Estimating the Bioconcentration Factors of Hydrophobic Organic Compounds from Biotransformation Rates Using Rainbow Trout Hepatocytes

Determining the biotransformation potential of commercial chemicals is critical for estimating their persistence in the aquatic environment. In vitro systems are becoming increasingly important as screening methods for assessing the potential for chemical metabolism. Depletion rate constants (k_d) for several organic chemicals with high octanol-water partition coefficient (K_ow) values (9-methylanthracene, benzo(a)pyrene, chrysene, and PCB-153) in rainbow trout hepatocytes were determined to estimate biotransformation rate constants (k_MET) that were used in fish bioconcentration factor (BCF) models. Benzo[a]pyrene was rapidly biotransformed when incubated singly; however, its depletion rate constant (k_d) declined 79% in a mixture of all four chemicals. Chrysene also exhibited significant biotransformation and its depletion rate constant declined by 50% in the mixture incubation. These data indicate that biotransformation rates determined using single chemicals may overestimate metabolism in environments containing chemical mixtures. Incubations with varying cell concentrations were used to determine whether cell concentration affected k_d estimates. No statistically significant change in depletion rate constants were seen, possibly due to an increase in nonspecific binding of hydrophobic chemicals as cell density increased, decreasing overall biotransformation. A new model was used to estimate BCFs from k_MET values calculated from empirically derived k_d values. The inclusion of k_MET in models resulted in significantly lower BCF values (compared k_MET = 0). Modelled BCF values were consistent with empirically derived BCF values from the literature.

The safety, tolerability, and pharmacokinetic profile of GSK2838232, a novel 2nd generation HIV maturation inhibitor, as assessed in healthy subjects

This work aimed to assess the safety, tolerability, pharmacokinetics (PK), and relative bioavailability of GSK2838232, an investigational HIV maturation inhibitor. GSK2838232 was administered over four dose-escalation studies in healthy subjects which assessed single oral doses (5-250 mg) and repeat doses (up to 200 mg once or twice daily) ±100 mg ritonavir (RTV) once daily. GSK2838232 administration (up to 250 mg) to 124 subjects across four studies resulted in few mild adverse events (AEs) with similar frequencies to placebo. There were no clearly identified drug-related AEs. GSK2838232 tested fasted was quickly absorbed with a tmax of 2-3 hours. With food, the absorption was delayed and more variable, with ~60% increase in AUC and Cmax. Overall, following single doses GSK2838232 AUC and Cmax generally exhibited proportional PK from 50 to 100 mg dose without RTV and from 50 to 250 mg with RTV and following repeated doses of 20-200 mg with RTV. In relative bioavailability studies, a micronized formulation was found to be suitable for development. At steady state, RTV increased GSK2838232 AUC and Cmax by 10- and 3-fold, respectively. Half-life was prolonged from ~17 hours nonboosted to ~34 hours with RTV. This boosting effect was also seen in repeat-dose GSK2838232 studies, which achieved the targeted plasma exposure with GSK2838232 as a once-daily regimen of up to 200 mg with RTV. The results of these studies demonstrated a favorable safety and PK profile for GSK2838232 and support its investigation for the treatment of HIV infection.

The detoxification process, bioaccumulation and damage effect in juvenile white shrimp Litopenaeus vannamei exposed to chrysene

This study aimed to evaluate the effect of chrysene (CHR) on detoxification enzymes, bioaccumulation and effect of CHR on biomolecule damage in different organs of the juvenile white shrimp Litopenaeus vannamei. In this study, juvenile white shrimp L. vannamei were exposed to CHR for 21 days at four different concentrations as 0, 0.3, 2.1 and 14.7 μg/L. Results showed that CHR bioaccumulation increased rapidly at first then reached a plateau. The activities of aryl hydrocarbon hydroxylase (AHH), 7-ethoxyresorufin O-deethylase (EROD), epoxide hydrolase (EH), glutathione-S-transferase (GST), sulfotransferase (SULT) and uridinediphosphate glucuronyltransferase (UGT) were induced and then became stable gradually. Moreover, 2.1 and 14.7 μg/L CHR treatments increased activity of superoxide dismutase (SOD) in gills and hepatopancreas, while total antioxidant capacity (T-AOC) and GSH/GSSG were suppressed after CHR exposure. Additionally, lipid peroxidation (LPO) levels, protein carbonyl (PC) contents and DNA damage were induced throughout the exposure period, and different trends were detected with time of exposure. Overall, these novel findings of CHR bioaccumulation and resulted toxicity demonstrate that CHR could affect the physical status of L. vannamei. This study will form a solid basis for a realistic extrapolation scientific data for aquaculture water monitoring and food security.

A comparative study on the uptake of polycyclic aromatic hydrocarbons by Anodonta californiensis

Uptake of polycyclic aromatic hydrocarbons (PAHs) by the freshwater bivalve mollusc Anodonta californiensis was examined in the presence and absence of surfactant in order to gain further insight into mixture toxicity and to predict whether certain mixtures have negative and/or positive effects on aquatic organisms. In the presence of surfactant, the uptake of anthracene or chrysene was higher than that of naphthalene, given the same concentration in the solution. In the absence of surfactant, the trend was similar, but the uptakes were increased by approximately 100% compared to those in the presence of surfactant. On the uptake of naphthalene, the presence of anthracene showed only minor influence. The uptake of anthracene was affected by both naphthalene and chrysene. The uptake of chrysene was influenced by neither naphthalene nor anthracene. There was no observable displacement of divalent cations from the surface of the gill membrane by any of the PAHs studied.

