In vitro antioxidative potential of lactoferrin and black tea polyphenols and protective effects in vivo on carcinogen activation, DNA damage, proliferation, invasion, and angiogenesis during experimental oral carcinogenesis.

The present study was designed to evaluate the in vitro antioxidant potential of bovine lactoferrin (bLF) and black tea polyphenols [Polyphenon-B (P-B)] as well as in vivo inhibitory effects on the development of 7,12-dimethylbenz[a]anthracene (DMBA)-induced hamster buccal pouch (HBP) carcinomas. Antioxidant activity was screened using a panel of assays including 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2'-azinobis-(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS), hydroxyl radical anion (OH*), superoxide anion (O2*-), and nitric oxide (NO) radical scavenging assays as well as assay for reducing power. The chemopreventive potential of bLF and P-B was assessed in the HBP model based on the modulatory effects on DMBA-induced oxidative DNA damage as well as the expression of proteins associated with carcinogen activation (CYP1A1, CYP1B1), cell proliferation [cyclin D1, proliferating cell nuclear antigen (PCNA), glutathione S-transferase pi (GST-P)], angiogenesis [vascular endothelial growth factor (VEGF), VEGF receptor 1 (VEGFR1)], and invasion and metastasis [matrix metalloproteinase-9 (MMP-9) and tissue inhibitors of MMP-2 (TIMP-2)]. Both bLF and P-B showed high radical scavenging activity and reductive potential. Although administration of bLF and P-B alone suppressed DMBA-induced HBP tumors, combined administration of bLF and P-B was more effective in inhibiting HBP carcinogenesis by inhibiting oxidative DNA damage, carcinogen activation, cell proliferation, invasion, and angiogenesis. Our study suggests that the antioxidative property of bLF and P-B may be responsible for chemoprevention of HBP carcinogenesis by modulating multiple molecular targets.

Pretreatment with black tea polyphenols modulates xenobiotic-metabolizing enzymes in an experimental oral carcinogenesis model.

The objective of this study was to evaluate the chemopreventive potential of the black tea polyphenols Polyphenon-B and BTF-35 during the preinitiation phase of 7,12-dimethylbenz[a]anthracene (DMBA)-induced hamster buccal pouch (HBP) carcinogenesis. Hamsters were divided into six groups. Animals in groups 2 and 3 received diet containing Polyphenon-B and BTF-35, respectively, 4 weeks before carcinogen administration when they were 6 weeks of age and continued until the final exposure to carcinogen. At 10 weeks of age, animals in groups 1, 2, and 3 were painted with 0.5% DMBA three times a week for 14 weeks. Animals in groups 4 and 5 were given Polyphenon-B and BTF-35 alone, respectively, as in groups 2 and 3. Animals in group 6 served as control. All the animals were sacrificed after an experimental period of 18 weeks. Phase I and phase II xenobiotic-metabolizing enzymes and 8-hydroxy-deoxyguanosine (8-OH-dG) in the buccal pouch and liver were used as biomarkers of chemoprevention. Hamsters painted with DMBA showed increased expression of 8-OH-dG and enhanced activities of phase I (CYP450; total as well as CYP1A1, 1A2, and 2B isoforms and cytochrome b5) and phase II (GST and quinone reductase) xenobiotic-metabolizing enzymes with increased immunohistochemical expression of CYP1A1, and CYP1B1 isoforms in the buccal pouch. This was accompanied by increased phase I and decreased phase II enzyme activities in the liver. Administration of Polyphenon-B and BTF-35 significantly decreased tumor incidence, oxidative DNA damage, phase I enzyme activities as well as expression of CYP1A1 and CYP1B1 isoforms, while enhancing phase II enzyme activities in the buccal pouch and liver. Our results provide a mechanistic basis for the chemopreventive potential of black tea polyphenols. Furthermore, the greater efficacy of BTF-35 in chemoprevention of HBP carcinomas via inhibition of oxidative DNA damage and modulation of xenobiotic-metabolizing enzymes may have a major impact in human oral cancer prevention.

CYP2C subfamily, primarily CYP2C9, catalyses the enantioselective demethylation of the endocrine disruptor pesticide methoxychlor in human liver microsomes: use of inhibitory monoclonal antibodies in P450 identification.

