In situ absorption and protein binding characteristics of CDRI-85/92, an antiulcer pharmacophore

CDRI-85/92, a new antiulcer drug, acts as a proton pump inhibitor arresting the secretion of acid in the stomach. The absorption kinetics of CDRI-85/92 was evaluated in situ using rat intestinal recirculation perfusion method. The experiment was conducted at pH 2.6 and 7.4 representing the acidic and the mild alkaline environment, which the drug experiences through the GIT during oral treatment. The rate of absorption was the same (0.12h(-1)) at pH 2.6 and 7.4, thus suggesting equal absorption profile of the CDRI-85/92 throughout the GIT irrespective of the pH. Equal rates of absorption can also be correlated with the presence of acidic and basic groups in the structure of CDRI-85/92.Protein binding studies of CDRI-85/92 using ultrafiltration were conducted in vitro and in vivo. Protein binding was found to be in the range of 31.49-32.91% both in in vitro and in vivo (employing 5-min post dose samples of rat serum after 20 mgkg(-1) i.v. treatment of CDRI-85/92). The binding was found to be linear in the concentration range of 156.25-2000 ngml(-1) (r(2)>0.99).     

Comparative studies of the in vitro metabolism and covalent binding of 14C-benzene by liver slices and microsomal fraction of mouse, rat, and human

Metabolism of benzene by the liver has been suggested to play an important role in the hepatotoxicity of benzene. The role of the different benzene metabolites and the causes of species differences in benzene hepatotoxicity are, however, not known. The metabolism and covalent binding of 14C-benzene by liver microsomal fractions and liver slices from rat, mouse, and human subjects have been studied. Rat microsomal fraction formed phenol at a rate of 0.32 nmol/min/mg of protein; mouse microsomal fraction formed phenol at 0.64 nmol/min/mg and hydroquinone at 0.03 nmol/min/mg; and human microsomal fraction formed phenol at 0.46 nmol/min/mg and hydroquinone at 0.07 nmol/min/mg. Covalent binding of 14C-benzene metabolites to rat, mouse, and human liver microsomal protein was 29, 113, and 169 pmol/min/mg of protein, respectively. The rates of metabolite formation from benzene by liver slices in nmol/min/g of tissue were: rat, phenol 0.15, hydroquinone 0.26, and phenylsulfate 1.22; mouse: phenol 0.13, hydroquinone 0.29, phenylsulfate 1.37, and phenylglucuronide 1.34; and human: phenol 0.16, hydroquinone 0.27, phenylsulfate 0.83, and phenylglucuronide 0.52. trans,trans-Muconic acid formation was not detected with liver slices of any species. Covalent binding of 14C-benzene metabolites to rat, mouse, and human liver slices was 8.2, 79.7, and 27.3 pmol/min/g liver, respectively. There was no correlation between ascorbic acid levels in the human liver slices and covalent binding of 14C-benzene metabolites. The results show that phenol and hydroquinone found in extrahepatic tissues, including bone marrow, of animals exposed to benzene could originate from the liver. There was no evidence for the release of highly reactive benzene metabolites such as trans,trans-muconaldehyde or p-benzoquinone from liver cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

LC determination of the anti-ulcer agent CDRI-85/92 in rat serum

CDRI-85/92 is a new potent anti-ulcer compound developed by Central Drug Research Institute, Lucknow (India). This compound is in advanced stage of preclinical trials. A High-performance liquid chromatographic (HPLC) method was developed and validated for the analysis of CDRI-85/92 using rat serum. The HPLC analysis, applicable to 0.5-ml volumes of serum, involved protein precipitation of serum samples with acetonitrile (1:3 v/v) followed by centrifugation and separation on a C-18 column and the use of UV detector at the wavelength 250 nm. The method was sensitive with a lowest limit of quantitation (LLOQ) of 1.25 ng ml⁻¹ in rat serum and the recovery was more than 96%. The linearity was satisfactory as indicated by correlation of >0.99, in addition to the visual examination of the calibration curves. The precision and accuracy were acceptable as indicated by relative standard deviation (R.S.D.) ranging from 4.15 to 8.21%, bias values ranging from 2.96 to 11.18%. In-process stability evaluation showed the stability of the compound in processed samples lasted up to 168 h. The method was applied for analysing CDRI-85/92 in rat serum after administration of single oral or iv bolus dose of 20 mg kg⁻¹. The robustness/ruggedness of the HPLC procedure was tested using different HPLC instrumentation and column of different make. The assay was found to be sensitive (limit of quantification was 1.25 ng ml⁻¹), specific (retention time for CDRI-85/92 is 7.5 min), accurate (% bias is <12%), precise (% R.S.D. is <10%), robust (no significant change in peak profile in two HPLC Instruments) and reliable for use in pharmacokinetic or toxicokinetic studies.     