A model for estimating the potential biomagnification of chemicals in a generic food web: preliminary development

GOAL, SCOPE AND BACKGROUND

Bioaccumulation and biomagnification of organic pollutants have been increasingly assessed and modeled during the last years. Due to the complexity of these processes and the large variability of food webs, setting generic assessments for these parameters is really difficult. Equilibrium models, based on a compound's lipophylicity, are the main tool in regulatory proposals, such as for identifying Persistent, Bioaccumulative and Toxic Substances (PBTs), although a refinement has been claimed by the scientific community. Toxicokinetic studies offer an alternative for these estimations, where biomagnification is modeled as a succession of bioaccumulation processes, each one regulated by toxicokinetic parameters.

METHODS

A review of kinetic models covering species belonging to different trophic levels and with different ecological behavior has been conducted. The results were employed for setting a conceptual model for estimating the biomagnification potential in a generic food web, which was mathematically implemented through system dynamic models developed under data sheet software. Crystal Ball was then employed for allowing Monte Carlo based probabilistic calculations. Bioaccumulation laboratory assays have been performed to estimate toxicokinetic parameters in mussels (Mytilus edulis) with two PAHs (chrysene and benzo[a]pyrene). The contamination was delivered via food. The exposure period lasted more than one month followed then by a depuration phase. The contaminant content was determined on an individual basis on five replicates.

RESULTS AND DISCUSSION

. The reviewed information suggested the development of a tiered conceptual biomagnification model, starting with a simplified food chain which can be refined to more realistic and complex models in successive levels.

CONCLUSIONS

The mathematical implementation of the conceptual model offers tools for estimating the potential for bioaccumulation and biomagnification of chemicals under very different conditions. The versatility of the model can be used for both comparative estimations and for validating the model.

RECOMMENDATIONS AND PERSPECTIVES

Since bioaccumulation and biomagnification processes are crucial elements for a proper risk assessment of chemicals, their estimation by mathematical models has been widely tested. However, inregulatory assessments, too simplistic models are still being used quite often. The biomagnification model presented in this study should be amore accurate alternative to these models. In comparison to other previously published biomagnification models, the present one covers the time variation of bioaccumulation using just a few toxicokinetic parameters.

Comparative metabolism of chrysene and 5-methylchrysene by rat and rainbow trout liver microsomes

We have investigated the metabolism of chrysene (CHR) and 5-methychyrsene (5-MeCHR) by Shasta rainbow trout (Oncorhyncus mykiss) and Long Evans rat liver microsomes to assess the effect of a non-benzo ring methyl substituent on the reactions involved in the metabolism of polycyclic aromatic hydrocarbons (PAHs). Trout as well as rat liver microsomes metabolized both CHR and 5-MeCHR at essentially similar rates, indicating that the methyl substituent does not alter the substrate specificity of the cytochrome P450(s) involved in the metabolism of the two PAHs. Dihydrodiols were the major CHR metabolites formed by both trout and rat liver microsomes, whereas the trout liver microsomes formed a considerably higher proportion of 5-MeCHR phenols compared to diols, indicating that 5-methyl substitution alters the substrate specificity of trout microsomal epoxide hydrolase for 5-MeCHR epoxides. Unlike trout liver microsomes, rat liver microsomes formed a much greater proportion of 5-MeCHR diols compared to 5-MeCHR phenols, suggesting that 5-MeCHR epoxides are better substrates for the microsomal epoxide hydrolase present in rat liver than for the enzyme in trout liver. Both trout and rat liver microsomes are more efficient at attacking the bay-region bond versus the non-bay-region double bond in chrysene. In contrast the reverse is true in the case of 5-MeCHR, indicating that a non-benzo ring methyl substituent alters the regioselectivity of the enzymes involved in the oxidative metabolism of PAHs.

Pyrene and chrysene fate in surface soil and sand microcosms

Polycyclic aromatic hydrocarbons (PAHs) are major components of wastes from municipal gas plants and many wood preservatives. Soil contaminated with these wastes is a potential threat to human health because of the carcinogenicity of many PAHs. This study follows the fate of two four-ring PAHs, pyrene and chrysene, in three matrices: an adapted soil (obtained from a site contaminated with PAHs for more than 75 years), an uncontaminated soil (with and without an inoculum of adapted soil), and sand mixed with an inoculum of adapted soil. Radiolabeled pyrene, chrysene, and salicylic acid (a metabolite of PAH biodegradation) were used to trace the mineralization, transformation, extractability, and formation of an unextractable residual over time. Linear approximations of the rates of these processes were made. High-performance liquid chromatography (HPLC) analysis of extracts from inoculated soil showed the transient formation of two known metabolites: 1-hydroxypyrene (from pyrene) and 1-hydroxy-2-naphthoic acid (from chrysene). The amount of extractable label diminished steadily over the course of the study in systems that were not inhibited with sodium azide, whereas the amount of extractable label remained relatively constant in inhibited systems. Correspondingly, the amount of nonextractable residual label generally increased during each incubation in uninhibited systems, whereas the amount of this residual label remained relatively constant in inhibited systems. In contrast, the rate and extent of mineralization varied widely across matrix types. This suggests that alterations of the PAH that impact extractability and residual formation are common, in contrast to mineralization, which was apparently limited to adapted communities.