1. The endocrine disruptor pesticide methoxychlor undergoes O-demethylation by mammalian liver microsomes forming chiral mono-phenolic (1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane, i.e. mono-OH-M) and achiral bis-phenolic oestrogenic metabolites. Human liver microsomes (HLM) generated primarily the S-mono-OH-M. 2. Inhibitory monoclonal antibodies (MAb) identified those P450s catalysing the enantioselective O-demethylation of methoxychlor. In HLM, O-demethylation was inhibited by MAb anti-2C9 (30-40%), diminishing the per cent of S-mono-OH-M from about 80 to 55-60%. MAb anti-CYP1A2, 2A6, 2B6, 2C8, 2C19, 2D6 and 3A4 did not affect the demethylation rate in HLM. Nevertheless, MAb anti-CYP1A2 decreased the formation of R-mono-OH-M from 21-23 to 10-17%, indicating that CYP1A2 exhibits a role in generating the R-enantiomer. 3. Among cDNA-expressed human P450s (supersomes), CYP2C19 was the most active in demethylation, but in HLM, CYP2C19 appeared inactive (no inhibition by MAb anti-CYP2C19). There was a substantial difference in the per cent inhibition of demethylation by MAb anti-CYP2C9 and anti-rat CYP2C (MAb inhibiting all human CYP2C forms) and in altering the enantioselectivity, suggesting that demethylation by combined CYP2C8, 2C18 and 2C19 was significant (20-30%). 4. Polymorphism of methoxychlor demethylation was examined with supersomes and HLM-expressing CYP2C9 allelic variants. CYP2C9*1 and 2C9*2 were highly active; however, CYP2C9*3 appeared inactive.

Monoclonal antibodies specific and inhibitory to human cytochromes P450 2C8, 2C9, and 2C19.

Hybridomas were isolated that produce 13 monoclonal antibodies (mAbs) that are specific and highly inhibitory to members of the human P450 2C subfamily, 2C8, 2C9, 2C9*2, and 2C19. Many of the mAbs to P450 2C8, 2C9, and 2C19 are specific and exhibit potent inhibitory activity (85-95%). mAb 281-1-1 specifically binds, immunoblots, and strongly inhibits the activity of P450 2C8. mAb 763-15-5 specifically binds and strongly inhibits the activity of P450 2C9. mAb 1-7-4-8 specifically binds and strongly inhibits the activity of P450 2C19. The other mAbs bind and inhibit sets and subsets of the P450 2C family. The single and the combinatorial use of the mAbs can "reaction phenotype", i.e., determine the metabolic contribution and interindividual variation of a P450 isoform for the metabolism of a drug or nondrug xenobiotic in human liver microsomes. The utility of the mAb-based analytic system was examined with the model substrates Taxol (paclitaxel), diazepam, tolbutamide, diclofenac, mephenytoin, and imipramine. The mAb system can identify drugs metabolized by a common P450 or several P450s and polymorphic P450s. The mAb system identifies drugs or drug metabolic pathways that are catalyzed by a single P450 and thus may be used for in vivo phenotyping. The mAb system can identify whether a particular drug is metabolized by a single P450 that may exhibit polymorphic expression in humans. The mAb system offers large potential for studies of cytochrome P450 function useful in drug discovery and reduces the possibility of adverse drug reactions due to polymorphisms and drug interactions.

Cytochromes P450 2A6, 2E1, and 3A and production of protein-aldehyde adducts in the liver of patients with alcoholic and non-alcoholic liver diseases.

