The species differences of intestinal drug absorption and first-pass metabolism between cynomolgus monkeys and humans

In order to elucidate the causes of the species differences in the oral bioavailability (BA) between cynomolgus monkeys and humans, the contributions of first-pass metabolism and intestinal absorption were investigated. Typical substrates of cytochrome P450 enzymes, UDP-glucuronosyltransferase enzymes and efflux transporters were selected, and the BA, the hepatic availability (Fh) and the fraction dose absorbed from gastro-intestinal tract (Fa*Fg) were calculated from pharmacokinetic analysis after oral and intravenous administration in cynomolgus monkeys. In addition, in vitro metabolism was investigated using liver and intestinal microsomes to evaluate the relationship between in vivo and in vitro results. The BA of cynomolgus monkeys was low compared with that in humans with most of the drugs tested, and not only Fh but also Fa*Fg contributed significantly to the low BA in cynomolgus monkeys. When Fh was evaluated in in vitro experiments, it correlated well with the in vivo Fh. However, although the metabolic activities of CYP3A4 substrates were high in cynomolgus monkey intestinal microsomes, those of the other substrates were low or not detected. These findings suggested that the species differences and low BA in cynomolgus monkeys could be mostly attributed not only to hepatic first-pass metabolism but also to the intestinal absorption process. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4343–4353, 2009     

Predicting drug-drug interactions involving the inhibition of intestinal CYP3A4 and P-glycoprotein

Recently, interest has grown in drug-drug interactions (DDIs) involving the inhibition of intestinal CYP3A4, P-glycoprotein (P-gp), and other drug efflux transporters. The criteria for intestinal DDIs are described in the draft guidances of the US Food and Drug Administration (FDA) and European Medicines Agency (EMA). Substrate drugs with small fraction absorbed (Fa) and/or low intestinal availability (Fg) as a result of intestinal efflux transport and metabolism are important as "victim" drugs because these substrates are likely to show considerable interactions. The susceptibility of a victim drug to intestinal interactions can be evaluated from its FaFg. In this review, methods for estimating the FaFg of substrate drugs are discussed. The nonlinear pharmacokinetics of substrate drugs caused by the saturation of intestinal CYP3A4/P-gp is also discussed. The methods for predicting intestinal DDIs caused by inhibitor drugs are then summarized. Because the prediction accuracy of intestinal DDIs also depends on the inhibition constant (Ki) estimated in in vitro studies, these in vitro methods of estimating Ki are also discussed. Standardized methods for predicting intestinal DDIs have not yet been established. Further studies are required to establish more accurate and standardized prediction methods.     

Impact of curcumin-induced changes in P-glycoprotein and CYP3A expression on the pharmacokinetics of peroral celiprolol and midazolam in rats.

The aim of this study was to evaluate whether curcumin could modulate P-glycoprotein (P-gp) and CYP3A expression, and in turn modify the pharmacokinetic profiles of P-gp and CYP3A substrates in male Sprague-Dawley rats. Intragastric gavage of the rats with 60 mg/kg curcumin for 4 consecutive days led to a down-regulation of the intestinal P-gp level. There was a concomitant upregulation of hepatic P-gp level, but the renal P-gp level was unaffected. Curcumin also attenuated the CYP3A level in the small intestine but induced CYP3A expression in the liver and kidney. Regular curcumin consumption also caused the C(max) and area under the concentration-time curve (AUC(0-8) and total AUC) of peroral celiprolol (a P-gp substrate with negligible cytochrome P450 metabolism) at 30 mg/kg to increase, but the apparent oral clearance (CL(oral)) of the drug was reduced. Similarly, rats treated with curcumin for 4 consecutive days showed higher AUC (AUC(0-4) and total AUC) and lower CL(oral) for peroral midazolam (a CYP3A substrate that does not interact with the P-gp) at 20 mg/kg in comparison with vehicle-treated rats. In contrast, curcumin administered 30 min before the respective drug treatments did not significantly modify the pharmacokinetic parameters of the drugs. Analysis of the data suggests that the changes in the pharmacokinetic profiles of peroral celiprolol and midazolam in the rat model were contributed mainly by the curcumin-mediated down-regulation of intestinal P-gp and CYP3A protein levels, respectively.     

