黄豆苷元对不同基因型健康受试者氯沙坦药代动力学的影响

目的 探讨黄豆苷元对不同基因型健康受试者氯沙坦药代动力学的影响.方法 筛选中国男性健康受试者18名(CYP2C9*1/*1、CYP2C9*1/*3、CYP2C9*1/*13基因型各6名).执行双周期、自身前后对照方案,第一阶段随机分为对照组和试验组,受试者分别口服黄豆苷元片或安慰剂15 d,第15天两组受试者均口服氯沙...     

Influence of CYP2C9*2 genetic polymorphism on pharmacokinetics of losartan and its active metabolite E-3174 on the background of CYP3A4 wild genotype in healthy Chinese Hui subjects

In the present study, we aimed to investigate the influence of CYP2C9*2 genetic polymorphism on pharmacokinetics of losartan and its active metabolite E-3174 on the background of CYP3A4 wild genotype in healthy Chinese Hui subjects. Blood samples were collected from subjects for CYP2C9 and CYP3A4 genotyping using a polymerase chain reaction-restriction fragmentlength polymorphism (PCR-RFLP) assay. A pharmacokinetic study was then carried out in two groups with CYP2C9*1/*1 (n = 8) andCYP2C9*1/*2 (n = 6) genotypes at the same time, and all the 14 subjects were CYP3A4 wildgenotype. Plasma levels of losartan and E-3174 were determined by high-performance liquid chromatography-fluorescence (HPLC-FLD) method before and after a single oral dose of 50-mg dose of losartan in tablet form. The pharmacokinetic parameters were calculated by DAS 2.0 software and analyzed by SPSS 16.0 software. Pharmacokinetic parameters, including area under the curve from 0 h to the last measured point 24 h (AUC_0–24), area under the curve from 0 h to infinite time (AUC_0–∞), peak plasma concentration (C_max), time to reach C_max (t_max), oral clearance (CL), oral volume of distribution (V_d) and elimination half-life (t_1/2), were determined. Compared with the CYP2C9*1/*2 group, the AUC_0–24, AUC_0–∞ and C_max of E-3174 in CYP2C9*1/*1 group of Hui subjects were respectively 1.36, 1.32 and 1.64 times more, and the statistic differences were significant (P<0.05). The CYP2C9*2 mutant allele played an important role in the pharmacokinetics of losartan after oral administration, and itmight decrease the generationof E-3174. However, large-sample clinical trials are required to validate whether the dose adjustment according to CYP2C9 genotype is necessary.     

The role of CYP2C9 genotype in the metabolism of diclofenac in vivo and in vitro

INTRODUCTION: The polymorphic cytochrome P450 enzyme 2C9 (CYP2C9) catalyses the metabolism of many drugs including S-warfarin, acenocoumarol, phenytoin, tolbutamide, losartan and most of the non-steroidal anti-inflammatory drugs. Diclofenac is metabolised to 4'-hydroxy (OH), the major diclofenac metabolite, 3'-OH and 3'-OH-4'-methoxy metabolites by CYP2C9. The aim of the present study was to clarify the impact of the CYP2C9 polymorphism on the metabolism of diclofenac both in vivo and in vitro. SUBJECTS, MATERIALS AND METHODS: Twenty healthy volunteers with different CYP2C9 genotypes [i.e. CYP2C9*1/ *1 (n = 6), *1/*2 (n = 3), *1,/*3 (n = 5), *2/*3 (n = 4), *21*2 (n = 1), *31*3 (n = 1)] received a single 50-mg oral dose of diclofenac. Plasma pharmacokinetics [peak plasma concentration (Cmax), half-life (t 1/2) and area under the plasma concentration-time curve (AUCtotal)] and urinary recovery of diclofenac and its metabolites were compared between the genotypes. Diclofenac 4'-hydroxylation was also analysed in vitro in 16 different samples of genotyped [i.e. CYP2C9*1/*1 (n = 7), *1/*2 (n=2), *1/*3 (n = 2), *2/*3 (n = 2), *2/*2 (n = 2), *31/*3 (n = 1)] human liver microsomes. RESULTS: Within each genotype group, a high variability was observed in kinetic parameters for diclofenac and 4'-OH-diclofenac (6- and 20-fold, respectively). No significant differences were found between the different genotypes either in vivo or in human liver microsomes. No correlation was found between the plasma AUC ratio of diclofenac/4'-OH-diclofenac and that of losartan/ E-3174, previously determined in the same subjects. CONCLUSION: No relationship was found between the CYP2C9 genotype and the 4'-hydroxylation of diclofenac either in vivo or in vitro. This, together with the lack of correlation between losartan oxidation and diclofenac hydroxylation in vivo raises the question about the usefulness of diclofenac as a CYP2C9 probe.     