A phase I and pharmacological study of protracted infusions of crisnatol mesylate in patients with solid malignancies

This Phase I and pharmacological study was performed to assess the feasibility of administering the polycyclic aromatic hydrocarbon crisnatol in increasingly prolonged continuous i.v. infusions to patients with advanced solid malignancies. The study also sought to characterize the-principal toxicities of crisnatol on this schedule, to recommend doses for subsequent disease-directed studies, and to characterize possible associations between pharmacological parameters and toxicity. Sixteen patients were treated with 40 courses of crisnatol administered as a continuous i.v. infusion. The initial dose-schedule was 750 mg/m2/day for 6 days, and the duration of the infusion was to be progressively increased by 3-day increments to 9, 12, 15, 18, and 21. Courses were to be repeated every 4 weeks. Moderate to severe central nervous system (CNS) toxicity precluded the administration of crisnatol 750 mg/m2/day for longer than 6 days, and, therefore, the dose of crisnatol was reduced to 600 mg/m2/day. At this dose, three of five patients receiving a 12-day infusion experienced dose-limiting toxicity, which consisted of pulmonary thromboembolism (two patients) and grade 4 thrombocytopenia (one patient). None of the six patients completing a 9-day infusion at 600 mg/m2/day developed dose-limiting toxicity during the first or second course of crisnatol. At this dose level, the plasma concentrations at steady state (Css) averaged 1607.8+/-261.1 ng/ml, which exceeds minimal inhibitory concentrations for most tumors in vitro (1000 ng/ml). In fact, the administration of crisnatol at a dose of 600 mg/m2/day for 9 days resulted in the longest duration that biologically relevant plasma crisnatol concentrations have been sustained. Plasma Css values were significantly higher in patients who experienced severe CNS toxicity compared with those who did not (2465.3+/-1213.5 versus 1342+/-447.3 ng/ml; P = 0.04). There were no relationships evident between the clearance of crisnatol and indices reflecting renal and hepatic functions. One patient with a glioblastoma multiforme experienced a partial response lasting 14 months. The relative lack of intolerable CNS toxicity at the recommended dose for Phase II studies of crisnatol, 600 mg/m2/day for 9 days, as well as the magnitude of the Css values achieved and the antitumor activity observed at this dose, are encouraging. However, the mechanisms for the apparently increased thrombogenicity observed in this trial are unclear and require further elucidation.

Specificity of murine glutathione S-transferase isozymes in the glutathione conjugation of (-)-anti- and (+)-syn-stereoisomers of benzo[g]chrysene 11,12-diol 13,14-epoxide

Specificities of murine glutathione (GSH) S-transferase (GST) isozymes mGSTA1-1, mGSTA2-2, mGSTA3-3 and mGSTA4-4 (alpha class), mGSTP1-1 (pi class) and mGSTM1-1 (mu class) for GSH conjugation of (-)-anti- and (+)-syn-stereoisomers of benzo[g]chrysene 11, 12-diol 13,14-epoxide (B[g]CDE), the activated metabolites of the environmental pollutant benzo[g]chrysene (B[g]C), have been determined. When GST activity was determined as a function of varying (-)-anti- or (+)-syn-B[g]CDE concentration (10-320 microM) at a fixed saturating concentration of GSH (2 mM), each isozyme obeyed Michaelis-Menten kinetics. mGSTA1-1 was significantly more efficient than other murine GSTs in the GSH conjugation of not only (-)-anti-stereoisomer but also (+)-syn-B[g]CDE. For example, the catalytic efficiency (k(cat)/K(m)) of mGSTA1-1 towards (-)-anti-B[g]CDE was approximately 2.3- to 16.6-fold higher compared with other murine GSTs. Likewise, mGSTA1-1 was approximately 2.7-, 6.7-, 4.4- and 12.4-fold more efficient than mGSTA2-2, mGSTA3-3, mGSTP1-1 and mGSTM1-1, respectively, in catalyzing the GSH conjugation of (+)-syn-B[g]CDE. Interestingly, mGSTA4-4, which also belongs to class alpha, was virtually inactive towards both stereoisomers of B[g]CDE. The results of the present study indicate that murine GSTs, especially alpha class isozymes, significantly differ in their ability to detoxify B[g]CDE stereoisomers and that mGSTA1-1 plays a major role in the detoxification of both (-)-anti- and (+)-syn-B[g]CDE, which among four B[g]CDE stereoisomers are formed from the carcinogen B[g]C as major DNA binding metabolites.

Role of human N-acetyltransferases, NAT1 or NAT2, in genotoxicity of nitroarenes and aromatic amines in Salmonella typhimurium NM6001 and NM6002

Human NAT1 and NAT2 genes were subcloned into pACYC184 vector and the plasmids thus obtained were introduced into Salmonella typhimurium O-acetyltransferase-deficient strain NM6000 (TA1538/1, 8-DNP/pSK1002), establishing new strains NM6001 and NM6002, respectively. We compared the sensitivities of these two strains with those of NM6000 towards carcinogenic nitroarenes and aromatic amines in the SOS/umu response. The induction of umuC gene expression by these chemicals in the presence and absence of the S9 fraction was assayed by measuring the cellular beta-galactosidase activity expressed by the umuC"lacZ fusion gene in the tester strains. 2-Nitrofluorene and 2-aminofluorene induced umuC gene expression more strongly in the NM6001 strain than in the NM6002 strain. In contrast, induction of umuC gene expression by 1, 8-dinitropyrene, 6-aminochrysene and 2-amino-3,5-dimethylimidazo[4, 5-f]quinoline was weaker in the NM6001 strain than in the NM6002 strain. 1-Nitropyrene, 2-amino-6-methyl-dipyrido[1,2-a:3', 2'-d]imidazole, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole, 3-amino-1-methyl-5H-pyrido[4,3-b]indole, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3-methyl-9H-pyrido[2,3-b]indole were found to induce umuC gene expression at similar extents in both strains. These results suggest that the newly developed strains can be employed for the studies on mechanisms of genotoxicity of a variety of nitroarenes and aromatic amines, along with the assessment of cancer risk to humans.

