Interrelationship between substrates and inhibitors of human CYP3A and P-glycoprotein

Hepatic disposition of fexofenadine: influence of the transport inhibitors erythromycin and dibromosulphothalein

PURPOSE

To examine the disposition of fexofenadine in the isolated perfused rat liver and the influence of erythromycin and dibromosulphthalein (DBSP) on the hepatic uptake and biliary excretion of fexofenadine.

METHODS

Livers from four groups of rats were perfused in a recirculatory manner with fexofenadine HCl added as a bolus (125, 250, 500, or 1000 microg) to perfusate. Livers from another three groups of rats were perfused with 250 microg of fexofenadine HCl. With one group as control, erythromycin (4.0 microg/ml) or DBSP (136 microg/ml) was added to the perfusate of the other groups. In all experiments, perfusate and bile were collected for 60 min; in addition, livers from the second experiment were retained for assay. Fexofenadine was determined in perfusate, bile, and homogenized liver by HPLC.

RESULTS

The area under the curve (AUC) of fexofenadine was linearly related to concentration. It was unchanged from control (12,800 +/- 200 ng x h/ml) by erythromycin (14,400 +/- 2000 ng x h/ml), but was increased 95% by DBSP (25,000 +/- 2600 ng x h/ml, P <0.001). The ratios of the concentrations of fexofenadine in liver/perfusate were decreased significantly by DBSP; those for bile/liver were increased by erythromycin.

CONCLUSIONS

Erythromycin reduced the canalicular transport of fexofenadine into bile, whereas DBSP reduced uptake across the sinusoidal membrane.

Transporter-mediated permeation of drugs across the blood-brain barrier

Drug distribution into the brain is strictly regulated by the presence of the blood-brain barrier (BBB) that is formed by brain capillary endothelial cells. Since the endothelial cells are connected to each other by tight junctions and lack pores and/or fenestrations, compounds must cross the membranes of the cells to enter the brain from the bloodstream. Therefore, hydrophilic compounds cannot cross the barrier in the absence of specific mechanisms such as membrane transporters or endocytosis. So, for efficient supply of hydrophilic nutrients, the BBB is equipped with membrane transport systems and some of those transporter proteins have been shown to accept drug molecules and transport them into brain. In the present review, we describe mainly the transporters that are involved in drug transfer across the BBB and have been molecularly identified. The transport systems described include transporters for amino acids, monocarboxylic acids, organic cations, hexoses, nucleosides, and peptides. Most of these transporters function in the direction of influx from blood to brain; the presence of efflux transporters from brain to blood has also been demonstrated, including P-glycoprotein, MRPs, and other unknown transporters. These efflux transporters seem to be functional for detoxication and/or prevention of nonessential compounds from entering the brain. Various drugs are transported out of the brain via such efflux transporters, resulting in the decrease of CNS side effects for drugs that have pharmacological targets in peripheral tissues or in the reduction of efficacy in CNS because of the lower delivery by efflux transport. To identify the transporters functional at the BBB and to examine the possible involvement of them in drug transports by molecular and physiological approaches will provide a rational basis for controlling drug distribution to the brain.

Role of metabolic enzymes and efflux transporters in the absorption of drugs from the small intestine

It has been established that the absorption of many drugs from the small intestine is hindered by the detoxification systems which are present in this epithelial tissue. In this article, we will summarize the significant role of small intestine in reducing the oral bioavailability of drugs, particularly focusing on the role of metabolic enzymes and efflux transporters. Since the role of cytochrome P450 3A (CYP3A) and MDR1 P-glycoprotein (P-gp) in intestinal drug disposition has been highlighted, the disposition of CYP3A substrates, P-gp substrates and CYP3A/P-gp bisubstrates are summarized. Moreover, it is plausible that conjugative enzymes and/or carboxyesterases act synergistically with efflux transporters of organic anions, affecting the intestinal availability, i.e. many xenobiotics and ester-type prodrugs are metabolized to the corresponding glucuronide and sulfate conjugates and carboxylates (active drugs), respectively, followed by cellular extrusion. The characteristics of the efflux transporters of organic anions across the apical and basal membrane of enterocytes and Caco-2 cells are also summarized from this point of view.