BACKGROUND/AIMS: Interaction between CYP2E1, ethanol metabolites, and enhanced lipid peroxidation is linked to the pathogenesis of alcoholic liver disease. This study was conducted to compare the expression of various cytochrome enzymes and the appearance of aldehyde adducts in humans. METHODS: Acetaldehyde- and lipid peroxidation-derived protein adducts and CYP2A6, 2E1, and 3A4/5 were examined immunohistochemically from liver specimens of 12 alcohol abusers with either mild (n=7) or severe (n=5) liver disease, and from nine non-drinking patients with non-alcoholic steatosis (n=4), or hepatitis (n=5). RESULTS: Ethanol-inducible CYP2E1 was present in all alcoholic livers. While CYP2A6 in zone 3 hepatocytes was also abundant in the alcoholic patients with various degrees of liver disease, CYP3A415 was most prominent in alcoholic cirrhosis. The sites of CYP2E1 and CYP2A6 immunoreactivity co-localized with fatty deposits, and with the sites of acetaldehyde and lipid peroxidation-derived protein adducts. The CYP enzymes were also abundant in the centrilobular hepatocytes of patients with fatty liver due to obesity or diabetes. CONCLUSIONS: Alcohol-induced liver damage is associated with a generalized induction of CYP2A6, CYP2E1 and CYP3A4 and generation of acetaldehyde and lipid peroxidation-derived protein-aldehyde adducts. However, CYP induction also occurred in patients with non-alcoholic steatosis.

An inhibitory monoclonal antibody to human cytochrome P450 that specifically binds and inhibits P4502C9II, an allelic variant of P4502C9 having a single amino acid change Arg144 Cys.

A monoclonal antibody (MAb 292-2-3) has been isolated that binds specifically to a single allele of three expressed human cytochrome P4502C9 alleles. The MAb binds to 2C9Cys144 (II), and does not bind to the wild-type 2C9Arg144 (I), or the third allele 2C9Ile-->Leu359 (III) and thus the MAb detects an allele with > 99% homology and differing from the wild-type 2C9Arg144 (I) by a single amino acid. The MAb 292-2-3 does not bind to the other 2C isoforms (2C8, 2C18, 2C19) or the other human cytochrome P450s, 1A1, 1A2, 2A6, 2B6, 2C8, 2D6, 2E1 or 3A4/5. MAb 292-2-3 inhibits the metabolism of tolbutamide, diclofenac and phenanthrene by the target 2C9Cys144 (II) allele by > 90% and does not inhibit the catalytic activity of the wild-type 2C9Arg144 (I), or 2C9Ile-->Leu359 (III) the other 2C isoforms 2C8, 2C18, 2C19, or the other non-2C human P450s listed above. The MAb 292-2-3 is thus a prototype of an ideal and extraordinarily specific reagent for the detection and measurement of the metabolic role of highly related isoforms and polymorphic alleles of human cytochrome P450s. MAbs of high specificity can also determine the amount of phenotypic expression of polymorphic alleles and their metabolic role in drug and non-drug xenobiotic metabolism in heterozygote individuals. The inhibitory MAb might also identify allele-specific substrates of polymorphic human cytochrome P450s.

Assessment of specificity of eight chemical inhibitors using cDNA-expressed cytochromes P450.

1. The selectivity of eight chemical inhibitors has been extensively evaluated with 10 cDNA-expressed human cytochrome P450 isoforms (CYP). The results indicate that sulphaphenazole, quinidine and alpha-naphthoflavone are selective inhibitors of CYP2C9 (IC50 = 0.5-0.7 microM), CYP2D6 (0.3-0.4 microM) and CYP1A (0.05-5 microM) respectively on the basis of the IC50, which are much lower than those of other P450 isoforms (> 10-fold). 2. Ketoconazole exhibited potent inhibition of both CYP3A4-catalysed metabolism of phenanthrene, testosterone, diazepam (IC50 = 0.03-0.5 microM) and CYP1A1-catalysed deethylation of 7-ethoxycoumarin (0.33 microM). The selectivity of ketoconazole for other P450s was highly related to the concentration used. 3. Diethyldithiocarbamate, orphenadrine and furafylline were shown separately to be less selective inhibitors of CYP2E1, CYP2B6 and CYP1A isoforms by a broad range of IC50 that overlap those observed with other P450 isoforms. 4. Furafylline, quinidine and alpha-naphthoflavone activated CYP3A4-catalysed phenanthrene metabolism by 1.7-, 2- and 15-fold respectively. 5. The selectivity of orphenadrine and ketoconazole was further examined by using inhibitory monoclonal antibodies (MAb). Inhibitory MAb specific for the individual P450 isoforms may be of greater value than chemical inhibitors.