Prediction of Nonlinear Intestinal Absorption of CYP3A4 and P-Glycoprotein Substrates from their <Emphasis Type="Italic">In Vitro</Emphasis> Km Values

Purpose  

CYP3A4 and P-glycoprotein (P-gp) are present in the human intestine and mediate intestinal first-pass metabolism and the efflux of oral drugs, respectively. We aimed to predict whether intestinal CYP3A4/P-gp is saturated in a therapeutic dose range.     

Characterization of Intestinal and Hepatic CYP3A-Mediated Metabolism of Midazolam in Children Using a Physiological Population Pharmacokinetic Modelling Approach

Purpose

Changes in drug absorption and first-pass metabolism have been reported throughout the pediatric age range. Our aim is to characterize both intestinal and hepatic CYP3A-mediated metabolism of midazolam in children in order to predict first-pass and systemic metabolism of CYP3A substrates.

Methods

Pharmacokinetic (PK) data of midazolam and 1-OH-midazolam from 264 post-operative children 1–18 years of age after oral administration were analyzed using a physiological population PK modelling approach. In the model, consisting of physiological compartments representing the gastro-intestinal tract and liver,intrinsic intestinal and hepatic clearances were estimated to derive values for bioavailability and plasma clearance.

Results

The whole-organ intrinsic clearance in the gut wall and liver were found to increase with body weight, with a 105 (95% confidence interval (CI): 5–405) times lower intrinsic gut wall clearance than the intrinsic hepatic clearance (i.e. 5.08 L/h (relative standard error (RSE) 10%) versus 527 L/h (RSE 7%) for a 16 kg individual, respectively). When expressed per gram of organ, intrinsic clearance increases with increasing body weight in the gut wall, but decreases in the liver, indicating that CYP3A-mediated intrinsic clearance and local bioavailability in the gut wall and liver do not change with age in parallel. The resulting total bioavailability was found to be age-independent with a median of 20.8% in children (95%CI: 3.8–50.0%).

Conclusion

In conclusion, the intrinsic CYP3A-mediated gut wall clearance is substantially lower than the intrinsic hepatic CYP3A-mediated clearance in children from 1 to 18 years of age, and contributes less to the overall first-pass metabolism compared to adults.

     

Enzyme-catalyzed processes of first-pass hepatic and intestinal drug extraction

Oral bioavailability of pharmacologically effective drugs is often limited by first-pass biotransformation. In humans, both hepatic and intestinal enzymes can catalyze the metabolism of a drug as it transits between the gastrointestinal lumen and systemic blood for the first time. Although a spectrum of drug biotransformations can occur during first-pass, the most common are oxidations catalyzed by cytochromes P450. It is the isozymes CYP2D6, CYP3A4, CYP1A2, CYP2C9 and CYP2C19 that are most often implicated in first-pass drug elimination. For any given substrate, enzyme specificity, enzyme content, substrate binding affinity and sensitivity to irreversible catalytic events all play a role in determining the overall efficiency, or intrinsic clearance, of elimination. Several models have been proposed over the past twenty-five years that mathematically describe the process of drug extraction across the liver. The most widely used, the well-stirred model, has also been considered for depiction of first-pass drug elimination across the intestinal wall. With these models it has been possible to examine sources of interindividual variability in drug bioavailability including, variable constitutive enzyme expression (both genetic and environmentally determined), enzyme induction by drugs, disease and diet, and intrinsic or acquired differences in plasma protein binding and organ blood flow (food and drug effects). In recent years, the most common application of hepatic clearance models has been the determination of maximum organ availability of a drug from in vitro derived estimates of intrinsic metabolic clearance. The relative success of the in vitro-in vivo approach for both low and highly extracted drugs has led to a broader use by the drug industry for a priori predictions as part of the drug selection process. A considerable degree of effort has also been focused on gut wall first-pass metabolism. Important pathways of intestinal Phase II first-pass metabolism include the sulfation of terbutaline and isoproterenol and glucuronidation of morphine and labetalol. It is also clear that some of the substrates for CYP3A4 (e.g., cyclosporine, midazolam, nifedipine, verapamil and saquinavir) undergo significant metabolic extraction by the gut wall. For example, the first-pass extraction of midazolam by the intestinal mucosa appears, on average, to be comparable to extraction by the liver. However, many other CYP3A substrates do not appear susceptible to a gut wall first-pass, possibly because of enzyme saturation during first-pass or a limited intrinsic metabolic clearance. Both direct biochemical and indirect in vivo clearance data suggest significant inter-individual variability in gut wall CYP3A-dependent metabolism. The source of this constitutive variability is largely unknown. Because of their unique anatomical location, enzymes of the gut wall may represent an important and highly sensitive site of metabolically-based interactions for orally administered drugs. Again, interindividual variability may make it impossible to predict the likelihood of an interaction in any given patient. Hopefully, though, newer models for studying human gut wall metabolic extraction will provide the means to predict the average extraction ratio and maximum first-pass availability of a putative substrate, or the range of possible inhibitory or inductive changes for a putative inhibitor/inducer.     

