Comparing coordination uranyl(vi) complexes with 2-(1H-imidazo[4,5-b]phenazin-2-yl)phenol and derivatives

Four derivatives of 2-(1H-imidazo[4,5-b]phenazin-2-yl)phenol have been synthesized and characterized structurally using X-ray crystallography. Coordination complexes with uranyl (UO₂²⁺) and copper (Cu²⁺) were prepared and absorption/emission spectra detailed. We observed increased fluorescence upon uranyl binding, in stark contrast to rapid quenching observed with the addition of copper. These phenomena have been further examined by DFT computational methods.

Identification and determination of ephedrine and its congeners in urine by gas chromatography

A sensitive and specific assay for ephedrine-type compounds in urine is described. The procedure involves the removal of neutral and acidic compounds, then extraction of the bases with ether. This extract, after concentration, is injected into a flame ionisation gas chromatograph equipped with a 2% polyethylene glycol 6000, alkaline treated, column at 165°. Linear detector response between 5 and 100 μg base/ml urine was observed (peak height with respect to an internal marker) and replicate analyses indicated good reproducibility. No interfering compounds have been encountered nor any “on column” decomposition. The amines were further identified by chromatography of their acetone derivatives.

Some Aspects of The Storage and Testing of Sterilised Catgut

Results obtained during an examination of plain and chromicised surgical catgut stored in tubing fluids of differing water content, and the effect on diameter, tensile strength and flexibility are described. A fall in tensile strength and an increase in diameter which vary with water content and time of storage was observed, although gut does not deteriorate rapidly when immersed in water and several days elapse before significant changes occur. The official testing methods have been examined, and the results indicate that the surgeon's knot test gives a more reliable measurement of tensile strength than the straight pull. Also the present gauge method for measurement of diameter can lead to errors due to compression of the gut. The time of equilibration of the gut in air is important for the measurement of both tensile strength and diameter, particularly when tubing fluids contain a high water content. A relation has been shown between tensile strength, extensibility and softening, and it is suggested that this might provide an index of flexibility of the gut. The effect of some common disinfectant solutions on the gut has been investigated.

Disposition of debrisoquine and nortriptyline in Korean subjects in relation to CYP2D6 genotypes, and comparison with Caucasians

AIMS

To study the influence of the CYP2D6*10 allele on the disposition of debrisoquine and nortriptyline.

METHODS

The pharmacokinetics of debrisoquine and nortriptyline and their main metabolites were determined in ten Koreans with the CYP2D6*1/*1 (n = 5) and CYP2D6*1/*10 (n = 5) genotypes after single oral doses of 20 mg debrisoquine and 25 mg nortriptyline, respectively. The data were compared with previously published findings from 21 Caucasians with 0, one, two, three, four or 13 functional CYP2D6 genes.

RESULTS

The AUC0-8 of 4-hydroxydebrisoquine was significantly lower in Koreans with CYP2D6*1/*10 genotype compared with CYP2D6*1/*1[95% confidence interval (CI) for the ratio between means 1.17, 1.85]. No other genotype-related differences were found in the plasma kinetics of nortriptyline and debrisoquine, or their hydroxy metabolites. The AUCnortriptyline/AUC10-hydroxynortriptyline ratio did not differ between the *1/*1 and *1/*10 genotype groups (95% CI for the ratio of means 0.60, 1.26). Similarly, there was no difference between these genotypes with respect to the AUCdebrisoquine/AUC4-hydroxydebrisoquine ratio (95% CI for the ratio of mean values 0.38, 1.46). Both Korean genotype groups had similar AUCs and parent compound/metabolite AUC ratios of debrisoquine and nortriptyline to Caucasians with two functional CYP2D6 genes.

CONCLUSIONS

Heterozygosity for CYP2D6*10 decreases the CYP2D6-dependent elimination of nortriptyline and debrisoquine to only a limited degree. Further studies in subjects homozygous for CYP2D6*10 are required to elucidate fully the pharmacokinetic consequences of this CYP2D6 genotype in Orientals.

In-vivo phenotyping for CYP3A by a single-point determination of midazolam plasma concentration

We investigated whether a single plasma midazolam concentration could serve as an accurate predictor of total midazolam clearance, an established in-vivo probe measure of cytochrome P450 3A (CYP3A) activity. In a retrospective analysis of data from 224 healthy volunteers, non-compartmental pharmacokinetic parameters were estimated from plasma concentration-time curves following intravenous (IV) and/or oral administration. Based on statistical moment theory, the concentration at the mean residence time (MRT) should be the best predictor of the total area under the curve (AUC). Following IV or oral midazolam administration, the average MRT was found to be approximately 3.5 h, suggesting that the optimal single sampling time to predict AUC was between 3 and 4 h. Since a 4-h data point was common to all studies incorporated into this analysis, we selected this time point for further investigation. The concentrations of midazolam measured 4 h after an IV or oral dose explained 80 and 91% of the constitutive interindividual variability in midazolam AUC, respectively. The 4-h midazolam measurement was also an excellent predictor of drug-drug interactions involving CYP3A induction and inhibition. Compared with baseline values, the direction and magnitude of change in midazolam AUC and the 4-h concentration were completely concordant for all study subjects. We conclude that a single 4-h midazolam concentration following IV or oral administration represents an accurate marker of CYP3A phenotype under constitutive and modified states. Moreover, the single-point approach offers an efficient means to phenotype and identify individuals with important genetic polymorphisms that affect CYP3A activity.

Identification and functional characterization of a new CYP2C9 variant (CYP2C9*5) expressed among African Americans

CYP2C9 is a polymorphic gene for which there are four known allelic variants; CYP2C9*1, CYP2C9*2, CYP2C9*3, and CYP2C9*4. In the present study, DNA from 140 European Americans and 120 African Americans was examined by single-strand conformational polymorphism and restriction fragment length polymorphism analyses, resulting in the identification of a new CYP2C9 variant, CYP2C9*5. This variant is derived from a C1080G transversion in exon 7 of CYP2C9 that leads to an Asp360Glu substitution in the encoded protein. The CYP2C9*5 variant was found to be expressed only in African Americans, such that approximately 3% of this population carries the CYP2C9*5 allele. The variant was expressed in, and purified from, insect cells infected with a recombinant baculovirus. Comparative kinetic studies using the purified wild-type protein CYP2C9*1; the Ile359Leu variant, CYP2C9*3; and the Asp360Glu variant, CYP2C9*5 were carried out using (S)-warfarin, diclofenac, and lauric acid as substrates. The major effect of the Asp360Glu mutation was to increase the K(m) value relative to that of CYP2C9*1 for all three substrates: 12-fold higher for (S)-warfarin 7-hydroxylation, 5-fold higher for the 4'-hydroxylation of diclofenac, and 3-fold higher for the omega-1 hydroxylation of lauric acid. V(max) values differed less than K(m) values between the CYP2C9*1 and CYP2C9*5 proteins. In vitro intrinsic clearances for CYP2C9*5, calculated as the ratio of V(max)/K(m), ranged from 8 to 18% of CYP2C9*1 values. The corresponding ratio for CYP2C9*3 was 4 to 13%. Accordingly, the in vitro data suggest that carriers of the CYP2C9*5 allele would eliminate CYP2C9 substrates at slower rates relative to persons expressing the wild-type protein.