Can oral midazolam predict oral cyclosporine disposition?

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

Mdr1 limits CYP3A metabolism in vivo

We determined whether the drug efflux protein P-glycoprotein (Pgp) could influence the extent of CYP3A-mediated metabolism of erythromycin, a widely used model substrate for CYP3A. We compared CYP3A metabolism of erythromycin (a Pgp substrate) using the erythromycin breath test in mice proficient and deficient of mdr1 drug transporters. We first injected mdr1(+/+) mice with [(14)C]N-methyl erythromycin and measured the rate of appearance of (14)CO(2) in the breath as a measure of hepatic CYP3A activity. Animals treated with CYP3A inducers or inhibitor showed accelerated or diminished (14)CO(2) in the breath, respectively. The erythromycin breath test was next administered to mdr1a(-/-) and mdr1a/1b(+/+) and (-/-) mice. These animals had equivalent levels of immunoreactive CYP3A and CYP3A activity as measured by erythromycin N-demethylase activity in liver microsomes. Nevertheless, the rate of (14)CO(2) appearance in the breath showed no relationship with these measurements of CYP3A, but changed proportionally to expression of mdr1. The average breath test (14)CO(2) area under the curves were 1.9- and 1.5-fold greater in mdr1a/1b(-/-) and mdr1a(-/-) mice, respectively, compared with (+/+) mice, and CER(max) was 2-fold greater in mdr1a/1b(-/-) compared with (+/+) mice. We conclude that Pgp, by limiting intracellular substrate availability can be an important determinant of CYP3A metabolism of numerous medications that are substrates for CYP3A and Pgp.

Effect of an oral contraceptive preparation containing ethinylestradiol and gestodene on CYP3A4 activity as measured by midazolam 1'-hydroxylation

AIMS

To characterize the effect of an oral contraceptive (OC) containing ethinylestradiol and gestodene on the activity of CYP3A4 in vivo as measured by the 1'-hydroxylation of midazolam.

METHODS

In this randomised, double-blind, cross-over trial nine healthy female subjects received either a combined OC (30 microg ethinylestradiol and 75 microg gestodene) or placebo once daily for 10 days. On day 10, a single 7.5 mg dose of midazolam was given orally. Plasma concentrations of midazolam and 1'-hydroxymidazolam were determined up to 24 h and the effects of midazolam were measured with three psychomotor tests up to 8 h.

RESULTS

The combined OC increased the mean AUC of midazolam by 21% (95% CI 2% to 40%; P = 0.03) and decreased that of 1'-hydroxymidazolam by 25% (95% CI 10% to 41%; P = 0.01), compared with placebo. The metabolic ratio (AUC of 1'-hydroxymidazolam/AUC of midazolam) was 36% smaller (95% CI 19% to 53%; P = 0.01) in the OC phase than in the placebo phase. There were no significant differences in the Cmax, tmax, t(1/2) or effects of midazolam between the phases.

CONCLUSIONS

A combined OC preparation caused a modest reduction in the activity of CYP3A4, as measured by the 1'-hydroxylation of midazolam, and slightly increased the AUC of oral midazolam. This study suggests that, at the doses used, ethinylestradiol and gestodene have a relatively small effect on CYP3A4 activity in vivo.

Inhibitory potencies of 1,4-dihydropyridine calcium antagonists to P-glycoprotein-mediated transport: comparison with the effects on CYP3A4

PURPOSE

Recently, we clarified the inhibitory effects of 13 kinds of 1,4-dihydropyridine calcium antagonists on human cytochrome P450 (CYP) 3A4. It has been reported that the substrates and/or inhibitors are overlapped between CYP3A4 and P-glycoprotein (P-gp). The purpose of this study was to investigate the inhibitory effects of 13 kinds of 1,4-dihydropyridine calcium antagonists on P-gp-mediated transport in order to evaluate the overlapping specificity of the inhibitors between P-gp and CYP3A4.

