Iron is not involved in oxidative stress-mediated cytotoxicity of doxorubicin and bleomycin

BACKGROUND AND PURPOSE

The anticancer drugs doxorubicin and bleomycin are well-known for their oxidative stress-mediated side effects in heart and lung, respectively. It is frequently suggested that iron is involved in doxorubicin and bleomycin toxicity. We set out to elucidate whether iron chelation prevents the oxidative stress-mediated toxicity of doxorubicin and bleomycin and whether it affects their antiproliferative/proapoptotic effects.

EXPERIMENTAL APPROACH

Cell culture experiments were performed in A549 cells. Formation of hydroxyl radicals was measured in vitro by electron paramagnetic resonance (EPR). We investigated interactions between five iron chelators and the oxidative stress-inducing agents (doxorubicin, bleomycin and H(2)O(2)) by quantifying oxidative stress and cellular damage as TBARS formation, glutathione (GSH) consumption and lactic dehydrogenase (LDH) leakage. The antitumour/proapoptotic effects of doxorubicin and bleomycin were assessed by cell proliferation and caspase-3 activity assay.

KEY RESULTS

All the tested chelators, except for monohydroxyethylrutoside (monoHER), prevented hydroxyl radical formation induced by H(2)O(2)/Fe(2+) in EPR studies. However, only salicylaldehyde isonicotinoyl hydrazone and deferoxamine protected intact A549 cells against H(2)O(2)/Fe(2+). Conversely, the chelators that decreased doxorubicin and bleomycin-induced oxidative stress and cellular damage (dexrazoxane, monoHER) were not able to protect against H(2)O(2)/Fe(2+).

CONCLUSIONS AND IMPLICATIONS

We have shown that the ability to chelate iron as such is not the sole determinant of a compound protecting against doxorubicin or bleomycin-induced cytotoxicity. Our data challenge the putative role of iron and hydroxyl radicals in the oxidative stress-mediated cytotoxicity of doxorubicin and bleomycin and have implications for the development of new compounds to protects against this toxicity.

The effect of new lipophilic chelators on the activities of cytosolic reductases and P450 cytochromes involved in the metabolism of anthracycline antibiotics: studies in vitro

A major obstacle to the therapeutic use of anthracyclines, highly effective anticancer agents, is the fact that their administration results in dose-dependent cardiomyopathy. According to the currently accepted hypothesis, anthracyclines injure the heart by generating oxygen free radicals. The ability of pyridoxal isonicotinoyl hydrazone (PIH) and salicylaldehyde isonicotinoyl hydrazone (SIH) -- new iron chelators -- to protect against peroxidation as well as their suitable biological, physical and chemical properties make the compounds promising candidates for pre-clinical and clinical studies. Activities of carbonyl reductase CR (1.1.1.184), dihydrodiol dehydrogenase DD2 (1.3.1.20), aldehyde reductase ALR1 (1.1.1.2) and P450 isoenzymes (CYP1A1, CYP1A2, CYP2B, CYP3A) involved in the metabolism of daunorubicin, doxorubicin and other drugs or xenobiotics were studied. Various concentrations of the chelators were used either alone or together with daunorubicin or doxorubicin for in vitro studies in isolated hepatocytes. A significant decrease of activity was observed for all enzymes only at PIH and SIH concentrations higher than those presumed to be used for therapy. The results show that PIH and SIH have no effect on the activities of the enzymes studied in vitro and allow us to believe that they will not interfere with the metabolism of co-administered drugs and other xenobiotics. Daunorubicin (Da) and doxorubicin (Dx) significantly reduce cytochrome P450 activity, but the addition of SIH and PIH chelators (50 microM) reverses the reduction and restores the activity to 70-90 % of the activity of relevant controls.