Role of P-glycoprotein in the hepatic metabolism of tacrolimus

The main objective was to determine the potential effect of P-glycoprotein (P-gp) modulation on hepatic metabolism of tacrolimus, a P-gp and cytochrome P450(CYP)3A4 substrate, and to investigate various potential factors that may contribute to the interaction between P-gp and CYP. An isolated perfused rat liver system was used to study the hepatic disposition of tacrolimus in the presence of a P-gp inhibitor, GF120918, and a comprehensive pharmacokinetic analysis was conducted. GF120918 significantly decreased mean intrinsic metabolic clearance (by 86 and 41% based on the well-stirred and tube models, respectively) as well as hepatic clearance (from 47.3 to 44.2 ml min(-1)). Potential factors that might contribute to these observations, such as the effects of GF120918 on hepatic metabolism or on distribution of tacrolimus, were investigated and found to be negligible. In conclusion, it was shown that P-gp inhibition by GF120918 reduces hepatic clearances of its substrate drugs although the mechanism is yet to be determined.     

Unchanged cytochrome P450 3A (CYP3A) expression and metabolism of midazolam, triazolam, and dexamethasone in mdr(-/-) mouse liver microsomes

P-Glycoprotein (P-gp) and cytochrome P450 3A (CYP3A) share common substrates and expression properties, but the relationship of mdrl deficiency to CYP3A-mediated metabolism and protein expression is not established. The in vitro kinetic parameters of CYP3A-mediated metabolism of midazolam (MDZ), triazolam (TRZ), and dexamethasone (DEX) were studied in liver microsomes from three mrdrla(-/-) mice, one mdrla/b(-/-) mouse, and mdrla/b(+/+) controls. The kinetic profiles of CYP3A-mediated MDZ 4-hydroxylation were not significantly different between mdrl-deficient animals and controls. Overall mean (+/- SEM, N = 8) values were: Vmax, 0.74+/-0.05 nmol/min/mg protein; Km, 28.2+/-2.7 microM; and estimated intrinsic clearance, 0.026+/-0.003 mL/min/mg protein. Likewise, rates of formation of alpha-OH- and 4-OH-TRZ (from 500 microM TRZ), and of DEX metabolites sensitive to ketoconazole inhibition, M1 and M5 (from 20 microM DEX), did not differ between mdrl-deficient and control animals. Immunoquantified microsomal CYP3A protein levels in mdrla(-/-), mdrla/b(-/-), and mdrla/b(+/+) mice were not different, with overall mean immunoreactive protein levels of 2.68+/-0.09 pmol/microg protein. Although CYP3A and P-gp share aspects of activity and expression, disruption of the mdrl genes does not affect CYP3A-mediated metabolism or protein expression in the mouse.     

Interplay of metabolism and transport in determining oral drug absorption and gut wall metabolism: a simulation assessment using the "Advanced Dissolution, Absorption, Metabolism (ADAM)" model