Potent estrogenic metabolites of bisphenol A and bisphenol B formed by rat liver S9 fraction: their structures and estrogenic potency.

We previously demonstrated that the estrogenicity of either bisphenol A [BPA; 2,2-bis(4-hydroxyphenyl)propane] or bisphenol B [BPB; 2,2-bis(4-hydroxyphenyl)butane] was increased several times after incubation with rat liver S9 fraction (Yoshihara et al., 2001). This metabolic activation, requiring both microsomal and cytosolic fractions, was observed with not only rat liver, but also human, monkey, and mouse liver S9 fractions. To characterize the active metabolites of BPA and BPB, we investigated the structures of the isolated active metabolites by negative mode LC/MS/MS and GC/MS. The active metabolite of BPA gave a negative mass peak at [M-H](-) 267 on LC/MS and a single daughter ion at m/z 133 on MS/MS analysis, suggesting an isopropenylphenol dimer structure. Finally, this active metabolite was confirmed to be identical with authentic 4-methyl-2,4-bis(p-hydroxyphenyl)pent-1-ene (MBP) by means of various instrumental analyses. The corresponding peaks of the BPB metabolite were [M-H](-) 295 and m/z 147, respectively, suggesting an isobutenylphenol dimer structure. Further, coincubation of BPA and BPB with rat liver S9 afforded an additional active metabolite(s), which gave a negative mass peak at [M-H](-) 281 and two daughter ion peaks at m/z 133 and m/z 147 on MS/MS analysis. These results strongly suggest that the active metabolite of either BPA or BPB might be formed by recombination of a radical fragment, a one-electron oxidation product of carbon-phenyl bond cleavage. It is noteworthy that the estrogenic activity of MBP, the active metabolite of BPA, is much more potent than that of the parent BPA in several assays, including two reporter assays using a recombinant yeast expressing human estrogen receptor alpha and an MCF-7-transfected firefly luciferase plasmid.     

Disposition of flavonoids via enteric recycling: enzyme stability affects characterization of prunetin glucuronidation across species, organs, and UGT isoforms

We characterized the in vitro glucuronidation of prunetin, a prodrug of genistein that is a highly active cancer prevention agent. Metabolism studies were conducted using expressed human UGT isoforms and microsomes/S9 fractions prepared from intestine and liver of rodents and humans. The results indicated that human intestinal microsomes were more efficient than liver microsomes in glucuronidating prunetin, but rates of metabolism were dependent on time of incubation at 37 degrees C. Human liver and intestinal microsomes mainly produced metabolite 1 (prunetin-5- O-glucuronide) and metabolite 2 (prunetin-4'- O-glucuronide), respectively. Using 12 human UGT isoforms, we showed that UGT1A7, UGT1A8, and UGT1A9 were mainly responsible for the formation of metabolite 1, whereas UGT1A1, UGT1A8, and UGT1A10 were mainly responsible for the formation of metabolite 2. This isoform-specific metabolism was consistent with earlier results obtained using human liver and intestinal microsomes, as the former (liver) is UGT1A9-rich whereas the latter is UGT1A10-rich. Surprisingly, we found that the thermostability of the microsomes was isoform- and organ-dependent. For example, human liver microsomal UGT activities were much more heat-stable (37 degrees C) than intestinal microsomal UGT activities, consistent with the finding that human UGT1A9 is much more thermostable than human UGT1A10 and UGT1A8. The organ-specific thermostability profiles were also evident in rat microsomes and mouse S9 fractions, even though human intestinal glucuronidation of prunetin differs significantly from rodent intestinal glucuronidation. In conclusion, prunetin glucuronidation is species-, organ-, and UGT-isoform-dependent, all of which may be impacted by the thermostability of specific UGT isoforms involved in the metabolism.     

