The use of 3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-methoxy-4-methylcoumarin (AMMC) as a specific CYP2D6 probe in human liver microsomes

Recently, a novel nonfluorescent probe 3-[2-(N,N-diethyl-N-methylammonium)-ethyl]-7-methoxy-4-methylcoumarin (AMMC), which produces a fluorescent metabolite AMHC (3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-hydroxy-4-methylcoumarin) was used with microsomes containing recombinant enzymes (rCYP) to monitor CYP2D6 inhibition in a microtiter plate assay. This article describes the studies that were performed in human liver microsomes (HLM) to establish the selectivity of AMMC toward CYP2D6. Metabolism studies in HLM showed that AMMC was converted to one metabolite identified by mass spectrometry as AMHC. Kinetic studies indicated an apparent K(m) of 3 microM with a V(max) of 20 pmol/min. mg of protein for the O-demethylation reaction. The O-demethylation of AMMC in HLM was inhibited significantly in the presence of a CYP2D6 inhibitory antibody. Using a panel of various HLM preparations (n = 12), a good correlation (r(2) = 0.95) was obtained between AMMC O-demethylation and bufuralol metabolism, a known CYP2D6 substrate, but not with probes for the other major xenobiotic metabolizing CYPs. Finally, only rCYP2D6 showed detectable metabolism in experiments conducted with rCYPs using AMMC at a concentration of 1.5 microM (near K(m)). However, at a concentration of 25 microM AMMC, rCYP1A also contributed significantly to the formation of AMHC. Knowing the experimental conditions under which AMMC was selective for CYP2D6, a microtiter assay was developed to study the inhibition of various compounds in HLM using the fluorescence of AMHC as an indication of CYP2D6 activity. The inhibition potential of various chemicals was found to be comparable to those determined using the standard CYP2D6 probe, bufuralol, which requires high-performance liquid chromatography separation for the analysis of its CYP2D6-mediated 1'-hydoxylated metabolite.

Human cell mutagenicity of chlorinated and unchlorinated water and the disinfection byproduct 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX)

Extracts of three water samples--humic acid-enriched water-both peatland water and drinking water, both with and without chlorination were tested for mutagenicity at the tk locus in MCL-5 cells, a line of human B-lymphoblastoid cells that express cytochrome P450 enzymes and microsomal epoxide hydrolase. Our results show that chlorination caused a 5.5-fold increase (P<0.0001) in the mutagenicity of the humic acid-enriched water. The unchlorinated peatland water was mutagenic at the two highest doses (240 and 480 microg equivalent total organic carbon (TOC)/ml), possibly due to polycyclic aromatic hydrocarbons (PAH) that were measured in the peat. In contrast, the chlorinated peatland water was non-mutagenic at low doses, while at the highest dose (240 microg equivalent TOC/ml) the sample was so toxic that an insufficient number of cells survived treatment to allow plating. The chlorinated and unchlorinated drinking water were both non-mutagenic. 3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a potent bacterial mutagen and chlorine-disinfection byproduct, was also tested in MCL-5 cells as well as in two other human B-lymphoblastoid cell-lines, AHH-1 TK+/- and h1A1v2 cells, which differ from each other and from MCL-5 cells in the amounts of cytochrome P450 enzymes they can express. MX was mutagenic to all three cell-lines, but there was no apparent correlation between cytochrome P450 enzyme expression and the mutagenicity of MX. Overall, our results show that samples of chlorinated humic acid-enriched water and MX, a model chlorine-disinfection byproduct, are moderately mutagenic to human cells.

Potential role for P-glycoprotein in the non-proportional pharmacokinetics of UK-343,664 in man

1. UK-343,664 is a potent and specific PDE5 inhibitor. Following single oral doses to human volunteers, it exhibited non-proportional pharmacokinetics over the dose range 30-800 mg. Over this 27-fold dose range, Cmax and AUCt increased 247- and 287-fold respectively. The half-life (4-6 h) was similar at all doses. No systemic exposure was quantifiable at doses <10 mg. 2. UK-343,664 is a lipophilic molecule (log D7.4 = 3.1) and as such is expected to be cleared mainly by metabolism. Based on studies with expressed human P450 enzymes it was concluded that the metabolism of UK-343,664 was predominantly mediated by CYP3A4. With a moderate Km = 76 microM for this enzyme, saturation of first-pass metabolism alone was considered unlikely to account for the non-proportional pharmacokinetics. 3. UK-343,664 showed high affinity for P-glycoprotein in vitro, with a Km = 7.3 microM. In transport studies in LLC-PK1 cell monolayers transfected with P-glycoprotein, UK343,664 showed marked polarized transport which was concentration dependent. 4. The high affinity of UK-343,664 for P-glycoprotein is considered to be the primary source of the non-proportional pharmacokinetic profile observed in man.

Fluorometric high-throughput screening for inhibitors of cytochrome P450

Rapid screening for cytochrome P450 inhibitors is part of the current paradigm for avoiding development of drugs likely to give clinical pharmacokinetic drug-drug interactions and associated toxicities. We have developed microtiter plate-based, direct, fluorometric assays for the activities of the principal human drug-metabolizing enzymes, CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4, as well as for CYP2A6, which is an important enzyme in environmental toxicology. These assays are rapid and compatible with existing high-throughput assay instrumentation. For CYP1A2, CYP2C8, CYP2C9, CYP2C19, and CYP2D6, the potency of enzyme inhibition (IC50) is consistent regardless of the probe substrate or assay method employed. In contrast, CYP3A4 inhibition for an individual inhibitor shows significant differences in potency (>300-fold) depending on the probe substrate being used. We have investigated these differences through the use of several structurally distinct fluorescent substrates for CYP3A4 and several classical substrate probes (e.g., testosterone, nifedipine, and midazolam), with a panel of known, clinically significant, CYP3A4 inhibitors. The use of multiple probe substrates appears to be needed to characterize the inhibition potential of xenobiotics for CYP3A4.

Substrate-dependent modulation of CYP3A4 catalytic activity: analysis of 27 test compounds with four fluorometric substrates

Inhibition of cytochrome P450 catalytic activity is a principal mechanism for pharmacokinetic drug-drug interactions. Rapid, in vitro testing for cytochrome P450 inhibition potential is part of the current paradigm for identifying drug candidates likely to give such interactions. We have explored the extent that qualitative and quantitative inhibition parameters are dependent on the cytochrome P450 (CYP) 3A4 probe substrate. Inhibition potential (e.g., IC(50) values from 8-point inhibition curves) or activation potential for most compounds varied dramatically depending on the fluorometric probe substrates for CYP3A4 [benzyloxyresorufin (BzRes), 7-benzyloxy-4-trifluoromethylcoumarin (BFC), 7-benzyloxyquinoline (BQ), and dibenzylfluorescein (DBF)]. For 21 compounds that were primarily inhibitors, the range of IC(50) values for the four substrates varied from 2.1- to 195-fold with an average of 29-fold. While the rank order of sensitivity among the fluorometric substrates varied among the individual inhibitors, on average, BFC dealkylation was the most sensitive to inhibition, while BQ dealkylation was least sensitive. Partial inhibition was observed with BzRes and BQ but not for BFC and DBF. BzRes was more prone to activation, whereas dramatic changes in IC(50) values were observed when the BQ concentration was below the S(50). Three different correlation analyses indicated that IC(50) values with BFC, BQ, and DBF correlated well with each other, whereas the response with BzRes correlated more weakly with the other substrates. One of these correlation analyses was extended to the percent inhibition of 10 microM inhibitor with the standard CYP3A4 probe substrates testosterone, midazolam, and nifedipine. In this analysis the responses with BQ, BFC and DBF correlated well with testosterone and midazolam but more poorly with nifedipine. In the aggregate, BFC and DBF appear more suitable as an initial screen for CYP3A4 inhibition. However, the substrate-dependent effects reported here and by others indicate that all CYP3A4 inhibition data should be interpreted with caution.