The Role of CYP2C8 and CYP2C9 Genotypes in Losartan‐Dependent Inhibition of Paclitaxel Metabolism in Human Liver Microsomes

The aim of the present study was to further investigate a previously identified metabolic interaction between losartan and paclitaxel, which is one of the marker substrates of CYP2C8, by using human liver microsomes (HLMs) from donors with different CYP2C8 and CYP2C9 genotypes. Although CYP2C8 and CYP2C9 exhibit genetic linkage, previous studies have yet to determine whether losartan or its active metabolite, EXP‐3174 which is specifically generated by CYP2C9, is responsible for CYP2C8 inhibition. Concentrations of 6α‐hydroxypaclitaxel and EXP‐3174 were measured by high‐performance liquid chromatography after incubations with paclitaxel, losartan or EXP‐3174 in HLMs from seven donors with different CYP2C8 and CYP2C9 genotypes. The half maximal inhibitory concentration (IC₅₀) values were not fully dependent on CYP2C8 genotypes. Although the degree of inhibition was small, losartan significantly inhibited the production of 6α‐hydroxypaclitaxel at a concentration of 1 μmol/L in only HL20 with the CYP2C8*3/*3 genotype. HLMs with either CYP2C9*2/*2 or CYP2C9*1/*3 exhibited a lower losartan intrinsic clearance (V_max/K_m) than other HLMs including those with CYP2C9*1/*1 and CYP2C9*1/*2. Significant inhibition of 6α‐hydroxypaclitaxel formation by EXP‐3174 could only be found at levels that were 50 times higher (100 μmol/L) than the maximum concentration generated in the inhibition study using losartan. These results suggest that the metabolic interaction between losartan and paclitaxel is dependent on losartan itself rather than its metabolite and that the CYP2C8 inhibition by losartan is not affected by the CYP2C9 genotype. Further study is needed to define the effect of CYP2C8 genotypes on losartan–paclitaxel interaction.     

Inhibitory effect of 5-fluorouracil on cytochrome P450 2C9 activity in cancer patients

Drug interactions have been reported between 5-fluorouracil and cytochrome P450 2C9 (CYP2C9) substrates, S-warfarin and phenytoin. This study was performed to determine the influence of 5-fluorouracil on cytochrome P450 2C9 (CYP2C9) activity in colorectal cancer patients (n=17) receiving 5-fluorouracil. Losartan was used as a marker to assess CYP2C9 activity. Losartan and its CYP2C9 dependent metabolite, E-3174, were determined in urine. The ratios of urinary losartan/E-3174 before and after the 5-fluorouracil treatment were compared for each patient. Genotyping was performed to detect the CYP2C9*2 and CYP2C9*3. At the end of the first cycle of 5-fluorouracil, losartan/E-3174 ratio was increased by 28.0% compared to the pre-treatment values (P=0.15). In five patients recruited for phenotyping after three 5-fluorouracil cycles, the metabolic ratio was increased significantly by 5.3 times (P=0.03). The results suggest that in most patients 5-fluorouracil inhibited CYP2C9 activity. This inhibition was more pronounced when the total administered dose increased. This finding may help explain the mechanism of interaction between 5-fluorouracil and CYP2C9 substrates.     