Identification of functionally variant MDR1 alleles among European Americans and African Americans

MDR1 (P-glycoprotein) is an important factor in the disposition of many drugs, and the involved processes often exhibit considerable interindividual variability that may be genetically determined. Single-strand conformational polymorphism analysis and direct sequencing of exonic MDR1 deoxyribonucleic acid from 37 healthy European American and 23 healthy African American subjects identified 10 single nucleotide polymorphisms (SNPs), including 6 nonsynonymous variants, occurring in various allelic combinations. Population frequencies of the 15 identified alleles varied according to racial background. Two synonymous SNPs (C1236T in exon 12 and C3435T in exon 26) and a nonsynonymous SNP (G2677T, Ala893Ser) in exon 21 were found to be linked (MDR1*2 ) and occurred in 62% of European Americans and 13% of African Americans. In vitro expression of MDR1 encoding Ala893 (MDR1*1 ) or a site-directed Ser893 mutation (MDR1*2 ) indicated enhanced efflux of digoxin by cells expressing the MDR1-Ser893 variant. In vivo functional relevance of this SNP was assessed with the known P-glycoprotein drug substrate fexofenadine as a probe of the transporter's activity. In humans, MDR1*1 and MDR1*2 variants were associated with differences in fexofenadine levels, consistent with the in vitro data, with the area under the plasma level-time curve being almost 40% greater in the *1/*1 genotype compared with the *2/*2 and the *1/*2 heterozygotes having an intermediate value, suggesting enhanced in vivo P-glycoprotein activity among subjects with the MDR1*2 allele. Thus allelic variation in MDR1 is more common than previously recognized and involves multiple SNPs whose allelic frequencies vary between populations, and some of these SNPs are associated with altered P-glycoprotein function.

In vivo inhibition of human CYP1A2 activity by oltipraz

PURPOSE

Oltipraz is currently undergoing clinical evaluation as a cancer chemopreventive agent, especially with respect to aflatoxin-associated hepatocarcinogenesis. The agent's ability to induce phase II xenobiotic enzymes that detoxify the ultimate carcinogen formed in vivo is thought to be an important mechanism by which disease risk may be attenuated. However, an additional mechanism could be a reduction in the activation of environmental procarcinogens by certain cytochrome P450 (CYP) isoforms. This hypothesis was tested with respect to CYP1A2, by using the clearance of caffeine by N-demethylation as a phenotypic trait measurement of the isoform's catalytic activity.

METHODS

Subjects received a single oral dose of caffeine (200 mg) on five separate occasions: on the day prior to oltipraz administration (day 0), 2 h after the first (day 1) of eight daily oral doses of oltipraz (125 mg) and 2 h after the last dose (day 8). In addition, CYP1A2 activity was also measured 2 and 14 days (days 10 and 22, respectively) after discontinuation of oltipraz administration. Plasma concentrations of caffeine and its N-demethylated metabolite, paraxanthine, over 24 h after drug administration, were determined by HPLC.

RESULTS

A single 125-mg dose of oltipraz markedly reduced CYP1A2 activity by 75 +/- 13% in nine healthy subjects, resulting in a higher caffeine plasma level and prolongation of the in vivo probe's elimination half-life. Daily administration of 125 mg oltipraz for 8 days resulted in further inhibition so that only 19 +/- 13% of the original baseline level of activity was present. However, 2 days after discontinuation of oltipraz treatment, CYP1A2 activity had returned to 66 +/- 33% of its original level and complete recovery was achieved within 14 days of the chemopreventive agent being stopped.

CONCLUSIONS

These results demonstrate that oltipraz is a potent, in vivo inhibitor of CYP1A2 in humans and, because this isoform is importantly involved in procarcinogen activation, they also indicate that such inhibition probably contributes to oltipraz's cancer-chemopreventive effect. In addition, the findings also suggest the likelihood of significant drug interactions between oltipraz and drugs whose metabolism is mediated by CYP1A2.

CYP3A activity in European American and Japanese men using midazolam as an in vivo probe

OBJECTIVE

To investigate putative differences in CYP3A activity between European American and Japanese subjects using midazolam as an in vivo probe.

METHODS

Midazolam was administered orally (2 mg) to 22 young healthy Japanese men and, on a separate occasion, to 19 of these by the intravenous route (1 mg). The disposition of the drug and its 1'-hydroxy metabolite were determined and compared with data collected in a similar fashion in 20 young healthy European American men.

RESULTS

Plasma concentrations of midazolam, especially those attained soon after drug administration, were higher after intravenous injection in Japanese subjects than those in European American men. This observation was associated with smaller initial (2.5-fold) and steady-state (1.8-fold) volumes of distribution for the drug; normalization for body weight only modestly reduced these differences. The systemic clearance value of midazolam was 25% lower (P < .03) in Japanese subjects, but this difference was not apparent after accounting for the smaller body weights of that group. No statistical differences were noted in the elimination half-life (t 1/2) of midazolam between European American and Japanese subjects. Much greater interindividual variability was observed after oral administration compared with intravenous administration, but significant differences were not found between the 2 groups with respect to the maximum midazolam plasma level or its oral clearance. Absolute oral bioavailability and its associated gastrointestinal and hepatic extraction ratios also showed no statistically significant interracial differences.

CONCLUSIONS

On average, hepatic CYP3A, as measured by the metabolism of midazolam, is lower in young healthy Japanese men compared with similar European Americans. However, there is considerable interindividual variability, and body size appears to be an important determinant. After oral administration, even greater variability in the plasma level-time profile of midazolam is present, and no statistically significant or clinically important interracial/ethnic difference is present. Possibly because of smaller body mass and differences in body composition, midazolam has a smaller distribution volume(s) in Japanese men than in European American men that might be an important factor when drugs are administered intravenously.

Evidence of a source of HIV type 1 within the central nervous system by ultraintensive sampling of cerebrospinal fluid and plasma

Defining the source of HIV-1 RNA in cerebrospinal fluid (CSF) will facilitate studies of treatment efficacy in the brain. Four antiretroviral drug-naive adults underwent two 48-hr ultraintensive CSF sampling procedures, once at baseline and again beginning on day 4 after initiating three-drug therapy with stavudine, lamivudine, and nelfinavir. At baseline, constant CSF HIV-1 RNA concentrations were maintained by daily entry of at least 10(4) to 10(6) HIV-1 RNA copies into CSF. Change from baseline to day 5 ranged from -0.38 to -1.18 log(10) HIV-1 RNA copies/ml in CSF, and from -0.80 to -1.33 log(10) HIV-1 RNA copies/ml in plasma, with no correlation between CSF and plasma changes. There was no evidence of genotypic or phenotypic viral resistance in either CSF or plasma. With regard to pharmacokinetics, mean CSF-to-plasma area-under-the-curve (AUC) ratios were 38.9% for stavudine and 15.3% for lamivudine. Nelfinavir and its active M8 metabolite could not be accurately quantified in CSF, although plasma M8 peak level and AUC(0-8hr) correlated with CSF HIV-1 RNA decline. This study supports the utility of ultraintensive CSF sampling for studying HIV-1 pathogenesis and therapy in the CNS, and provides strong evidence that HIV-1 RNA in CSF arises, at least in part, from a source other than plasma.

CYP3A activity in African American and European American men: population differences and functional effect of the CYP3A4*1B5'-promoter region polymorphism

OBJECTIVE

Cytochrome P4503A (CYP3A) activity exhibits considerable interindividual variability. Possible differences in CYP3A activity were investigated in European American and African American men with the use of midazolam as an in vivo probe.

METHODS

Midazolam was simultaneously administered intravenously (1 mg, [15N3]-labeled) and orally (2 mg, unlabeled in capsule form) to 15 young healthy European American men and a similar group of men of African American descent. Plasma concentration-time curves were measured. The subjects were subsequently genotyped with respect to the CYP3A4*B1 polymorphism (A-290G) in the 5'-promoter (nifedipine-specific element) region.

