The in vivo induction of rat hepatic cytochrome P450-dependent enzyme activities and their maintenance in culture

Cytochrome P450-dependent enzyme activities were measured in hepatocytes from adult male rats, induced in vivo with phenobarbitone, beta-naphthoflavone, dexamethasone or isoniazid: the stability of the induced activities in culture was also determined. Each inducer produced a characteristically different pattern of enzyme activities with dexamethasone, isoniazid and beta-naphthoflavone selectively inducing erythromycin N-demethylase, p-nitrophenol hydroxylase and ethoxyresorufin O-dealkylase respectively. In general, the induced activities were maintained for 24 hr in culture. This indicates the feasibility of an in vivo induction-hepatocyte culture system for the study of metabolism-mediated toxicity.     

A microtiterplate-based screening assay to assess diverse effects on cytochrome P450 enzyme activities in primary rat hepatocytes by various compounds

During the development of potential drugs it is useful to identify pharmacological and/or toxicological side effects of a compound as early as possible in order to exclude them from further development for reasons of time and cost. Activation or inactivation of members of the cytochrome P450-dependent monooxygenase system (CYP450) might indicate potential undesired effects of a given compound. However, results using CYP450 assay systems are often inconsistent because of different experimental settings. Therefore, it was the goal of the present study to optimize the CYP450 assay in primary rat hepatocytes with respect to the time point of addition of and duration of exposure to alpha-naphthoflavone (ANF) and beta-naphthoflavone (BNF) as well as trans-resveratrol (RES), which have well-described stimulatory and inhibitory effects on CYP450 enzymes of the 1A and 2B family, respectively. Hepatocytes were also treated with putative lipoxygenase (LOX)/cyclooxygenase (COX) inhibitors with unknown impact on CYP450 enzyme activity in order to detect potential side effects. Cells were cultured for up to 7 days on 96-well microtiter plates, and enzyme activity was determined by a conventional fluorescence spectroscopy assay. ANF and BNF, given to the cells after 4 days of culture, stimulated CYP1A and 2B activities significantly in a concentration-dependent fashion after long-term exposure for at least 1 day. However, during short-term exposure for 1-6 h, CYP1A activity was inhibited, while CYP2B was increased weakly by ANF but not BNF. RES inhibited CYP1A activity during short- and long-term exposure without affecting CYP2B activity. From the results it was concluded that primary rat hepatocytes should be cultured for at least 3-4 days but no longer prior to the assay. The assay should be performed at two different time points of exposure, i.e., 6 h for short-term and 24 h for long-term exposure. The compounds under investigation should be applied at two different concentrations, e.g., at one time and 10 times higher concentrations, which should be oriented to the ED50, provided it is known for the respective substance. Under these assay conditions the LOX/COX inhibitors tested activated CYP1A enzyme activity in long-term but instead inhibited it in short-term experiments. CYP2B activity was stimulated during short- and long-term exposure. These results indicated drug side effects recommending exclusion of the compounds from the drug developmental process. Hence, in order to assess the pharmacological potential of novel compounds it is adequate to perform both short- and long-term experiments to concisely describe the effect of a compound on the CYP450 system.     

Effects of prototypical microsomal enzyme inducers on cytochrome P450 expression in cultured human hepatocytes.