Oxidation of 5-methoxy-N,N-diisopropyltryptamine in rat liver microsomes and recombinant cytochrome P450 enzymes

The oxidative metabolism of 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT), a tryptamine-type designer drug, was studied using rat liver microsomal fractions and recombinant cytochrome P450 (CYP) enzymes. 5-MeO-DIPT was biotransformed mainly into a side-chain N-deisopropylated metabolite and partially into an aromatic ring O-demethylated metabolite in liver microsomal fractions from untreated rats of both sexes. This metabolic profile is different from our previous findings in human liver microsomal fractions, in which the aromatic ring O-demethylation was the major pathway whereas the side-chain N-deisopropylation was minor [Narimatsu S, Yonemoto R, Saito K, Takaya K, Kumamoto T, Ishikawa T, et al. Oxidative metabolism of 5-methoxy-N,N-diisopropyltryptamine (Foxy) by human liver microsomes and recombinant cytochrome P450 enzymes. Biochem Pharmacol 2006;71:1377-85]. Kinetic and inhibition studies indicated that the side-chain N-dealkylation is mediated by CYP2C11 and CYP3A2, whereas the aromatic ring O-demethylation is mediated by CYP2D2 and CYP2C6 in untreated male rats. Pretreatment of male rats with beta-naphthoflavone (BNF) produced an aromatic ring 6-hydroxylated metabolite. Recombinant rat and human CYP1A1 efficiently catalyzed 5-MeO-DIPT 6-hydroxylation under the conditions used. These results provide valuable information on the metabolic fate of 5-MeO-DIPT in rats that can be used in the toxicological study of this designer drug.     

Nicorandil metabolism in rat myocardial mitochondria

The in vitro study using rats was carried out to clarify the hypothesis that nicorandil is denitrated and then may produce nitric oxide (NO) in myocardial mitochondria. In the presence of a NADPH-generating system, [14C]nicorandil, which was incubated in mitochondrial and microsomal fractions of the lung, heart, or liver, was converted to its main denitrated metabolite, SG-86 and other metabolites. Apparent Km and Vmax for nicorandil in mitochondrial and microsomal fractions of the heart were considerably similar to those of the lung, but completely different from those of the liver. It seems that glutathione-S-transferases (GSTs) are not primarily involved in the conversion of nicorandil to SG-86, because a known GST inhibitor, indomethacin, did not affect the nicorandil degradation in the mitochondrial fraction. Nitrite, the stable metabolite of NO, was measured by the Griess reaction. In the presence of an NADPH-generating system, nicorandil significantly increased nitrite production in myocardial mitochondria, but SG-86 did not. These data strongly indicate that nicorandil is metabolized to SG-86 in myocardial mitochondria, then releasing NO, and that GSTs are not primarily responsible for the conversion of nicorandil to SG-86.     

In vitro metabolic products of RWJ-34130, an antiarrythmic agent, in rat liver preparations

The in vitro metabolism of RWJ-34130, an antiarrhythmic agent, was conducted using rat hepatic 9000 x g supernatant (S9) and microsomes in an NADPH-generating system, and the rat liver perfusion. The 100 and 20 microg ml(-1) concentrations of RWJ-34130 aqueous solution were used for microsomal incubation and liver perfusion, respectively. Unchanged RWJ-34130 (approximately 77-78% of the sample in both S9 and microsomes) plus a major metabolite, RWJ-34130 sulfoxide (20% of the sample in both S9 and microsomes) were profiled, isolated and identified from both hepatic S9 and microsomal incubates (60 min) using HPLC and mass spectrometry (MS), and by comparison to a synthetic RWJ-34130 sulfoxide, which was synthesized by reacting RWJ-34130 with MCPBA (meta-chloroperoxy benzoic acid). No unchanged RWJ-34130 was detected in the 3 h liver perfusate, however, 1-phenyl-2-oxo-pyrrolidine was profiled, isolated and identified as a major hydrolyzed metabolite of liver perfusate. RWJ-34130 is not extensively metabolized in vitro in rat hepatic S9 and microsomes. All HPLC metabolic profiles of hepatic S9 and microsomal samples (30 min, 60 min) were qualitatively and nearly quantitatively identical.     