Effect of silymarin on the pharmacokinetics of losartan and its active metabolite E-3174 in healthy Chinese volunteers

Purpose  To investigate the effects of silymarin on the pharmacokinetics of losartan and its active metabolite E-3174 and its relationship with CYP2C9 genotypes. Methods  Twelve healthy adult men of known CYP2C9 genotype (six CYP2C9*1/*1 and six CYP2C9*1/*3) were recruited in a two-phase randomized crossover design study. The pharmacokinetics of losartan and E-3174 were measured before and after a 14-day treatment with 140 mg of silymarin three times daily. Results  The area under the plasma concentration–time curve (AUC) of losartan increased significantly following a 14-day silymarin treatment in subjects with the CYP2C9*1/*1 genotype, but not in those with the CYP2C9*1/*3 genotype. The AUC of E-3174 decreased significantly with a silymarin pretreatment in both CYP2C9*1/*1 and the CYP2C9*1/*3 subjects. The metabolic ratio of losartan (ratio of of E-3174 to of losartan) decreased significantly after a 14-day treatment with silymarin in individuals with the CYP2C9*1/*1 genotype (p < 0.05), but not in those with the CYP2C9*1/*3 genotype (p = 0.065). Conclusion  Silymarin inhibits the metabolism of losartan to E-3174, with the magnitude of the interaction differing in individuals with different CYP2C9 genotypes.     

Inhibitory effects of angiotensin receptor blockers on CYP2C9 activity in human liver microsomes

We investigated the inhibitory effects of the angiotensin receptor blockers (ARBs), candesartan, irbesartan, losartan, losartan active metabolite (EXP-3174), olmesartan, telmisartan and valsartan (0.3-300 microM), on the CYP2C9 activity in human liver microsomes using (S)-(-)-warfarin as a typical CYP2C9 substrate. Except for olmesartan and valsartan, these ARBs inhibited the activity of 7-hydroxylation of (S)-(-)-warfarin with IC50 values of 39.5-116 microM. Of six synthetic derivatives of olmesartan, five compounds which possess either alkyl groups or a chloro group at the same position as that of the hydroxyisopropyl group in olmesartan inhibited CYP2C9 activity with IC50 values of 21.7-161 microM. Olmesartan and the olmesartan analogue, RNH-6272, both having a hydroxyisopropyl group, showed no inhibition, indicating that the hydrophilicity of this group greatly contributes to the lack of CYP2C9 inhibition by these two compounds. A three-dimensional model for docking between EXP-3174 and CYP2C9 indicated that the chloro group of EXP-3174 is oriented to a hydrophobic pocket in the CYP2C9 active site, indicating that the lipophilicity of the group present in ARBs at the position corresponding to that of the hydroxyisopropyl group in olmesartan is important in inhibiting CYP2C9 activity.     

<Emphasis Type="Italic">CYP2C9</Emphasis> genetic variants and losartan oxidation in a Turkish population

Objective Cytochrome P₄₅₀ 2C9 (CYP2C9) is a polymorphic enzyme catalysing the metabolism of several important drugs. Losartan has recently been suggested as a selective probe for CYP2C9 metabolic activity. The aim of the study was to determine the activity of CYP2C9, using losartan as a probe drug, in relation to CYP2C9 genotype in healthy Turkish subjects.Methods A single oral dose of 25 mg losartan was given to 85 Turkish unrelated subjects. Concentrations of losartan and its carboxylic acid metabolite, E3174, were analysed by means of high-performance liquid chromatography in urine collected for 8 h. The CYP2C9 genotypes were determined in 85 subjects using polymerase chain reaction-based endonuclease digestion methods specific for CYP2C9*2 and *3. Losartan oxidation was also studied in vitro, using human CYP2C8 and CYP2C9 enzymes expressed in yeast.Results The frequencies of the allelic variants CYP2C9*2 and CYP2C9*3 were 0.100 and 0.088, respectively. The urinary losartan/E3174 ratio was significantly higher in subjects with CYP2C9*1/*3 genotype (median 2.35, n=12) than in subjects with CYP2C9*1/*1 (0.71, n=58) and *1/*2 (0.85, n=10) genotypes (P<0.05). In contrast to CYP2C9, no E3174 was formed by CYP2C8 in vitro.Conclusion The urinary losartan to E3174 metabolic ratio after a 25-mg losartan dose was found to be a safe and useful phenotyping assay for CYP2C9 activity in vivo. CYP2C9*3 variant allele is a major determinant of the enzyme activity, and it decreases losartan metabolism significantly, while CYP2C9*2 allele has less impact on enzyme function.     