Comparison between cytochrome P450 (CYP) content and relative activity approaches to scaling from cDNA-expressed CYPs to human liver microsomes: ratios of accessory proteins as sources of discrepancies between the approaches

Relative activity factors (RAFs) and immunoquantified levels of cytochrome P450 (CYP) isoforms both have been proposed as scaling factors for the prediction of hepatic drug metabolism from studies using cDNA-expressed CYPs. However, a systematic comparison of the two approaches, including possible mechanisms underlying differences, is not available. In this study, RAFs determined for CYPs 1A2, 2B6, 2C19, 2D6, and 3A4 in 12 human livers using lymphoblast-expressed enzymes were compared to immunoquantified protein levels. 2C19, 2D6, and 3A4 RAFs were similar to immunoquantified enzyme levels. In contrast, 1A2 RAFs were 5- to 20-fold higher than CYP1A2 content, and the RAF:content ratio was positively correlated with the molar ratio of NADPH:CYP oxidoreductase (OR) to CYP1A2. The OR:CYP1A2 ratio in lymphoblast microsomes was 92-fold lower than in human liver microsomes. Reconstitution experiments demonstrated a 10- to 20-fold lower activity at OR:CYP1A2 ratios similar to those in lymphoblasts, compared with those in human livers. CYP2B6-containing lymphoblast microsomes had 29- and 13-fold lower OR:CYP and cytochrome b(5):CYP ratios, respectively, than did liver microsomes and yielded RAFs that were 6-fold higher than CYP2B6 content. Use of metabolic rates from cDNA-expressed CYPs containing nonphysiologic concentrations of electron-transfer proteins (relative to human liver microsomes) in conjunction with hepatic CYP contents may lead to incorrect predictions of liver microsomal rates and relative contributions of individual isoforms. Scaling factors used in bridging the gap between expression systems and liver microsomes should not only incorporate relative hepatic abundance of individual CYPs but also account for differences in activity per unit enzyme in the two systems.

Analysis of drug transport and metabolism in cell monolayer systems that have been modified by cytochrome P4503A4 cDNA-expression

Human CYP3A4, the major human, intestinal, drug metabolizing cytochrome P450, has been introduced into three mammalian cell lines (Caco-2, MDCK and LLC-PK1) suitable for making drug permeability measurements. The levels and stability of expression were analyzed by enzyme assays (testosterone 6beta-hydroxylase and nifedipine oxidase). Long term, stable CYP3A4 expression/cell growth rate was obtained in MDCK cells. In the LLC-PK1 system, shorter term, stable expression was achieved. However, in Caco-2 cells, derivatives with better properties than those previously reported could not be obtained. The highest level of CYP3A4 catalytic activity was obtained in LLC-PK1 cells. In this system, CYP3A4 activity levels appeared comparable to median level human intestinal microsomes. Metabolite formation and inhibition kinetics were examined in cell monolayers. Nifedipine was found to be extensively metabolized (19%) during passage across cell monolayers. In general, affinity related parameters (apparent Km and apparent Ki) were 1.5- to three-fold higher under conditions of flux through the monolayers relative to steady-state conditions. These systems should be useful for examining the role of intestinal CYP3A4 in first-pass metabolism and drug-drug interactions.

Selective inhibition of human cytochrome P450 3A4 by N-[2(R)-hydroxy-1(S)-indanyl]-5-[2(S)-(1, 1-dimethylethylaminocarbonyl)-4-[(furo[2, 3-b]pyridin-5-yl)methyl]piperazin-1-yl]-4(S)-hydroxy-2(R)-phenylmethy lpentanamide and P-glycoprotein by valspodar in gene transfectant systems

Our previous report showed that L754.394 and valspodar (PSC833) are potent inhibitors of midazolam hydroxylation in human jejunum microsomes and vectorial transport of vinblastine in Caco-2 cells, respectively. In the present study, to directly examine the interactions of these compounds as well as other substrates with CYP3A4 and P-glycoprotein (P-gp), we performed in vitro inhibition studies using recombinant CYP3A4-expressed microsomes and an MDR1-transfected cell line, LLC-MDR1, respectively. In CYP3A4-expressed microsomes, both L754.394 and ketoconazole, at a concentration less than 0.5 microM, are the most potent inhibitors of the formation of 1'-hydroxymidazolam, a major metabolite of midazolam formed by CYP3A4. The greatest inhibitory effect on the transcellular transport of digoxin in LLC-MDR1 cells was observed in the presence of valspodar (<0.1 microM), followed by verapamil. From a comparison of the IC(50) values, it was shown that L754.394 and valspodar exhibited the highest selectivity for CYP3A4 and P-gp, respectively. To demonstrate such specificity, both midazolam hydroxylation and digoxin transport were observed in CYP3A4 transfected Caco-2 cells, which coexpress both P-gp and CYP3A4, in the presence or absence of L754.394 (0.5 microM) and valspodar (1.0 microM). L754.394 almost completely inhibited midazolam hydroxylation, but not digoxin transport, whereas almost complete inhibition of digoxin transport was observed in the presence of valspodar, but inhibition of the hydroxylation was minimal. Thus, the present study has demonstrated that L754.394 has a specific inhibitory effect on CYP3A4, whereas valspodar is specific for P-gp.

Fluorometric screening for metabolism-based drug--drug interactions

Inhibition of cytochromes P-450 (CYP) is a principal mechanism for metabolism-based drug interactions. In vitro methods for quantitatively measuring the extent of CYP inhibition are well-established. Classical methods use drug molecules as substrates and HPLC-based analysis. However, methodologies, which do not require HPLC separations for data acquisition generally offer higher throughputs and lower costs. Multiwell plate-based, direct, fluorometric assays for the activities of the five principal drug-metabolizing enzymes are available and parameters for the use of these substrates to measure CYP inhibition have been established. This methodology is quantitative, rapid, reproducible, and compatible with common high throughput screening instrumentation. This article describes approaches to establishing this methodology in a drug-discovery support program.

Differential protection by rat UDP-glucuronosyltransferase 1A7 against Benzo[a]pyrene-3,6-quinone- versus Benzo[a]pyrene-induced cytotoxic effects in human lymphoblastoid cells

UDP-glucuronosyltransferase 1A7 (UGT1A7) is a polyaromatic hydrocarbon (PAH)-inducible UGT with activity toward various benzo[a]pyrene (B[a]P) metabolites. To investigate the influence of rat UGT1A7 on B[a]P-induced cytotoxicity, human lymphoblastoid L3 cells were transfected with pMF6 (control expression vector), p167Dtk2 (microsomal epoxide hydrolase expression vector), or p167Dtk2-1A7 (epoxide hydrolase/UGT1A7 coexpression vector), and the cell populations were compared for sensitivity to B[a]P-induced effects. B[a]P inhibited cell proliferation and decreased relative cell survival of p167Dtk2 and p167Dtk2-1A7 cells to a similar extent. Metabolism studies using [(3)H]B[a]P revealed increased formation of glucuronide conjugates of B[a]P-4,5-diol, 3-OH-, or 9-OH-B[a]P and an unidentified metabolite by p167Dtk2-1A7 cells, but the presence of unconjugated metabolites suggested that glucuronidation capacity may be limited. No differences between p167Dtk2 and p167Dtk2-1A7 L3 cells were observed in the growth inhibitory effects of 3-OH-B[a]P or B[a]P-7,8-diol, but p167Dtk2-1A7-expressing cells were found to be less sensitive to B[a]P-3,6-quinone-induced effects on cell proliferation and relative cell survival. The effect was also observed in AHH-1 lymphoblastoid cells expressing UGT1A7 without epoxide hydrolase. The UGT1A7-expressing AHH-1 cells were also less sensitive to growth inhibition by B[a]P-1,6-quinone and B[a]P-6,12-quinone. Flow cytometric analysis of vehicle and B[a]P-3, 6-quinone-exposed cell populations showed an association between UGT1A7 expression and resistance to B[a]P-3,6-quinone-induced apoptosis and loss of cell viability. These data suggest that UGT1A7 may be preferentially active toward B[a]P-quinones and that UGT1A7 may represent the PAH-inducible UGT activity previously implicated in protection against toxic redox cycling by B[a]P-3,6-quinone.

CYP3A4 allelic variants with amino acid substitutions in exons 7 and 12: evidence for an allelic variant with altered catalytic activity

OBJECTIVE

To determine the existence of mutant and variant CgammaP3A4 alleles in three racial groups and to assess functions of the variant alleles by complementary deoxyribonucleic acid (cDNA) expression.

METHODS

A bacterial artificial chromosome that contains the complete CgammaP3A4 gene was isolated and the exons and surrounding introns were directly sequenced to develop primers to polymerase chain reaction (PCR) amplify and sequence the gene from lymphocyte DNA. DNA samples from Chinese, black, and white subjects were screened. Mutating the affected amino acid in the wild-type cDNA and expressing the variant enzyme with use of the baculovirus system was used to functionally evaluate the variant allele having a missense mutation.