Reductive metabolism of the anticonvulsant agent zonisamide, a 1,2-benzisoxazole derivative.

1. The metabolism of zonisamide in vitro was characterized through aerobic and anaerobic incubations with rat liver subcellular fractions and cultured gastrointestinal microflora. 2. Zonisamide reacted with rat hepatic microsomal cytochrome P-450 and exhibited a Type I binding spectrum. 3. Metabolism of zonisamide in vitro by hepatic subcellular fractions and cultured gastrointestinal flora produced a single metabolite, 2-(sulphamoylacetyl)-phenol (2-SMAP), by reductive cleavage of the 1,2-benzisoxazole ring. 4. The reductive metabolism of zonisamide was primarily mediated by microsomal cytochrome P-450. The soluble fraction enhanced reduction when combined with the microsomal fraction but itself possessed only weak reductive activity. 5. Reduction of zonisamide by the most enzymically active liver fractions required NADPH, was stimulated by FMN and SKF-525A, and was inhibited by CO or air, as well as by n-octylamine. 6. Unlike their involvement in the reduction of numerous nitro, azo, and N-oxide compounds, cultured aerobic and anaerobic intestinal flora were not principally involved in the reduction of zonisamide.     

Use of human liver S9 in the Ames test: assay of three procarcinogens using human S9 derived from multiple donors

The purpose of the present study was to examine the inter-individual variation in the mutagenicity of chemicals using a variety of human S9 fractions. For this purpose, three procarcinogens, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), benzo[a]pyrene (BP), and dimethylnitrosamine (DMN), were selected for the Ames test and their mutagenicity was examined using human liver S9 fractions prepared from 18 different donors and one pooled liver S9 fraction prepared from 15 different donors. In addition, rat S9 fraction prepared from male rats pretreated with phenobarbital and 5,6-benzoflavone (PB/BF) was used as reference in order to examine the mutagenic differences between human and rat (PB/BF) S9 fractions. The data demonstrate a large inter-individual diversity in the mutagenic response to procarcinogens. The mutagenicity of IQ and BP in the presence of a human liver S9 fraction (lot HLS-014) was equal to that observed in the presence of rat (PB/BF) S9 fraction. The mutagenicity of IQ and BP in the presence of a pooled human liver S9 fraction was lower (90 and 95%, respectively) than that observed in the presence of rat (PB/BF) S9. On the contrary, the mutagenicity of DMN in the presence of either a selected human liver S9 fraction (lot HLS-014) or pooled fraction was 8-fold higher than that found in the presence of rat (PB/BF) S9 fraction. Human liver S9 fraction (lot HLS-014) had one of the highest cytochrome P450 enzyme activities among the 18 different donors and higher than the pooled human liver S9 fraction. These results suggest that the use of both selected human liver S9 fractions with high metabolic activity (e.g., lot HLS-014 as used in this study) and a pooled S9 fraction with moderate metabolic activity could be used as a means to evaluate the inter-individual variability in mutagenic response to chemicals and to confirm positive responses from studies completed with rodent S9.     

Mutagen activation of 1,2-dibromo-3-chloropropane by cytosolic glutathione S-transferases and microsomal enzymes