RESULTS

The oral bioavailability of midazolam was about equally determined by intestinal and hepatic extraction with CYP3A activity at the former site exhibiting greater variability. Oral bioavailability was related to intestinal metabolism (r = 0.98), whereas hepatic CYP3A activity contributed little to the interindividual variability (r = 0.03). A lower systemic clearance (265+/-54 versus 310+/-56 mL/min; P = .04), but not oral clearance, was observed in African Americans. With one exception, the African Americans possessed a variant CYP3A4*1B allele (4 heterozygotes A/G and 10 homozygote G/G), whereas all of the European Americans were wild-type homozygotes (A/A). Hepatic CYP3A activity and the systemic clearance of midazolam were about 30% lower in G/G homozygotes than in A/A homozygotes (252+/-53 versus 310+/-54 mL/min; P = .02), and a gene-dose effect was present (P = .01). There was no genotype/phenotype relationship with respect to the oral clearance of midazolam.

CONCLUSION

Comparison of CYP3A activity between populations is complicated by frequency distribution differences in the regulatory CYP3A4*1B polymorphism and lower hepatic CYP3A activity associated with the variant allele. However, this reduction is modest; therefore no major and clinically important difference in CYP3A activity is present between Americans of African or European descent.

Pharmacological inhibition of P-glycoprotein transport enhances the distribution of HIV-1 protease inhibitors into brain and testes

HIV protease inhibitors have proven remarkably effective in treating HIV-1 infection. However, some tissues such as the brain and testes (sanctuary sites) are possibly protected from exposure to HIV protease inhibitors due to drug entry being limited by the membrane efflux transporter P-glycoprotein, located in the capillary endothelium. Intravenous administration of the novel and potent P-glycoprotein inhibitor LY-335979 to mice (1-50 mg/kg) increased brain and testes concentration of [(14)C]nelfinavir, up to 37- and 4-fold, respectively, in a dose-dependent fashion. Similar effects in brain levels were also observed with (14)C-labeled amprenavir, indinavir, and saquinavir. Because [(14)C]nelfinavir plasma drug levels were only modestly increased by LY-335979, the increase in brain/plasma and testes/plasma ratios of 14- to 17- and 2- to 5-fold, respectively, was due to increased tissue penetration. Less potent P-glycoprotein inhibitors like valspodar (PSC-833), cyclosporin A, and ketoconazole, as well as quinidine and verapamil, had modest or little effect on brain/plasma ratios but increased plasma nelfinavir concentrations due to inhibition of CYP3A-mediated metabolism. Collectively, these findings provide "proof-of-concept" for increasing HIV protease inhibitor distribution into pharmacologic sanctuary sites by targeted inhibition of P-glycoprotein using selective and potent agents and suggest a new therapeutic strategy to reduce HIV-1 viral replication.

Assessment of CYP2D6 and CYP2C19 activity in vivo in humans: a cocktail study with dextromethorphan and chloroguanide alone and in combination

OBJECTIVES

Dextromethorphan and chloroguanide (INN, proguanil) are used as prototypic phenotyping substrates of polymorphically expressed CYP2D6 and CYP2C19 in humans. We determined whether the dextromethorphan/dextrorphan and chloroguanide/cycloguanil metabolic ratios, obtained after administration of the parent drugs either alone or in combination, are equivalent.

METHODS

Thirty-six healthy male volunteers received single oral doses of 80 mg dextromethorphan and 200 mg chloroguanide during a three-period, randomized crossover study. Plasma and urine were collected to calculate metabolic ratios and analyze the disposition kinetics of the probe drugs.

RESULTS

All subjects were extensive metabolizers for both CYP2D6 and CYP2C19. Chloroguanide kinetics and urinary metabolic ratio were not altered after dextromethorphan administration. Dextromethorphan urinary metabolic ratio increased from -2.52 +/- 0.67 to -2.03 +/- 0.58 (P < .001) in the presence of chloroguanide. This was caused by an increase of dextromethorphan without a significant change of dextrorphan in both urine and plasma. Inhibition of CYP3A-dependent biotransformation of dextromethorphan to methoxymorphinan did not appear to be responsible for this change because the log(dextromethorphan/methoxymorphinan) urinary ratio, an index of CYP3A activity, did not significantly change during chloroguanide coadministration. The chloroguanide and dextromethorphan metabolic ratio determined from urine collection correlated with the corresponding metabolic ratio determined from plasma obtained 3 hours after oral administration.

CONCLUSION

When CYP2D6 and CYP2C19 activity are assessed, dextromethorphan and chloroguanide cannot be associated in a cocktail because chloroguanide increases the dextromethorphan metabolic ratio. CYP2D6 and CYP2C19 activity can be determined from a blood sample drawn 3 hours after oral administration of dextromethorphan and chloroguanide, respectively.

Modulation by drugs of human hepatic sodium-dependent bile acid transporter (sodium taurocholate cotransporting polypeptide) activity

Adequate bile flow, maintained in part by the efficient enterohepatic recirculation of bile acids, is critical for normal liver function. One important component of this process is the uptake of bile acids from the portal circulation into hepatocytes by the bile acid uptake transporter sodium taurocholate cotransporting polypeptide (NTCP). Thus, the expression and functional activity of this transporter may affect the rate of bile acid removal from the portal circulation. Accordingly, we assessed NTCP mRNA expression from human livers using a sensitive RNase protection assay. In addition, the ability of various bile acids and drugs to inhibit NTCP activity was determined using a recombinant vaccinia expression system. A 40-fold interindividual variability was found in NTCP mRNA levels determined in eight liver samples of disease-free donors. Expressed NTCP exhibited high-affinity, sodium-dependent uptake of taurocholate, and as expected, this was markedly inhibited by bile acids and organic anions. A number of drugs, including peptidomimetic renin inhibitors, propranolol, cyclosporin, and progesterone, were found to be potent inhibitors, whereas antiarrhythmic agents, including bupivicaine, lidocaine, and quinidine, were found to enhance NTCP activity. Accordingly, these results indicate that large interindividual variability exists in NTCP mRNA level and that a number of drugs currently in clinical use have the potential to interact with and alter NTCP activity, thereby affecting hepatic bile acid uptake.

Disposition of debrisoquine in Caucasians with different CYP2D6-genotypes including those with multiple genes

Debrisoquine is a major prototypic in-vivo probe used to assess polymorphic CYP2D6 activity in humans, based on the 0-8 h urinary excretion of unchanged drug and its 4-hydroxy metabolite (the so-called metabolic ratio). The primary purpose of the study was to investigate further the relationship between genotype and phenotype by determining the overall disposition characteristics of the drug in selected groups of healthy Swedish Caucasian individuals. Debrisoquine (20 mg) was orally administered to five poor metabolizers with no functional CYP2D6 gene, five heterozygous extensive metabolizers, five homozygous extensive metabolizers, five ultrarapid metabolizers with duplicated/triplicated CYP2D6*2 genes and one individual with 13 copies of CYP2D6*2. Peak plasma levels of debrisoquine and 4-hydroxydebrisoquine were attained within 2-4 h and then declined in a multi-exponential fashion over 96 h. However, the post 8-h period of the elimination process was characterized by irregular fluctuations that prevented formal pharmacokinetic analysis. Nevertheless, marked differences were apparent in the compounds' plasma level-time profiles between the CYP2D6 genotypes. For example, in the case of debrisoquine, the mean ratio of the AUC(0-8) values was 22:22:7:6:1, corresponding to 0, 1, 2, 3/4 and 13 genes and, for 4-hydroxydebrisoquine, the respective values were 1:7:19:28:17. The 0-96 h urinary recovery of debrisoquine differed 100-fold between the genotypes, being essentially complete in poor metabolizers and zero in the individual with 13 CYP2D6*2 genes. 4-hydroxydebrisoquine excretion increased according to the number of functional CYP2D6 genes. A highly significant correlation (r(s) = 0.95, P < 0.001) was observed between the plasma AUC(0-8) ratio for debrisoquine to 4-hydroxydebrisoquine and the 0-8 h urinary metabolic ratio. This study demonstrates that the number of functional CYP2D6 alleles is critically important in the plasma concentration-time curves of debrisoquine and its CYP2D6-mediated 4-hydroxy metabolite. Concentration-related pharmacologic effects would be expected to be similarly affected by gene dosage and it is likely that the same situation also applies to other drugs whose elimination is importantly determined by this enzyme; for example, many antidepressants and neuroleptics, antiarrhythmic agents, beta-adrenoceptor antagonists and opiates.