Bioavailability of orally administered drugs can be influenced by a number of factors including release from the formulation, dissolution, stability in the gastrointestinal (GI) environment, permeability through the gut wall and first-pass gut wall and hepatic metabolism. Although there are various enzymes in the gut wall which may contribute to gut first pass metabolism, Cytochrome P450 (CYP) 3A has been shown to play a major role. The efflux transporter P-glycoprotein (P-gp; MDR1/ABCB1) is the most extensively studied drug efflux transporter in the gut and might have a significant role in the regulation of GI absorption. Although not every CYP3A substrate will have a high extent of gut wall first-pass extraction, being a substrate for the enzyme increases the likelihood of a higher first-pass extraction. Similarly, being a P-gp substrate does not necessarily pose a problem with the gut wall absorption however it may reduce bioavailability in some cases (e.g. when drug has low passive permeability). An on-going debate has focused on the issue of the interplay between CYP3A and P-gp such that high affinity to P-gp increases the exposure of drug to CYP3A through repeated cycling via passive diffusion and active efflux, decreasing the fraction of drug that escapes first pass gut metabolism (F(G)). The presence of P-gp in the gut wall and the high affinity of some CYP3A substrates to this transporter are postulated to reduce the potential for saturating the enzymes, thus increasing gut wall first-pass metabolism for compounds which otherwise would have saturated CYP3A. Such inferences are based on assumptions in the modelling of oral drug absorption. These models should be as mechanistic as possible and tractable using available in vitro and in vivo information. We review, through simulation, this subject and examine the interplay between gut wall metabolism and efflux transporters by studying the fraction of dose absorbed into enterocytes (F(a)) and F(G) via systematic variation of drug characteristics, in accordance with the Biopharmaceutics Classification System (BCS) within one of the most physiological models of oral drug absorption currently available, respectively ADAM. Variables studied included the intrinsic clearance (CLint) and the Michaelis-Menten Constant (Km) for CYP3A4 and P-gp (C(Lint-CYP3A4) and K(m-CYP3A4), CL(int-P-gp) and K(m-P-gp)). The impact of CYP3A4 and P-gp intracellular topography were not investigated since a well-stirred enterocyte is assumed within ADAM. An increased CLint-CYP3A4 resulted in a reduced F(G) whereas an increase in C(Lint-P-gp) resulted in a reduced F(a), but interestingly decreased F(G) too. The reduction in FG was limited to certain conditions and was modest. Non-linear relationships between various parameters determining the permeability (e.g. P(app), C(Lint-P-gp,) and K(m-P-gp)) and gut wall metabolism (e.g. C(Lint-CYP3A4,) K(m-CYP3A4)) resulted in disproportionate changes in F(G) compared to the magnitude of singular effects. The results suggest that P-gp efflux decreases enterocytic drug concentration for drugs given at reasonably high dose which possess adequate passive permeability (high P(app)), by de-saturating CYP3A4 in the gut resulting in a lower F(G). However, these findings were observed only in a very limited area of the parameters space matching very few therapeutic drugs (a group with very high metabolism, high turn-over by efflux transporters and low F(a)). The systematic approach in this study enabled us to recognise the combination of parameters values where the potential interplay between metabolising enzymes and efflux transporters is expected to be highest, using a realistic range of parameter values taken from an intensive literature search.     

Effect of dexamethasone on the intestinal first-pass metabolism of indinavir in rats: evidence of cytochrome P-450 3A [correction of P-450 A] and p-glycoprotein induction .

Indinavir, a potent and specific inhibitor of HIV protease, is a known substrate of cytochrome P-450 (CYP) 3A and p-glycoprotein. The purpose of this study is to investigate and compare the inducing effect of dexamethasone (DEX) on CYP3A and p-glycoprotein in the hepatic and intestinal first-pass metabolism of indinavir in rats. Pretreatment of rats with DEX had little effect on the pharmacokinetics (Cl and T(1/2)) after i.v. administration of indinavir, whereas DEX markedly altered the peak concentration (C(max)) and bioavailability of indinavir after oral dosing. The C(max) decreased from 2.8 microM in control rats to 0.28 microM in DEX-treated rats, and bioavailability decreased from 28 to 12.4%. The decreased bioavailability after DEX pretreatment was due mainly to an increase in first-pass metabolism. Intestinal first-pass metabolism (E(G)) increased from 6% in control rats to 34% in DEX-treated rats, and hepatic first-pass metabolism (E(H)) increased from 65 to 82%. Analysis of in vitro kinetic data revealed that the increased intestinal and hepatic metabolism by DEX was attributed to an increase in the V(max), as a result of CYP3A induction, without a significant change in the K(m) values. DEX pretreatment also induced p-glycoprotein in the intestine and liver of rats. p-Glycoprotein appeared to increase the intestinal metabolism of indinavir whereas it had little effect on the hepatic metabolism of indinavir. Although it has been suggested that the role of intestinal metabolism for some drugs is quantitatively greater than that of hepatic metabolism in the overall first-pass metabolism, the contribution of intestinal metabolism to the overall first-pass metabolism of indinavir in rats is not quantitatively as important as the hepatic metabolism, regardless of DEX induction.     