Losartan and E3174 pharmacokinetics in cytochrome P450 2C9*1/*1, *1/*2, and *1/*3 individuals

STUDY OBJECTIVE: To determine if differences in the pharmacokinetics of losartan and its pharmacologically active E3174 metabolite exist among individuals expressing the cytochrome P450 (CYP) 2C9*1/*1, *1/*2, and *1/*3 genotypes. DESIGN: Single-dose pharmacokinetic study. SETTING: University general clinical research center. SUBJECTS: Fifteen healthy volunteers, five from each genotype: CYP2C9*1/*1, *1/*2, and *1/*3. INTERVENTION: A single oral dose of losartan 50 mg. MEASUREMENTS AND MAIN RESULTS: Plasma and urine samples were collected for 24 hours, and losartan and E3174 pharmacokinetic data were compared across the three genotypes. Orthostatic blood pressure was measured over 12 hours after dosing. No significant differences were observed among the three groups in losartan or E3174 area under the plasma concentration-time curve, losartan or E3174 elimination half-life, or losartan oral clearance. A significant association between CYP2C9 genotype and losartan to E3174 formation clearance was observed, such that 50% of the variability was accounted for by the genotype. No significant relationship between that genotype and blood pressure was observed at any time. CONCLUSION: Differences in the pharmacokinetics of losartan and its active E3174 metabolite were not observed in healthy subjects with the genotype of CYP2C9*1/*2 and *1/*3 compared with those expressing *1/*1. Alterations in losartan dosing in CYP2C9*1/*2 and *1/*3 individuals does not appear necessary.     

Role of human liver cytochrome P450 2C9 in the metabolism of a novel alpha4beta1/alpha4beta7 dual antagonist, TR-14035

The metabolism of a novel dual antagonist for alpha4beta1/alpha4beta7 integrin, TR-14035, and the role of polymorphic enzyme responsible for this metabolism were investigated. Human liver microsomes catalyzed the NADPH-dependent metabolism of TR-14035 to a primary metabolite, O-desmethyl TR-14035. This formation was completely blocked by both sulfaphenazole, a selective CYP2C9 inhibitor, and CYP2C9 antibody, whereas potent inhibitors selective for other CYPs exhibited little effects. Of 12 recombinant CYPs examined, O-desmethyl metabolite was principally formed by CYP2C9. CYP1A1, an extrahepatic enzyme, also had this activity (about one-fourth of CYP2C9). Utilizing recombinant CYP2C9*1, K(m) and V(max)/K(m) values of 23.3 microM and 0.284 microL/min/pmol CYP2C9, respectively, were obtained for the O-desmethyl formation, which were quite similar to those in CYP2C9*2 enzyme. In contrast, V(max)/K(m) value in recombinant CYP2C9*3 was approximately one-sixth of CYP2C9*1 and *2. In agreement, kinetics studies using human liver microsomes with CYP2C9*1/*1, *2/*2 and *3/*3 genotypes revealed that the V(max)/K(m) value in *2/*2 microsomes was comparable to that in wild type microsomes, in contrast, that in *3/*3 microsomes was reduced. These results demonstrate CYP2C9 is a primary enzyme mediating the O-desmethylation of TR-14035 in human liver. In homozygotes of CYP2C9*3, the metabolic clearance of TR-14035 should be decreased compared with homozygotes of CYP2C9*1 or 2.     