Catalytic efficiencies of allelic variants of human glutathione S-transferase P1-1 toward carcinogenic anti-diol epoxides of benzo[c]phenanthrene and benzo[g]chrysene

Four allelic variants of glutathione (GSH) S-transferase P1-1 (hGSTP1-1) that differ in their structures at amino acid(s) in position(s) 104 and/or 113 are known to exist in human populations. However, the physiological significance of hGSTP1-1 polymorphism is not fully understood. In this communication, we report that the I104,A113 allele of hGSTP1-1, which is most frequent in human populations, is also most efficient in the GSH conjugation of carcinogenic anti-diol epoxides of benzo[g]chrysene and benzo[c]phenanthrene (anti-BGCDE and anti-BCPDE, respectively). The catalytic efficiency of hGSTP1-1(I104,A113) isoform toward anti-BGCDE, 0.36 mM(-1) x s(-1), was approximately 1.7-fold higher (P < 0.05) compared with hGSTP1-1(V104,V113). Interestingly, the frequency of codon 104-valine alleles is significantly higher in certain cancers compared with codon 104-isoleucine alleles. Like anti-BGCDE, the catalytic efficiency of hGSTP1-1(I104,A113) isoform toward anti-BCPDE was higher by about 1.4- to 2.2-fold (P < 0.05) than those of other hGSTP1-1 variants. These observations are interesting because we have shown previously (Hu, X. et al., Biochem. Biophys. Res. Commun., 238: 397-402, 1997) that the V104,V113 variant, not the I104,A113 isoform, is most efficient in the GSH conjugation of bay-region anti-diol epoxide of benzo(a)pyrene (anti-BPDE), which, unlike anti-BGCDE or anti-BCPDE, is a planar molecule. In conclusion, our results suggest that hGSTP1-1 polymorphism may be an important factor in differential susceptibility of humans to cancers where polycyclic aromatic hydrocarbons are etiological factors and that I104,A113 variant may play a major role in the detoxification of nonplanar, sterically hindered fjord-region diol epoxides (e.g., anti-BGCDE).

Synthesis of fjord region tetraols and their use in hepatic biotransformation studies of dihydrodiols of benzo[c]chrysene, benzo[g]chrysene and dibenzo[a,l]pyrene

Metabolic activation of the racemic benzo[c]chrysene-trans-9,10-, benzo[g]chrysene-trans-11,12- and dibenzo[a,l]pyrene-trans-11,12-dihydrodiols to fjord region syn- and anti-dihydrodiol epoxides by microsomes of Aroclor 1254-treated Sprague-Dawley rats has been examined. Since the fjord region dihydrodiol epoxides were hydrolytically unstable under the experimental conditions, their enzymatic formation was determined by analyzing the tetraols as their products of acidic hydrolysis upon addition of perchloric acid. The various stereoisomeric tetraols formed were separated by HPLC and identified by co-chromatography with authentic tetraols, which had been prepared by acidic hydrolysis of synthetically available syn- and anti-dihydrodiol epoxides and characterized by NMR and UV spectroscopy. Under standardized conditions the acidic hydrolysis of syn-dihydrodiol epoxides of benzo[c]chrysene, benzo[g]chrysene and dibenzo[a,l]pyrene resulted in the formation of two tetraols with cis/trans ratios of 81:19, 77:23 and 80:20, respectively, whereas the anti-dihydrodiol epoxides underwent almost exclusively trans hydrolysis. The proportion of the stereoisomeric tetraols obtained from microsomal incubations indicates that all three dihydrodiols are predominantly oxidized at the adjacent olefinic double bond to the anti-diastereomers of the corresponding fjord region dihydrodiol epoxides accounting for 4-35% of the ethyl acetate-extractable metabolites. To allow quantitative assessment of the metabolites 3H-labeled trans-dihydrodiols were synthesized by reduction of the corresponding o-quinones with sodium borotritide. Metabolic conversion of benzo[c]chrysene-trans-9,10- and dibenzo[a,l]pyrene-trans-11,12-dihydrodiol by rat liver microsomes were in a similar low range during the first 10 min of incubation (6.2 +/- 1.2 and 3.4 +/- 1.0 nmol substrate/nmol cytochrome P450/10 min, respectively), whereas the conversion of benzo[g]chrysene-trans-11,12-dihydrodiol was much higher (20.6 +/- 2.2 nmol substrate/nmol cytochrome P450/10 min). Given the strong intrinsic mutagenic and carcinogenic activity of the fjord region dihydrodiol epoxides, our data indicate that their formation, even at a relatively low level, may contribute significantly to the biological activity of the parent hydrocarbons.

Metabolic activation of benzo[g]chrysene in the human mammary carcinoma cell line MCF-7

Benzo[g]chrysene (BgC) is an environmental pollutant, and recent studies have demonstrated that anti- BgC-11,12-dihydrodiol 13,14-epoxide (anti-BgCDE) is a potent mammary carcinogen in rats. To determine whether BgC can be metabolically activated to anti-BgCDE in human cells, the human mammary carcinoma cell line MCF-7 was treated with BgC and with the racemic trans-3,4- and 11,12-dihydrodiols. The DNA adducts formed in these experiments were examined using 32P-postlabeling, and specific adducts were identified through comparisons with adducts obtained by the reaction of the racemic syn- and anti-BgCDEs with calf thymus DNA and with purine deoxyribonucleoside-3'-phosphates in vitro. It was found that BgC is metabolically activated in MCF-7 cells to form major DNA adducts through both the syn- and anti-11,12-dihydrodiol 13,14-epoxide metabolites. BgC is therefore a potential environmental risk to humans. The major BgC-DNA adducts formed from both the dihydrodiol-epoxide diastereomers were deoxyadenosine adducts. Thus, BgC has DNA-binding properties that are very similar to those of the potent mammary carcinogens 7,12-dimethylbenz[a]anthracene and dibenzo[a,l]pyrene.