Allelic, genotypic and phenotypic distributions of S-mephenytoin 4'-hydroxylase (CYP2C19) in healthy Caucasian populations of European descent throughout the world

Impaired S-mephenytoin 4'-hydroxylation is a well-described genetic polymorphism affecting drug metabolism in humans. The reported population prevalence of the CYP2C19 poor metabolizer phenotype in Caucasians of European descent has been described as ranging from 0.9% to 7.7%. To address the question of whether the difference in the frequency of poor metabolizers represents an ethnic genetic microheterogeneity in the structure and expression of the CYP2C19 gene in Caucasian individuals, we performed a pooled analysis of available studies. Combined data from the 22 homogeneous studies showed that the frequency of poor metabolizers in healthy unrelated Caucasians determined by phenotyping was 2.8% (110 of 3990; 95% confidence interval 2.3-3.3). Data obtained from eight homogeneous studies that determined the frequency of poor metabolizers by genotyping showed that the genotypic frequency of poor metabolizers was 2.1% (28 of 1356; 95% confidence interval 1.3-2.8), consistent with the poor metabolizer frequency determined by phenotyping. In the extensive metabolizers, 26% (471 of 1786; 95% confidence interval 24.4-28.4) were heterozygotes. The observed frequencies of the three Mendelian genotypes were 73% for wt/wt, 26% for wt/m, and 2.1% for m/m. Based on the overall phenotypic poor metabolizer frequency of 2.8%, the expected genotypic frequencies were 69% for wt/wt, 28% for wt/m and 2.8% for m/m, which are in good agreement to the observed values. However, in the 84 Caucasian phenotyped and genotyped poor metabolizers, approximately 10% of the putative poor metabolizer alleles (17 of 168) were unknown. This study provides a systematic overview of the population distribution of the CYP2C19 poor metabolizer phenotype and CYP2C19 alleles and genotypes in healthy Caucasians living in different geographical areas, and shows a similar polymorphic pattern in the structure and expression of the CYP2C19 gene in the worldwide Caucasian populations.

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.

Genetic polymorphism of (S)-mephenytoin 4'-hydroxylation in populations of African descent

AIMS

The frequency of CYP2C19 poor metabolizers (PMs) in populations of African descent has been reported to range from 1.0% to 35.4%. In order to determine with greater certainty the frequency of CYP2C19 PMs in such black populations we have performed a meta-analysis of the studies.

METHODS

Relevant data on the frequency of both the PM phenotype of probe drugs (mephenytoin, omeprazole, and proguanil), and the distribution frequencies of CYP2C19 alleles and genotypes in black populations were summarized and reanalysed using a meta-analytical approach.

RESULTS

Of nine reported studies two were excluded because of significant heterogeneity (chi2=115, P<0.0001). The combined data from the remaining seven studies showed that the frequency of the PM phenotype in 922 healthy unrelated black Africans and black Americans ranged from 1.0% to 7.5% (n=7 for combined data) with an overall frequency being 3.9% (36 of 922; 95%CI: 2.7%-5.2%). The frequency of the PM genotypes in blacks was 3.7% (36 of 966; 95%CI: 2.5%-4.9%), in agreement with the frequency of the PM phenotype. In the extensive metabolizers (EMs) 29% (271 of 930) were heterozygotes (wt/m ). The observed frequencies of the three Mendelian genotypes were 0.68 for wt/wt, 0.28 for wt/m, and 0.04 for m/m. The allelic distribution was appropriate at 82.3% (95%CI: 80.5%-83.9%) for CYP2C19*1, 17.3% (95%CI:15.7%-19.0%) for CYP2C19*2 (m1 ), and 0.4% (95%CI: 0.1%-0.7%) for CYP2C19*3 (m2 ) in these populations.

CONCLUSIONS

We conclude that subjects of African ancestry have a low frequency of the CYP2C19 PM phenotype and genotype; that the defective CYP2C19 alleles are uncommon, and that a small proportion of heterozygotes exists in the EM subpopulation.

P-glycoprotein and cytochrome P-450 3A inhibition: dissociation of inhibitory potencies

Many P-glycoprotein (P-gp) inhibitors studied in vitro and in vivo are also known or suspected to be substrates and/or inhibitors of cytochrome P-450 3A (CYP3A). Such overlap raises the question of whether CYP3A inhibition is an intrinsic characteristic of P-gp inhibitors, a matter of concern in the development and rational use of such agents. Thus, the purpose of the present study was to determine whether the ability to inhibit P-gp and CYP3A is, in fact, linked and whether specific P-gp inhibitors with limited ability to inhibit CYP3A can be identified. Therefore, the potency of a series of 14 P-gp inhibitors was assessed by measuring their inhibition of the transepithelial flux across Caco-2 cells of digoxin, a prototypical P-gp substrate. CYP3A inhibition was determined from the impairment of nifedipine oxidation by human liver microsomes. Determination of the apparent Ki values for CYP3A inhibition and the IC50s for P-gp and CYP3A inhibition allowed comparison of the relative inhibitory potency of the compounds on the two proteins' function. The IC50s for P-gp inhibition ranged from 0.04 to 3.8 microM. All compounds inhibited CYP3A with apparent Ki values of between 0.3 and 76 microM and IC50s between 1.5 and 50 microM. However, no correlation was found between the extent of P-gp inhibition and CYP3A inhibition, and the ratio of the IC50 for CYP3A inhibition to the IC50 for P-gp inhibition varied from 1.1 to 125. These results demonstrate that, although many P-gp inhibitors are potent inhibitors of CYP3A, a varying degree of selectivity is present. The development and use of P-gp inhibitors with minimal or absent CYP3A inhibitory effects should decrease the impact of drug interactions on the therapeutic use of such compounds.

A novel transversion in the intron 5 donor splice junction of CYP2C19 and a sequence polymorphism in exon 3 contribute to the poor metabolizer phenotype for the anticonvulsant drug S-mephenytoin

Cytochrome P-450 (CYP) 2C19 is responsible for the metabolism of a number of therapeutic agents such as S-mephenytoin, omeprazole, proguanil, certain barbiturates, diazepam, propranolol, citalopram and imipramine. Genetic polymorphisms in this enzyme are responsible for the poor metabolizers (PM) of mephenytoin, which represent approximately 13-23% of Asians and 3-5% of Caucasians. Several polymorphisms contribute to this phenotype. We have isolated two new allelic variants that contribute to the PM phenotype in Caucasians. CYP2C19*7 contained a single T --> A nucleotide transversion in the invariant GT at the 5' donor splice site of intron 5. The second PM allele, CYP2C19*8, consisted of a T358C nucleotide transition in exon 3 that results in a Trp120Arg substitution. In a bacterial expression system, CYP2C198 protein exhibited a dramatic (approximately 90% and 70%) reduction in the metabolism of S-mephenytoin and tolbutamide, respectively, when compared with the wild-type CYP2C191B protein. Restriction fragment length polymerase chain reaction tests were developed to identify the new allelic variants.