Ketobemidone is a substrate for cytochrome P4502C9 and 3A4, but not for P-glycoprotein

The role of the major drug-metabolizing cytochrome P450 (CYP) enzymes as well as P-glycoprotein (PGP) was investigated in the disposition of ketobemidone in vitro. Formation of norketobemidone from ketobemidone was studied and compared with the activities of 11 major CYP enzymes in human liver microsomes. The formation of norketobemidone from ketobemidone (1 microM) correlated best with CYP2C9 activity, measured as losartan oxidation (rs = 0.82, n = 19, p < 0.001), but there was also a strong correlation with CYP3A4 activity. Additionally, a good correlation was observed with CYP2C19, CYP2C8 and CYP2B6 at a ketobemidone concentration of 50 microM. Inhibition studies confirmed the involvement of CYP2C9 and CTP3A4 in the formation of norketobemidone. The formation rate of norketobemidone was three times higher in the CYP2C9*1*1 genotype group compared with the CYP2C9*1*2, CYP2C9*1*3 and CYP2C9*3*3 genotypes (p < 0.01). Treatment with verapamil as a PGP inhibitor did not affect the transport of ketobemidone in Caco-2 cells, indicating that PGP is not involved. The data suggest that CYP2C9 and CYP3A4 play a major role in the formation of norketobemidone at clinically relevant concentrations.     

Bioactivation of cyclophosphamide: the role of polymorphic CYP2C enzymes

Several-fold differences have been observed among patients in the biotransformation of cyclophosphamide. The aim of this study was to investigate the contribution of CYP2C9 and CYP2C19 and their polymorphisms to the variability of cyclophosphamide activation. The formation of 4-hydroxycyclophosphamide was studied in microsomes from a total of 32 different genotyped human livers, as well as in yeast microsomes expressing different genetic variants of CYP2C9 and CYP2C19. The kinetic data obtained in the yeast system revealed that the intrinsic clearance (V(max)/K(m)) of cyclophosphamide by CYP2C9.2 and CYP2C9.3 samples was approximately threefold lower than that by CYP2C9.1. However, in liver microsomes, there were no statistically significant differences in the intrinsic clearance of 4-hydroxycyclophosphamide formation between the group of seven CYP2C9*1/*1 livers and the remaining nine with one or two variant CYP2C9 alleles ( P>0.7). We found a statistically significant correlation ( r(s)=0.65, P=0.003) between 4-hydroxylation of cyclophosphamide and 5'-hydroxylation of R-omeprazole, a measure of CYP2C19 activity in human liver microsomes ( n=19). No correlation was found between 4-hydroxylation of cyclophosphamide and the formation rate of hydroxycelecoxib, mainly catalysed by CYP2C9 ( r(s)=0.17, P=0.55, n=32). In conclusion, based on the correlation with the formation of R-5'-hydroxyomeprazole, CYP2C19 may partly contribute to the bioactivation of cyclophosphamide in human liver microsomes, while the role of CYP2C9 appears minor.     

Effect of the single CYP2C9*3 allele on pharmacokinetics and pharmacodynamics of losartan in healthy Japanese subjects

Objective Losartan is metabolized to the active carboxylic acid metabolite EXP3174 by CYP2C9. In this study, we determined the effects of the single CYP2C9*3 variant on the pharmacokinetics and pharmacodynamics of losartan.Methods Seven healthy Japanese subjects (CYP2C9*1/*1, n=4 and CYP2C9*1/*3, n=3) were phenotyped with a single dose of losartan (25 mg). Blood and urine samples were collected and assayed for losartan and EXP3174. Blood pressure and pulse rate were also measured using a sphygmomanometer.Results The maximum plasma concentration of EXP3174 was significantly (P<0.05) lower in the CYP2C9*1/*3 (n=3) group than in the CYP2C9*1/*1 (n=4) group. Diastolic blood pressure in the CYP2C9*1/*1 group, but not that in the CYP2C9*1/*3 group except for at 6 h and 8 h, was reduced from 1.5 h to 12 h compared with the baseline level. Systolic blood pressure in the CYP2C9*1/*1 group, but not that in the CYP2C9*1/*3 group, was reduced from 1 h to 12 h compared with the baseline level. The metabolic ratio (MR) of EXP3174 concentration to the losartan concentration in plasma at 6 h post-dosing and the 4-h to 8-h urinary EXP3174/losartan MR were significantly lower in the CYP2C9*1/*3 group than in the CYP2C9*1/*1 group. The plasma 6-h MR and the 4-h to 8-h urinary MR were significantly (P<0.05) correlated with the plasma AUC ratio (AUC_EXP3174/AUC_losartan), with Spearman rank correlation coefficients of 0.75 and 0.89, respectively.Conclusion The single CYP2C9*3 variant reduces the metabolism of losartan and its hypotensive effect. Plasma MR, as well as urine MR, may be useful for phenotyping assays of CYP2C9 activity.     