[Preparation of baculovirus transfer vectors for in vitro production of the P4501A1 and P4503A4 isoenzymes]

The aim of this work is the construction of an expression system for in vitro synthesis of microsomal monooxygenases P4501A1 and P4503A4, which catalyze oxidative transformations of most xenobiotics in both animal and human organisms. cDNAs encoding both proteins were obtained following the UBMTA protocol by the courtesy of holders, and amplified by established methods. Baculovirus transfer vectors were used to clone these cDNAs. These vectors contain a strong polyhedrine promoter surrounded by sequences homologous to that of baculovirus DNA, allowing the recombination of the vector with the viral DNA, and hence the production of a protein. Established methods and PCR were used to insert cDNA into the vectors, and the insertion was verified by the PCR method with specific primers and using restriction endonucleases.

Effect of ivermectin on activities of cytochrome P450 isoenzymes in mouflon (Ovis musimon) and fallow deer (Dama dama)

Ivermectin is an antiparasitic drug widely used in veterinary and human medicine. We have found earlier that repeated treatments of rats with high doses of this drug led to significant increase of cytochrome P450-dependent 7-methoxyresorufin O-demethylase (MROD) and 7-ethoxyresorufin O-deethylase (EROD) activities in hepatic microsomes. In the present study, the effects of ivermectin on cytochrome P450 (CYP) activities were investigated in mouflon (Ovis musimon) and fallow deer (Dama dama). This study was conducted also to point out general lack of information on both basal levels of CYP enzymes and their inducibilities by veterinary drugs in wild ruminants. Liver microsomes were prepared from control animals, mouflons, after single or repeated (six doses in six consecutive days) treatments with therapeutic doses of ivermectin (0.5 mg kg(-1) of body weight), and fallow deer exposed to repeated doses of ivermectin under the same conditions. Alkyloxyresorufins, testosterone and chlorzoxazone were used as the specific substrate probes of activities of the CYP isoenzymes. A single therapeutic dose of ivermectin significantly induced (300-400% of the control group) the activities of all alkyloxyresorufin dealkylases tested in mouflon liver microsomes. Repeated doses of ivermectin also caused an increase of these activities, but due to fair inter-individual differences, this increase was not significant. The administration of ivermectin led to an induction (170-210% of the control) of the testosterone 6beta- and 16alpha-hydroxylase activities in mouflon liver but no significant modulation of chlorzoxazone hydroxylase (CZXOH) activity was found in mouflon liver. CYP-dependent activities in hepatic microsomes were generally higher in fallow deer than in mouflons. However, with the exception of slight increase in the 7-benzyloxyresorufin O-dealkylase (BROD) activities, no significant modulation of the other activities was observed. The induction of CYP3A-like isoenzyme was confirmed by immunoblotting only in the microsomes from mouflons administered with repeated doses of ivermectin; however, no significant increase of CYP1A isoenzymes was observed due to a weak cross-reactivity of anti-rat CYP1A1/2 polyclonal antibodies used in the study. The results indicate that ivermectin should be considered as an inducer of several cytochrome P450 isoenzymes, including CYP1A, 2B and 3A subfamilies, in mouflons. The comparison of induction effect of ivermectin in rat, mouflon and fallow deer also demonstrates the inter-species differences in inducibility of CYP enzymes.

The main metabolic pathway of oracin, a new potential cytostatic drug, in human liver microsomes and cytosol: stereoselectivity of reoxidation of the principal metabolite 11-dihydrooracin to oracin

Chirality is a prominent feature of most biological processes. The intrinsic asymmetry of receptors, enzymes, and other endogenous macromolecules represents the basis for biological discrimination between the stereoisomeric forms of all foreign compounds in organism. Stereoselectivity and stereospecificity, two principal chiral attributes of enzyme activity, play important role in biotransformation process of drugs and other xenobiotics. The stereospecificity of enzymes leads to the preferential formation of certain enantiomer, the stereoselectivity of enzymes, on the other hand, expresses the preference of one stereoisomer form of substrate for subsequent biotransformation. An approach to the study of different conditions for the formation of the two enantiomers of principal metabolite of potential cytostatic drug oracin in Man in vitro is described. The futile cycle, in which the principal metabolite is converted to the parent drug, is also discussed. The results emphasise the fact that the stereospecificity of enzymes in Man is often distinct from other laboratory species studied.