METHODS

The transcellular transports of [3H]daunorubicin or [3H]digoxin by monolayers of LLC-GA5-COL150 cells in which P-gp was overexpressed were measured in the presence or absence of the 1,4-dihydropyridine calcium antagonists.

RESULTS

The transport of [3H]daunorubicin was strongly inhibited by manidipine, barnidipine, benidipine, (-)-efonidipine, nicardipine, (+)-efonidipine, and amlodipine with the IC50 values of 4.6, 8.6, 9.5, 17.3, 17.5, 20.6, and 22.0 microM, respectively. The transport of [3H]digoxin was strongly inhibited by benidipine, nicardipine, barnidipine, and manidipine.

CONCLUSIONS

It was clarified that 13 kinds of 1,4-dihydropyridine calcium antagonists have different inhibitory potencies and substrate specificities to the transport of [3H]daunorubicin or [3H]digoxin. Some compounds did not demonstrate the overlapping specificity for inhibition between P-gp and CYP3A4. It was also clarified that nicardipine, benidipine, manidipine, and barnidipine were strong inhibitors of P-gp as well as CYP3A4.

Lipids, lipophilic drugs, and oral drug delivery-some emerging concepts

Lipid-based dose forms, which encompass a wide variety of compositional and functional characteristics, can be advantageously utilized for the formulation of lipophilic drugs. There has been a traditional reluctance to develop lipid-based dose forms due to potential problems of chemical and physical instability, and a paucity of knowledge regarding formulation design algorithms and technology transfer issues. However, there is a current resurgence of interest in lipid-based dose forms due to potential commercial and pharmaceutical benefits, and the industry trend towards the discovery/development of increasingly hydrophobic (and potent) new chemical entities. This mini-review describes some emerging formulation and biopharmaceutic strategies that hold promise for better understanding how to design and evaluate lipid-based dose forms.

Evaluation of the genetic component of variability in CYP3A4 activity: a repeated drug administration method

The CYP3A4 enzyme contributes to the disposition of more than 60 therapeutically important drugs and displays marked person-to-person variability of the catalytic function. However, the extent of genetic contribution to variability in CYP3A4 activity remains elusive. Recently, we showed that a comparison of between- (SDb2) and within-person (SDW2) variances provides an estimate of the genetic component of variability in drug disposition. The aim of the present analysis was to assess the genetic control of CYP3A4 activity in vivo. A computerized literature search was conducted covering 1966 to September 1999 to identify studies reporting repeated administration of CYP3A4 substrates. The genetic contribution (rGC) to disposition of each CYP3A4 substrate was obtained by the formula (SDb2-SDW2)/SDb2. The rGC values approaching 1.0, point to overwhelming genetic control, whereas those close to zero suggest that environmental factors dominate. A total of 16 studies with 10 different CYP3A4 substrates were identified (n = 161 subjects). The rGC for hepatic CYP3A4 activity as measured by midazolam plasma clearance or the erythromycin breath test was 0.96 (0.92-0.98) (95% Cl) and 0.89 (0.65-0.98), respectively (P < 0.05). The point estimates of rGC for composite (hepatic + intestinal) CYP3A4 activity measured after oral administration of cyclosporine, ethinylestradiol, ethylmorphine, nifedipine and nitrendipine, ranged from 0.66-0.98 (median: 0.83) (P < 0.05). Cyclosporine data suggested a higher genetic control of CYP3A4 at night than during the day. These data indicate that further molecular genetic investigations are warranted to identify genetic variants at CYP3A4 or elsewhere in the genome which contribute to regulation of CYP3A4 activity.