RESULTS

To investigate the existence of mutant and variant CgammaP3A4 alleles in humans, all 13 exons and the 5'-flanking region of the human CgammaP3A4 gene in three racial groups were sequenced and four alleles were identified. An A-->G point mutation in the 5'-flanking region of the human CgammaP3A4 gene, designated CgammaP3A4*1B, was found in the three different racial groups. The frequency of this allele in a white population was 4.2%, whereas it was 66.7% in black subjects. The CgammaP3A4*1B allele was not found in Chinese subjects. A second variant allele, designated CgammaP3A4*2, having a Ser222Pro change, was found at a frequency of 2.7% in the white population and was absent in the black subjects and Chinese subjects analyzed. Baculovirus-directed cDNA expression revealed that the CYP3A4*2 P450 had a lower intrinsic clearance for the CYP3A4 substrate nifedipine compared with the wild-type enzyme but was not significantly different from the wild-type enzyme for testosterone 6beta-hydroxylation. Another rare allele, designated CgammaP3A4*3, was found in a single Chinese subject who had a Met445Thr change in the conserved heme-binding region of the P450.

CONCLUSIONS

These are the first examples of potential function polymorphisms resulting from missense mutations in the CgammaP3A4 gene. The CgammaP3A4*2 allele was found to encode a P450 with substrate-dependent altered kinetics compared with the wild-type P450.

The use of heterologously expressed drug metabolizing enzymes--state of the art and prospects for the future

The first report of the functional, heterologous expression of a mammalian cytochrome P450 (CYP) enzyme occurred more than a decade ago. In the intervening years, these expression systems have been optimized with regard to the specific requirements for production of catalytically active enzymes. In this review, we discuss the strengths and limitations of heterologously expressed enzymes as they affect in vitro drug metabolism studies. Emphasis is given to new applications (screens for CYP inhibition and novel enzyme mixtures) that have been enabled by high level, functional expression of CYP enzymes.

Mutagenicity of C24H14 PAH in human cells expressing CYP1A1

Relatively little is known about the mutagenicity of C24H14 PAH, a diverse group of five- and six-ring PAH, some of which are present at trace levels in the environment. To better understand the mutagenicity of this class of compounds, 11 C24H14 PAH, including benzo[a]perylene, benzo[b]perylene, dibenzo[a,e]fluoranthene, dibenzo[a,f]fluoranthene, dibenzo[j,l]fluoranthene, dibenzo[a,h]pyrene, dibenzo[a,i]pyrene, dibenzo[e,l]pyrene, naphtho[1,2-b]fluoranthene, naphtho[2,3-a]pyrene, and naphtho[2,3-e]pyrene, were tested in a mutagenicity assay based on human h1A1v2 cells. h1A1v2 cells are a line of human B-lymphoblastoid cells that have been engineered to express cytochrome P4501A1 (CYP1A1), an enzyme capable of metabolizing promutagenic PAH. Mutagenicity was measured at the thymidine kinase (tk) locus following a 72-h exposure period. Our results show that nine of the compounds were mutagenic. Benzo[a]perylene, dibenzo[a,e]fluoranthene, dibenzo[a,i]pyrene, and naphtho[2,3-a]pyrene were the most potent mutagens, having minimum mutagenic concentrations (MMC) (i.e., the dose at which the induced response was twice that of the negative controls) in the 1-5 ng/ml range. Benzo[b]perylene, dibenzo[a,h]pyrene, dibenzo[a,f]fluoranthene, and naphtho[2,3-e]pyrene were somewhat less potent mutagens, having MMC in the 10-30 ng/ml range. Dibenzo[e,l]pyrene, which had an MMC of 280 ng/ml, was the least potent mutagen. Dibenzo[j,l]fluoranthene and naphtho[1,2-b]fluoranthene were not mutagenic at the doses tested (1-3000 ng/ml). The most mutagenic compounds were also quite toxic. At the highest doses tested, benzo[a]perylene, dibenzo[a,e]fluoranthene, dibenzo[a,i]pyrene, dibenzo[a,h]pyrene, and dibenzo[a,f]fluoranthene induced > 60% killing, and naphtho[2,3-a]pyrene and naphtho[2,3-e]pyrene induced > 50% killing. Benzo[b]perylene, dibenzo[e,l]pyrene, dibenzo[j,l]fluoranthene, and naphtho[1,2-b]fluoranthene induced < 50% killing at the highest doses tested. Comparing these results to a previous study in which nine other C24H14 PAH were tested for mutagenicity in this same assay, it was found that dibenzo[a]pyrene isomers were generally more mutagenic than the other groups of C24H14 PAH tested. These observations are discussed with emphasis given to identifying C24H14 PAH that may be important environmental mutagens.

Transport and metabolic characterization of Caco-2 cells expressing CYP3A4 and CYP3A4 plus oxidoreductase

PURPOSE

To further characterize CYP3A4-transfected Caco-2 cells with regard to morphological, transport, and metabolic properties, and to evaluate a different Caco-2 cell strain transfected with both CYP3A4 and oxidoreductase (OR).

METHODS

Transfected Caco-2 cells, Caco-2 TC7 cells, and wild-type Caco-2 cells grown onto Millicell were used. We determined the morphological characteristics of transfected cell monolayers using light and transmission electron microscope. We determined the transport and metabolic capabilities of the transfected cells, TC7 cells, and wild-type cells with a variety of drugs, nutrients, and marker compounds.

RESULTS

The transfected Caco-2 cells formed a tight monolayer with TEER values and mannitol transport similar to the untransfected parent cell strain (wild type). However, the transfected cells (grown onto Millicell) reached maturity approximately 33% faster than the wild-type cells. Permeabilities of propranolol, nifedipine, testosterone, linopirdine, mannitol, and cephalexin were similar in transfected and wild-type Caco-2 cells. On the other hand, the transfected cells of early passages were much more metabolically active, and metabolized standard CYP3A4 substrates (e.g., testosterone and nifedipine) as much as 100 times faster than untransfected cells. In addition, metabolism of standard substrates was inhibitable by ketoconazole and TAO. Using comparable data, the transfected cells metabolized testosterone the fastest, followed by linopirdine and nifedipine (approximate ratio: 10:6:2). The metabolites of standard substrates were generally preferably excreted to the apical membrane.

CONCLUSION

The monolayers of newly transfected cells (CYP3A4 + OR) have a significantly increased level of CYP3A4 activities compared to untransfected cells. These cell monolayers also have desirable morphological and transport characteristics that are similar to untransfected cells.

Specific dehydrogenation of 3-methylindole and epoxidation of naphthalene by recombinant human CYP2F1 expressed in lymphoblastoid cells

3-Methylindole (3MI) is a naturally occurring pulmonary toxin that requires metabolic activation. Previous studies have shown that 3MI-induced pneumotoxicity resulted from cytochrome P-450-catalyzed dehydrogenation of 3MI to an electrophilic methylene imine (3-methyleneindolenine), which covalently bound to cellular macromolecules. Multiple cytochrome P-450s are capable of metabolizing 3MI to several different metabolites, including oxygenated products. In the present study, the role of human CYP2F1 in the metabolism of 3MI was examined to determine whether it catalyzes dehydrogenation rather than hydroxylation or ring oxidation. Metabolism was examined using microsomal fractions from human lymphoblastoid cells that expressed the recombinant human CYP2F1 P-450 enzyme. Expression of CYP2F1 in the lymphoblastoid cells proved to be an appropriate expression system for this enzyme. Products were analyzed using HPLC and the mercapturate, 3-[(N-acetylcystein-S-yl)methyl]indole, of the reactive intermediate was identified and quantified. Product analysis showed that human CYP2F1 efficiently catalyzed the dehydrogenation of 3MI to the methylene imine without detectable formation of indole-3-carbinol or 3-methyloxindole. High substrate concentrations of 3MI strongly inhibited production of the dehydrogenated product, a result that may indicate the existence of mechanism-based inhibition of CYP2F1 by 3MI. Recombinant CYP2F1 demonstrated remarkable selectivity for the bioactivation of 3MI to the putative dehydrogenated reactive electrophile. Bioactivation of naphthalene to its pneumotoxic epoxide by CYP2F1 was also demonstrated.