It is not clear whether glutathione (GSH) conjugation to 1,2-dibromo-3-chloropropane (DBCP) results in genotoxic activation. Therefore S9, cytosolic, and microsomal fractions from uninduced rat liver were evaluated for their relative ability to activate DBCP in a modified Ames system. The S9 enzymes, either alone or in combination with exogenous GSH, did not enhance the mutagenicity of DBCP; identical results were obtained with cytosolic enzymes. Significant mutagenic activation of DBCP was produced by either S9 or microsomal fractions in the presence of NADPH. Activation was proportional to cytochrome P-450 concentrations, and was diminished by exogenous GSH. The protection against genotoxicity exerted by GSH did not require cytosolic glutathione S-transferases (GST). Thus, mutagenic activation of DBCP as obtained with S9 fractions is primarily due to biotransformation by microsomal rather than by cytosolic enzymes. Kinetic studies of cytosol-catalyzed conjugation of GSH to DBCP revealed tissue-specific differences in apparent Km and Vmax. Renal and testicular GSTs were associated with 28-46% smaller Vmax values when compared to hepatic GSTs (31.2 +/- 1.9 nmol/min X mg protein). However, renal and testicular GSTs had relatively higher affinities for DBCP. Thus, extrahepatic tissues possess significant capacity to conjugate GSH to DBCP. DBCP-GSH conjugates may undergo enzymatic modification by extrahepatic peptidase and beta-lyase to yield other sulfur-containing moieties that perhaps mediate DBCP's extrahepatic toxicity.     

Stereo- and regioselectivity account for the diversity of dehydroepiandrosterone (DHEA) metabolites produced by liver microsomal cytochromes P450.

The purpose of this study was to quantify the oxidative metabolism of dehydroepiandrosterone (3beta-hydroxy-androst-5-ene-17-one; DHEA) by liver microsomal fractions from various species and identify the cytochrome P450 (P450) enzymes responsible for production of individual hydroxylated DHEA metabolites. A gas chromatography-mass spectrometry method was developed for identification and quantification of DHEA metabolites. 7alpha-Hydroxy-DHEA was the major oxidative metabolite formed by rat (4.6 nmol/min/mg), hamster (7.4 nmol/min/mg), and pig (0.70 nmol/min/mg) liver microsomal fractions. 16alpha-Hydroxy-DHEA was the next most prevalent metabolite formed by rat (2.6 nmol/min/mg), hamster (0.26 nmol/min/mg), and pig (0.16 nmol/min/mg). Several unidentified metabolites were formed by hamster liver microsomes, and androstenedione was produced only by pig microsomes. Liver microsomal fractions from one human demonstrated that DHEA was oxidatively metabolized at a total rate of 7.8 nmol/min/mg, forming 7alpha-hydroxy-DHEA, 16alpha-hydroxy-DHEA, and a previously unidentified hydroxylated metabolite, 7beta-hydroxy-DHEA. Other human microsomal fractions exhibited much lower rates of metabolism, but with similar metabolite profiles. Recombinant P450s were used to identify the cytochrome P450s responsible for DHEA metabolism in the rat and human. CYP3A4 and CYP3A5 were the cytochromes P450 responsible for production of 7alpha-hydroxy-DHEA, 7beta-hydroxy-DHEA, and 16alpha-hydroxy-DHEA in adult liver microsomes, whereas the fetal/neonatal form CYP3A7 produced 16alpha-hydroxy and 7beta-hydroxy-DHEA. CYP3A23 uniquely formed 7alpha-hydroxy-DHEA, whereas other P450s, CYP2B1, CYP2C11, and CYP2D1, were responsible for 16alpha-hydroxy-DHEA metabolite production in rat liver microsomal fractions. These results indicate that the stereo- and regioselectivity of hydroxylation by different P450s account for the diverse DHEA metabolites formed among various species.     

Activation of styrene to styrene oxide in hepatocytes and subcellular fractions of rat liver

The oxidation of styrene to styrene oxide and the hydration of this metabolite to styrene glycol was investigated in hepatocytes, 9000 x g supernatant (S9) and the microsomal fraction from rat liver. Similar amounts of free styrene oxide were found in microsomes, hepatocytes and S9. However, on the basis of the formation of styrene glycol and the depletion of glutathione (GSH), it appeared that hepatocytes were the most active system in the metabolism of styrene, followed by S9 and microsomes.     