OATP and P-glycoprotein transporters mediate the cellular uptake and excretion of fexofenadine

Fexofenadine, a nonsedating antihistamine, does not undergo significant metabolic biotransformation. Accordingly, it was hypothesized that uptake and efflux transporters could be importantly involved in the drug's disposition. Utilizing a recombinant vaccinia expression system, members of the organic anion transporting polypeptide family, such as the human organic anion transporting polypeptide (OATP) and rat organic anion transporting polypeptides 1 and 2 (Oatp1 and Oatp2), were found to mediate [(14)C]fexofenadine cellular uptake. On the other hand, the bile acid transporter human sodium taurocholate cotransporting polypeptide (NTCP) and the rat organic cation transporter rOCT1 did not exhibit such activity. P-glycoprotein (P-gp) was identified as a fexofenadine efflux transporter, using the LLC-PK1 cell, a polarized epithelial cell line lacking P-gp, and the derivative cell line (L-MDR1), which overexpresses P-gp. In addition, oral and i.v. administration of [(14)C]fexofenadine to mice lacking mdr1a-encoded P-gp resulted in 5- and 9-fold increases in the drug's plasma and brain levels, respectively, compared with wild-type mice. Also, a number of drug inhibitors of P-gp were found to be effective inhibitors of OATP. Because OATP transporters and P-gp colocalize in organs of importance to drug disposition such as the liver, their activity provides an explanation for the heretofore unknown mechanism(s) responsible for fexofenadine's disposition and suggests potentially similar roles in the disposition of other xenobiotics.

Genetic and dietary predictors of CYP2E1 activity: a phenotyping study in Hawaii Japanese using chlorzoxazone

Cytochrome P4502E1 (CYP2E1) is considered to play an important role in the metabolic activation of procarcinogens such as N-nitrosoamines and low molecular weight organic compounds. An RsaI polymorphism is present in the 5'-flanking region of the CYP2E1 gene, which could possibly affect its transcription. However, the relationship between genotype and the phenotypic catalytic activity of the enzyme has not been defined. Also, the effects in humans of specific dietary factors, other than ethanol, which have been shown in animal and in vitro studies to modulate CYP2E1 activity, are unknown. Accordingly, the CYP2E1-mediated metabolism of chlorzoxazone to its 6-hydroxy metabolite was investigated in 50 healthy Japanese of both sexes in Hawaii. The oral clearance of the in vivo probe, the trait measure of CYP2E1 activity, was smaller than that reported in European-Americans. Significantly, after adjustment for age and sex, the oral clearance of chlorzoxazone decreased with the number of variant c2 alleles, and its mean in the c2/c2 genotype (147 ml/min) was statistically lower (P < or = 0.05) than that for either the homozygous wild-type (238 ml/min) or the heterozygote (201 ml/min) genotypes. Stepwise multiple regression analysis indicated that body weight was a major contributor to the interindividual variability in the oral clearance of chlorzoxazone, accounting for 43% of the variance. Consumption of lettuce, broccoli, and black tea explained additional components of the variability (7, 5, and 6%, respectively), as did medication use (3%), age (4%), and CYP2E1 genotype (5%). Overall, 73% of the variance could be accounted for by these variables. Body weight, lettuce, and use of medications were associated with increased CYP2E1 activity, and the other covariates were associated with reduced enzyme function. Because of the role that CYP2E1 plays in procarcinogen activation, especially of N-nitrosamines involved in lung cancer, the identified factors may account in part for observed differences in individual susceptibility to disease and may also have implications for cancer prevention.

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

PURPOSE

CYP3A and P-gp both function to reduce the intracellular concentration of drug substrates, one by metabolism and the other by transmembrane efflux. Moreover, it has been serendipitously noted that the two proteins have many common substrates and inhibitors. In order to test this notion more fully, systematic studies were undertaken to determine the P-gp-mediated transport and inhibitory characteristics of prototypical CYP substrates.

METHODS

L-MDR1, LLC-PK1, and Caco-2 cells were used to evaluate established CYP substrates as potential P-gp substrates and inhibitors in vitro, and mdr1a deficient mice were used to assess the in vivo relevance of P-gp-mediated transport.

RESULTS

Some (terfenadine, erythromycin and lovastatin) but not all (nifedipine and midazolam) CYP3A substrates were found to be P-gp substrates. Except for debrisoquine, none of the prototypical substrates of other common human CYP isoforms were transported by P-gp. Studies in mdr1a disrupted mice confirmed that erythromycin was a P-gp substrate but the CYP3A-inhibitor ketoconazole was not. In addition, CYP3A substrates and inhibitors varied widely in their ability to inhibit the P-gp-mediated transport of digoxin.

CONCLUSIONS

These results indicate that the overlap in substrate specificities of CYP3A and P-gp appears to be fortuitous rather than indicative of a more fundamental relationship.

Inhibition of P-glycoprotein-mediated drug transport: A unifying mechanism to explain the interaction between digoxin and quinidine [seecomments]

BACKGROUND

Although quinidine is known to elevate plasma digoxin concentrations, the mechanism underlying this interaction is not fully understood. Digoxin is not extensively metabolized, but it is known to be transported by the drug efflux pump P-glycoprotein, which is expressed in excretory tissues (kidney, liver, intestine) and at the blood-brain barrier. Accordingly, we tested the hypothesis that inhibition of P-glycoprotein-mediated digoxin transport by quinidine contributes to the digoxin-quinidine interaction.

METHODS AND RESULTS

First, we demonstrated active transcellular transport of both digoxin and quinidine in cultured cell lines that express P-glycoprotein in a polarized fashion. In addition, 5 micromol/L quinidine inhibited P-glycoprotein-mediated digoxin transport by 57%. Second, the effect of quinidine on digoxin disposition was studied in wild-type and in mdr1a(-/-) mice, in which the gene expressing the major digoxin-transporting P-glycoprotein has been disrupted. Because the in vitro data showed that quinidine itself is a P-glycoprotein substrate, quinidine doses were reduced in mdr1a(-/-) mice to produce plasma concentrations similar to those in wild-type control animals. Quinidine increased plasma digoxin concentrations by 73.0% (P=0.05) in wild-type animals, compared with 19.5% (P=NS) in mdr1a(-/-) mice. Moreover, quinidine increased digoxin brain concentrations by 73.2% (P=0.05) in wild-type animals; by contrast, quinidine did not increase digoxin brain concentrations in mdr1a(-/-) mice but rather decreased them (-30.7%, P<0.01).

CONCLUSIONS

Quinidine and digoxin are both substrates for P-glycoprotein, and quinidine is a potent inhibitor of digoxin transport in vitro. The in vivo data strongly support the hypothesis that inhibition of P-glycoprotein-mediated digoxin elimination plays an important role in the increase of plasma digoxin concentration occurring with quinidine coadministration in wild-type mice and thus support a similar mechanism in humans.

Human MRP3 transporter: identification of the 5'-flanking region, genomic organization and alternative splice variants

In humans, at least six members of the multidrug resistance-associated protein (MRP) family are thought to exist. Here we report the molecular cloning of two splice variants of MRP3 from human liver. In addition, MRP3 genomic organization including the 5'-flanking region and a major portion of the MRP3 intron-exon organization are identified and characterized.

Selective effect of liver disease on the activities of specific metabolizing enzymes: investigation of cytochromes P450 2C19 and 2D6

BACKGROUND AND OBJECTIVES

Drug metabolism is influenced by liver disease because of the central role that the liver plays in metabolic activities in the body. However, it is still unclear how activities of specific drug-metabolizing enzymes are influenced by the presence and severity of liver disease. As a consequence, alteration in metabolism of specific drugs cannot be easily predicted or appropriate dosage adjustment recommendations made.