Development of a substrate-activity based approach to identify the major human liver P-450 catalysts of cyclophosphamide and ifosfamide activation based on cDNA-expressed activities and liver microsomal P-450 profiles

The contributions of specific human liver cytochrome P-450 (CYP) enzymes to the activation, via 4-hydroxylation, of the oxazaphosphorine anticancer prodrugs cyclophosphamide (CPA) and ifosfamide (IFA) were investigated. Analysis of a panel of 15 human P-450 cDNAs expressed in human lymphoblasts and/or baculovirus-infected insect cells (Supersomes) demonstrated that CYPs 2A6, 2B6, 3A4, 3A5, and three CYP2C enzymes (2C9, 2C18, 2C19) exhibited significant oxazaphosphorine 4-hydroxylase activity, with 2B6 and 3A4 displaying the highest activity toward CPA and IFA, respectively. CYP2B6 metabolized CPA at a approximately 16-fold higher in vitro intrinsic clearance (apparent Vmax/Km) than IFA, whereas 3A4 demonstrated approximately 2-fold higher Vmax/Km toward IFA. A relative substrate-activity factor (RSF)-based method was developed to calculate the contributions of individual P-450s to total human liver microsomal metabolism based on cDNA-expressed P-450 activity data and measurements of the liver microsomal activity of each P-450 form. Using this method, excellent correlations were obtained when comparing measured versus predicted (calculated) microsomal 4-hydroxylase activities for both CPA (r = 0. 96, p <.001) and IFA (r = 0.90, p <.001) in a panel of 17 livers. The RSF method identified CYP2B6 as a major CPA 4-hydroxylase and CYP3A4 as the dominant IFA 4-hydroxylase in the majority of livers, with CYPs 2C9 and 2A6 making more minor contributions. These predicted P-450 enzyme contributions were verified using an inhibitory monoclonal antibody for 2B6 and the P-450 form-specific chemical inhibitors troleandomycin for 3A4 and sulfaphenazole for 2C9, thus validating the RSF approach. Finally, Western blot analysis using anti-2B6 monoclonal antibody demonstrated the presence of 2B6 protein at a readily detectable level in all but one of 17 livers. These data further establish the significance of human liver CYP2B6 for the activation of the clinically important cancer chemotherapeutic prodrug CPA.

Effect of methanol, ethanol, dimethyl sulfoxide, and acetonitrile on in vitro activities of cDNA-expressed human cytochromes P-450

The effects of methanol, ethanol, dimethyl sulfoxide (DMSO), and acetonitrile were studied in vitro on nine individual, cDNAexpressed cytochrome P-450 activities (phenacetin O-deethylase for CYP1A1 and CYP1A2, coumarin 7-hydroxylase for CYP2A6, testosterone 6beta-hydroxylase for CYP3A4, 7-ethoxy-4-trifluoromethylcoumarin deethylase for CYP2B6, paclitaxel 6alpha-hydroxylase for CYP2C8, diclofenac 4'-hydroxylase for CYP2C9, S-mephenytoin 4-hydroxylase for CYP2C19, and (+/-)-bufuralol 1'-hydroxylase for CYP2D6) in commercially available human lymphoblastoid microsomes. These data show that specific solvents have enzyme-selective effects on P-450 activities. Methanol did not substantially inhibit (</=10%) any of the activities at 0.3%, but did inhibit CYP1A1, CYP2B6, CYP2C9, and CYP2D6 by 12 to 26% at 1%. In contrast, 0.1% ethanol inhibited CYP1A1, CYP2B6, and CYP2C19 by 20 to 30%. Ethanol at 1% did not inhibit CYP1A2, CYP3A4, CYP2C8, and CYP2C9. DMSO inhibited CYP3A4, CYP2C19, and CYP2D6 by 15 to 25% at 0.1%. However, DMSO had little effect on CYP1A2, CYP2A6, and CYP2C8. Acetonitrile, like methanol, did not inhibit any P-450 activity at 0.3% solvent except for CYP1A1 (26%) and CYP2B6 (13%). At 1%, acetonitrile decreased activities of CYP1A1 and CYP2B6 by 40 to 60%, and inhibited CYP2A6, CYP3A4, CYP2C19, and CYP2D6 activity by 10 to 20%. Acetonitrile also increased CYP2C9 activity by 10 to 15% above control values at 1 to 3% solvent. Excluding solubility considerations, methanol and acetonitrile appear to be the most suitable solvents for the introduction of substances to cytochrome P-450 incubations for in vitro metabolism studies.

Higher-throughput screening with human cytochromes P450

Recent progress has been made in increasing the throughput of measurements of cytochrome P450-mediated metabolism of drug candidates. This review focuses on two innovations: (i) the reduction of the number of samples for HPLC-based analyses by simultaneous measurement multiple analytes in a single injection (N in one analysis) and (ii) screens for the inhibition of cytochrome P450-mediated metabolism (a mechanism for drug-drug interactions) using fluorometric assays. Both of these innovations substantially increase the throughput and decrease the resources needed for conducting these routine preclinical studies.

Detection and characterization of novel polymorphisms in the CYP2E1 gene

To investigate whether interindividual variation in CYP2E1 levels can be explained by genetic polymorphism, we analysed DNA samples from 40 healthy individuals by single-strand conformational polymorphism analysis for polymorphisms in the CYP2E1 coding sequence and promoter region. DNA sequencing of samples showing mobility shifts on single-strand conformational polymorphism detected polymorphisms at positions -316 (A to G), -297 (T to A), -35 (G to T), 1107 (G to C; intron 1), 4804 (G to A Val179Ile; exon 4) and 10157 (C to T; exon 8). All individuals positive for either A(-316)G, G(-35)T, G(4804)A or the previously described RsaI polymorphism at -1019 were also positive for T(-297)A, which had the highest allele frequency of the observed polymorphisms (0.20). A(-316)G, G(-35)T and G(4804)A were detected at allele frequencies of 0.022, 0.052 and 0.013, respectively. The functional significance of the upstream polymorphisms was examined by preparing constructs of positions -549 to +3 of CYP2E1 containing the observed combinations of the polymorphisms fused to luciferase reporter genes and transfecting HepG2 cells. For the G(-35)T/T(-297)A construct, a 1.8-fold increase in luciferase activity compared with the wild-type sequence (P = 0.06) and 2.5-fold compared with T(-297)A only (P = 0.025) was observed. No significant difference in activity was observed between the other constructs. The significance of the predicted Val179Ile base change from G(4804)A was determined by expression of the wild-type and mutated full length cDNAs in lymphoblastoid cells. No significant difference in kinetic constants for chlorzoxazone hydroxylation between mutant and wild-type was observed. In summary, this study demonstrated six novel CYP2E1 polymorphisms, including three upstream of the promoter, but with the possible exception of G(-35)T, none appeared to be of functional significance.

Use of cDNA-expressed human cytochrome P450 enzymes to study potential drug-drug interactions

Complementary DNA (cDNA)-expressed human cytochrome P450 enzymes provide a reproducible, consistent source of single enzymes for many types of studies. The use of single enzymes systems, relative to multienzyme systems, has distinct advantages and disadvantages depending on the specific application. cDNA-expressed materials have advantages in the analysis of cytochrome P450 form-selective metabolism of a drug or drug candidate. This analysis can be accomplished by direct incubation of the drug with microsomes prepared from cells expressing a single cytochrome P450 form coupled with analysis of either metabolite formation or loss of parent compound. This approach allows the unambiguous assignment of specific biotransformations to specific enzymes. However, extending these data to the balance of enzymes present in human liver microsomes can be problematic. New approaches for relating rates of metabolism for cDNA-expressed enzymes to human liver microsomes metabolism are being developed (Crespi, 1995). In addition, cDNA-expressed enzymes can be used to study the cytochrome P450 form-selective inhibition by drugs or drug candidates. This analysis is accomplished through the study of the inhibition of the metabolism of a model substrate by the drug or drug candidate. Through these analyses, apparent Ki values can be obtained and compared to Ki values for known, clinically significant inhibitors of the same enzyme. For this application, cDNA-expressed, single enzyme systems have distinct advantages because of greater flexibility in the choice of model substrates and the lack of competing pathways of metabolism. Specific data for the use of cDNA-expressed CYP2C9, CYP2D6, and CYP3A4 are presented.

Human cytochrome P4502B6: interindividual hepatic expression, substrate specificity, and role in procarcinogen activation

The level of expression and interindividual variation in human hepatic microsomal cytochrome P450 (CYP) 2B6 was characterized using a polyclonal antibody (WB-2B6) raised against rat CYP2B1. Immunoblot analysis using cDNA-expressed human CYPs revealed strong cross-reactivity of this antibody with CYP2B6 (limit of detection < 0.05 pmol) and only minor cross-reactivities with human CYP2A6, CYP2D6, and CYP2E1, all of which could be resolved from CYP2B6 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis of human liver microsomes using this antibody revealed immunodetectable CYP2B6 protein in a majority of individual liver samples, with levels up to 74 pmol/mg protein in the CYP2B6-positive samples. Kinetic analysis of cDNA-expressed CYPs identified many of these enzymes as catalysts of 7-ethoxy-4-trifluoromethylcoumarin (7EFC) O-deethylation, but with significantly different apparent K(M) values (CYP1A2 < CYP2B6 approximately CYP1A1 < CYP2C19 < CYP2C9 < CYP2E1 < CYP2A6). By assaying liver microsomal 7EFC O-deethylase activity at a low 7EFC concentration (5 microM) and preincubating human liver microsomes with anti-CYP1A, anti-CYP2C, and anti-CYP2E1 antibodies, we were able to monitor CYP2B6-dependent 7EFC O-deethylase activity in a panel of 17 human liver microsomes and observe a significant correlation (r2 = 0.80) between this activity and CYP2B6 protein content. The ability of CYP2B6 to activate prodrugs and procarcinogens was examined using gene locus mutation assays in CYP2B6-expressing human lymphoblast cells. CYP2B6-expressing cells were found to be more sensitive than control cells to the cytotoxicity and mutagenicity of cyclophosphamide, aflatoxin B1, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. CYP2B6 is thus a widely expressed human liver microsomal CYP that can contribute to a broad range of drug metabolism and procarcinogen activation reactions.