Multixenobiotic resistance as a cellular defense mechanism in aquatic organisms

Multixenobiotic resistance in aquatic organisms exposed to natural toxins or anthropogenic contaminants is a phenomenon analogous to multidrug resistance in mammalian tumor cell lines tolerant of anti-cancer drugs. Multidrug resistance is commonly due to the elevated expression of transmembrane P-glycoproteins (P-gp) which actively transport a wide variety of structurally and functionally diverse compounds. The purpose of this review is to place aquatic ecotoxicological data in context of the larger multidrug resistance field of study. Information on P-glycoproteins structure, mechanism of transport, and substrate specificity gained through traditional mammalian and cell culture models is examined in conjunction with recent work on aquatic species exposed to xenobiotics both in the field and in the laboratory. The physiological function of P-glycoproteins is explored through studies of gene knockout models and expression patterns in normal tissues and tumors. The effect of xenobiotic exposures on P-gp activity and protein titer is examined in wild and captive populations of aquatic invertebrates and vertebrates. Substrate overlap and evidence of co-expression of phase I detoxification enzymes (e.g. cytochromes P450) and P-gp are presented. The role of P-gp chemosensitizers as environmental pollutants and the ecotoxicological consequences of P-gp inhibition are highlighted. The overwhelming evidence suggests that P-glycoproteins provide aquatic organisms with resistance to a wide range of natural and anthropogenic toxins.

Inhibition of debrisoquine hydroxylation with quinidine in subjects with three or more functional CYP2D6 genes

AIMS

To study whether the CYP2D6 capacity in ultrarapid metabolizers of debrisoquine due to duplication/multiduplication of a functional CYP2D6 gene, can be 'normalised' by low doses of the CYP2D6 inhibitor quinidine and whether this is dose-dependent.

METHODS

Five ultrarapid metabolizers of debrisoquine with 3, 4 or 13 functional CYP2D6 genes were given single oral doses of 5, 10, 20, 40, 80 and 160 mg quinidine. Four hours after quinidine intake, 10 mg debrisoquine was given. Urine was collected for 6 h after debrisoquine administration. Debrisoquine and its 4-hydroxymetabolite were analysed by h.p.l.c. and the debrisoquine metabolic ratio (MR) was calculated.

RESULTS

Without quinidine the MR in the ultrarapid metabolizers ranged between 0.01 and 0.07. A dose-effect relationship could be established for quinidine with regard to the inhibitory effect on CYP2D6 activity. To reach an MR of 1-2, subjects with 3 or 4 functional genes required a quinidine dose of about 40 mg, while the sister and brother with 13 functional genes required about 80 mg quinidine. After 160 mg quinidine, the MRs, in the subjects with 3, 3, 4, 13 and 13 functional genes, were 12.6, 10.1, 9.2, 2.4 and 2.2, respectively.

CONCLUSIONS

A dose-effect relationship could be established for quinidine inhibition of CYP2D6 in ultrarapid metabolizers. The clinical use of low doses of quinidine as an inhibitor of CYP2D6 might be considered in ultrarapid metabolizers taking CYP2D6 metabolized drugs rather than giving increased doses of the drug. Normalizing the metabolic capacity of CYP2D6, by giving a low dose of quinidine, may solve the problem of 'treatment resistance' caused by ultrarapid metabolism.

Effects of the antifungal agents on oxidative drug metabolism: clinical relevance