Effect of atorvastatin on CYP2C9 metabolic activity as measured by the formation rate of losartan metabolite in hypercholesterolaemic patients

HMG-CoA reductase inhibitors (statins) have a potential to interact with substrates of the drug-metabolizing enzyme cytochrome P450 2C9 (CYP2C9). This may lead to concentration-dependent toxicity such as skeletal muscle side effects. Atorvastatin, a widely used statin, is presently inadequately investigated in vivo with regard to effects on CYP2C9 activity in human beings. The aim of this study was to determine the effect of atorvastatin on the activity of CYP2C9 in a group of Turkish hypercholesterolaemic patients. We prospectively investigated the atorvastatin effect on CYP2C9 activity in a sample of Turkish hypercholesterolaemia patients (11 women, 7 men) who commenced atorvastatin (10 mg/day). Losartan was used as a probe drug to determine CYP2C9 metabolic activity. A single 25-mg oral dose of losartan was given to the patients before, on the first day and after the fourth week of the atorvastatin treatment. Urinary concentrations of losartan and its metabolite, E3174, were measured by high-pressure liquid chromatography (HPLC). Urinary losartan/E3174 ratios were used as an index of CYP2C9 activity. As the baseline enzyme activity may influence the extent of drug-drug interactions, the CYP2C9*2 and 2C9*3 alleles were identified by using PCR-RFLP. In the patients with the CYP2C9*1*1 genotype (n = 12), atorvastatin treatment did not cause a significant change in losartan/E3174 ratios (medians; 95% CI) neither after the first day (0.73; 0.34-1.61) nor at the fourth week (0.71; 0.36-1.77) of the treatment as compared with the baseline activity (0.92; 0.57-1.74, p = 0.38). Similarly, no significant change in the baseline CYP2C9 activity (0.91; 0.30-1.60) was observed in patients with the CYP2C9*1*2 genotype as compared with those of the first day (1.08; 0.08-2.72) and fourth week (0.64; 0.0-3.82) of the atorvastatin treatment (n = 4, p = 0.86). These observations in a hypercholesterolaemic patient sample suggest that atorvastatin does not have a significant effect on enzymes encoded by the CYP2C9*1*1 and CYP2C9*1*2 genotypes when co-administered with a CYP2C9 substrate, losartan.     

Catalytic roles of CYP2C9 and its variants (CYP2C9*2 and CYP2C9*3) in lornoxicam 5'-hydroxylation.

The effects of allelic variants of CYP2C9 (CYP2C9*2 and CYP2C9*3) on lornoxicam 5'-hydroxylation were studied using the corresponding variant protein expressed in baculovirus-infected insect cells and human liver microsomes of known genotypes of CYP2C9. The results of the baculovirus expression system showed that CYP2C9.3 gives higher K(m) and lower V(max) values for lornoxicam 5'-hydroxylation than does CYP2C9.1. In contrast, K(m) and V(max) values of CYP2C9.1 and CYP2C9.2 for the reaction were comparable. Lornoxicam 5'-hydroxylation was also determined in liver microsomes of 12 humans genotyped for the CYP2C9 gene (*1/*1, n = 7; *1/*2, n = 2; *1/*3, n = 2; *3/*3, n = 1). A sample genotyped as *3/*3 exhibited 8- to 50-fold lower intrinsic clearance for lornoxicam 5'-hydroxylation than did samples genotyped as *1/*1. However, the values for intrinsic clearance for *1/*3 were within the range of values exhibited by samples of *1/*1. In addition, no appreciable differences were observed in kinetic parameters for lornoxicam 5'-hydroxylation between *1/*1 and *1/*2. In conclusion, this study showed that lornoxicam 5'-hydroxylation via CYP2C9 was markedly decreased by the substitution of Ile359Leu (CYP2C9.3), whereas the effect of the substitution of Arg144Cys (CYP2C9.2) was nonexistent or negligible. Additional in vivo studies are required to confirm that individuals with homologous CYP2C9*3 allele exhibit impaired lornoxicam clearance.     