Quantitative RT-PCR measurement of cytochromes p450 1A1, 1B1, and 2B7, microsomal epoxide hydrolase, and NADPH oxidoreductase expression in lung cells of smokers and nonsmokers

Bronchial epithelial cells (BEC) are the progenitors of bronchogenic carcinomas and are exposed to polycyclic aromatic hydrocarbon (PAH) procarcinogens through inhalation of combustion products. PAH are converted to carcinogenic molecules through a combination of monoxygenation by cytochrome p450 (CYP) enzymes in the presence of NADPH oxidoreductase (OR) and hydrolysis by microsomal epoxide hydrolase (mEH). In artificial systems, the relative expression of these genes determines whether carcinogenic or noncarcinogenic species are generated during metabolism. This relationship was explored in humans by using quantitative competitive reverse transcriptase polymerase chain reaction amplification to determine the range of expression of CYP1A1, CYP1B1, mEH, and NADPH OR in BEC recovered from 10 nonsmokers and 9 smokers. CYP2B7 expression was evaluated because, although little is known of its substrate specificity, it is expressed at high levels in human lung tissue. CYP1A1 and CYP1B1 were expressed in BEC at significantly different levels (P < 0.05) in the 9 smokers at 1.4 +/- 2.3 x 10(4) and 2.4 +/- 3.2 x 10(3) molecules/10(6) beta-actin molecules (mean +/- STD), respectively, but each was measurable in only one of the 10 nonsmokers. There was significant inter-individual variation (P < 0.05) in both CYP1A1 and CYP1B1 expression among the subjects for whom sufficient data were obtained. The inducibility of human BEC CYP1A1 gene by PAH exposure was confirmed in vitro by incubating cultured immortalized human BEC with beta-naphthoflavone and observing a > 6-fold induction of CYP1A1 after 24 h. In contrast to BEC, alveolar macrophages expressed CYP1A1 at low (30-70 molecules/10(6) beta-actin molecules) to unmeasurable levels in both smokers and nonsmokers. There was no significant difference in expression of mEH, CYP2B7, or NADPH OR in smokers compared with nonsmokers. The inter-individual variation in absolute and relative expression of PAH metabolism enzymes in BEC reported here supports the hypothesis that inter-individual variation in ability to activate/inactivate inhaled PAH carcinogens accounts for at least some of the inter-individual variation in risk for bronchogenic carcinoma.

The R144C change in the CYP2C9*2 allele alters interaction of the cytochrome P450 with NADPH:cytochrome P450 oxidoreductase

We have examined the kinetics of substrate metabolism by cDNA-expressed human CYP2C9 and the R144C variant. Both enzymes exhibited similar apparent K(m) values for (S)-warfarin 7-hydroxylation, diclofenac 4'-hydroxylation and lauric acid 11-hydroxylation. In contrast, the R144C variant (relative to CYP2C9) had slower rates of metabolism for all three substrates. The difference was most pronounced for (S)-warfarin. Surprisingly, the magnitude of the difference was found to be dependent on the cytochrome P450 to NADPH-cytochrome P450 reductase (OR) ratio in the system (the difference being more pronounced at higher OR to P450 ratios) implying that the R144C change affects interaction of the P450 with OR. The rates of (S)-warfarin 7-hydroxylation by CYP2C9 and the R144C variant also exhibited differential dependence on salt concentration which further supported a difference in interaction with OR. When OR was bypassed and the hydroxylation was supported by cumene hydroperoxide, no difference in the rates of diclofenac 4'-hydroxylation was observed for CYP2C9 and the R144C variant regardless of OR to P450 ratio. However, for (S)-warfarin 7-hydroxylation, some OR-dependence was maintained even when the reaction was supported by cumene hydroperoxide. Finally, we compared CYP2C9 activity and CYP2C9 protein levels for human lymphoblast expressed (high OR to P450 ratio) to human liver microsomes using immunoblotting and enzyme selective substrates. Human liver microsomal CYP2C9 and human lymphoblast-expressed CYP2C9 showed comparable amounts of activity per unit enzyme. This final observation indicates that the high OR to P450 ratio is the preferred model and predicts that the R144C change in human liver microsomal CYP2C9 should markedly reduce the rates of substrate metabolism. The implications of these observations for the interpretation of results with cDNA-expressed enzymes is discussed.

Coexpression of cytochrome P4502A6 and human NADPH-P450 oxidoreductase in the baculovirus system

Heterologous expression using baculovirus vectors has become a popular method for the production of catalytically active cytochrome P450s (CYPs). We have systematically optimized the multiplicity of infection (MOI) for a coinfection approach for the coexpression of CYP2A6 (viral vector designated v2A6) and NADPH-P450 oxidoreductase (OR; viral vector designated vOR) using Sf9 insect cells. A 3000-fold range of MOI was examined in stationary culture and stirred suspension culture. Surprisingly, our results indicate that the best CYP2A6 catalytic activity (850-1300 pmol/ min/mg total lysate protein as measured by coumarin 7-hydroxylase activity) was obtained only when using a low MOI of v2A6 (1.5-3 x 10(-2)) and a vOR of 10- to 20-fold less. This activity was approximately 7- to 11-fold higher than the best activity obtained when infecting cells with v2A6 alone. At this level of coinfection, the P450 content ranged from 180 to 250 pmol/mg total lysate protein, and the NADPH cytochrome c reductase activity ranged from 350 to 520 nmol/min/mg total lysate protein. Increasing the MOI of both viruses to 50-fold higher resulted in lower overall activity with the optimum (250 pmol/min/mg total lysate protein) being seen earlier postinfection (60 vs. 72 hr). Increasing the MOI of vOR to levels comparable with those of v2A6, decreased coumarin 7-hydroxylase activity 14-fold. These results suggest that the best CYP2A6 catalytic activity depends on properly posttranslationally modified proteins accumulating in a right ratio as a result of primary, secondary, and possibly tertiary infection of both viruses. These results also suggest that high OR expression results in degradation of P450.

Mutagenicity of the atmospheric transformation products 2-nitrofluoranthene and 2-nitrodibenzopyranone in Salmonella and human cell forward mutation assays

The mutagenicity of the atmospheric transformation products 2-nitrofluoranthene (2-NF) and 2-nitrodibenzopyranone (2-NDBP), as well as a related isomer 3-nitrodibenzopyranone (3-NDBP), was measured in quantitative forward mutation assays with bacteria (Salmonella typhimurium TM677) and in two metabolically competent human cell lines (MCL-5 and h1A1v2) that differ in their complement of cytochrome P450s and microsomal epoxide hydrolase. 2-NF was a potent mutagen in Salmonella TM677 both in the absence and presence of rat liver postmitochondrial supernatant (PMS). 2-NDBP was non-mutagenic in the absence of PMS, but was mutagenic in its presence. The converse result was obtained for 3-NDBP. The mutagenic potency series in Salmonella in the absence of PMS, expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml, was: 2-NF, 2.5; 3-NDBP, 16.9; and 2-NDBP, > 415. With PMS, the potency series was: 2-NF, 1.2; 2-NDBP, 15.1; 3-NDBP, 208. Neither 2-NDBP nor 3-NDBP were mutagenic at the tk locus in MCL-5 or h1A1v2 cells at up to 200 nmol/ml. 2-NF was also inactive in MCL-5 cells, but was a potent mutagen in h1A1v2 cells with an MDMC of 0.02 nmol/ml. Cytochrome P450 CYP1A1, present constitutively only in h1A1v2 cells, was implicated in 2-NF activation because mutagenicity was reduced by 55-80% when alpha-naphthoflavone (ANF) was present during incubation. The lack of mutagenicity in MCL-5 cells was attributed to the inability of 2-NF to induce CYP1A1 activity in this cell line. These data indicate a primary role for ring oxidation in 2-NF activation. Previous emphasis placed upon 2-NDBP as a major mutagen in ambient air may need to be modified in view of the negative results for this compound in the human cell assays and in the absence of PMS in Salmonella TM677. However, these findings support the concern that 2-NF may be a risk to human health.