Cultured human hepatocytes are a valuable in vitro system for evaluating new molecular entities as inducers of cytochrome P450 (P450) enzymes. The present study summarizes data obtained from 62 preparations of cultured human hepatocytes that were treated with vehicles (saline or dimethylsulfoxide, 0.1%), beta-naphthoflavone (33 microM), phenobarbital (100 or 250 microM), isoniazid (100 microM) and/or rifampin (20 or 50 microM), and examined for the expression of P450 enzymes based on microsomal activity toward marker substrates, or in the case of CYP2C8, the level of immunoreactive protein. The results show that CYP1A2 activity was markedly induced by beta-naphthoflavone (on average 13-fold, n = 28 preparations), and weakly induced by phenobarbital (1.9-fold, n = 25) and rifampin (2.3-fold, n = 22); CYP2A6 activity tended to be increased with phenobarbital (n = 7) and rifampin (n = 3) treatments, but the effects were not statistically significant; CYP2B6 was induced by phenobarbital (6.5-fold, n = 13) and rifampin (13-fold, n = 14); CYP2C8 was induced by phenobarbital (4.0-fold, n = 4) and rifampin (5.2-fold, n = 4); CYP2C9 was induced by phenobarbital (1.8-fold, n = 14) and rifampin (3.5-fold, n = 10); CYP2C19 was markedly induced by rifampin (37-fold, n = 10), but relatively modestly by phenobarbital (7-fold, n = 9); CYP2D6 was not significantly induced by phenobarbital (n = 5) or rifampin (n = 5); CYP2E1 was induced by phenobarbital (1.7-fold, n = 5), rifampin (2.2-fold, n = 5), and isoniazid (2.3-fold, n = 5); and, CYP3A4 was induced by phenobarbital (3.3-fold, n = 42) and rifampin (10-fold, n = 61), but not by beta-naphthoflavone. Based on these observations, we generalize that beta-naphthoflavone induces CYP1A2 and isoniazid induces CYP2E1, whereas rifampin and, to a lesser extent phenobarbital, tend to significantly and consistently induce enzymes of the CYP2A, CYP2B, CYP2C, CYP2E, and CYP3A subfamilies but not the 2D subfamily.     

Presence of hexobarbital in primary cultures of rat hepatocytes maintains cytochrome P-450 levels and drug metabolizing enzyme activities

Addition of hexobarbital (1 mM) to the culture medium of rat hepatocytes protected against the rapid decline in the level of cytochrome P-450 and the activities of various drug metabolizing enzymes. While the hepatocytes cultured for 72 hr without hexobarbital had only 30% of their original level of cytochrome P-450, the cells maintained with hexobarbital had 75% of the initial level of the hemoprotein. After 72 hr in culture, the activities of aminopyrine N-demethylase and biphenyl 4-hydroxylase were 22-24% of the original rate for the nontreated cells and 73-78% for the hexobarbital treated cells. The activities of 7-ethoxycoumarin O-deethylase and aryl hydrocarbon hydroxylase in the cultures of treated cells were even higher than those of the freshly isolated hepatocytes. Additions of other substrates of hepatic mixed function oxidase to the culture medium did not protect against the loss of cytochrome P-450 and enzyme activities.     

Effects of a hormone-supplemented medium on cytochrome P-450 content and mono-oxygenase activities of rat hepatocytes in primary culture

Cytochrome P-450 content and associated mono-oxygenation activities (7-ethoxycoumarin-O-deethylase, biphenyl-4-hydroxylase and 7-ethoxyresorufin-O-deethylase) of rat hepatocytes were found to decrease during the first 48 hr in primary culture in control (WOBA-M₂) medium. However, by culturing the hepatocytes in a hormone-supplemented medium (AB medium), all of these enzymes were maintained at higher levels after 12. 24 and 48 hr in culture. In particular. 7-ethoxyresorufin-O-deethylase activity was markedly enhanced after 12 and 24 hr in culture in AB medium to levels greater than that in isolated hepatocytes. Metabolic capacities of the cytochromes P-450 present in hepatocytes cultured in WOBA-M₂ medium vs AB medium were also quantitatively different at 12, 24 and 48 hr when specific activities/pmole of hemoprotein were compared. Sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) experiments further suggested that a cytochrome P-450-related protein was maintained to a greater extent in AB medium than in WOBA-M₂ medium. It is proposed that AB medium may maintain a higher cytochrome P-450 concentration in cultured primary rat hepatocytes by increasing both the rate of hcme synthesis and the synthesis of a cytochrome P-450-related protein.     