METHODS

The activities of cytochromes P450 (CYP) 2C19 and 2D6 were investigated in a group of patients with mild or moderate liver disease (n = 18) and a group of healthy control subjects (n = 10). The disposition of racemic mephenytoin for CYP2C19 and debrisoquin for CYP2D6 were characterized in plasma and urine samples collected over 192 hours.

RESULTS

The elimination of S-mephenytoin was severely reduced among patients with liver disease, resulting in a 79% decrease in plasma clearance for all patients combined. This reduction was related to the severity of disease, patients with moderate disease being affected more severely than patients with mild disease. Similar differences were observed in the urinary excretion of 4'-hydroxymephenytoin metabolite. By contrast, there was no effect on the disposition of R-mephenytoin or debrisoquin.

CONCLUSION

These results show selectivity in the effect of liver disease on activities of specific metabolizing enzymes, CYP2C19 being more sensitive than CYP2D6. They suggest that recommendations for modification in drug dosage in the presence of liver disease should be based on knowledge of the particular enzyme involved in metabolism of the drug. The results emphasize the need for further studies of each specific drug-metabolizing enzyme in the presence of liver disease.

In vitro and in vivo drug interactions involving human CYP3A

Cytochrome P4503A (CYP3A) is importantly involved in the metabolism of many chemically diverse drugs administered to humans. Moreover, its localization in high amounts both in the small intestinal epithelium and liver makes it a major contributor to presystemic elimination following oral drug administration. Drug interactions involving enzyme inhibition or induction are common following the coadministration of two or more CYP3A substrates. Studies using in vitro preparations are useful in identifying such potential interactions and possibly permitting extrapolation of in vitro findings to the likely in vivo situation. Even if accurate quantitative predictions cannot be made, several classes of drugs can be expected to result in a drug interaction based on clinical experience. In many instances, the extent of such drug interactions is sufficiently pronounced to contraindicate the therapeutic use of the involved drugs.

An additional defective allele, CYP2C19*5, contributes to the S-mephenytoin poor metabolizer phenotype in Caucasians

The metabolism of the anticonvulsant drug mephenytoin exhibits a genetic polymorphism in humans. This polymorphism exhibits marked racial heterogeneity, with the poor metabolizer PM phenotype representing 13-23% of oriental populations, but only 2-5% of Caucasian populations. Two defective CYP2C19 alleles (CYP2C19*2 and CYP2C19*3) have been described, which account for more than 99% of Oriental poor metabolizer alleles but only approximately 87% of Caucasian poor metabolizer alleles. Therefore, additional defects presumably contribute to the poor metabolizer in Caucasians. Recent studies have found a third mutation CYP2C19*4, which accounts for approximately 3% of Caucasian poor metabolizer alleles. A fourth rare mutation (CYP2C19*5A) (C99,A991,Ile331;C1297T,Arg433-->Trp) resulting in an Arg433 to Trp substitution in the heme-binding region has been reported in a single Chinese poor metaboliser outlier belonging to the Bai ethnic group. The present study identifies a second variant allele CYP2C19*5B (C99-->T; A991-->G, Ile331-->Val; C1297-T, Arg433-->Trp in one of 37 Caucasian poor metabolizers. The frequency of the CYP2C19*5 alleles is low in Chinese (approximately 0.25% in the Bai ethnic group) and Caucasians (< 0.9%). However, these alleles contribute to the poor metabolizer phenotype in both ethnic groups and increases the sensitivity of the genetic tests for identifying defective alleles to approximately 100% in Chinese poor metabolizers and 92% in Caucasian poor metabolizers genotyped in our laboratory. The Arg433 to Trp mutation in the heme-binding region essentially abolishes activity of recombinant CYP2C19*5A toward S-mephenytoin and tolbutamide, which is consistent with the conclusion that CYP2C19*5 represents poor metabolizer alleles.

The drug transporter P-glycoprotein limits oral absorption and brain entry of HIV-1 protease inhibitors

Currently available HIV-1 protease inhibitors are potent agents in the therapy of HIV-1 infection. However, limited oral absorption and variable tissue distribution, both of which are largely unexplained, complicate their use. We tested the hypothesis that P-glycoprotein is an important transporter for these agents. We studied the vectorial transport characteristics of indinavir, nelfinavir, and saquinavir in vitro using the model P-glycoprotein expressing cell lines L-MDR1 and Caco-2 cells, and in vivo after intravenous and oral administration of these agents to mice with a disrupted mdr1a gene. All three compounds were found to be transported by P-glycoprotein in vitro. After oral administration, plasma concentrations were elevated 2-5-fold in mdr1a (-/-) mice and with intravenous administration, brain concentrations were elevated 7-36-fold. These data demonstrate that P-glycoprotein limits the oral bioavailability and penetration of these agents into the brain. This raises the possibility that higher HIV-1 protease inhibitor concentrations may be obtained by targeted pharmacologic inhibition of P-glycoprotein transport activity.

A new genetic defect in human CYP2C19: mutation of the initiation codon is responsible for poor metabolism of S-mephenytoin

The 4'-hydroxylation of the S-enantiomer of the anticonvulsant drug mephenytoin exhibits a genetic polymorphism in humans. This polymorphism shows marked interracial heterogeneity, with the poor metabolizer (PM) phenotype representing 2 to 5% of Caucasian and 13 to 23% of Asian populations. Two defective CYP2C19 alleles, CYP2C19*2 and CYP2C19*3, have been described which account for approximately 87% of Caucasian and > 99% of Oriental PM alleles. The present study identifies a new allele (CYP2C19*4) in Caucasian PMs which contains an A-->G mutation in the initiation codon. A new polymerase chain reaction-restriction fragment length polymorphism genotyping test was developed, and the incidence of this allele was examined in a European Caucasian population which had been phenotyped for mephenytoin metabolism. One of nine putative PMs was heterozygous for CYP2C19*2/CYP2C19*4, which suggests that CYP2C19*4 represents a defective allele. Six of the seven remaining putative PMs available for genotyping were explained by CYP2C19*2. The frequency of the CYP2C19*4 allele in Caucasians was 0.6%. An additional Caucasian PM from a separate study was also heterozygous for CYP2C19*2 and CYP2C19*4. To verify that CYP2C19*4 represented a defective CYP2C19 allele, the initiation codon of the normal CYP2C19*1 cDNA was mutated to a GTG, and both cDNAs were expressed in yeast. Recombinant CYP2C19 protein was detected by Western blot analysis of colonies transformed with CYP2C19*1 cDNA, but not in those transformed with CYP2C19*4 cDNA. The two cDNAs were also used in an in vitro coupled transcription/translation assay. CYP2C19 protein was translated only from the CYP2C19*1 allele. These data indicate that CYP2C19*4 represents a new PM allele.

Mephenytoin disposition and serum bile acids as indices of hepatic function in chronic viral hepatitis

BACKGROUND AND OBJECTIVES

The effect of chronic viral hepatitis on liver function may vary from none to hepatic failure. Changes in function are usually the result of impaired hepatocyte function or altered vascular flow and architecture. Conventional liver function tests usually cannot distinguish contributions from these mechanisms or indicate degree of hepatic metabolic dysfunction. An alternative approach is to measure the hepatic metabolism of a highly extracted compound whose oral clearance and systemic bioavailability are dependent on both hepatocyte function and degree of portosystemic shunt.

METHODS

The stereoselective metabolism of racemic mephenytoin (100 mg oral dose) was investigated in 35 patients with chronic viral hepatitis and compared with 153 healthy subjects. The mephenytoin R/S enantiomeric ratio and cumulative excretion of the 4'-hydroxymephenytoin metabolite in a 0- to 8-hour urine sample were used in addition to serum bile acid levels and pathologic examination of biopsy specimens to assess the severity of hepatic dysfunction and portosystemic shunting.