Use of inhibitory monoclonal antibodies to assess the contribution of cytochromes P450 to human drug metabolism.

Three inhibitory monoclonal antibodies specific to cytochrome P450 3A4/5 (CYP3A4/5), CYP2C8/9/19 and CYP2E1, respectively, were used to assess the contribution of the P450s to the metabolism of seven substrates in liver microsomes from 18 human donors, as measured by monoclonal antibody inhibition phenotyping of the substrate conversion to product(s). Metabolism of seven substrates by recombinant cytochromes P450 and human liver microsomes was performed in the presence of monoclonal antibodies and their metabolites were analyzed by high-performance liquid chromatography (HPLC) or gas chromatography-mass spectrophotometry (GC-MS) to measure the magnitude of inhibition. Our results showed that CYP3A4/5 contributes to testosterone 6beta-hydroxylation, taxol phenol formation, diazepam 3-hydroxylation, diazepam N-demethylation, and aflatoxin B1 3-hydroxylation in human liver by 79.2%, 81.5%, 73. 2%, 34.5% and 80%, respectively. CYP2E1 contributes to chlorzoxazone 6-hydroxylation, p-nitroanisole O-demethylation, and toluene hydroxylation by 45.8%, 27.7% and 44.2% respectively, and CYP2C8/9/19 contribute to diazepam N-demethylation by 30.6%. The additive contribution (75.3%) of human CYP3A and CYP2C to diazepam N-demethylation was also observed in the presence of both anti-CYP3A4/5 and anti-CYP2C8/9/19 monoclonal antibodies. The contribution of individual P450s to the specific metabolic reaction in human liver varies greatly in the individual donors and the substrates examined. Thus, inhibitory monoclonal antibodies could play a unique role in defining the single or subfamily of cytochrome P450 that is responsible for the metabolism of specific drugs.

Targeted disruption of the microsomal epoxide hydrolase gene. Microsomal epoxide hydrolase is required for the carcinogenic activity of 7,12-dimethylbenz[a]anthracene.

Microsomal epoxide hydrolase (mEH) is a conserved enzyme that is known to hydrolyze many drugs and carcinogens, and a few endogenous steroids and bile acids. mEH-null mice were produced and found to be fertile and have no phenotypic abnormalities thus indicating that mEH is not critical for reproduction and physiological homeostasis. mEH has also been implicated in participating in the metabolic activation of polycyclic aromatic hydrocarbon carcinogens. Embryonic fibroblast derived from the mEH-null mice were unable to produce the proximate carcinogenic metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), a widely studied experimental prototype for the polycylic aromatic hydrocarbon class of chemical carcinogens. They were also resistant to DMBA-mediated toxicity. Using the two-stage initiation-promotion skin cancer bioassay, the mEH-null mice were found to be highly resistant to DMBA-induced carcinogenesis. In a complete carcinogenesis bioassay, the mEH mice were totally resistant to tumorigenesis. These data establish in an intact animal model that mEH is a key genetic determinant in DMBA carcinogenesis through its role in production of the ultimate carcinogenic metabolite of DMBA, the 3,4-diol-1,2-epoxide.

An inhibitory monoclonal antibody to human cytochrome P450 2A6 defines its role in the metabolism of coumarin, 7-ethoxycoumarin and 4-nitroanisole in human liver.