Do men and women differ in proximal small intestinal CYP3A or P-glycoprotein expression?

The higher systemic clearance of some CYP3A4 [whether also P-glycoprotein (P-gp)] drug substrates in women versus men is attributed in part to a higher hepatic CYP3A4 content in women. This, combined with the general paucity of reported sex differences in the apparent oral clearance of CYP3A4 substrates, suggested a sex-dependent expression of CYP3A4 in the intestine, but in a pattern opposite to that in the liver. Accordingly, duodenal biopsies obtained from healthy men (n = 46) and women (n = 45) were analyzed, by Western blot, for relative CYP3A4, as well as for CYP3A5 and P-gp, expression levels. Among all subjects, CYP3A4 and P-gp varied 8- and 10-fold, respectively. CYP3A5, which was readily detected in 27% of these predominantly white individuals, varied 7-fold. For all three proteins, a sex difference was not detected (p >/= 0.55). The lack of a difference remained for CYP3A4 and P-gp when the analysis was restricted to white individuals (n = 74) or to individuals with undetectable CYP3A5. Comparing the 21 premenopausal women (all were aged <45 years) with the 43 men aged <45 years, again no sex differences were detected in CYP3A4 and P-gp. Comparing the pre- with postmenopausal women, mean CYP3A4 content was 20% lower in the postmenopausal individuals (p = 0.01). The lack of a sex-dependent difference in proximal intestinal CYP3A4 could account, in part, for the lack of reported sex differences in the oral, relative to systemic, clearance of some CYP3A4 substrates. Ramifications of lower intestinal CYP3A4 content in post- versus premenopausal women require further investigation.     

Quantitative contribution of CYP2D6 and CYP3A to oxycodone metabolism in human liver and intestinal microsomes.

Oxycodone undergoes N-demethylation to noroxycodone and O-demethylation to oxymorphone. The cytochrome P450 (P450) isoforms capable of mediating the oxidation of oxycodone to oxymorphone and noroxycodone were identified using a panel of recombinant human P450s. CYP3A4 and CYP3A5 displayed the highest activity for oxycodone N-demethylation; intrinsic clearance for CYP3A5 was slightly higher than that for CYP3A4. CYP2D6 had the highest activity for O-demethylation. Multienzyme, Michaelis-Menten kinetics were observed for both oxidative reactions in microsomes prepared from five human livers. Inhibition with ketoconazole showed that CYP3A is the high affinity enzyme for oxycodone N-demethylation; ketoconazole inhibited >90% of noroxycodone formation at low substrate concentrations. CYP3A-mediated noroxycodone formation exhibited a mean K(m) of 600 +/- 119 microM and a V(max) that ranged from 716 to 14523 pmol/mg/min. Contribution from the low affinity enzyme(s) did not exceed 8% of total intrinsic clearance for N-demethylation. Quinidine inhibition showed that CYP2D6 is the high affinity enzyme for O-demethylation with a mean K(m) of 130 +/- 33 microM and a V(max) that ranged from 89 to 356 pmol/mg/min. Activity of the low affinity enzyme(s) accounted for 10 to 26% of total intrinsic clearance for O-demethylation. On average, the total intrinsic clearance for noroxycodone formation was 8 times greater than that for oxymorphone formation across the five liver microsomal preparations (10.5 microl/min/mg versus 1.5 microl/min/mg). Experiments with human intestinal mucosal microsomes indicated lower N-demethylation activity (20-50%) compared with liver microsomes and negligible O-demethylation activity, which predict a minimal contribution of intestinal mucosa in the first-pass oxidative metabolism of oxycodone.     

Effect of oral ketoconazole on intestinal first-pass effect of midazolam and fexofenadine in cynomolgus monkeys.