Activation of chemically diverse procarcinogens by human cytochrome P-450 1B1

A human cytochrome P-450 (P450) 1B1 cDNA was expressed in Saccharomyces cerevisiae and the microsomes containing P450 1B1 were used to examine the selectivity of this enzyme in the activation of a variety of environmental carcinogens and mutagens in Salmonella typhimurium TA1535/pSK1002 or NM2009 tester strains, using the SOS response as an end point of DNA damage. We also determined and compared these activities of P450 1B1 with those catalyzed by recombinant human P450s 1A1 and 1A2, which were purified from membranes of Escherichia coli. The carcinogenic chemicals tested included 27 polycyclic aromatic hydrocarbons and their dihydrodiol derivatives, 17 heterocyclic and aryl amines and aminoazo dyes, three mycotoxins, two nitroaromatic hydrocarbons, N-nitrosodimethylamine, vinyl carbamate, and acrylonitrile. Among the three P450 enzymes examined here, P450 lB1 was found to have the highest catalytic activities for the activation of 11,12-dihydroxy-11,12-dihydrodibenzo[a,l]pyrene, 1,2-dihydroxy-1,2-dihydro-5-methylchrysene, (+)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene, 11,12-dihydroxy-11,12-dihydrobenzo[g]chrysene, 3,4-dihydroxy-3,4-dihydrobenzo[c]phenanthrene, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole, 2-aminoanthracene, 3-methoxy-4-aminoazobenzene, and 2-nitropyrene. P450 1B1 also catalyzed the activation of 2-amino-3,5-dimethylimidazo[4,5-f]quinoline, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, 2-amino-3-methylimidazo[4,5-f]quinoline, 2-aminofluorene, 6-aminochrysene and its 1,2-dihydrodiol, (-)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene, 1,2-dihydroxy-1,2-dihydrochrysene, 1,2-dihydroxy-1,2-dihydro-5,6-dimethylchrysene, 2,3-dihydroxy-2,3-dihydrofluoranthene, 3,4-dihydroxy-3,4-dihydro-7,12-dimethylbenz[a]anthracene, and 6-nitrochrysene to appreciable extents. However, P450 1B1 did not produce genotoxic products from benzo[a]pyrene, trans- 3,4-dihydroxy-3,4-dihydrobenzo[a]anthracene, trans-8,9-dihydroxy-8,9-dihydrobenzo[a]anthracene, 7,12-dimethylbenz[a]anthracene and its cis-5,6-dihydrodiol, 5-methylchrysene, 11,12-dihydroxy-11,12-dihydro-3-methylcholanthrene, 1,2-dihydroxy-1,2-dihydro-6-methylchrysene, benzo[c]phenanthrene, 2-amino-6-methyldipyridol[1,2-a:3',2'-d]imidazole, 2-acetylaminofluorene, benzidine, 2-naphthylamine, aflatoxin B1, aflatoxin G1, sterigmatocystin, N-nitrosodimethylamine, vinyl carbamate, or acrylonitrile in this assay system. P450 1B1 is expressed constitutively in extrahepatic organs, including fetal tissue samples, and is highly inducible in various organs by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds in experimental animal models. Thus, activation of procarcinogens by P450 lB1 may contribute to human tumors of extrahepatic origin.

Metabolism of 5-methylchrysene and 6-methylchrysene by human hepatic and pulmonary cytochrome P450 enzymes

The metabolism of environmentally occurring methylated polynuclear aromatic hydrocarbons by human cytochrome P450 (P450) enzymes has not been examined previously. We compared the metabolism of the tobacco smoke constituents 5-methylchrysene (5-MeC), a strong carcinogen, and 6-MeC, a weak carcinogen, in 18 hepatic and 11 pulmonary human microsomes. Major metabolites of 5-MeC were its proximate carcinogen trans-1,2-dihydroxy-1,2-dihydro-5-methylchrysene (5-MeC-1,2-diol), as well as 5-MeC-7,8-diol, bay region dihydrodiols, and phenols. 5-MeC-1,2-diol and 5-MeC-7,8-diol were formed stereoselectively, with the R,R enantiomers predominating. Major metabolites of 6-MeC were 6-MeC-1,2-diol, bay region dihydrodiols, phenols, and 6-(hydroxymethyl)chrysene. 6-MeC-1,2-diol was also formed stereoselectively in the 1R,2R configuration. All human liver samples formed the proximate carcinogenic 1,2-diols (0.2-2.3 pmol/mg protein/min for 5-MeC and 0.3-3.1 pmol/mg protein/min for 6-MeC). Comparable results were obtained in pulmonary microsomes, but the extent of metabolism was less than in the hepatic samples, and only 4 of 11 samples showed activity. Catalytic activities known to be associated with specific P450s were analyzed in each hepatic sample and correlated with levels of 5-MeC and 6-MeC metabolites in the same samples. The results of the correlation analysis indicated that P450s 1A1 and 1A2 were active in the formation of 5-MeC-1,2-diol and 6-MeC-1,2-diol, as well as several other metabolites resulting from ring oxidation. The formation of the hydroxymethyl metabolites was catalyzed by P450 3A4 (for 5-MeC) or P450s 3A4 and 1A2 (for 6-MeC). Experiments with chemical inhibitors and antibodies supported these results. The metabolism of 5-MeC and 6-MeC was also investigated using purified recombinant human P450s 1A1, 1A2, 2C10, 2D6, 2E1, 3A4, and 3A5. P450s 1A1, 1A2, and 2C10 had higher activities than the other enzymes for ring oxidation of 5-MeC and 6-MeC, whereas P450s 1A2 and 3A4 were more active than the other enzymes for methyl hydroxylation of 6-MeC. Only P450 3A4 showed substantial catalytic activity for methyl hydroxylation of 5-MeC. Collectively, the results of these studies demonstrate that P450s 1A2 and 2C10 are important catalysts of the metabolic activation of 5-MeC and 6-MeC in human liver, whereas P450 1A1 plays a major role in the metabolic activation of these compounds in human lung.