3-甲基胆蒽对大鼠肝细胞细胞色素P450 1A的诱导作用

目的　研究 3 甲基胆蒽 (3 methylcholanthrene,3 MC)对原代培养大鼠肝细胞细胞色素P450 1A(CYP 1A)酶活力的诱导作用 ,并探讨其时间 效应和剂量 效应关系。方法　原位 2步胶原酶灌流法分离大鼠肝细胞 ,无血清条件下培养细胞 ,并用不同剂量的 3 MC诱导肝细胞 2 4h或 48h。乙氧基试卤灵 (ethoxyresorufin,EOR)为CYP 1A酶活力探针药 ,高效液相色谱法 (HPLC)测定试卤灵 (resorufin,RSF)浓度。结果　实验条件下对照组和 3 MC诱导组的RSF均可被检测 ,与对照细胞相比 ,除 0 0 0 5μmol L的 3 MC对原代培养大鼠肝细胞EROD无显著诱导作用外 ,其余剂量 (0 0 5 ,0 5 ,5μmol L)的 3 MC均明显诱导酶活力 ,酶活力分别被上调至对照组的 5 0 8,2 3 3 ,33 6倍 (P <0 0 1 ) ,3 MC的诱导作用在 2 4h达最强 ,而其在 48h的诱导作用与 2 4h相似。3 MC对原代培养大鼠肝细胞CYP 1A酶活力的诱导作用呈明显的剂量依赖性。结论　 3 MC对大鼠原代培养肝细胞CYP 1A酶活力有明显的诱导作用     

慢性间断性低氧对大鼠肝脏细胞色素P450的影响

目的观察慢性间断性低氧对大鼠肝脏P450同工酶的影响。方法♂SD大鼠随机分为对照组和实验组,实验组分别低氧3、7、14、28d。采用酶法测定血清丙氨酸氨基转移酶(ALT)和天冬氨酸氨基转移酶(AST)活性,分光光度法测定大鼠肝微粒体红霉素N-脱甲基酶(ERD)、苯胺羟化酶(ANH)活性,半定量逆转录聚合酶链式反应(RT-PCR)检测大鼠肝脏细胞色素P4503A2、2E1的mRNA表达水平。结果慢性间断性低氧对血清ALT和AST活性无明显影响;低氧7d后,大鼠肝脏ERD和ANH活性明显升高,28d时诱导率分别为155·5%和42·2%;同时CYP3A2和CYP2E1mR-NA的表达水平,也分别增加了220·5%和102·8%。结论慢性间断性低氧能明显增加大鼠肝脏ERD(CYP3A2)和ANH(CYP2E1)活性,其机制可能与其在转录水平上提高肝脏CYP3A2和CYP2E1的基因表达水平有关。     

Imipramine- and mianserin-induced acute cell injury in primary cultured rat hepatocytes: implication of different cytochrome P450 enzymes

The antidepressants, imipramine and mianserin, have been reported to cause liver damage. We investigated a role of cytochrome P450 (CYP)-mediated formation of a reactive metabolite in antidepressant-induced acute cell injury using hepatocytes isolated from male and female Wistar rats, and male Dark Agouti rats, which have different relative abundance of CYP enzymes. Culture of the hepatocytes with imipramine and mianserin caused a marked decrease in glutathione followed by protein thiol, which preceded lactate dehydrogenase leakage. The decreases in glutathione and protein thiol contents by imipramine were significantly slower in hepatocytes from male Dark Agouti rats than those from male Wistar rats, whereas no significant sex difference in Wistar rats was observed. The decrease in thiol by mianserin was significantly slower in hepatocytes from female Wistar than those from male Wistar rats, whereas no significant differences were found between Wistar and Dark Agouti males. Results consistent with alteration of the thiols were obtained for lactate dehydrogenase leakage induced by imipramine and mianserin. These findings indicated that CYP-mediated metabolic activation was involved in acute cell injury induced by the antidepressants; namely a CYP2D enzyme(s), which is deficient in Dark Agouti rats, and a male specific CYP enzyme(s) were suggested to be responsible for the cytotoxicity of imipramine and mianserin, respectively. Received: 6 January 1999 / Accepted: 22 February 1999     