Development of a human lymphoblastoid cell line constitutively expressing human CYP1B1 cDNA: substrate specificity with model substrates and promutagens

An AHH-1 TK+/- cell derivative was developed that stably expresses human cytochrome P4501B1 (CYP1B1) cDNA in an extrachromosomal vector which confers resistance to 1-histidinol and co-expresses NADPH cytochrome P450 oxidoreductase (OR). The CYP1B1-expressing cell line was designated h1B1/OR. Microsomes prepared from CYP1B1 cDNA expressing cells exhibit elevated levels of 7-ethoxy-resorufin deethylase (EROD), 7-ethoxy-4-trifluoromethyl-coumarin deethylase (EFCD), benzo(alpha)pyrene hydroxylase (BPH), bufuralol 1'-hydroxylase, testosterone hydroxylase activities and spectrally quantifiable cytochrome P450. CYP1B1-containing microsomes did not contain detectable coumarin 7-hydroxylase, p-nitrophenol hydroxylase, lauric acid hydroxylase, (S)-mephenytoin 4'-hydroxylase or diclofenac 4'-hydroxylase activities. Kinetic parameters for selected substrates were compared among CYP1B1 and the two additional members of the CYP1 family, CYP1A1 and CYP1A2. For BPH and EFCD, the rank order of rates of substrate metabolism were CYP1A1 > CYP1B1 > CYP1A2. For EROD, the rank order of substrate metabolism was CYP1A1 > CYP1A2 > CYP1B1. For both EROD and EFCD the apparent K(m) values for CYP1B1 were more similar to CYP1A1 than to CYP1A2. In order to begin to characterize the promutagen activating ability of CYP1B1, the mutagenicity of selected chemicals was examined in h1B1/OR cells; there was increased sensitivity (CYP1B1-expressing relative to control cells) to the mutagenicity of benzo(a)pyrene, cyclopenta(c,d)pyrene, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and aflatoxin B1 (AFB). CYP1B1, expressed in this system, appears to be particularly efficient at activating AFB.

Human cell mutagenicity of oxygenated, nitrated and unsubstituted polycyclic aromatic hydrocarbons associated with urban aerosols

Polycyclic aromatic compounds (PAC) are ubiquitous pollutants in urban air that may pose risks to human health. In order to better assess the health risks associated with this class of compounds, a total of 67 PAC that either have been identified (55) or are suspected to be present (12) in urban aerosol samples were tested for mutagenicity in a forward mutation assay based on human B-lymphoblastoid cells. The cell line used (designated h1A1v2) constitutively expresses the cytochrome P4501A1, which is known to be necessary for the metabolism of many promutagens. The PAC tested included 39 polycyclic aromatic hydrocarbons (PAH). 19 oxygen-containing PAH (oxy-PAH) and nine NO2-substituted PAH (nitro-PAH). A total of 26 PAH were mutagenic. In comparing the minimum mutagenic concentrations of the mutagenic PAH with that of benzo[a]pyrene (B[a]P) it was found that dibenzo[a,l]pyrene (DB[al]P), cyclopenta[c,d]pyrene (CPP), naphtho[2,1-a]pyrene, dibenzo[a,e]pyrene (B[a]P) and 1-methylbenzo[a]pyrene were 24 +/- 21, 6.9 +/- 4.2, 3.2 + 3.0, 2.9 +/- 2.9 and 1.6+/- 1.4 times, respectively, more mutagenic than B[a]P, and that dibenzo[a,k]fluoranthene and B[a]P were approximately equally mutagenic. The 19 other mutagenic PAH were between approximately 2 and approximately 1800 times less mutagenic than B[a]P. Of the oxy-PAH tested only phenalenone, 7H-benz[d,e]anthracen-7-one, 3-nitro-6H-dibenzo[b,d]pyran-6-one, cyclopenta[c,d]pyren-3(4H)-one, 6H-benzo[c,d]pyren-6-one (BPK) and anthanthrenequinone were mutagenic; however, with the exception of BPK, these were over 50 times less active than B[a]P, BPK was approximately 3 times less active than B[a]P. Seven of the nitro-PAH were mutagenic including 9-nitroanthracene, 1-nitropyrene, 2-nitrofluoranthene, 3-nitrofluoranthene, 1,3-dinitropyrene, 1,6-dinitropyrene (1,6-DNP) and 1,8-dinitropyrene. 1,6-DNP was approximately 4 times less active than B[a]P; the six other mutagenic nitro-PAH were between 20 and 380 times less active than B[a]P. These results are discussed in terms of their relevance for determining the most important mutagens in ambient air. Based on reported concentrations of PAC in ambient aerosols, it is possible that CPP, DB[ae]P, DB[al]P and BPK could account for a greater proportion of the mutagenicity than B[a]P in some aerosols.

Development of Caco-2 cells expressing high levels of cDNA-derived cytochrome P4503A4

PURPOSE

To develop Caco-2 cell derivatives expressing high levels of human cytochrome P450 drug metabolizing enzymes.

METHODS

The cDNAs for two cytochrome P450 forms, CYP2A6 and CYP3A4, were introduced into an extrachromosomal vector under control of the cytomegalovirus early intermediate promoter. Vector-bearing cells were selected via resistance to hygromycin B.

RESULTS

Transfected cells exhibited high levels of cDNA-derived protein as measured by Western blot, spectrophotometric P450 determination and/or cytochrome P450 form-selective enzyme assay. CYP3A4 and CYP2A6 catalytic activities were about 100 fold higher than in control cells. cDNA-expressing cells were found to form tight monolayers and were suitable for study of xenobiotic transport and metabolism. The permeabilities of cephalexin, phenylalanine, mannitol and propranolol across transfected monolayers were found to be similar to those across untransfected monolayers. The appropriate transfected monolayers metabolized the CYP2A6 substrate coumarin and the CYP3A4 substrates testosterone and nifedipine.

CONCLUSIONS

A Caco-2 cell system to simultaneously study drug transport and metabolism has been developed.

Comparison of substrate metabolism by wild type CYP2D6 protein and a variant containing methionine, not valine, at position 374

We have analysed kinetic parameters of cDNA-derived CYP2D6 proteins derived from the original CYP2D6 cDNA isolate (Gonzalez FJ et al. Nature 1988: 331, 442-446) which contains methionine at position 374 (CYP2D6-Met) and a modified cDNA which contains valine at position 374 (CYP2D6-Val). This latter protein is predicted from the CYP2D6 genomic sequence. Several quantitative differences, but no qualitative differences in metabolism were observed. CYP2D6-Met was found to have a two-fold lower Km and a three-fold lower turnover rate for (R)(+)-bufuralol 1'-hydroxylation as compared to CYP2D6-Val. In contrast, CYP2D6-Met and CYP2D6-Val had a similar Km for debrisoquine 4-hydroxylation while CYP2D6-Val had an 18-fold higher turnover rate. CYP2D6-Val and CYP2D6-Met had similar Kms for metoprolol but CYP2D6-Val showed a three-fold higher capacity for the O-demethylation reaction compared to alpha-hydroxylation which is more similar to that seen in human liver. In the case of sparteine, CYP2D6-Val and CYP2D6-Met showed similar capacities for formation of the 2-dehydrosparteine metabolite but the Km value for CYP2D6-Met was six-fold higher than that for CYP2D6-Val. Kinetic differences between CYP2D6-Met and CYP2D6-Val were further probed by examination of apparent Ki for inhibition of (R,S)(+/-)-bufuralol 1'-hydroxylation. Similar Ki values (within a factor of three) were observed for perhexiline and (R,S)-propranolol while quinidine and dextromethorphan were 8.5-fold and 21-fold more effective inhibitors of CYP2D6-Val relative to CYP2D6-Met. An allele specific polymerase chain reaction assay was developed for the CYP2D6-Met allele. The CYP2D6-Met allele was not found among 83 individuals. In the aggregate, these data indicated that the CYP2D6-Val allele is the more common allele in human populations. The quantitative kinetic differences between these two enzymes appears most pronounced for substrates/inhibitors with rigid structures. CYP2D6-Val more often has a substantially lower Km and/or a substantially higher capacity to metabolize those substrates.

