P450 enzyme for metabolic clearance. Integration of results obtained from in vitro metabolism studies and in vivo pharmacokinetic evaluations enabled the modulation of site-specific CYP3A-mediated metabolism, yielding analogs with improved overall metabolic profiles.

Human pregnane X receptor: genetic polymorphisms, alternative mRNA splice variants, and cytochrome P450 3A metabolic activity

Human pregnane X receptor (hPXR) gene polymorphisms (spanning exon 2 to exon 5) and alternative mRNA splicing were investigated as possible contributors to individual variability in CYP3A metabolic activity measured both in vivo and in vitro. None of the 9 variants evaluated, including the 2 most common nonsynonymous variants (Pro27Ser and Gly36Arg), was found to be associated with midazolam 1'-hydroxylation rate measured in a bank of human livers (48 European Americans, 4 African Americans, 2 Hispanics). In contrast, 3 linked hPXR variants (g.252A > G, g.275A > G, and g.4760G > A) were significantly (P < .05) associated with oral midazolam clearance in a mixed race/ethnicity population (n = 26) and the African American subpopulation (n = 14) but not in European Americans (n = 9). Although the amount of hPXR mRNA normally spliced at the exon 4-5 junction correlated well with midazolam 1'-hydroxylation activities (P < .05), none of the 6 hPXR mRNA splice variants identified was associated with midazolam 1'-hydroxylation. In conclusion, several hPXR polymorphisms have been identified that may have predictive value for oral midazolam clearance, particularly in African Americans.