Effects of propofol on human hepatic microsomal cytochrome P450 activities

1. The potential of propofol to inhibit the activity of major human cytochrome P450 enzymes has been examined in vitro using human liver microsomes. Propofol produced inhibition of CYP1A2 (phenacetin O -deethylation), CYP2C9 (tolbutamide 4-hydroxylation), CYP2D6 (dextromethorphan O -demethylation) and CYP3A4 (testosterone 6 beta hydroxylation) activities with IC = 40, 49, 213 and 32 mu M respectively. K for propofol against all of these enzymes with the exception of CYP2D6, where propofolishowed little inhibitory activity, was 30, 30 and 19 mu M respectively for CYPs 1A2, 2C9 and 3A4. 2. Furafylline, sulphaphenazole, quinidine and ketoconazole, known selective inhibitors of CYPs 1A2, 2C9, 2D6 and 3A4 respectively, were much more potent than propofol having IC = 0.8, 0.5, 0.2 and 0.1 mu M; furafylline and sulphaphenazole yielded K = 0.6 and 0.7 mu M respectively. i 3. The therapeutic blood concentration of propofol (20 mu M; 3-4 mu?g ml) together with the in vitro K estimates for each of the major human P450 enzymes have been used to i estimate the extent of cytochrome P450 inhibition, which may be produced in vivo by propofol. This in vitro - in vivo extrapolation indicates that the degree of inhibition of CYP1A2, 2C9 and 3A4 activity which could theoretically be produced in vivo by propofol is relatively low (40-51%); this is considered unlikely to have any pronounced clinical significance. 4. Although propofol has now been used in 190 million people since its launch in 1986,thereare onlysinglereportsofpossible druginteractions between propofoland either alfentanil or warfarin. Consequently, it is difficult to conclude from either the published literature or the ZENECA safety database whether there is any evidence to indicate that propofol produces clinically significant drug interactions through inhibition of cytochrome P450-related drug metabolism.     

环孢素A与盐酸小檗碱合用对大鼠肝脏和小肠药物代谢酶的影响

目的:研究盐酸小檗碱(berberine chloride,Ber)与环孢素A(CsA)合用对大鼠肝脏和小肠药物代谢酶的影响.方法:采用分光光度法测定各给药组大鼠肝脏和小肠微粒体中红霉素N-脱甲基酶(ERD)、氨基比林N-脱甲基酶(ADM)和谷胱甘肽转移酶(GST)的活性.结果:Ber组和Ber CsA组能明显抑制肝微粒体中ERD、ADM和GST酶活性(P< 0.05 ),而且Ber CsA组较CsA单用组有更明显的抑制作用(P< 0.05 ).Ber CsA组还能明显抑制肠微粒体中ERD、ADM和GST酶活性(P< 0.05 ).结论:Ber与CsA合用时,能显著抑制大鼠肝脏和小肠药物代谢酶活性,这可能是Ber增加CsA血药浓度的一个重要机制.     

Effect of troglitazone on cytochrome P450 enzymes in primary cultures of human and rat hepatocytes