This article reviews the metabolic pharmacokinetic drug-drug interactions with the systemic antifungal agents: the azoles ketoconazole, miconazole, itraconazole and fluconazole, the allylamine terbinafine and the sulfonamide sulfamethoxazole. The majority of these interactions are metabolic and are caused by inhibition of cytochrome P450 (CYP)-mediated hepatic and/or small intestinal metabolism of coadministered drugs. Human liver microsomal studies in vitro, clinical case reports and controlled pharmacokinetic interaction studies in patients or healthy volunteers are reviewed. A brief overview of the CYP system and the contrasting effects of the antifungal agents on the different human drug-metabolising CYP isoforms is followed by discussion of the role of P-glycoprotein in presystemic extraction and the modulation of its function by the antifungal agents. Methods used for in vitro drug interaction studies and in vitro-in vivo scaling are then discussed, with specific emphasis on the azole antifungals. Ketoconazole and itraconazole are potent inhibitors of the major drug-metabolising CYP isoform in humans, CYP3A4. Coadministration of these drugs with CYP3A substrates such as cyclosporin, tacrolimus, alprazolam, triazolam, midazolam, nifedipine, felodipine, simvastatin, lovastatin, vincristine, terfenadine or astemizole can result in clinically significant drug interactions, some of which can be life-threatening. The interactions of ketoconazole with cyclosporin and tacrolimus have been applied for therapeutic purposes to allow a lower dosage and cost of the immunosuppressant and a reduced risk of fungal infections. The potency of fluconazole as a CYP3A4 inhibitor is much lower. Thus, clinical interactions of CYP3A substrates with this azole derivative are of lesser magnitude, and are generally observed only with fluconazole dosages of > or =200 mg/day. Fluconazole, miconazole and sulfamethoxazole are potent inhibitors of CYP2C9. Coadministration of phenytoin, warfarin, sulfamethoxazole and losartan with fluconazole results in clinically significant drug interactions. Fluconazole is a potent inhibitor of CYP2C19 in vitro, although the clinical significance of this has not been investigated. No clinically significant drug interactions have been predicted or documented between the azoles and drugs that are primarily metabolised by CYP1A2, 2D6 or 2E1. Terbinafine is a potent inhibitor of CYP2D6 and may cause clinically significant interactions with coadministered substrates of this isoform, such as nortriptyline, desipramine, perphenazine, metoprolol, encainide and propafenone. On the basis of the existing in vitro and in vivo data, drug interactions of terbinafine with substrates of other CYP isoforms are unlikely.

Pharmacokinetic-pharmacodynamic consequences and clinical relevance of cytochrome P450 3A4 inhibition

Drug interactions occur when the efficacy or toxicity of a medication is changed by administration of another substance. Pharmacokinetic interactions often occur as a result of a change in drug metabolism. Cytochrome P450 (CYP) 3A4 oxidises a broad spectrum of drugs by a number of metabolic processes. The location of CYP3A4 in the small bowel and liver permits an effect on both presystemic and systemic drug disposition. Some interactions with CYP3A4 inhibitors may also involve inhibition of P-glycoprotein. Clinically important CYP3A4 inhibitors include itraconazole, ketoconazole, clarithromycin, erythromycin, nefazodone, ritonavir and grapefruit juice. Torsades de pointes, a life-threatening ventricular arrhythmia associated with QT prolongation, can occur when these inhibitors are coadministered with terfenadine, astemizole, cisapride or pimozide. Rhabdomyolysis has been associated with the coadministration of some 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors ('statins') and CYP3A4 inhibitors. Symptomatic hypotension may occur when CYP3A4 inhibitors are given with some dihydropyridine calcium antagonists, as well with the phosphodiesterase inhibitor sildenafil. Excessive sedation can result from concomitant administration of benzodiazepine (midazolam, triazolam, alprazolam or diazepam) or nonbenzodiazepine (zopiclone and buspirone) hypnosedatives with CYP3A4 inhibitors. Ataxia can occur with carbamazepine, and ergotism with ergotamine, following the addition of a CYP3A4 inhibitor. Beneficial drug interactions can occur. Administration of a CYP3A4 inhibitor with cyclosporin may allow reduction of the dosage and cost of the immunosuppressant. Certain HIV protease inhibitors, e.g. saquinavir, have low oral bioavailability that can be profoundly increased by the addition of ritonavir. The clinical importance of any drug interaction depends on factors that are drug-, patient- and administration-related. Generally, a doubling or more in plasma drug concentration has the potential for enhanced adverse or beneficial drug response. Less pronounced pharmacokinetic interactions may still be clinically important for drugs with a steep concentration-response relationship or narrow therapeutic index. In most cases, the extent of drug interaction varies markedly among individuals; this is likely to be dependent on interindividual differences in CYP3A4 tissue content, pre-existing medical conditions and, possibly, age. Interactions may occur under single dose conditions or only at steady state. The pharmacodynamic consequences may or may not closely follow pharmacokinetic changes. Drug interactions may be most apparent when patients are stabilised on the affected drug and the CYP3A4 inhibitor is then added to the regimen. Temporal relationships between the administration of the drug and CYP3A4 inhibitor may be important in determining the extent of the interaction.