Fluconazole but not itraconazole decreases the metabolism of losartan to E-3174

Objective: Losartan is metabolised to its active metabolite E-3174 by CYP2C9 and CYP3A4 in vitro. Itraconazole is an inhibitor of CYP3A4, whereas fluconazole affects CYP2C9 more than CYP3A4. We wanted to study the possible interaction of these antimycotics with losartan in healthy volunteers. Methods: A randomised, double-blind, three-phase crossover study design was used. Eleven healthy volunteers ingested orally, once a day for 4 days, either itraconazole 200 mg, fluconazole (400 mg on day 1 and 200 mg on days 2–4) or placebo (control). On day 4, a single 50-mg oral dose of losartan was ingested. Plasma concentrations of losartan, E-3174, itraconazole, hydroxy-itraconazole and fluconazole were determined over 24 h. The blood pressure and heart rate were also recorded over 24 h. Results: The mean peak plasma concentration (C_max) and area under the curve [AUC(0∞)] of E-3174 were significantly decreased by fluconazole to 30% and to 47% of their control values, respectively, and the t_1/2 was increased to 167%. Fluconazole caused only a nonsignificant increase (23–41%) in the AUC and t_1/2 of the unchanged losartan. Itraconazole had no significant effect on the pharmacokinetic variables of losartan or E-3174. The ratio AUC(0∞)_E-3174/AUC(0∞)_losartan was 60% smaller during the fluconazole than during the placebo and itraconazole phases. No clinically significant changes in the effects of losartan on blood pressure and heart rate were observed between fluconazole, itraconazole and placebo phases. Conclusion: Fluconazole but not itraconazole interacts with losartan by inhibiting its metabolism to the active metabolite E-3174. This implicates that, in man, CYP2C9 is a major enzyme for the formation of E-3174 from losartan. The clinical significance of the fluconazole–losartan interaction is unclear, but the possibility of a decreased therapeutic effect of losartan should be kept in mind. Received: 4 June 1997 / Accepted in revised form: 10 September 1997     

Lack of polymorphism of the conversion of losartan to its active metabolite E-3174 in extensive and poor metabolizers of debrisoquine (cytochrome P450 2D6) and mephenytoin (cytochrome P450 2C19)

Objective: Losartan was given to subjects with known phenotypes of the polymorphic enzymes CYP2D6 and CYP2C19 to study any possible influence on the metabolism of the drug. Methods: Plasma concentrations of losartan and E-3174 were studied after oral intake of 50 mg losartan in 24 healthy, male, Swedish Caucasian subjects who were extensive or poor metabolizers (EM/PM) of debrisoquine [cytochrome P450 2D6 (CYP2D6)] or mephenytoin [cytochrome P450 2C19 (CYP2C19)]. Results: The areas under the curve (AUC_∞) of losartan and E-3174 did not differ between poor and extensive metabolizers of debrisoquine or mephenytoin, respectively. Conclusion: About 14% of the antihypertensive drug losartan is metabolized to the active carboxylic acid metabolite E-3174, which contributes to the effect of losartan. The present study suggests that CYP2D6 and CYP2C19 are not involved to any major extent in the in vivo conversion of losartan to E-3174. Received: 21 April 1998 / Accepted in revised form: 16 December 1998     

The impact of CYP2C9 genetics and oral contraceptives on cytochrome P450 2C9 phenotype.