Bioactivation of 6-aminochrysene by animal and human hepatic preparations: contributions of microsomal and cytosolic enzyme systems

6-Aminochrysene was converted into mutagen(s), in the Ames test in the presence of Aroclor 1254-induced hepatic S9, microsomal and cytosolic fractions, the first being the least and the last the most efficient activation system. The cytosolic activation of 6-aminochrysene decreased in the presence of increasing amounts of microsomes. The Aroclor 1254-induced rat microsomal and cytosolic systems differed markedly in a number of properties, including their cofactor requirements and responses to prototype inducers of the cytochrome P450-dependent mixed-function oxidase system. The cytosolic activation system could also convert 2-aminochrysene to mutagens but not 2- and 6-methylchrysene. Human hepatic cytosol could convert 6-aminochrysene and 2-aminoanthracene to mutagens in the Ames test. It is concluded that a hepatic cytosolic oxygenase exists, totally different from the microsomal oxygenases, which metabolizes aminopolycyclic aromatic hydrocarbons to mutagens, presumably through N-oxidation. This oxygenase activity appears to be present in human hepatic cytosol.

Caloric restriction profoundly inhibits liver tumor formation after initiation by 6-nitrochrysene in male mice

Caloric restriction (CR) inhibited strongly the incidence of chemically-induced tumors in the neonatal B6C3F1 mouse tumorigenicity bioassay, when begun 3 months after treatment with the potent carcinogen 6-nitrochrysene. These data indicate that CR can have a profound inhibitory effect on tumor development even long after metabolic activation and DNA repair have occurred.

Characterization of cytochrome P-450 2B6 in human liver microsomes

A cytochrome P-450 (P-450) enzyme of the CYP2B subfamily was partially purified from human liver microsomes and characterized with respect to immunochemical properties, N-terminal amino acid sequence, and catalytic activities toward typical P-450 substrates. P-450 enzymes were monitored in chromatographic fractions by immunoblotting analysis using antibodies raised against a monkey P-450 2B, as well as several purified human P-450 enzymes. The final P-450 2B preparation thus obtained was contaminated with P-450 3A4, but an N-terminal amino acid sequence matching the sequence predicted from the CYP2B6 cDNA was obtained. The apparent M(r) of this protein was 48 kDa, and the migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was the same as that of the P-450 2B6 protein expressed in a human lymphoblast cell line. Immunoblotting analysis of 50 human liver samples revealed that the protein band considered to be P-450 2B6 was detected in only 12 samples, with four of these having relatively high levels. Several activities toward typical P-450 substrates were determined in a reconstituted monooxygenase system containing partially purified P-450 2B6 and compared with those obtained using a highly purified preparation of P-450 3A4 enzyme; we found that most of the activities were similar in these preparations, except that the partially purified P-450 2B6 showed high rates of activation of the mutagens 6-aminochrysene and 3-methoxy-4-aminoazobenzene to genotoxic metabolites in Salmonella typhimurium NM2009 strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Roles of different forms of cytochrome P450 in the activation of the promutagen 6-aminochrysene to genotoxic metabolites in human liver microsomes

We reported previously that the potent mutagen 6-aminochrysene is catalyzed principally by rat liver microsomal P4501A and P4502B enzymes to reactive metabolites that induce umu gene expression in O-acetyltransferase-over-expressing strain Salmonella typhimurium NM2009; the proposal was made that there are different mechanisms in the formation of reactive N-hydroxylated and diolepoxide metabolites by P450 enzymes (Yamazaki, H. and Shimada, T., Biochem. Pharmacol., 44, 913-920, 1992). Here we further examined the roles of human liver P450 enzymes and the mechanism of activation of 6-aminochrysene by rat and human P450 enzymes in the Salmonella tester strains. Liver microsomes from 18 different human samples catalyzed activation of 6-aminochrysene more efficiently in S. typhimurium NM2009 than in the original strain of S. typhimurium TA1535/pSK1002. The rates of 6-aminochrysene activation in 18 human liver samples showed good correlation to the contents of P4502B6 as well as contents of P4503A4 and the respective mono-oxygenase activities catalyzed by P4503A4. Among purified P450 enzymes examined, P4501A2 as well as P4503A4 were highly active in transforming 6-amino-chrysene to reactive metabolites, suggesting the involvement of different human P450 enzymes in the reaction. Four human samples that contained relatively high levels of particular P450 enzymes in their microsomes were selected and used for further characterization. Liver microsomes from human samples HL-13 and HL-4 that contained the highest levels of P4502B6 and P4503A4 respectively, were sensitive to the respective antibodies raised against monkey P4502B and human P4503A4; the activity in sample HL-16 having the highest level of P4501A2 was inhibited by anti-P4501A2 IgG. alpha-Naphthoflavone enhanced the activation of 6-aminochrysene very significantly in human liver microsomes enriched in P4503A4 and P4502B6 enzymes. Pentachlorophenol, an inhibitor of acetyltransferase activity, suppressed the activation of 6-aminochrysene in liver microsomes from phenobarbital-treated rats and from human samples HL-4, HL-13 and HL-18 but not HL-16. In contrast, 1,1,1-trichloropropane-2,3-oxide, an inhibitor of epoxide hydrolase activity, enhanced the activation of 6-aminochrysene catalyzed by liver microsomes from beta-naphthoflavone-treated rats and from human samples HL-16 but not HL-4, HL-13 and HL-18. Inclusion of purified rat epoxide hydrolase to the reconstituted system containing rat and human P4501A enzymes caused a decrease in the rates of 6-aminochrysene activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Genotoxicity characteristics of reverse diol-epoxides of chrysene