Mutagenicity of benzo[a]pyrene and dibenzopyrenes in the Salmonella typhimurium TM677 and the MCL-5 human cell forward mutation assays

The mutagenicity of benzo[a]pyrene (B[a]P), dibenzo[ae]pyrene (DB[ae]P), dibenzo[ah]pyrene (DB[ah]P), dibenzo[ai]pyrene (DB[ai]P), and dibenzo[al]pyrene (DB[al]P) was measured in quantitative forward mutation assays with bacteria (Salmonella typhimurium TM677) and a metabolically competent cell line derived from human B-lymphoblastoid cells (MCL-5) that contained activity for five cytochrome P450s and microsomal epoxide hydrolase found in human liver. DB[al]P and B[a]P, both potent animal carcinogens, were the most mutagenic substances in both assays. DB[al]P was nearly 50-fold more potent than B[a]P in human cells, but only 60% more mutagenic in Salmonella. The carcinogenic isomer DB[ah]P, though nonmutagenic in bacteria, was active in human cells. The following mutagenic potency series, expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml, was obtained with Salmonella in the presence of rat liver postmitochondrial supernatant (PMS): DB[al]P (3.7), B[a]P (5.8), DB[ae]P (6.9), DB[ai]P (14.9), DB[ah]P (> 100). None of the compounds were mutagenic in the absence of PMS. In human MCL-5 cells the potency series was: DB[al]P (3.1 x 10(-4)), B[a]P (1.5 x 10(-2)), DB[ae]P (2.5 x 10(-2)), DB[ah]P (0.5), DB[ai]P (3.2). The human cell assay thus exhibited over a 10,000-fold range between the most mutagenic and least mutagenic compound, whereas in the bacterial assay there was only a corresponding four-fold difference if the nonmutagenic DB[ah]P was excluded. The results were discussed in terms of their concordance with animal carcinogenicity studies.

Human cell mutagenicity of mono- and dinitropyrenes in metabolically competent MCL-5 cells

Nitropyrenes are ubiquitous environmental pollutants that may pose a human health hazard because some are highly potent mutagens and carcinogens. The mutagenicity (trifluorothymidine resistance at the thymidine kinase locus) of 1-, 2-, and 4-nitropyrene (1-, 2-, and 4-NP), 1,3-, 1,6-, and 1,8-dinitropyrene (1,3-, 1,6-, and 1,8-DNP), and pyrene was assessed in a quantitative forward mutation assay using a metabolically competent line (MCL-5) of human B-lymphoblastoid cells. These cells contain endogenous cytochrome P450 activity (CYP1A1) and two plasmids that express cDNAs for four additional P450s (CYP1A2, CYP2A6, CYP2E1, CYP3A4) and microsomal epoxide hydrolase found in human liver. The major finding is that 2-NP and 1,3-DNP, both potent bacterial mutagens, were nonmutagenic in this assay. The following mutagenic potency series, expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml, was obtained: 1,6-DNP (0.8), 1,8-DNP (1.5), 4-NP (3.1), 1-NP (9.1), 2-NP (> 81), 1,3-DNP (> 86), pyrene (> 494). There was over an 11-fold difference between the most potent (1.6-DNP) and the least potent (1-NP) mutagen. 1,6-DNP was approximately twice as mutagenic as 1,8-DNP, which was almost twice as mutagenic as 4-NP, which, in turn was nearly three times as potent as 1-NP. This is the first report on the testing of 2-NP and 4-NP for mutagenicity in mammalian cell cultures. The human cell mutagenicity of these compounds was discussed in terms of potency series of nitropyrenes obtained from animal carcinogenicity experiments and other mammalian cell mutagenicity assays.

Development of a human lymphoblastoid cell line constitutively expressing human CYP1A1 cDNA: substrate specificity with model substrates and promutagens

AHH-1 TK+/- cell derivatives were developed that stably express human CYP1A1 cDNA, and an AHH-1 TK+/- derivative expressing higher levels of CYP1A2 cDNA in extrachromosomal vectors which confer resistance to 1-histidinol. The CYP1A1-expressing cell lines, designated h1A1 and h1A1v2, differ by containing one and two CYP1A1 cDNA expression units per vector. The CYP1A2-expressing cell line, designated h1A2v2, also has two CYP1A2 cDNA expression units per vector. Microsomes prepared from CYP1A1 cDNA expressing cells exhibit high, constitutive levels of 7-ethoxyresorufin deethylase (EROD), 7-ethoxycoumarin deethylase (ECD), 7-ethoxy-4-trifluoromethylcoumarin deethylase (EFCD), benzo[a]-pyrene hydroxylase (BPH) activities and spectrally quantifiable cytochrome P450. Kinetic comparisons between cDNA-expressed CYP1A1 and CYP1A2 indicate that CYP1A1 is more active than CYP1A2 for EROD, ECD, EFCD and BPH. CYP1A2 was more active than CYP1A1 for acetanilide hydroxylation and activation of aflatoxin B1 (AFB1). The mutagenicity of selected promutagens were examined in h1A1 cells and control cells. Relative to control cells, the h1A1 cell line exhibits increased sensitivity to the mutagenicity of benzo[a]pyrene, cyclopenta[c,d]pyrene, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and AFB1.

Regio- and stereo-selective metabolism of phenanthrene by twelve cDNA-expressed human, rodent, and rabbit cytochromes P-450

The regio- and stereoselective metabolism of phenanthrene (PA) by seven cDNA-expressed human and five rodent and rabbit cytochromes P-450 has been examined using reverse-phase and chiral stationary phase high-pressure liquid chromatography (HPLC). Turnover numbers ranged from 0.2 to 55 nmol/min per nmol. Using vaccinia virus expression of P-450 enzymes in Hep G2 cells, m1A1 and m1A2 were found to be the most active P-450s. Of the human P-450s, 1A2 and 2B6 have the highest activity and 2C9 has moderate activity. Using cytochrome P-450s expressed in a lymphoblastoid cell line in presence of epoxide hydrolase (EH), human 1A1 had approximately twice the activity of human 1A2. Regioselectivities for PA metabolism were found to be both isozyme and species-dependent. Stereochemical analysis revealed that the P-450s 1A1, m1A1, m1A2, r2A1, r2B1, PB- and 3MC-treated rat liver microsomes preferentially formed 3R,4R-diol enantiomer (88-97%), whereas rabbit 4B formed the 3S,4S-diol enantiomer (72%). Eleven P-450s, 3MC and PB microsomes preferentially formed 1R,2R-diol enantiomer (80-96%). This is the same stereochemistry as the precursors to some diol epoxides that are potent carcinogens.

N-nitrosodiethylamine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone induced morphological transformation of C3H/10T1/2CL8 cells expressing human cytochrome P450 2A6

Transfection of specific genes into cells capable of expressing chemically induced morphological cell transformation provides a valuable approach to study the mechanisms of action of carcinogens. A human cytochrome P450 isozyme, CYP2A6, has been successfully expressed from a retroviral vector in transformable C3H/10T1/2 (10T1/2) mouse embryo fibroblasts and these resulting 10T1/2 clones were evaluated for the cytotoxic and transforming activities of two nitrosamines, 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosodiethylamine (DEN). 10T1/2 clone 29 cells, which expressed high levels of CYP2A6 activity, were responsive to the cytotoxic and morphological transforming effects of DEN or NNK on a concentration-related basis. In 10T1/2 clone 29 cells, DEN at 600 micrograms/ml decreased cell survival to 67%, and induced 0.5 type II&III foci/dish. NNK at 400 micrograms/ml administered to 10T1/2 clone 29 cells decreased survival to 57% and induced 0.43 type II&III foci/dish. Wild-type 10T1/2 cells and 10T1/2 clone 4 cells (infected with the vector but not expressing the CYP2A6 activity) were unresponsive. These results indicate that expression of a cDNA coding for cytochrome P450 in 10T1/2 cells can provide information about the role of the enzyme in the activities of chemical carcinogens and also increase the sensitivity of 10T1/2 cells to a larger number of classes of chemical carcinogens.