Cytochrome P450 (CYP) 2A6 is an important enzyme catalysing the metabolism of many drugs, procarcinogens and promutagens. Its role in human liver metabolism of coumarin, 4-nitroanisole, 4-nitrophenol and 7-ethoxycoumarin was analysed with an inhibitory monoclonal antibody (MAb) to CYP2A6. MAbs were derived from a panel of 16 hybridomas which yielded positive enzyme-linked immunosorbent assay (ELISA) results or immunoblots against CYP2A6. The hybridomas were selected from more than 500 clones generated by the fusion of myeloma cells with spleen cells of mice immunized with purified baculovirus-expressed human CYP2A6. The MAbs obtained from four of the 16 hybridomas exhibited strong inhibitory activity to CYP2A6-catalysed phenanthrene metabolism. MAb 151-45-4 was positive and highly specific to CYP2A6 as determined by ELISA and immunoblot, and showed no cross-reactivity with recombinant human CYP 1A1, 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4 and 3A5, as tested with ELISA and immunoblot analyses. MAb 151-45-4 specifically inhibited CYP2A6-catalysed metabolism of phenanthrene, 4-nitroanisole, 4-nitrophenol, coumarin and 7-ethoxycoumarin each by 94-99% and did not inhibit their metabolism catalysed by 10 other human CYPs. The potent inhibitory effect of MAb 151-45-4 was used to define the contribution of human CYP2A6 to the metabolism of coumarin, 4-nitroanisole and 7-ethoxycoumarin in seven human liver microsome samples. Coumarin metabolism in all of the seven samples was inhibited by greater than 94% by MAb 151-45-4 which indicates that essentially all microsome mediated coumarin metabolism in human liver is catalysed only by CYP2A6. Inhibition of 4-nitroanisole and 7-ethoxycoumarin metabolism by anti 2A6 MAb ranged from 22-65% and 8-24%, respectively. The degree of inhibition defines the contribution of CYP2A6 activity to the 4-nitroanisole and 7-ethoxycoumarin metabolism in human liver and the range reflects the variability among samples. The inhibitory antibody to CYP2E1 was used to determine its role in 4-nitroanisole and 7-ethoxycoumarin metabolism in seven human liver samples. The addition of both MAbs to CYP2A6 and 2E1 to the microsome samples defined combinatorially the relative role of CYP2A6 and 2E1 in the metabolism of 4-nitroanisole and 7-ethoxycoumarin.

Eight inhibitory monoclonal antibodies define the role of individual P-450s in human liver microsomal diazepam, 7-ethoxycoumarin, and imipramine metabolism.

Eight inhibitory monoclonal antibodies (MAbs) individually specific to human cytochrome P-450 (P-450) 1A1, 1A2, 2A6, 2B6, 2C subfamily (2C8, 2C9, 2C18 and 2C19), 2D6, 2E1, and 3A4/5 were used to define the role of single P-450s in the metabolism of diazepam (DZ), 7-ethoxycoumarin (7-EC), and imipramine (IMI) in human liver microsomes (HLM). The MAbs were added combinatorially to six HLM samples. With DZ as a substrate, more than 80% of temazepam (TMZ) formation was inhibited in all six samples by the addition of MAb to 3A4/5, indicating an 80% contribution of 3A4/5 to TMZ formation. Nordiazepam formation was inhibited with MAbs to 2B6 (6-23%), 2C subfamily (12-61%) and 3A4/5 (14-45%). The MAbs to 1A1, 1A2, 2A6, 2D6, and 2E1 did not inhibit TMZ or nordiazepam formation; this indicates their noninvolvement in DZ metabolism. The MAb-defined P-450 contribution to 7-EC Odeethylation in six HLM samples was 17 to 60% for 2E1, 15 to 46% for 2A6, and 5 to 22% for 1A2, reflecting the role and variation of each P-450 in this activity. MAbs to 1A1, the 2C subfamily, 2D6, and 3A4/5 did not affect 7-EC metabolism in the HLM samples. IMI is metabolized mainly to 2-hydroxyimipramine by expressed 2C19 and 2D6, and desipramine (DIM) by expressed 1A2, 2C18, 2C19 and 2D6. Expressed 1A1, 2C9, and 3A4 showed low activities for the formation of DIM. Of six HLM samples, five showed IMI hydroxylation activity (0.35-2.6 nmol/min/nmol P-450) while one (HL43) lacked hydroxylation activity. All six HLM samples showed N-deethylation activity (0.74-1.4 nmol/min/nmol P-450). The MAb-determined contribution of 2D6 and 2C19 to 2-hydroxyimipramine formation ranged from 47 to 90% and from 0 to 49%, respectively, while HL43 did not show 2-hydroxylation. The role of P-450s involved in DIM formation varied for 2C19 (13-50%), 1A2 (23-41%), and 3A4 (8-26%). These studies demonstrate a system for identifying the quantitative metabolic role of single P-450s and their interindividual variability in a tissue containing multiple P-450s. The system using inhibitory MAbs is simple, precise, and applicable to any P-450-mediated catalytic activity including that for drugs, carcinogens, mutagens, toxic chemicals and endobiotics.