1. Troglitazonewas the first thiazolidinedione approved for clinical use in the treatment of non-insulin-dependent diabetes mellitus. During clinical investigations of drug-drug interactions with therapeutics (terfenadine and cyclosporine) known to be metabolized by CYP3A4, pharmacokinetic interactions were noted upon troglitazone multiple-dose treatments. The nature of the interactions suggested induction of CYP3A enzymes. 2. Primary cultures of human hepatocytes were used to investigate the induction potential of troglitazone with respect to CYP3A4, CYP2B6 and CYP1A1/2. In human hepatocytes, troglitazone induced both immunoreactive CYP3A4 protein and testosterone 6beta-hydroxylase activity in a dose-dependent fashion (EC₅₀ = 5-10 muM), accompanied by an increase in CYP3A4 mRNA. The capacity of troglitazone to induce CYP3A4 was between that of rifampin (EC₅₀ = ~0.8 muM) and dexamethasone (40-50 muM). Troglitazone increased CYP2B6 immunoreactive protein but did not significantly effect CYP1A1/2 activity, immunoreactive protein or mRNA. 3. Troglitazone produced significant increases in CYP3A message, protein and activity in primary rat hepatocytes, a slight increase in CYP2B1/2 activity and no change in CYP1A1/2 message or activity. 4. These results provide evidence that troglitazone can induce CYP3A and CYP2B enzymes while apparently not altering CYP1A. This provides a rationale for the clinically observed interactions of troglitazone with selected CYP3A4 substrates.     

Induction of cytochrome P-450c and P-450d by metyrapone in the primary culture of rat hepatocytes

The effects of metyrapone on qualitative changes in cytochrome P-450-dependent drug metabolizing activities in primary cultures of rat hepatocytes were investigated. Metyrapone apparently increased benzo(a)pyrene hydroxylation and maintained both ethoxycoumarin-O-deethylation and propoxycoumarin-O-depropylation, whereas it had little effect on methoxycoumarin-O-demethylation. Furthermore, P-450d (high spin type of P-448) as well as P-450c (low spin type of P-448) were induced by metyrapone, while P-450b and P-450e were not. In conclusion, metyrapone act as a 3-methylcholanthrene-like inducer in the primary cultures of rat hepatocytes.     

Effect of troglitazone on cytochrome P450 enzymes in primary cultures of human and rat hepatocytes

1. Troglitazone was the first thiazolidinedione approved for clinical use in the treatment of non-insulin-dependent diabetes mellitus. During clinical investigations of drug-drug interactions with therapeutics (terfenadine and cyclosporine) known to be metabolized by CYP3A4, pharmacokinetic interactions were noted upon troglitazone multiple-dose treatments. The nature of the interactions suggested induction of CYP3A enzymes. 2. Primary cultures of human hepatocytes were used to investigate the induction potential of troglitazone with respect to CYP3A4, CYP2B6 and CYP1A1/2. In human hepatocytes, troglitazone induced both immunoreactive CYP3A4 protein and testosterone 6beta-hydroxylase activity in a dose-dependent fashion (EC50 = 5-10 microM), accompanied by an increase in CYP3A4 mRNA. The capacity of troglitazone to induce CYP3A4 was between that of rifampin (EC50 = 0.8 microM) and dexamethasone (40-50 microM). Troglitazone increased CYP2B6 immunoreactive protein but did not significantly effect CYP1A1/2 activity, immunoreactive protein or mRNA. 3. Troglitazone produced significant increases in CYP3A message, protein and activity in primary rat hepatocytes, a slight increase in CYP2B1/2 activity and no change in CYP1A1/2 message or activity. 4. These results provide evidence that troglitazone can induce CYP3A and CYP2B enzymes while apparently not altering CYP1A. This provides a rationale for the clinically observed interactions of troglitazone with selected CYP3A4 substrates.     

Metabolism of ipecac alkaloids cephaeline and emetine by human hepatic microsomal cytochrome P450s, and their inhibitory effects on P450 enzyme activities.