RESULTS

The patients as a group excreted less 4'-hydroxymephenytoin and had a smaller R/S enantiomeric ratio of mephenytoin. The two measures were discriminatory between the patient groups classified by either serum cholylglycine level or pathologic examination of biopsy specimens. Combination of the two measures of mephenytoin metabolism allowed the patients to be classified into three groups: normal hepatocyte function without portosystemic shunt, normal hepatocyte function with portosystemic shunt, and low hepatocyte function with or without portosystemic shunt.

CONCLUSION

This study has shown the potential usefulness of mephenytoin metabolism as a sensitive indicator of hepatic pathologic condition with an ability to discriminate between contributory alternative mechanisms.

The relationship between cytochrome P4502E1 activity and plasma fluoride levels after sevoflurane anesthesia in humans

We determined whether the perianesthetic plasma fluoride levels after sevoflurane anesthesia in humans were correlated with the metabolic ratio (MR) of 6-hydroxychlorzoxazone to chlorzoxazone, an in vivo probe for cytochrome P4502E1 (CYP2E1) activity. Thirty ASA physical status I or II patients scheduled for extraabdominal surgery were randomized to a chlorzoxazone (n = 20) or a control group (n = 10). Patients in the chlorzoxazone group received 500 mg chlorzoxazone orally on the morning of the day of surgery. Chlorzoxazone and its 6-hydroxymetabolite concentrations were measured in plasma 2 h after drug administration. Anesthesia was induced with propofol, fentanyl, and atracurium intravenously and maintained with sevoflurane (inspired concentration 1-3 vol%). Plasma fluoride concentrations were determined before the induction of anesthesia, at the cessation of sevoflurane, and 2, 4, 6, 10, and 24 h thereafter. The area under the plasma fluoride concentration-time curve (AUC) was calculated up to 24 h after sevoflurane cessation. MR correlated significantly with the plasma fluoride AUC (r2 = 0.28, P < 0.025), the elimination constant calculated for the postanesthetic 10- to 24-h period (r2 = 0.30, P < 0.025), and the plasma fluoride levels 24 h after the cessation of sevoflurane (r2 = 0.48, P < 0.05). A comparison between groups indicated that the administration of chlorzoxazone itself did not alter the postanesthetic fluoride kinetics. Thus, the interindividual variability in perianesthetic plasma fluoride levels after sevoflurane anesthesia is reflected by differences in the MR of chlorzoxazone and hence is related to the interindividual variability in CYP2E1 activity. We conclude that although the predictive value is limited, this study provides a reasonable basis for examining renal function after sevoflurane anesthesia in a subgroup of patients with a high preoperative metabolic ratio of chlorzoxazone.

IMPLICATIONS

CYP2E1 metabolizes sevoflurane as measured by the metabolic ratio of chlorzoxazone. Patients with a high ratio may be used to justify examining renal function in patients receiving sevoflurane.

The effects of diet, aging and disease-states on presystemic elimination and oral drug bioavailability in humans

Presystemic metabolism occurring in the intestinal epithelium and/or liver is frequently an important determinant of drug bioavailability after oral administration. Several factors are potentially involved in such a first-pass effect and their modulation may significantly contribute to intra- and interindividual variability in a drug's plasma concentration-time curve. For example, macronutrient intake and nutritional status may alter cytochrome P-450 (CYP) metabolism by the liver, and food per se in the form of a meal can also affect the first-pass metabolism of some drugs. More important changes, however, result from micronutrients and non-nutrients present in food. In the case of charcoal-broiled and smoked foods, polycyclic aromatic hydrocarbons formed during their preparation result in the induction of xenobiotic metabolizing enzymes, especially those regulated by the Ah-receptor, e.g. CYP1A, which are localized in the intestinal tract. A similar effect also occurs following the ingestion of cruciferous vegetables like brussels sprouts and cabbage, which contain indole-related phytochemicals. Such induction can markedly reduce a drug's oral bioavailability. By contrast, the glucosinolate breakdown products of other vegetables produce inhibition of drug metabolism. In the case of phenethyl isothiocyanate-containing watercress, CYP2E1 activity is markedly impaired; however, other organo-sulfur compounds present in, for example, garlic appear to have essentially no effect on drug metabolism. Constituents of grapefruit juice also result in reduced first-pass metabolism, especially for drugs that are CYP3A substrates. Again, this dietary effect is more pronounced in the intestinal epithelium than the liver. A similar, but more generalized, phenomenon also appears to be associated with eating piperine- and capsaicin-containing spices. Possible future applications of such metabolic inhibition include the use of active phytochemicals as bioavailability enhancers for drugs exhibiting a large first-pass effect, and also as cancer chemoprotective agents where CYP-mediated procarcinogen activation is a critical initial step in carcinogenesis. Aging results in a number of physiological changes that potentially can alter drug metabolism and presystemic elimination. By far the most important of these is a reduction in drug metabolizing enzyme activity. Unfortunately, the extent of this effect appears to be unpredictable, both with respect to a specific drug as well as a particular individual. However, the greatest age-related change in oral bioavailability and plasma concentrations is likely to occur with drugs that exhibit a significant first-pass effect (>80%) in young subjects. A similar situation also appears to apply when liver disease is present, especially when this is severe. A further complication in such patients is the presence of vascular shunting, which leads to drug-containing blood by-passing functional enzymes. As a result, plasma levels of drugs that normally exhibit marked first-pass metabolism may be many-fold higher in cirrhotic patients compared to those with normal liver function.

High affinity uptake by isolated rat hepatocytes of a linear pseudo-hexapeptide, ditekiren

The hepatic elimination of many oligopeptides is both rapid and extensive, and often limits their potential as therapeutic agents. The linear, hydrophobic pseudo-hexapeptide ditekiren, a renin inhibitor, is one such example. The mechanism(s) involved in its hepatic clearance are largely unknown; accordingly, the characteristics of ditekiren's transport into isolated rat hepatocytes was investigated. In addition to a concentration-independent, linear process, uptake also involved a carrier-mediated component (Km = 0.2 +/- 0.05 microM; Vmax = 11.6 +/- 0.6 pmol (mg protein)[-1] min[-1]). Phenobarbital pretreatment in vivo resulted in marked induction of such transport. Negative results from cis-inhibition studies with substrates and/or inhibitors of well-established hepatic transport systems, e.g., sodium-dependent bile acid, sodium-independent multispecific bile acid and cation carriers, ruled out their involvement in ditekiren's uptake. By contrast, a number of cyclic and linear oligopeptides inhibited the uptake process to varying extents and in the case of EMD-59121, the most inhibitory compound, the interaction was competitive in nature. Collectively, these data suggest the presence of a novel high affinity, low capacity transporter in rat hepatocytes with specific affinity for ditekiren and possibly other oligopeptides.

Characterization of the hepatic canalicular membrane transport of a model oligopeptide: ditekiren

Many small oligopeptides are rapidly excreted unchanged into bile, which requires vectorial transport across the hepatocyte. To characterize the involved carrier system(s) at the canalicular membrane, studies were undertaken with vesicle preparations from the rat and the model pseudohexapeptide ditekiren. The initial uptake rate into inside-out-oriented vesicles was found to be ATP- and temperature-dependent and saturable. Kinetic analysis indicated the involvement of three processes: (1) an ATP-dependent carrier-mediated process (Km = 19.1 +/- 4.26 microM; mean +/- S.E.M.), Vmax = 140 +/- 29.4 pmol/mg of protein/15 sec), (2) an ATP-independent carrier-mediated transporter (Km = 17.2 +/- 9.58 microM, Vmax = 62.9 +/- 24.5 pmol/mg of protein/15 sec) and (3) a nonsaturable component. ATP-dependent uptake was inhibited by several other oligopeptides, which in the case of EMD 51921 was competitive. Cis-inhibition studies with known substrates for the canalicular bile salt (taurocholate), multispecific organic anion (glutathione disulfide) and P-glycoprotein (daunomycin, nicardipine, cyclosporin A) transporters indicated a major role for the latter carrier system. Inhibition of the initial uptake rate of ditekiren by daunomycin was found to be competitive in nature (Ki = 16 microM). These findings indicate that the biliary excretion of ditekiren and possibly other hydrophobic oligopeptides is mediated, in part, by P-glycoprotein and suggest a possible physiological role for this hepatic transporter.