Nitroglycerin metabolism in subcellular fractions of rabbit liver. Dose dependency of glyceryl dinitrate formation and possible involvement of multiple isozymes of glutathione S-transferases

The hepatic transformation of glyceryl trinitrate (GTN), commonly known as nitroglycerin, was studied in subcellular fractions prepared from rabbit livers. Both the cytosolic and microsomal fractions show activity toward GTN metabolism. Moreover, the formation of glyceryl dinitrates (GDNs) seems to be governed by different enzymatic processes in the two fractions. 1,2-GDN was preferentially formed in cytosolic fractions, whereas in microsomal fractions, 1,3-GDN was the predominant product. In cytosolic fractions, increasing starting concentrations of GTN led to a decrease in both the GTN degradation rate and the GDN ratio (1,2-GDN/1,3-GDN), which was mainly accounted for by saturation of the 1,2-GDN formation pathway. Various glutathione S-transferase (GST) inhibitors affected the rate of GDN formation differentially. In cytosolic fractions, 1-chloro-2,4-dinitrobenzene and iodomethane caused no change in the GDN ratio, while sulfobromophthalein, ethacrynic acid, and p-nitrobenzyl chloride decreased the GDN ratio, suggesting that different GST isozymes are inhibited by these agents. In microsomal fractions, no dose-dependent GTN metabolism and related change in the GDN ratios could be observed. With the exception of ethacrynic acid, addition of GST inhibitors did not decrease GDN metabolite production, and even in this case, no change in the GDN ratio was observed. The results suggest that different GTN metabolic pathways are present in the liver, most likely involving different GST isozymes.     

Detection of nimesulide metabolites in rat plasma and hepatic subcellular fractions by HPLC-UV/DAD and LC-MS/MS studies.

Nimesulide (4-nitro-2-phenoxymethanesulfonanilide) is an atypical NSAID lacking a carboxylic acid moiety. It has a good gastric tolerability due to selective inhibition of COX-2. The study objectives in the present work were to characterize the metabolism of nimesulide in rat plasma at certain time intervals. In vitro studies were also carried out to examine if nitroreduction takes place in vitro using rat hepatic subcellular fractions (microsomal and S9 fraction) besides aromatic hydroxylation. This communication describes detection and characterization of nimesulide metabolites isolated from plasma and hepatic subcellular post-incubates by the use of HPLC-UV/diode array and LC-MS/MS. Hydroxynimesulide was the major metabolite both in vivo and in vitro whereas nitroreduction was observed only in vitro with subcellular fractions under anaerobic conditions.     

The subcellular distribution of 14C-lidamidine and its metabolites

Examination of the subcellular distribution of 14C-lidamidine and its metabolites in rat liver showed that the majority of radioactivity appeared in the postmicrosomal supernatant fraction, with lysosomes and microsomes having the highest relative specific activity (RSA) of the particulate fractions. When the subcellular distribution pattern was corrected for cross-contamination, based on the distribution of subcellular fraction marker enzymes, there was a significant decrease in the lysosomal RSA with an attendant increase in the microsomal RSA. Thin-layer chromatography of subcellular fraction extracts revealed different distribution patterns for lidamidine and metabolites in each fraction. The whole homogenate and cytosol fraction contained mostly polar metabolites (76-91%), whereas the particulate fractions contained 37-50% of their radioactivity as polar metabolites. The highest percentages of unchanged lidamidine and its more pharmacologically active, demethylated metabolite were associated with the mitochondrial and microsomal fractions.     

Fractionation and subcellular localization of marker enzymes in rainbow trout liver

The subcellular fractionation of rainbow trout liver homogenates prepared in 0.25 M sucrose was investigated using marker enzymes to assess the homogeneity of the resulting fractions. In addition to the usual mitochondrial and microsomal fractions, an additional fraction was sedimented between 8000 g for 10 min and 13,300 g for 10 min. Of the four accepted hydrolytic “marker” enzymes for rat liver lysosomes, the high relative specific activity (R.S.A.) of acid phosphatase was indicative of enrichment in this fraction. The R.S.A. patterns of 5'-nucleotidase and alkaline phosphatase indicated that the plasma membranes of fish liver were sedimenting with “nuclear” and microsomal pellets. This latter fraction contained the highest percentage of the total glucose 6-phosphatase, benzopyrene hydroxylase and glucuronyl transferase assayed in the fish liver homogenate before fractionation. The R.S.A. of these same enzymes in the microsomal pellet indicated an enrichment in this fraction relative to other cellular fractions.     