Depentylation of [3H-pentyl]methyl-n-amylnitrosamine by rat esophageal and liver microsomes and by rat and human cytochrome P450 isoforms.

Methyl-n-amylnitrosamine (MNAN) induces esophageal cancer in rats, probably involving activation by cytochromes P450. We studied the metabolic depentylation of MNAN. [3H-4,5-pentyl]MNAN and [3H-2,3-pentyl]-MNAN were synthesized, purified, and incubated with rat esophageal microsomes (REM) or rat liver microsomes (RLM) to give [3H]pentaldehyde (depentylation), an indicator of MNAN activation. [3H]Pentaldehyde was determined by high-performance liquid chromatography of its 2,4-dinitrophenylhydrazone. Adding 5 mM semicarbazide to incubations increased the observed depentylation (except that due to CYP2E1) by >60%. MNAN depentylation by REM and uninduced and induced RLM showed Km values of 64, 610, and 170-330 microM, respectively (Vmax: 20, 220, and 160-1270 pmol/mg protein/min, respectively). The depentylation of 100 microM MNAN by REM was inhibited 98% by CO and 65% by coumarin preincubated for 15 min with REM (Ki, 120 microM) but was unaffected by antibodies inhibitory to various P450s. MNAN inhibited coumarin 7-hydroxylation by RLM and CYP2A6 (Ki, 3000 and 320 microM, respectively). REM showed slight coumarin 7-hydroxylase activity. MNAN depentylation by RLM was 41% inhibited by an antibody to CYP2C11. Km for rat CYP2E1, human CYP2E1, and human CYP2A6 was 210, 115, and 17 microM, respectively (Vmax: 900, 570, and 120 pmol/nmol P450/min, respectively). We conclude that MNAN activation by REM is probably due to a P450 related to CYP2A3, a rodent nasal P450.

Inhibitory monoclonal antibodies to human cytochrome P450 1A2: analysis of phenacetin O-deethylation in human liver.

Human cytochrome P450 1A2 metabolizes a large number of common drugs and engages in carcinogen metabolism and activation. Baculovirus-expressed 1A2 was used to immunize mice producing hybridomas yielding monoclonal antibodies (MAbs). Three of 2050 clones assayed yielded the MAbs, MAb 26-7-5, MAb 951-5-1, MAb 1812-2-4, which were specific for 1A2 as assessed by enzyme-linked immunosorbent assay and immunoblots. The three MAbs inhibited 1A2-catalysed metabolism of phenacetin, 7-ethoxycoumarin, chlorzoxazone and phenanthrene by more than 85%. The MAbs were highly specific to 1A2 and did not inhibit 11 other human P450s. The phenancetin O-deethylation activity varied from 0.44-2.49 nmol/min/nmol P450 in eight human liver microsomes samples. MAb 26-7-5 inhibited 1A2-dependent phenacetin O-deethylation in these samples by 64-84% indicating the amount of 1A2 contribution to this reaction and in addition a role for other P450s in the O-deethylation. Independent analysis of recombinant human P450s showed that 1A1, 1A2, 2A6 and 2C19 exhibited phenacetin O-deethylation activity, with 1A1 and 1A2 being the most active followed by 2C19 and 2A6. Eight other P450s were inactive towards phenacetin O-deethylation. The role of different P450 in eight liver samples was analysed with specific individual inhibitory MAbs. Inhibitory antibodies to 1A2, 2C8/9/18/19, 2A6, 2D6, 2E1, and 1A1 were combinatorially added to the microsomes. The O-deethylation activity was inhibited by antibodies to 1A2 (64-84%), to 2C19 (4.6-20%) and to 2A6 (0-8.8%). The total activity inhibited by antibodies to P450 2E1, 2D6 and 1A1 was less than 4.5%, indicating a minor role for these P450s in phenancetin metabolism in human liver microsomes. Thus, 1A2, 2C 9 and 2A6 are the dominant P450s for phenacetin O-deethylation. These studies demonstrate the use of inhibitory MAbs to P450s for a simple and precise assessment of the quantitative role of each P450 in the metabolism of substrates, including drugs, carcinogens, mutagens, environmental chemicals and endobiotics.