Because the expression of drug-metabolizing enzymes and drug efflux transporters has been shown in the intestine, the contribution of this tissue to the first-pass effect has become of significant interest. Consequently, a comprehensive understanding of the absorption barriers in key preclinical species would be useful for the precise characterization of drug candidates. In the present investigation, we evaluated the intestinal first-pass effect of midazolam (MDZ) and fexofenadine (FEX), typical substrates for CYP3A and P-glycoprotein (P-gp), respectively, with ketoconazole (KTZ) as a potent dual CYP3A/P-gp inhibitor in cynomolgus monkeys. When MDZ or FEX was administered i.v. at doses of 0.3 or 1 mg/kg, respectively, the plasma concentration-time profiles were not influenced by p.o. coadministration of KTZ (20 mg/kg). On the other hand, when MDZ or FEX was administered p.o. at doses of 1 or 5 mg/kg, respectively, concomitant with a dose p.o. of KTZ (20 mg/kg), significant increases were observed in the area under the plasma concentration-time curves of MDZ or FEX (22-fold in MDZ and 3-fold in FEX). These findings indicate that both CYP3A and P-gp play a key role in the intestinal barrier and that inhibition of intestinal CYP3A/P-gp activities contributes exclusively toward the drug-drug interactions (DDI) with KTZ. Additionally, the K(i) values of the antifungal agents, KTZ, itraconazole, and fluconazole, for MDZ 1'-hydroxylation in monkey intestinal and liver microsomes were comparable with those in the respective human samples. These results suggest that monkeys may be an appropriate animal species for evaluating the intestinal first-pass effect of p.o. administered drugs and predicting intestinal DDI related to CYP3A4 and P-gp in humans.     

Unmasking the dynamic interplay between efflux transporters and metabolic enzymes

Drug efflux by intestinal P-glycoprotein (P-gp) is known to decrease the bioavailability of many CYP3A4 substrates. We have demonstrated that the interplay between P-gp and CYP3A4 at the apical intestinal membrane can increase the opportunity for drug metabolism by determining bidirectional extraction ratios across CYP3A4-transfected Caco-2 cells for two dual P-gp/CYP3A4 substrates, K77 (an experimental cysteine protease inhibitor) and sirolimus, as well as two negative control, CYP3A4 only substrates, midazolam and felodipine. Studies were carried out under control conditions, with a P-gp inhibitor (GG918) and with a dual inhibitor (cyclosporine). Measurement of intracellular concentration changes is an important component in calculating the extraction ratios. We hypothesize that the inverse orientation of P-gp and CYP3A4 in the liver will result in an opposite interactive effect in that organ. In vivo rat intestinal perfusion studies with K77 and rat liver perfusion studies with tacrolimus under control conditions and with inhibitors of CYP3A4 (troleandomycin), P-gp (GG918) and both CYP3A4/P-gp (cyclosporine) lend support to our hypotheses. These results serve as a template for predicting enzyme-transporter (both absorptive and efflux) interactions in the intestine and the liver.     

Prediction methods of drug-drug interactions of non-oral CYP3A4 substrates based on clinical interaction data after oral administrations – Validation with midazolam,alfentanil, and verapamil after intravenous administration and prediction for blonanserin transdermal patch

Drug-drug interactions (DDI) have been examined for various drugs for oral use, but less for non-oral applications. This study provides DDI prediction methods for non-orally administered CYP3A4 substrates based on clinical DDI data of oral dosages. Gut availability (F_g) and fraction contribution of CYP3A4 to hepatic intrinsic clearance (fm_CYP3A4) were predicted by AUC ratio (AUCR) in oral DDI study with/without grapefruit juice, and alteration in intrinsic clearances with/without ketoconazole, respectively. AUCRs of non-orally administered CYP3A4 substrates with/without inhibitors or inducers were predicted with the estimated F_g, fm_CYP3A4 and changes in liver CYP3A4 activities with inhibitors/inducers predicted using Simcyp library. DDIs of intravenously administered midazolam and alfentanil with CYP3A4 inhibitors/inducers could be predicted well by this method with predicted AUCRs within ±64% of observed values. Moreover, maximum DDIs with strong CYP3A4 inducers could be predicted by comparing hepatic clearance with hepatic blood flow, as hepatic blood flow indicates the possible maximum hepatic clearance after strong enzyme induction. Predicted AUCRs of midazolam, alfentanil and R- and S-verapamil were less than, but not far from observed ratios, suggesting good conservative prediction. These methods were applied to blonanserin transdermal patch, suggesting much smaller interaction with CYP3A4 inhibitors/inducers compared to oral dosage of blonanserin.     