Trans-3,4-dihydroxy-3,4-dihydrochrysene (chrysene-3,4-diol), a major metabolite of chrysene, is further metabolized by rat liver enzymes to products which effectively revert the his- Salmonella typhimurium strain TA98 to histidine prototrophy, but are only weakly mutagenic in strain TA100 and in Chinese hamster V79 cells (acquisition of resistance to 6-thioguanine). The liver enzyme mediated mutagenicity of chrysene-3,4-diol is substantially enhanced in the presence of 1,1,1-trichloropropene 2,3-oxide, an inhibitor of microsomal epoxide hydrolase. The predominant metabolites of chrysene-3,4-diol, namely the anti- and syn-isomers of its 1,2-oxide (termed reverse diol-epoxides), proved to be extraordinarily effective mutagens in S.typhimurium strain TA98, but were only moderately active in strains TA100 and TA104, and in the SOS induction in Escherichia coli PQ37. These genotoxicity spectra in bacteria are completely different from those observed with the bay-region diol-epoxides of chrysene and 3-hydroxychrysene. In V79 cells, the reverse diol-epoxides formed low levels of DNA adducts and were very weak inducers of gene mutations. In M2 mouse prostate cells, however, high numbers of transformed foci were induced by chrysene-3,4-diol and its diastereomeric 1,2-oxides. Chrysene-3,4-diol was somewhat more potent than chrysene-1,2-diol. The potency of both reverse diol-epoxides was similar to that of the syn-diastereomers of the bay-region diol-epoxides of chrysene and 3-hydroxychrysene, but lower than that of their anti-diastereomers. The reverse diol-epoxides of chrysene, unlike the bay-region diol-epoxides, were inactivated by purified microsomal epoxide hydrolase. Noteworthy findings were also made with regard to the chemical stability of the diol-epoxides in buffer, determined from the decline in mutagenicity after preincubation in the absence of the target cells. Despite its lower delta Edeloc/beta value for the formation of the benzylic carbocation, anti-chrysene-3,4-diol 1,2-oxide was shorter-lived (t1/2 = 46 min) than anti-chrysene-1,2-diol 3,4-oxide (t1/2 = 74 min). Unlike other investigated diastereomeric pairs of diol-epoxides, it was also shorter-lived than its syn-diastereomer (t1/2 = 340 min).

Disposition, metabolism, and excretion of the anticancer agent crisnatol in the rat

Disposition and metabolism of crisnatol (14C-labeled), a novel antitumor agent, was examined after po and iv administration to rats. After both routes of drug administration, there was rapid elimination of the administered radioactivity in the urine (6-12% of the dose) and feces (81-92% of the dose). The drug appeared to be rapidly absorbed after oral dose and there was substantial "first-pass" metabolism. Analysis of the excreta indicated extensive metabolism of crisnatol by the rat, with the intact compound being the major radiolabeled component in the feces (17-20% of dose). Intact drug was not present in urine. Biotransformation of crisnatol by the rat mainly involves oxidation and conjugation pathways. Hydroxylation and dihydrodiol formation in the chrysene ring and oxidation of the propanediol side chain resulted in the formation of the three major fecal metabolites. The principal metabolite in the urine was also a dihydrodiol. Concentrations of intact drug in each tissue assayed exceeded those in plasma, and in the lungs the tissue/plasma ratio approached 300 and 82 at 2 hr after iv and po doses, respectively.

Crisnatol mesylate: phase I dose escalation by extending infusion duration

Crisnatol mesylate is a rationally designed cytotoxic arylmethylamino-propanediol with broad spectrum cytotoxic activity. A phase I study with an unconventional escalation scheme was developed using a constant drug infusion rate (mg/m2/hr) and prolonging the infusion duration from 6 to 96 hours. Sixty-five patients received crisnatol at doses from 18 mg/m2 in 6 hrs to 3400 mg/m2 in 72 hours. The dose-limiting toxicity in two of five patients at 2700 mg/m2 and two of three patients at 3400 mg/m2 was neurologic and consisted of a syndrome of confusion, agitation, and disorientation. Phlebitis mandated the use of a central line. The mean terminal phase half-life (T1/2 beta) was 3.3 hours with a total body clearance (CL) of 22.8 L/hr/m2 and a volume of distribution (Vdss) of 53 L/m2. The median steady-state peak plasma concentration (Css) at 2700 mg/m2/72 hours was 2.7 micrograms/ml and at 3400 mg/m2/72 hours was 3.8 micrograms/ml. No responses were seen. The maximum tolerated dose (MTD) on this schedule is 2700 mg/m2/72 hours in patients with no liver disease and good performance status.