In this study, we identified the metabolites and the CYP forms that are specifically involved in emetine O-demethylation in human liver microsomes, and cleared the inhibitory potential of cephaeline and emetine on the activity of the major drug-metabolizing CYP enzymes. Incubation of emetine with human liver microsomes yielded three metabolites identified by using HPLC by comparison of the retention time with the authentic sample of cephaeline, 9-O-demethylemetine and 10-O-demethylemetine. CYP3A4 and CYP2D6 were able to metabolize emetine to cephaeline and 9-O-demethylemetine, and CYP3A4 also participated in metabolizing emetine to 10-O-demethylemetine. Cephaeline and emetine inhibited probe substrates metabolism. IC50 for cephaeline against CYP2D6 and CYP3A4 were 121 and 1000 microM, respectively. For the emetine, CYP2D6 and CYP3A4 were 80 and 480 microM, respectively. Inhibition constants (Ki) for both compounds on the CYP2D6 and CYP3A4 activities were determined by graphic analysis of Dixon plots at various concentrations. The obtained Ki values of cephaeline for CYP2D6 and CYP3A4 were 54 and 355 microM, respectively, and the values of emetine were 43 and 232 microM, respectively. We concluded that these in vitro inhibitions of cephaeline and emetine would hardly increase plasma concentrations of co-administered drugs in clinical therapy.     

Time-course activities of Oct1, Mrp3, and cytochrome P450s in cultures of cryopreserved rat hepatocytes

The organic cation transporter 1 (Oct1) has been shown to be one of the most abundant uptake transporters responsible for the uptake of xenobiotics from the sinusoidal blood across the basolateral membrane of hepatocytes. On the same membrane the multidrug resistance-associated protein 3 (Mrp3) mediates the efflux of xenobiotics or their metabolites from the hepatocytes to the blood allowing their systemic exposure. In the present study we investigated the expression and activity of Oct1 and Mrp3 in suspensions and in monolayer- and sandwich cultures, and activities of CYP2B1/2, 2D1, and 3A1 in monolayer- and sandwich cultures of cryopreserved rat hepatocytes. Oct1-mediated active uptake of 10 μM [(3)H]-1-methyl-4-phenylpyridinium (MPP+) into hepatocytes was assessed in the presence of quinidine (1 mM). The results showed the presence of active uptake of MPP+ in suspended hepatocytes (~91 pmol/min/mg protein). In hepatocytes in cultures (monolayer and sandwich) a time-dependent decrease in MPP+ uptake was observed from day 0 to 4, from 80 to 90 pmol/min/mg protein at day 0 to ca. 17 pmol/min/mg protein at day 4. Mrp3 activity in suspensions and in monolayer- and sandwich cultures were investigated by measuring the efflux of [(3)H]-taurocholate from hepatocytes in the presence of the Mrp3 inhibitor taurolithocholate-3-sulfate (TLC-S) (500μM). Cells in suspensions showed efflux of taurocholate by an active transport mechanism indicating Mrp3 activity. Experiments in monolayer- and sandwich cultures also showed Mrp3 activity at day 0 and 1 in culture whereas experiments performed at day 2-4 showed no difference in efflux of taurocholate in the presence or absence of TLC-S, suggesting an absence of Mrp3 activity. The time-dependent decrease in Oct1 activity from day 0 to day 4 in cultures was confirmed by qPCR data also showing a time-dependent decrease in mRNA expression, whereas qPCR data did not support the observed time-dependent decrease in Mrp3 activity in cultures. Time-course activities of CYP2B1/2, 2D1, and 3A1 were also investigated by using bupropion, bufuralol, and midazolam as respective substrates. Activities of CYP2D1 and 3A1 were reduced by ~75% and ~80%, respectively, from day 0 to day 4 in cultures, whereas activity of CYP2B1/2 was reduced by ~50% from day 0 to day 4.     