A new approach to predicting human hepatic clearance of CYP3A4 substrates using monkey pharmacokinetic data

1. Focusing on the genetic similarity of CYP3A subfamily enzymes (CYP3A4 and CYP3A5) between monkeys and humans, we have attempted to provide a single-species approach to predicting human hepatic clearance (CL_h) of CYP3A4 substrates using pharmacokinetic parameters in cynomolgus monkeys following intravenous administrations.

2. Hepatic intrinsic clearance (CL_int,h) of six CYP3A4 substrates (alprazolam, clonazepam, diltiazem, midazolam, nifedipine, and quinidine), covering a wide range of clearance, in monkeys correlated well with that cited in literature for humans (R = 0.90) with a simple equation of Y = 0.165X (Y: human CL_int,h, X: monkey CL_int,h, represented in mL/min/kg).

3. To verify the predictability of human CL_int,h, monkey CL_int,h of a test set of CYP3A4 substrates cited in literature (dexamethasone, nifedipine, midazolam, quinidine, tacrolimus, and verapamil) was applied to the equation and human CL_int,h was calculated. The human CL_int,h of all the substrates was predicted within 3-fold error (fold error: 0.35–2.77).

4. The predictability of human CL_h by our method was superior to common in vivo prediction methods (allometry and liver blood flow method). These results suggest that human hepatic clearance of CYP3A4 substrates can be predicted by applying cynomolgus monkey CL_int,h obtained following intravenous administrations in each laboratory to the simple equation.     

Faster clearance of omeprazole in rats with acute renal failure induced by uranyl nitrate: contribution of increased expression of hepatic cytochrome P450 (CYP) 3A1 and intestinal CYP1A and 3A subfamilies

It has been reported that omeprazole is mainly metabolized via hepatic cytochrome P450 (CYP) 1A1/2, CYP2D1 and CYP3A1/2 in male Sprague-Dawley rats, and the expression of hepatic CYP3A1 is increased in male Sprague-Dawley rats with acute renal failure induced by uranyl nitrate (U-ARF rats). Thus, the metabolism of omeprazole would be expected to increase in U-ARF rats. After intravenous administration of omeprazole (20 mgkg(-1)) to U-ARF rats, the area under the plasma concentration-time curve from time zero to infinity (AUC) was significantly reduced (371 vs 494 microg min mL(-1)), possibly due to the significantly faster non-renal clearance (56.6 vs 41.2 mL min(-1) kg(-1)) compared with control rats. This could have been due to increased expression of hepatic CYP3A1 in U-ARF rats. After oral administration of omeprazole (40 mgkg(-1)) to U-ARF rats, the AUC was also significantly reduced (89.3 vs 235 microg min mL(-1)) compared with control rats. The AUC difference after oral administration (62.0% decrease) was greater than that after intravenous administration (24.9% decrease). This may have been primarily due to increased intestinal metabolism of omeprazole caused by increased expression of intestinal CYP1A and 3A subfamilies in U-ARF rats, in addition to increased hepatic metabolism.     

Methadone metabolism and clearance are induced by nelfinavir despite inhibition of cytochrome P4503A (CYP3A) activity

Background

Methadone plasma concentrations are decreased by nelfinavir. Methadone clearance and the drug interactions have been attributed to CYP3A4, but actual mechanisms of methadone clearance and the nelfinavir interaction are unknown. We assessed nelfinavir effects on methadone pharmacokinetics and pharmacodynamics, intestinal and hepatic CYP3A4/5 activity, and intestinal P-glycoprotein transport activity. CYP3A4/5 and transporters were assessed using alfentanil and fexofenadine, respectively.

Methods

Twelve healthy HIV-negative volunteers underwent a sequential crossover. On three consecutive days they received oral alfentanil plus fexofenadine, intravenous alfentanil, and intravenous plus oral methadone. This was repeated after nelfinavir. Plasma and urine analytes were measured by mass spectrometry. Opioid effects were measured by pupil diameter change (miosis).