A comparison between stereospecificity of oracin reduction and stereoselectivity of oxidation of 11-dihydrooracin enantiomers in vitro in rat and guinea pig

11-dihydrooracin (DHO) arises from the potential cytostatic drug oracin through the metabolic conversion of its prochiral centre (C11). The participation of reduction enzymes on production of DHO enantiomer under various incubation conditions were tested in rat and guinea pig microsomal and cytosolic fractions. Interesting differences in stereospecificity of oracin reduction enzymes were found. Reduction stereospecificity was further studied on rat and guinea pig isolated hepatocytes. The enantiomers were detected in rat and guinea pig hepatocytes in the (+)/(-) ratio 63/37 and 32/68 respectively. As the differences in the amounts of DHO enantiomers can be caused not only by stereospecificity of oracin reduction but also by subsequent conversion of the enantiomer, stereoselectivity of DHO oxidation to oracin was investigated. Synthetically prepared pure (+)- and (-)-DHO were incubated with rat or guinea pig microsomes and cytosol and with various coenzymes under aerobic or anaerobic conditions. Significant oxidation of DHO to oracin was observed in rat microsomes. This oxidation depends on NADPH and O2 and is stereoselective for (+)-DHO. The formation of oracin in the guinea pig was greater in cytosol than microsomes, but no significant preference for a particular DHO enantiomer was found.

Sex differences in stereospecificity of oracin reductases in rat in vitro and in vivo

In vitro and in vivo experiments to investigate possible stereospecific aspects of oracin reduction in relation to rat gender have been conducted. Incubation of oracin with rat microsomes, cytosol, and hepatocytes in the presence of various coenzymes and under aerobic or anaerobic conditions provided evidence for sex differences in the formation of 11-dihydrooracin (DHO) enantiomers. The greatest sex differences were seen in hepatocytes where females showed higher stereospecificity of the reductases than males. While female biotransformation enzymes preferentially generated approximately 82% of (+)-DHO, male enzymes gave only rise to 63% of (+)-DHO. Males displayed higher stereospecificity than females in the microsomal fraction. However, in the cytosolic fraction females exhibited higher stereospecificity than males. Similarly, in in vivo studies, the ratio of (+)- and (-)-DHO in faeces and urine gave no indication of the significant differences between the male and female rat. Enzyme stereospecificity has been defined as preferential formation of the (+)- or (-)-stereoisomer of 11-DHO by the respective enzyme. HPLC quantitative determinations of both enantiomers were performed using a Chiralcel OD-R column as the chiral stationary phase with excellent resolution and stability.

Effect of substituents on microsomal reduction of benzo(c)fluorene N-oxides

The potential benzo(c)fluorene antineoplastic agent benfluron (B) displays high activity against a broad spectrum of experimental tumours in vitro and in vivo. In order to suppress some of its undesirable properties, its structure has been modified. Benfluron N-oxide (B N-oxide) is one of benfluron derivatives tested. The main metabolic pathway of B N-oxide is its reduction to tertiary amine B. A key role of cytochrome P4502B and P4502E1 in B N-oxide reduction has been proposed in the rat. Surprisingly, B N-oxide is reduced also in the presence of oxygen although all other N-oxides undergo reduction only under anaerobic conditions. With the aim to determine the influence of the N-oxide chemical structure and its redox potential on reductase affinity, activity and oxygen sensitivity five relative benzo(c)fluorene N-oxides were prepared. A correlation between the redox potential measured and the non-enzymatic reduction ability of the substrate was found, but no effect of the redox potential on reductase activity was observed. Microsomal reductases display a high affinity to B N-oxide (apparent K(m) congruent with0. 2 mM). A modification of the side-chain or nitrogen substituents has led to only a little change in apparent K(m) values, but a methoxy group substitution on the benzo(c)fluorene moiety induced a significant K(m) increase (ten-fold). Based on kinetic study results, the scheme of mechanism of cytochrome P450 mediated benzo(c)fluorene N-oxides reduction have been proposed. All benzo(c)fluorene N-oxides under study were able to be reduced in the presence of oxygen. Changes in the B N-oxide structure caused an extent of anaerobic conditions preference. The relationship between the benzo(c)fluorene N-oxide structure and the profile of metabolites in microsomal incubation was studied and important differences in the formation of individual N-oxide metabolites were found.