Failure of erythromycin breath test to correlate with midazolam clearance as a probe of cytochrome P4503A

BACKGROUND

Cytochrome P4503A (CYP3A) activity exhibits considerable interindividual variability, and an in vivo probe to measure such differences would serve several purposes. The erythromycin breath test (ERBT) is an established approach that has proven useful in this regard, but it has several limitations. More recently, the hydroxylation of midazolam has been suggested as an alternative in vivo probe approach, because it is possible to estimate CYP3A activity in the intestinal epithelium as well as in the liver. The purpose of this study was to investigate the relationship, if any, between the ERBT and midazolam's CYP3A-mediated metabolism.

METHODS

Twenty healthy, medication-free young (24 to 46 years) European Americans (10 women) each received on separate days, in random order, either 3 microCi [14C-N-methyl]-erythromycin intravenously, 1 mg midazolam intravenously, or 2 mg midazolam orally. An ERBT value was determined 60 minutes after administration, and clearances were estimated after midazolam administration. In addition, an endogenous 0- to 4-hour urinary 6beta-hydroxycortisol/cortisol ratio was measured.

RESULTS

All three measured drug trait values varied approximately threefold to fivefold, whereas the endogenous phenotype measure exhibited far greater variability (>100-fold). No statistically significant (P < .05) correlations existed between any of the trait values, including the ERBT value, obtained after intravenous administration of the radiolabeled probe and the systemic clearance of midazolam, expressed in terms of either total or unbound drug, or on an absolute or a body weight-corrected basis (r = 0.03 to r = 0.24; P = .08 to P = .90). Substratification according to sex generally did not improve such relationships.

CONCLUSION

Although both erythromycin N-demethylation and the metabolism of midazolam by hydroxylation are mediated by CYP3A, the phenotypic trait measures associated with these two in vivo probe drugs do not provide the same information about the catalytic activity of the enzyme. An indirect measure such as the ERBT may reflect CYP3A activity and be useful for some purposes, but the estimation of the oral and intravenous clearance of midazolam has additional advantages, and they may be more applicable and have broader usefulness as quantitative estimates of CYP3A activity.

The effect of ketoconazole on the jejunal permeability and CYP3A metabolism of (R/S)-verapamil in humans

AIMS

The purpose of this human intestinal perfusion study was to investigate the effect of ketoconazole on the jejunal permeability and first-pass metabolism of (R)- and (S)-verapamil in humans.

METHODS

A regional single-pass perfusion of the jejunum was performed using a Loc-I-Gut(R) perfusion tube in six healthy volunteers. Each perfusion lasted for 200 min and was divided into two periods of 100 min each. The inlet concentration of (R/S)-verapamil was 120 mg l-1 in both periods, and ketoconazole was added at 40 mg l-1 in period 2. (R/S)-verapamil was also administered as a short intravenous infusion of 5 mg, over a period of 10 min. The appearance ratios of the CYP3A formed metabolites (R)- and (S)-norverapamil were also estimated in the outlet jejunal perfusate.

RESULTS

The effective jejunal permeability (Peff) of both (R)- and (S)-verapamil was unaffected by the addition of ketoconazole in period 2 suggesting that ketoconazole had no effect on the P-glycoprotein mediated efflux. However, the appearance ratio of both (R)- and (S)-norverapamil in the outlet jejunal perfusate decreased in the presence of ketoconazole. The rate of absorption into plasma of (R)- and (S)-verapamil increased despite the low dose of ketoconazole added, indicating an inhibition of the gut wall metabolism of (R/S)-verapamil by ketoconazole.

CONCLUSIONS

Ketoconazole did not affect the jejunal Peff of (R/S)-verapamil, but it did increase the overall transport into the systemic circulation (bioavailability), probably by inhibition of the gut wall metabolism of verapamil. This might be due to ketoconazole being less potent as an inhibitor of P-glycoprotein than of CYP3A4 in vivo in humans.