Dietary salt increases first-pass elimination of oral quinidine

BACKGROUND

Some cytochrome P450 (CYP) enzymes, including CYP3A, are expressed not only in the liver but also in the intestine; the latter may therefore be an important site of drug disposition. Animal data suggests that dietary salt modulates expression of renal CYPs. We therefore hypothesized that intestinal CYP3A may be similarly modulated by dietary salt.

METHODS

The effect of changes in dietary salt on the disposition of two CYP3A substrates, quinidine (administered orally and intravenously) and 14C-erythromycin (administered intravenously) were determined after normal volunteers were given high-salt (400 mEq/day) and low-salt (10 mEq/day) diets for 7 to 10 days each.

RESULTS

Plasma concentrations after oral quinidine were significantly lower during the high-salt phase, with the difference between the two treatments attributable to changes within the first 1 to 4 hours after administration. For example, the area under the plasma concentration-time curve for the first hour after drug administration was 0.56 +/- 0.38 microgram.hr/ml for the high-salt diet compared with 1.57 +/- 0.60 micrograms.hr/ml for the low-salt diet (p < 0.05). Similarly, the peak plasma concentration (Cmax) achieved was lower and the time to reach Cmax was later for the high-salt diet (p < 0.05). In contrast, the terminal phase elimination half-lives were similar for the two diets, and no differences in disposition were found with the intravenous drug. The erythromycin breath test was unaffected by the dietary treatments.

CONCLUSIONS

These results indicate an effect of dietary salt on the presystemic disposition of orally administered quinidine. Although the mechanism(s) of CYP3A activity modulation is unknown, this finding may be important in determining drug availability in conditions associated with abnormal salt homeostasis.

Modulation of CYP2E1 activity by isoniazid in rapid and slow N-acetylators

AIMS

An investigation was undertaken to compare the effects of isoniazid pretreatment on the CYP2E1-mediated 6-hydroxylation of chlorzoxazone in healthy subjects of known N-acetylator phenotype.

METHODS

CYP2E1 activity was estimated based on the 6-hydroxylation of chlorzoxazone following single dose (250 mg) oral administration to seven slow and eight rapid N-acetylators who were in good health. Separate studies were performed prior to and 14 days after the subjects received 300 mg isoniazid daily. Additional investigations were undertaken 2 and 16 days after discontinuing treatment with the antitubercular agent.

RESULTS

Concomitant administration of chlorzoxazone with the final dose of isoniazid resulted in reduced metabolism in both phenotypes; however, the extent of inhibition of 6-hydroxylation was greater in the slow N-acetylators-about 80% vs 60%. Two days after stopping isoniazid administration, chlorzoxazone's pharmacokinetic parameters had returned to their baseline values and remained constant for a further 14 days in the rapid acetylators. In contrast, chlorzoxazone's 6-hydroxylation in slow acetylators was increased by about 60% compared with baseline at 2 days after discontinuing isoniazid but had returned to its initial value 14 days later.

CONCLUSIONS

The interphenotypic difference in the time-dependent interactions of isoniazid with CYP2E1 probably reflect a higher drug exposure in slow acetylators. Inhibition of CYP2E1 activity occurs in both N-acetylator phenotypes but is less extensive in fast acetylators, during the time that effective levels of isoniazid are present in the body. Increased CYP2E1 activity reflective of enzyme induction, on the other hand, is only observable following isoniazid's elimination and is more extensive in slow than rapid acetylators. Even then, however, such induction is relatively modest and of short duration.

In vivo and in vitro characterization of CYP2E1 activity in Japanese and Caucasians

Chlorzoxazone's disposition after oral administration was determined in 20 young healthy Caucasian men and a similar group of Japanese men. The drug's plasma concentrations were significantly higher and its rate of elimination slower in Japanese compared to Caucasian men. Accordingly, chlorzoxazone's oral clearance was smaller (40%) in Japanese men and a similar difference (30%) was still apparent after normalizing for body weight (3.74 +/- 1.23 versus 5.05 +/- 1.41 ml.min-1.kg-1, P < .05). This slower elimination was associated with a reduced (fractional) clearance by 6-hydroxylation (2.34 +/- 1.04 ml.min-1.kg-1 versus 3.23 +/- 1.10, P < .05). Because such metabolism is mediated by cytochrome P4502E1 (CYP2E1), these findings suggest a lower level of the enzyme's catalytic activity in Japanese men. This was confirmed by in vitro studies with microsomes prepared from livers of individuals representative of the two racial groups. CYP2E1 levels were lower (61% P < .002) and CYP2E1-mediated chlorzoxazone 6-hydroxylase (22%, P < .001) and aniline 4-hydroylase (35%, P < .0001) activities were reduced in Japanese preparations compared to those from Caucasians. No relationships were found between measures of CYP2E1 activity, both in vivo and in vitro, and genomic polymorphisms in the CYP2E1 gene identified by Rsal/Pstl and Dral restriction fragment length polymorphisms. Collectively, these data show an interracial difference in CYP2E1 activity. Because this enzyme is importantly involved in the activation of environmental procarcinogens, such a difference may account, in part, for the lower rate of some cancers, e.g., lung cancer, in Japanese compared to Caucasians men.

Cytochrome P4503A (CYP3A) metabolism: prediction of in vivo activity in humans

CYP3A is one of the most important cytochrome P450 isoforms responsible for drug metabolism by humans because it is the major such enzyme in critical tissues such as the gastrointestinal tract and liver, and it is involved in the oxidative biotransformation of numerous clinically useful therapeutic agents. Many factors regulate CYP3A expression but these are being increasingly defined so that the disposition characteristics of a drug whose metabolism is importantly mediated by this isoform can be reasonably well predicted a priori. For example, metabolic clearance is distributed within a population in a unimodal fashion but marked (5- to 20-fold) interindividual variability is present as a consequence of both genetic and nongenetic factors. In addition, first-pass metabolism occurs following oral drug administration and this may be extensive so that bioavailability is low. CYP3A activity can also be readily modulated by inducers like rifampicin and several anticonvulsant agents, and many potent inhibitors exist such as azole antifungal agents and macrolide antibiotics. Accordingly, the potential for drug interactions with these drugs as well as other CYP3A substrates, when given concomitantly, is high. Metabolism involving CYP3A is also likely to be affected by liver disease as well as aging, and modest differences may be present between men and women but these are often clinically unimportant. Because of such predictability, knowledge of the role and importance of CYP3A in the metabolism of a putative drug candidate is becoming increasingly desirable at an early stage in the development process. In vitro studies using human liver preparations, including microsomes, cultured hepatocytes and heterologous expressed enzymes, can provide important insights in this regard. This is particularly the case for identifying potential drug interactions whose clinical significance can be subsequently assessed. Data with respect to terfenadine and cyclosporine obtained several years after their approval and marketing, indicate that, if available and applied during their development, the paradigm of using in vitro studies to rationally direct and prioritize clinical studies would have prospectively prevented the serious adverse effects and inefficacy that were only recognized during their empiric clinical use. Such examples, along with those associated with the genetic polymorphism of CYP2D6, provide strong justification for establishing the role and importance of individual CYP isoforms in a candidate drug's metabolism at an early stage.