The role of 12 cDNA-expressed human, rodent, and rabbit cytochromes P450 in the metabolism of benzo[a]pyrene and benzo[a]pyrene trans-7,8-dihydrodiol

The potent carcinogen benzo[a]pyrene (B[a]P) and its metabolite B[a]P trans-7,8-dihydrodiol (7,8-diol) require metabolic activation by the microsomal cytochrome P450s (P450s) to exert several adverse biological effects, including binding to DNA, toxicity, mutagenicity, and carcinogenicity. In the study reported here, we defined the role of each of 12 individual cDNA-expressed cytochrome P450s in the metabolism of B[a]P and 7,8-diol. Human P450s 1A1 and 1A2 were expressed in the absence or presence of epoxide hydrolase (EH) in a human lymphoblastoid cell line, and six human and five rodent and rabbit P450s were expressed from cDNA with vaccinia virus vectors in the hepatoma cell line Hep G2. B[a]P metabolism resulted in nine metabolites (three diols, three quinones, and three phenols), which were separated, identified, and quantitated by high-pressure liquid chromatography. In the human lymphoblastoid cells, human 1A1 metabolized B[a]P at a rate 4.5 times greater than that for 1A2. EH was shown to be directly involved in B[a]P activation, since increasing the amount of EH resulted in less 7-hydroxybenzo[a]pyrene and more 7,8-diol formation. Of the human P450s expressed with the vaccinia virus vectors in Hep G2 cells, 1A2 and 2C9 showed the highest activity and 2B6 showed moderate activity for B[a]P metabolism. Mouse 1A1 had activity 40 times higher than any human, rabbit, or rodent P450s, indicating the potential pitfalls of extrapolating P450 activity across species. Metabolism of the 7,8-diol resulted in six metabolites (four tetrols and two triols). In the lymphoblastoid cells, human 1A1 was shown to be 4.2 times more active than 1A2 for 7,8-diol metabolism. Among human P450s expressed from vaccinia virus, 1A2, 2E1, and 2C9 gave the highest activity, and 2C8 and 3A4 showed moderate activity for 7,8-diol metabolism to the diol epoxides. Again, mouse 1A1 was much more active than any other P450. These studies, in which we determined the capacity of individual P450 in the metabolism and activation of B[a]P and 7,8-diol, may thus lead to a better understanding of how P450s control the detoxification and activation of polycyclic aromatic hydrocarbons.

Differential activation of cyclophosphamide and ifosphamide by cytochromes P-450 2B and 3A in human liver microsomes

The present study identifies the specific human cytochrome P-450 (CYP) enzymes involved in hydroxylation leading to activation of the anticancer drug cyclophosphamide and its isomeric analogue, ifosphamide. Substantial interindividual variation (4-9-fold) was observed in the hydroxylation of these oxazaphosphorines by a panel of 12 human liver microsomes, and a significant correlation was obtained between these 2 activities (r = 0.85, P < 0.001). Enzyme kinetic analyses revealed that human liver microsomal cyclophosphamide 4-hydroxylation and ifosphamide 4-hydroxylation are best described by a 2-component Michaelis-Menten model composed of both low Km and high Km P-450 4-hydroxylases. To ascertain whether one or more human P-450 enzymes are catalytically competent in activating these oxazaphosphorines, microsomal fractions prepared from a panel of human B-lymphoblastoid cell lines stably transformed with individual P-450 complementary DNAs were assayed in vitro for oxazaphosphorine activation. Expressed CYP2A6, -2B6, -2C8, -2C9, and -3A4 were catalytically competent in hydroxylating cyclophosphamide and ifosphamide. Whereas CYP2C8 and CYP2C9 have the characteristics of low Km oxazaphosphorine 4-hydroxylases, CYP2A6, -2B6, and -3A4 are high Km forms. In contrast, CYP1A1, -1A2, -2D6, and -2E1 did not produce detectable activities. Furthermore, growth of cultured CYP2A6- and CYP2B6-expressing B-lymphoblastoid cells, but not of CYP-negative control cells, was inhibited by cyclophosphamide and ifosphamide as a consequence of prodrug activation to cytotoxic metabolites. Experiments with P-450 form-selective chemical inhibitors and inhibitory anti-P-450 antibodies were then performed to determine the contributions of individual P-450s to the activation of these drugs in human liver microsomes. Orphenadrine (a CYP2B6 inhibitor) and anti-CYP2B IgG inhibited microsomal cyclophosphamide hydroxylation to a greater extent than ifosphamide hydroxylation, consistent with the 8-fold higher activity of complementary DNA-expressed CYP2B6 with cyclophosphamide. In contrast, troleandomycin, a selective inhibitor of CYP3A3 and -3A4, and anti-CYP3A IgG substantially inhibited microsomal ifosphamide hydroxylation but had little or no effect on microsomal cyclophosphamide hydroxylation. By contrast, the CYP2D6-selective inhibitor quinidine did not affect either microsomal activity, while anti-CYP2A antibodies had only a modest inhibitory effect. Overall, the present study establishes that liver microsomal CYP2B and CYP3A preferentially catalyze cyclophosphamide and ifosphamide 4-hydroxylation, respectively, suggesting that liver P-450-inducing agents targeted at these enzymes might be used in cancer patients to enhance drug activation and therapeutic efficacy.

Human cell lines, derived from AHH-1 TK+/- human lymphoblasts, genetically engineered for expression of cytochromes P450

We are developing a panel of human B lymphoblastoid cells which have been engineered to express specific human cDNAs for cytochrome P450 and other xenobiotic metabolizing enzymes. The recipient cells are of a human B lymphoblastoid cell line, designated AHH-1 TK+/-. These cells are transfected using two extrachromosomal vectors both containing OriP sequences derived from Epstein Barr virus but containing independent means of selection in mammalian cells. Using this system, the level of cDNA expression is nearly always stable and consistent from one transfection to another. Thus, once the level of expression has been characterized, cell lines with potentially interesting combinations of xenobiotic-metabolizing enzymes can be predictably developed. cDNAs encoding the following human enzymes have been expressed in this system: CYP1A1, CYP1A2, CYP2A6, CYP2B8, CYP2C6, CYP2C9, CYP2D6, CYP2E1, CYP3A4 and microsomal epoxide hydrolase. We have expressed all of these enzymes individually and have developed cell lines which express combinations of the xenobiotic metabolizing enzymes. The expression of multiple enzymes is important for generalized use of engineered cells as toxicology screening tools. We have primarily used the cell lines in applications to toxicology focusing on procarcinogen activation as detected in assays for the induction of gene locus mutations. In this chapter we discuss the general properties of the system and applications to toxicology testing.

Retroviral mediated expression of human cytochrome P450 2A6 in C3H/10T1/2 cells confers transformability by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

In order to develop more efficient in vitro systems for the study of pro-mutagenic or pro-carcinogenic chemicals, we have produced transgenic C3H/10T1/2 cell lines expressing human cytochrome P450 (CYP) 2A6. A retroviral vector containing the cDNA was packaged in psi-2 cells, and used to infect C3H/10T1/2 cells. From 100 G418-resistant clones initially isolated, three cell lines were chosen for further study based upon their morphologies, growth rates and CYP2A6-dependent coumarin 7-hydroxylase activities. Infected clone 10T1/2-04, like the 10T1/2 cells, had no detectable CYP2A6 enzyme activity, while clones 10T1/2-10 and 10T1/2-29 had microsomal CYP2A6 enzyme activities within the range found in human liver microsomes. CYP2A6 protein levels were in agreement with the observed enzyme activities. Southern blots revealed that cells from clone 10T1/2-04 contained a vector lacking the CYP2A6 cDNA, while cells from clones 10T1/2-10 and 10T1/2-29 contained multiple full-length inserts. Southern analysis also indicated the presence of an endogenous CYP2A6 ortholog in the four cell lines. All cell lines exhibited about equal sensitivity to induction of cytotoxicity and conversion to ouabain resistance by the direct acting mutagen N-methyl-N'-nitro-N-nitrosoguanidine. The four lines were also about equally sensitive to transformation by benzo[a]pyrene, a chemical requiring metabolic activation. However, only clones 10T1/2-10 and 10T1/2-29, which express CYP2A6 activity, were mutated and morphologically transformed by the tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone.

Characterization of a human cell line expressing high levels of cDNA-derived CYP2D6

We have developed a human B-lymphoblastoid cell, designated h2D6v2, which expresses high levels of CYP2D6 cDNA. Microsomal P450 contents of 160 pmol mg-1 protein were observed. NADPH-fortified microsomes exhibited a substantial capacity to hydroxylate the prototype CYP2D6 substrates bufuralol and debrisoquine. Kinetic parameters, apparent Km, turnover number, Ki for quinidine inhibition and stereospecificity of bufuralol hydroxylation, observed with the human lymphoblast expressed enzyme were similar to those observed in human liver microsomes or purified liver CYP2D6 proteins. Therefore, the human lymphoblast expressed material appears to faithfully reflect the authentic protein. Relative to control cells, h2D6v2 cells were more sensitive to the cytotoxicity and mutagenicity of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), supporting our previous observation with a cell line expressing lower levels of CYP2D6. h2D6v2 microsomes were capable of metabolizing NNK and NNK metabolism and mutagenicity were markedly inhibited by the addition of quinidine, a CYP2D6 inhibitor. h2D6v2 cells coupled with control cells, represent a useful in vitro system for studying xenobiotic metabolism by the clinically important, polymorphic CYP2D6. The human lymphoblast system offers the desirable ability to couple metabolic transformation studies with toxicological endpoints such as cytotoxicity and mutagenicity.