Metabolism of trans, trans-muconaldehyde, a cytotoxic metabolite of benzene, in mouse liver by alcohol dehydrogenase Adh1 and aldehyde reductase AKR1A4

The reductive metabolism of trans, trans-muconaldehyde, a cytotoxic metabolite of benzene, was studied in mouse liver. Using an HPLC-based stopped assay, the primary reduced metabolite was identified as 6-hydroxy-trans, trans-2,4-hexadienal (OH/CHO) and the secondary metabolite as 1,6-dihydroxy-trans, trans-2,4-hexadiene (OH/OH). The main enzymes responsible for the highest levels of reductase activity towards trans, trans-muconaldehyde were purified from mouse liver soluble fraction first by Q-sepharose chromatography followed by either blue or red dye affinity chromatography. In mouse liver, trans, trans-muconaldehyde is predominantly reduced by an NADH-dependent enzyme, which was identified as alcohol dehydrogenase (Adh1). Kinetic constants obtained for trans, trans-muconaldehyde with the native Adh1 enzyme showed a Vmax of 2141+/-500 nmol/min/mg and a Km of 11+/-4 microM. This enzyme was inhibited by pyrazole with a KI of 3.1+/-0.57 microM. Other fractions were found to contain muconaldehyde reductase activity independent of Adh1, and one enzyme was identified as the NADPH-dependent aldehyde reductase AKR1A4. This showed a Vmax of 115 nmol/min/mg and a Km of 15+/-2 microM and was not inhibited by pyrazole.     

1,2-benzisoxazole phosphorodiamidates as novel anticancer prodrugs requiring bioreductive activation

Several 1,2-benzisoxazole phosphorodiamidates have been designed as prodrugs of phosphoramide mustard requiring bioreductive activation. Enzymatic reduction of 1,2-benziosoxazole moiety is expected to result in the formation of imine intermediate due to the cleavage of the N-O bond. The imine should then be spontaneously hydrolyzed to a ketone metabolite, thereby facilitating base-catalyzed beta-elimination of cytotoxic phosphoramide mustard. As expected, the proposed prodrugs 4, 9, and 12 were at least 3-5-fold more potent cytotoxins than control compounds 5 and 15, which lack in the phosphoramide mustard group. Upon incubation with phenobarb-induced rat liver S-9 fraction, compounds 4, 9, and 12 underwent extensive NADPH-dependent metabolism with concomitant generation of alkylating activity under both hypoxic and oxic conditions. Corresponding ketone metabolites were detected for 9 and 15. NADPH-dependent bioreduction of 15 to its ketone metabolite 16 was located in the microsomal fraction and inhibited by SKF-525A and pCMBA. Compared with phenobarb-induced rat liver microsomal fraction, incubation of 15 with rat or human p450 reductase microsomes showed moderate generation of 16. Microsomal cytochrome p450 and/or p450 reductase appear to be involved in the reductive metabolism of 1,2-benzisoxazole moiety under hypoxic as well as oxic conditions.     

Identification of a glutathione S-transferase associated with microsomes of tumor cells resistant to nitrogen mustards

Walker 256 rat mammary carcinoma cells resistant to chlorambucil (WR) exhibited an approximate 4-fold increase in glutathione S-transferase (GST) activity using 1-chloro-2,4-dinitrobenzene as compared to the sensitive parent cell line (WS). WR cells maintained without biannual exposure to chlorambucil (WRr) reverted to the sensitive phenotype and possessed GST levels equivalent to WS. Mitochondria, microsomes and cytosol were isolated from WS, WR and WRr cell lines and analyzed for their GST composition. GST activity in each subcellular compartment of resistant cells was increased over the sensitive cells. Antibodies raised against total rat liver cytosolic GST crossreacted in resistant cells with two microsomal proteins (25.7 kD and 29 kD). The 29 kD protein was not detected in microsomal fractions from either WS or WRr and this protein was found to be dissimilar from cytosolic GST subunits in its isoelectric point (pI 6.7) and migration on two-dimensional polyacrylamide gels. In addition, the 29 kD microsome-associated GST from WR cells was immunologically distinct from a 14 kD GST subunit previously identified in rat liver microsomes. These data implicate the induction of a specific microsomal GST subunit in WR cells following drug selection and suggest its potential involvement in the establishment of cellular resistance to chlorambucil.