Role of human cytochrome P450 3A4 and 3A5 in the metabolism of taxotere and its derivatives: enzyme specificity, interindividual distribution and metabolic contribution in human liver.

Taxotere, a promising anticancer agent, is metabolized almost exclusively in liver and excreted from bile in all species. To determine which cytochrome P450 is involved in taxotere biotransformation, 11 cDNA-expressed human cytochrome P450s were examined for their activity in the metabolism of taxotere and its derivatives. Of all P450s, cytochrome P450 3A4 and 3A5 were the most active for the oxidation of taxotere to the primary metabolite RPR104952 and for subsequent oxidation of RPR104952 to RPR111059 and RPR111026. RP70617, an epimer of taxotere was also metabolized by both P450 3A enzymes to form metabolite XII. The activity of 3A4/5 enzymes for these substrates was 4-50-fold greater than the other P450s examined. The Kms of 3A4 and 3A5 for taxotere were 0.91 and 9.28 microM, and Vmax for the formation of RPR104952 were 1.17 and 1.36 m(-1), respectively. The contribution of the 3A enzyme complex to the metabolism of taxotere in human livers from 21 individuals was assessed with the inhibitory monoclonal antibody and ranged from 64-93%. The primary oxidative metabolism of taxotere by human liver microsomes was well correlated with 3A4-dependent reactions for testosterone 6beta-hydroxylation (r2 = 0.84), taxol aromatic hydroxylation (r2 = 0.67) and aflatoxin B1 3alpha-hydroxylation (r2 = 0.63); whereas a poor correlation was found for reactions specifically catalysed by other P450s (all r2 < or =O.17). The extent of taxotere metabolism also closely correlated with levels of 3A4 enzyme in human livers quantified with immunoblot monoclonal antibody (r2 = 0.61). These results demonstrate that the P450 3A4 and 3A5 enzymes are major determinants in taxotere oxidation and suggest that care must be taken when administering this drug with other drugs that are also substrates for these enzymes.

Further characterization of the expression in liver and catalytic activity of CYP2B6.

Previous studies in this laboratory have determined the lack of specificity of several antibody and substrate probes of CYP2B6. The goals of the current study were to examine the expression of CYP2B6 in a bank of human liver microsome (HLM) samples using a new specific monoclonal antibody (MAb 49-10-20) and to further characterize the substrate specificity of CYP2B6. A 100-fold variability in expression of immunodetectable CYP2B6 was demonstrated in a bank of 19 HLM samples (0.7 pmol/mg protein to 71. 1 pmol/mg protein) using MAb 49-10-20. CYP2B6 levels were found to significantly (P < .0001) correlate with S-mephenytoin N-demethylation to nirvanol (r2 = 0.89), 7-hydroxy-4-trifluoromethylcoumarin formation (r2 = 0.81) and several markers of CYP3A levels and activity. The relationships between nirvanol formation and CYP3A levels or activity were found to depend on two HLM samples. Km (apparent) values were generated for benzyloxyresorufin O-deethylation (1.3 microM), benzphetamine N-demethylation (93.4 microM), 3-cyano 7-ethoxycoumarin O-deethylation (71.3 microM), midazolam 1'-hydroxylation (46.1 microM) and 4-chloromethyl-7-ethoxycoumarin O-deethylation (33.7 microM) using expressed CYP2B6. Testosterone 16beta-hydroxylation by expressed CYP2B6 resulted in atypical kinetics characteristic of substrate activation. The data best fit the Hill equation with a Km (apparent) of 50.5 microM and an n of 1.3 (n = number of sites bound by activator). In conclusion, the highly specific MAb 49-10-20 was used to provide further confirmation that S-mephenytoin N-demethylation to nirvanol is a CYP2B6 selective probe. Finally, some, but not all substrates of CYP2B6 demonstrate autoactivation.

Inhibitory monoclonal antibody to human cytochrome P450 2B6.