Results

Nelfinavir decreased intravenous and oral methadone plasma concentrations 40–50%. Systemic clearance, hepatic clearance, and hepatic extraction all increased 1.6- and 2-fold, respectively, for R- and S-methadone; apparent oral clearance increased 1.7- and 1.9-fold. Nelfinavir stereoselectively increased (S > R) methadone metabolism and metabolite formation clearance, and methadone renal clearance. Methadone bioavailability and P-glycoprotein activity were minimally affected. Nelfinavir decreased alfentanil systemic and apparent oral clearances 50 and 76%, respectively. Nelfinavir appeared to shift the methadone plasma concentration–effect (miosis) curve leftward and upward.

Conclusions

Nelfinavir induced methadone clearance by increasing renal clearance, and more so by stereoselectively increasing hepatic metabolism, extraction and clearance. Induction occurred despite 50% inhibition of hepatic CYP3A4/5 activity and more than 75% inhibition of first-pass CYP3A4/5 activity, suggesting little or no role for CYP3A in clinical methadone disposition. Nelfinavir may alter methadone pharmacodynamics, increasing clinical effects.     

An integrated pharmacokinetic model for the influence of CYP3A4 expression on the in vivo disposition of lopinavir and its modulation by ritonavir

Lopinavir, a human immunodeficiency virus protease inhibitor, has a very low oral bioavailability, which can be enhanced with a low dose of the CYPA4 inhibitor ritonavir. Our aim was to separately quantify the role of intestinal and hepatic cytochrome P450 3A (CYP3A4) expression on lopinavir disposition in a novel mouse model. Lopinavir and ritonavir were administered to mice selectively expressing human CYP3A4 in the intestine and/or liver. Using nonlinear mixed-effects modeling, we could separately quantify the effects of intestinal CYP3A4 expression, hepatic CYP3A4 expression, and the presence of ritonavir on both the absorption and elimination of lopinavir, which was previously not possible using noncompartmental methods. Intestinal, but not hepatic, CYP3A4-related first-pass metabolism was the major barrier for systemic entry of lopinavir. Relative oral bioavailability of lopinavir in mice expressing both hepatic and intestinal CYP3A4 was only 1.3% when compared with mice that were CYP3A deficient. In presence of ritonavir, relative bioavailability increased to 9.5% due to inhibiton of intestinal, but not due to inhibition of hepatic first-pass metabolism. Hepatic CYP3A4 related systemic clearance was inversely related to ritonavir exposure and not only hepatic but also intestinal CYP3A4 expression contributed to systemic clearance of lopinavir.     

Hepatic,gastric, and intestinal first-pass effects of vitexin in rats

Context: Recent research has demonstrated that vitexin exhibits a prominent first-pass effect. In this light, it is necessary to investigate the causes of this distinct first-pass effect.

Objective: The aim of this study was to evaluate hepatic, gastric, and intestinal first-pass effects of vitexin in rats and, furthermore, to investigate the role of P-glycoprotein (P-gp) and cytochrome P450 3A (CYP3A) in the absorption and secretion of vitexin in the duodenum.

Materials and methods: Vitexin was infused into rats intravenously, intraportally, intraduodenally, and intragastrically (30?mg/kg). In addition, verapamil (50?mg/kg), a common substrate/inhibitor of P-gp and CYP3A, was also instilled with vitexin into the duodenum to investigate the regulatory action of P-gp and CYP3A. The plasma concentrations of vitexin were measured by the HPLC method using hesperidin as an internal standard.

Results: The hepatic, gastric, and intestinal first-pass effects of vitexin in rats were 5.2%, 31.3%, and 94.1%, respectively. In addition, the total area under the plasma concentration–time curve from zero to infinity (AUC) of the vitexin plus verapamil group and of the normal saline group was 44.9 and 39.8?μg??min/mL, respectively.

Discussion and conclusion: The intestinal first-pass effect of vitexin was considerable, and gastric and hepatic first-pass effects also contribute to the low absolute oral bioavailability of vitexin. The AUC of the vitexin plus verapamil group was slightly higher than that of the vitexin plus normal saline group (by approximately 1.13-fold), suggesting that verapamil does not play an important role in the absorption and secretion of vitexin.