Evaluation of arylmethylaminopropanediols by a novel in vitro pharmacodynamic assay: correlation with antitumor activity in vivo

The pharmacodynamics of a new series of antitumor DNA intercalators, known as arylmethylaminopropanediols (AMAPs), has been evaluated in vitro against adherent (MCF-7 human breast cancer) and nonadherent (P388 murine leukemia) cell lines. Previous work had shown that the in vitro antitumor activity of the model AMAP crisnatol was a function of exposure (Cn x T), rather than concentration alone. A unique exposure parameter, the minimum C x T, was proposed as an end point for antitumor activity in cell culture. Comparison of crisnatol to several established agents by the minimum C x T versus the standard concentration producing 10% survival indicated that these end points were not equivalent. The current work examined the validity of the pharmacodynamic approach using AMAP isomers from three different ring systems that were known to exhibit a spectrum of activity against the P388 tumor in vivo. The results indicated that antiproliferative, but not cytotoxic, activity of AMAPs in the pharmacodynamic assay correlated with their differential activity in vivo, expressed as percentage of increase in life span. In contrast, the concentration producing 10% survival either at 1 h or after continuous exposure did not show a similar correlation. The pharmacodynamic assay also revealed that certain AMAPs, while equipotent by concentration alone, required significantly less time and therefore less overall exposure for efficacy. Finally, the activity of AMAP isomers in P388 cells differed from that in MCF-7 cells, which may indicate AMAP selectivity for certain tumor types. Since AMAP action was a function of exposure, drug effects on cellular targets could likewise depend on exposure rather than concentration. These findings emphasize the importance of relating drug mechanisms to the pharmacodynamics of anticancer agents.

The role of intestinal microflora in the metabolic activation of 6-nitrochrysene to DNA-binding derivatives in mice

6-Nitrochrysene has previously been shown to be a potent lung and liver carcinogen following i.p. administration to newborn mice and to be metabolically activated to DNA-binding derivatives by nitro-reduction or a combination of nitro-reduction and ring oxidation. In this study, we have examined fecal metabolites and DNA-carcinogen adducts in 5-week-old conventional and germfree Balb/c mice treated with [3H]6-nitrochrysene in order to determine if the metabolic activation pathway(s) for this compound in these mice differs from that observed in preweanling mice. We further evaluated the role of the intestinal microflora on the metabolism and generation of DNA-reactive metabolites in this system. The amount of 6-aminochrysene excreted in the feces of germfree mice within 48 h after treatment with a single i.p. dose of [3H]6-nitrochrysene (0.03 mumol/5 microliters/g body wt) was approximately 25% of that excreted in identically treated conventional mice. However, the levels of carcinogen-DNA adducts in the lungs and livers of conventional and germfree Balb/c mice were similar at the 24 and 48 h time points examined. HPLC analysis of hydrolysates of liver and lung DNA indicated that adducts derived from both N-hydroxy-6-aminochrysene and trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene metabolites were formed in the liver whereas only the latter adduct was detected in the lung. This contrasts with previous findings in preweanling mice where the adduct derived from the trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene metabolite was the single major adduct detected in both liver and lung DNA. The proportion of adducts derived from N-hydroxy-6-aminochrysene was significantly greater in the liver DNA of germfree mice than in the liver DNA of conventional mice.

Metabolic activation of 6-nitrochrysene in explants of human bronchus and in isolated rat hepatocytes

It has previously been shown that 6-nitrochrysene can be activated to electrophilic species capable of reacting with DNA through metabolic pathways that form N-hydroxy-6-aminochrysene or trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene as critical intermediates. Since the lung is a known target tissue for the carcinogenic action of polycyclic nitroaromatic hydrocarbons, we investigated the metabolism and DNA binding of [3H]6-nitrochrysene in 11 specimens of human bronchus. Analysis of medium from [3H]6-nitrochrysene-treated explants indicated the presence of trans-9,10-dihydroxy-9,10-dihydro-6-nitrochrysene (0.04-330 pmol/mg epithelial DNA), trans-1,2-dihydroxy-1,2-dihydro-6-nitrochrysene (12-1700 pmol/mg epithelial DNA), 6-aminochrysene (1.6-2200 pmol/mg epithelial DNA), and trans-1,2-dihydroxy-1,2-dihydro-6-aminochyrsene (3.6-610 pmol/mg epithelial DNA). Both the levels and the relative proportions of these metabolites varied widely in explants from different individuals. The amount of DNA recovered and the level of DNA modification were sufficient for adduct analysis in eight of the 11 cases for which metabolite data were obtained. Five additional bronchial specimens for which metabolite data were not obtained were also analyzed for carcinogen-DNA adducts. The levels of binding varied from 0.06 to 30.5 pmol [3H]6-nitrochrysene bound/mg DNA (two adducts per 10(8) nucleotides-10 adducts per 10(6) nucleotides). HPLC analyses of enzymatic hydrolysates of the explant DNA indicated that 11 of 13 cases contained adducts with retention times identical to those of adducts derived from trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene or N-hydroxy-6-aminochrysene. The adduct derived from trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene was the major adduct detected in eight of 13 cases. The reasons for the variation in metabolism and adduct formation observed in [3H]6-nitrochrysene-treated explants of bronchus from different donors are not known but may reflect differences in the activities of enzymes responsible for the metabolism of this compound. The influence of induction of drug metabolizing enzymes on the activation pathway of 6-nitrochrysene in an intact cell system was tested using rat hepatocytes. 6-Nitrochrysene was incubated with freshly isolated hepatocytes from rats that were either untreated or pretreated with phenobarbital, 3-methylcholanthrene or Aroclor 1254. Although the levels of adducts were similar in all cases, the pattern of DNA adducts formed in these hepatocytes was dependent on the nature of the pretreatment of the rats. As previously reported, hepatocytes from untreated rats contained adducts derived from N-hydroxy-6-aminochrysene.(ABSTRACT TRUNCATED AT 400 WORDS)