CYP2C9-dependent drug metabolism is subject to large interindividual variation. To some extent, this is explained by genetic polymorphism with expression of enzyme variants that differ in catalytic activity. The aim of this study was to characterize the variation in CYP2C9 phenotype in relation to genotype, with further analysis of the CYP2C9 gene in metabolic outliers. A study population of 126 healthy white subjects were recruited and genotyped for the variant alleles, CYP2C9*1-3. In CYP2C9 phenotyping with losartan, three subpopulations were distinguished that differed in the number of CYP2C9*3 alleles (0, 1, or 2). A three-fold higher metabolic ratio (MR; urinary losartan/carboxymetabolite) was found comparing CYP2C9*1/*3 (n = 20) to CYP2C9*1/*1 (n = 81), but there was considerable variation within each genotype. Subjects genotyped as CYP2C9*1/*1, but with an unexpectedly slow oxidation of losartan, were selected for DNA-sequencing analysis of the CYP2C9 gene. Interestingly, single nucleotide polymorphisms (SNPs) could not be identified either in the 5'-flanking region, the nine exons, or exon-intron boundaries. However, sequencing of the CYP2C9 gene was also carried out in patients genotyped as CYP2C9*1/*1 but with an exceptionally low steady-state clearance of S-warfarin. Here, five different SNPs were identified. In further analysis of the healthy volunteers, it became evident that women on oral contraceptives (OCs) had slower oxidation of losartan (MR of losartan: 1.7) than women without OCs (MR of losartan: 0.86). This novel finding was not explained by a different frequency of variant alleles. In summary, CYP2C9 genotype and oral contraceptives both contribute to a large interindividual variation in CYP2C9 activity.     

Effects of the CYP2C9*13 allele on the pharmacokinetics of losartan in healthy male subjects

1. The aim of the study was to determine the pharmacokinetics of losartan in relation to the CYP2C9*13 allele.

2. A single oral dose of 50?mg losartan was administrated to each of the 16 healthy male volunteers with a different genotype (CYP2C9*1/*1, n?=?6; CYP2C9*1/*13, n?=?4; and CYP2C9*1/*3, n?=?6). Blood samples were collected from pre-dose up to 24?h after the drug administration. Plasma losartan and E3174 (an active metabolite of losartan) were assayed by liquid chromatography-tandem mass spectrometry (LC-MS/MS).

3. All the subjects finished the study without adverse drug effects. In the present study, the frequencies of CYP2C9*13 and *13 alleles were 0.6% and 2.6% in Chinese healthy volunteers, respectively, and both alleles were in Hardy–Weinberg equilibrium. Compared with the subjects in the CYP2C9*1/*1 group, individuals carrying the CYP2C9*1/*13 genotype showed significantly a longer t_1/2 of losartan and E3174 and markedly increased the area under the curve (AUC) of losartan. Meanwhile, the CYP2C9*1/*3 genotype group had significant differences in t_1/2 and C_max of E3174 compared with the CYP2C9*1/*1 group. The ratio of AUC_E3174/AUC_losartan after losartan administration in the CYP2C9*1/*13 and CYP2C9*1/*3 groups was also statistically different from that in the CYP2C9*1/*1 group.

4. The data indicate that the presence of the CYP2C9*13 allele results in poor metabolism of losartan after a single oral dose.

     

The effects of fluvastatin, a CYP2C9 inhibitor, on losartan pharmacokinetics in healthy volunteers

Losartan is an angiotensin II receptor antagonist that is metabolized by CYP2C9 and CYP3A4 to a more potent antihypertensive metabolite, E3174. Interaction studies with inhibitors of CYP3A4 have not demonstrated significant changes in the pharmacokinetics of losartan or E3174. The authors assessed the steady-state pharmacokinetics of losartan and E3174 when administered alone and concomitantly with fluvastatin, a specific CYP2C9 inhibitor. A prospective, open-label, crossover study was conducted in 12 healthy volunteers with losartan alone and in combination with fluvastatin. The baseline phase was 7 days of losartan (50 mg QAM), and the inhibition phase was 14 total days of fluvastatin (40 mg QHS), with the final 7 days including losartan. The authors found that fluvastatin did not significantly change the steady-state AUC0-24 or half-life of losartan or E3174. Losartan apparent oral clearance was not affected by fluvastatin. Inhibition of losartan metabolism appears to require both CYP2C9 and CYP3A4 inhibition.