The effects of cimetidine, ranitidine and famotidine on rat hepatic microsomal cytochrome P-450 activities

It has been questioned whether the interaction of H2-antagonists with cytochrome P-450 that is observed in vitro is also relevant for the in vivo situation. Until now the possibility that cytochrome P-450 may function with different modes of action has been neglected in this respect. We studied the effect of cimetidine, ranitidine and famotidine on the monoxygenase, the oxidase and the peroxidase action of cytochrome P-450. Biotransformation catalyzed by the monoxygenase and oxidase action of cytochrome P-450 was affected by cimetidine (probably via its ligand interaction with cytochrome P-450), whereas metabolism by the peroxidase mode of action of cytochrome P-450 was hardly influenced. Ranitidine and famotidine (both pharmacodynamically more potent than cimetidine) only slightly affected cytochrome P-450 activities.     

六种大鼠肝微粒体细胞色素P450重组酶系对33种外来化合物的代谢

多氯联苯诱导的六种鼠肝微粒体细胞色素P450重组酶系A_1,A_2,B,C_1,C_2和D对33种外来化合物代谢的催化速率不同,其中以C_1酶系和C_2酶系催化活力最强,其次为A_1酶系,B酶系催化活力最弱,外来化合物的种类不同,经重组酶系催化的途径也不同,如卤代烷烃,卤代烯烃,苯及其同系物,亚硝胺类等化合物主要经P450C_1酶系代谢,而大多数有机磷酸酯,氨基甲酸酯类化合物及多环芳烃类致癌物则以P450 C_2酶系代谢为主。     

Inhibition of human liver microsomal cytochrome P450 activities by adefovir and tenofovir

Adefovir (PMEA) and tenofovir (PMPA) and their prodrugs, adefovir dipivoxil (bisPOM-PMEA) and tenofovir disoproxil (bisPOC-PMPA), were subjected to a detailed study of their potential to inhibit the activities of human liver microsomal cytochromes P450 (CYP). The inhibition of marker enzyme activities of CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1 and CYP3A4 was examined with high-performance liquid chromatography (HPLC) or spectroscopic (fluorescence, luminescence) detection. Adefovir and adefovir dipivoxil did not significantly influence activities of most CYP enzymes. The activity of CYP3A4 was inhibited by adefovir dipivoxil at concentrations over 100 microM. Adefovir and its prodrug inhibited CYP2C9 at concentrations below 100 microM; inhibition by adefovir was of the uncompetitive (at the lower inhibitor concentrations) or of the competitive nature with a Ki = 420 microM. Tenofovir and tenofovir disoproxil influenced the activity of CYP2C9, and competitive inhibition was found with Ki = 580 and 395 microM, respectively. Tenofovir disoproxil was shown to inhibit microsomal CYP2E1 activities by a mixed-type inhibition with Ki values at about 140 microM. The results indicate the possibility of an influence of the compounds tested on the respective CYP activities when used at high doses.     

Inhibition of human liver microsomal cytochrome P450 activities by adefovir and tenofovir

Adefovir (PMEA) and tenofovir (PMPA) and their prodrugs, adefovir dipivoxil (bisPOM-PMEA) and tenofovir disoproxil (bisPOC-PMPA), were subjected to a detailed study of their potential to inhibit the activities of human liver microsomal cytochromes P450 (CYP). The inhibition of marker enzyme activities of CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1 and CYP3A4 was examined with high-performance liquid chromatography (HPLC) or spectroscopic (fluorescence, luminescence) detection. Adefovir and adefovir dipivoxil did not significantly influence activities of most CYP enzymes. The activity of CYP3A4 was inhibited by adefovir dipivoxil at concentrations over 100?µM. Adefovir and its prodrug inhibited CYP2C9 at concentrations below 100?µM; inhibition by adefovir was of the uncompetitive (at the lower inhibitor concentrations) or of the competitive nature with a K_i?=?420?µM. Tenofovir and tenofovir disoproxil influenced the activity of CYP2C9, and competitive inhibition was found with K_i?=?580 and 395?µM, respectively. Tenofovir disoproxil was shown to inhibit microsomal CYP2E1 activities by a mixed-type inhibition with K_i values at about 140?µM. The results indicate the possibility of an influence of the compounds tested on the respective CYP activities when used at high doses.