The human cytochrome P450 2B6 metabolizes, among numerous other substrates, diazepam, 7-ethoxycoumarin, testosterone, and phenanthrene. A recombinant baculovirus containing the human 2B6 cDNA was constructed and used to express 2B6 in Sf9 insect cells. The 2B6 was present at 1.8 +/- 0.4% of the total cellular protein and was purified to a specific content of 13.3 nmol/mg protein. Mice were immunized with the purified 2B6, and a total of 811 hybridomas were obtained from the fusion of NS-1 myeloma cells and spleen cells of the immunized mice. Monoclonal antibodies (MAbs) from 24 of the hybrids exhibited immunobinding to 2B6 as determined by ELISA. One of the MAbs, 49-10-20, showed a strong immunoblotting activity and was highly inhibitory to 2B6 enzyme activity. MAb 49-10-20 inhibited cDNA-expressed 2B6-catalyzed metabolism of diazepam, phenanthrene, 7-ethoxycoumarin, and testosterone by 90-91%. MAb 49-10-20 showed extremely high specificity for 2B6 and did not bind to 17 other human and rodent P450s or inhibit the metabolism of phenanthrene catalyzed by human 1A2, 2A6, 2C8, 2C9, 2D6, 2E1, 3A4, and 3A5. MAb 49-10-20 was used to determine the contribution of 2B6 to the metabolism of phenanthrene and diazepam in human liver. In ten liver samples, MAb 49-10-20 inhibited phenanthrene metabolism variably by a wide range of 8-42% and diazepam demethylation by 1-23%. The degree of inhibition by the 2B6 specific MAb 49-10-20 defines the contribution of 2B6 to phenanthrene and diazepam metabolism in each human liver. This technique using inhibitory MAb 49-10-20 determines the contribution of 2B6 to the metabolism of its substrates in a human tissue containing multiple P450s. This study is a prototype for the use of specific and highly inhibitory MAbs to determine individual P450 function.

Role of cDNA-expressed human cytochromes P450 in the metabolism of diazepam.

The metabolic conversion of diazepam (DZ) to temazepam (TMZ, a C3-hydroxylation product of DZ) and N-desmethyldiazepam (NDZ, an N1-demethylation product of DZ) was studied using cDNA-expressed human cytochrome P450 (CYP) isozymes 1A2, 2B6, 2C8, 2C9, 2C9R144C, 2E1, 3A4, and 3A5 and human liver microsomes from five organ donors. Of the CYPs examined, 3A5, 3A4, and 2B6 exhibited the highest enzymatic activities with turnovers ranging from 7.5 to 12.5 nmol of product formed/min/nmol for the total metabolism of DZ, while 2C8, 2C9, and 2C9R144C showed lesser and moderate activities. 1A2 and 2E1 produced insignificant amounts of metabolites of DZ. The regioselectivity of CYPs was determined, and 2B6 was found to catalyze exclusively and 2C8, 2C9, and 2C9R144C preferentially the N1-demethylation of DZ to form NDZ. 3A4 and 3A5 catalyzed primarily the C3-hydroxylation of DZ, which was more extensive than the N1-demethylation. The ratios of TMZ to NDZ formed in the metabolism of DZ by 3A4 and 3A5 were approximately 4:1. Enzyme kinetic studies indicated that 2B6- and 2C9-catalyzed DZ metabolism followed Michaelis-Menten kinetics, whereas 3A4 and 3A5 displayed atypical and non-linear curves in Lineweaver-Burk plots. Human liver microsomes converted DZ to both TMZ and NDZ at a ratio of 2:1. Our results suggest that hepatic CYP3A, 2C, and 2B6 enzymes have an important role in the metabolism of DZ by human liver.

Effect of phenobarbital on hepatic CYP1A1 and CYP1A2 in the Ahr-null mouse.

Studies have suggested that phenobarbital (PB) induces members of the CYP1A subfamily by both transcriptional and post-transcriptional mechanisms. Using the Ahr -/- mice, we examined the induction of CYP1A1 and CYP1A2 by PB. CYP1A2 mRNA and protein were induced by PB in the null mice, suggesting that CYP1A2 is regulated by PB by a mechanism independent of the aryl hydrocarbon receptor (AHR). In contrast, the regulation of CYP1A1 is highly dependent on the AHR.