Inter-species comparison of microsomal reductive transformation of biologically active benfluron N-oxide

Benfluron N-oxide is an anti-neoplastic active metabolite of benfluron (B) /1/. It is generated by flavine-monooxygenase-catalysed reactions /2/ and immediately undergoes subsequent metabolic transformations, the most important of which are reductive reactions /3/. The products of reductive pathways catalysed by two different microsomal enzymatic systems are the tertiary amine benfluron (i.e. the original parent compound) and/or 7-dihydrobenfluron N-oxide. Our studies on the reductive transformation of B N-oxide in rat, mouse, guinea-pig, rabbit, mini-pig and human microsomes have revealed significant species differences both in the yields of respective reduced metabolites and in the conditions essential for the activity of the reductases involved. While B, the original tertiary amine, is the main product of aerobic incubation of B N-oxide with NADPH in rat, mouse and mini-pig, significantly higher activities of the enzymes catalysing the formation of 7-dihydro-B N-oxide have been detected in rabbit and human microsomes. In rat, mouse and mini-pig, NADPH rather than NADH is the preferred coenzyme for B formation, and NADPH is also the preferred coenzyme for the formation of 7-dihydro-B N-oxide in most of the species used. The yield of tertiary amine B is higher in anaerobic rather than aerobic conditions in most experimental species studied. Aerobic or anaerobic incubating conditions have an insignificant effect on the formation of 7-dihydro-B N-oxide. Based on the inhibitory effect of CO on the reductive transformation of B N-oxide, cytochromes P450 can be assumed to participate in the formation of B both in rat and mini-pig, and, in mini-pig only, also in the formation of 7-dihydro-B N-oxide. Inter-species comparison of the properties of the reductases participating in the transformation of B N-oxide shows that the rabbit is a suitable model to study reductive transformation of B N-oxide in man.

Induction and inhibition of cytochrome P450-catalysed reduction of biologically active benfluron N-oxide

Benfluron N-oxide [5-(2-N-oxo-2-N,N'-dimethylaminoethoxy)-7-oxo-7-H-benzo[c]fluorene] is a biologically active substance which displays a cytostatic effect on several experimental tumour cells. The main metabolic pathway of benfluron N-oxide in vitro and in vitro--its reduction to the parent tertiary amine benfluron--and the role of cytochrome P450 in this reduction were studied. The value of the benfluron N-oxide/benfluron redox potential as a criterion of suitability of the substrate for cytochrome P450 reductase activity was determined. Results of induction and inhibition studies on rats suggest that cytochromes P4502B and P4502E1 participate in microsomal reduction of benfluron N-oxide. Unlike most cytochrome P450 catalysed reactions, the reduction of benfluron N-oxide also occurs under aerobic conditions. Microsomes induced by phenobarbital, ethanol or beta-naphthoflavone showed no significantly greater inhibitory effect of oxygen on benfluron N-oxide reduction.

Role of cytochrome P4501A in biotransformation of the potential anticancer drug oracin

Metabolic fate of the potential anticancer drug oracin (I), was studied at microsomal level in rat using enzyme induction and inhibition. One of the main metabolites arising during incubation of hepatic microsomal fraction with oracin is 3-hydroxyoracin (III). Cytochromes P450 non-specific inhibitors (carbon monoxide, aminobenzotriazole, 1-benzylimidazole, proadifen hydrochloride, n-octylamine) diminished amount of III. Among several specific inducers of rat cytochromes P450 isoforms used, only 3-methylcholanthrene, inducer of cytochrome P4501A, caused a significant stimulation of 3-hydroxyoracin production. The amount of III was decreased to the level of controls when the microsomes prepared from 3-methylcholanthrene treated rats were incubated with substrate in the presence of specific P4501A inhibitor alpha-naphthoflavone. From the above mentioned results we can assume that metabolite III is formed from oracin by cytochrome P450 belonging to subfamily 1A.

Metabolic pathways of flobufen-a new antirheumatic and antiarthritic drug. Interspecies comparison

Metabolic transformations of flobufen, [4-(2',4'-difluoro-biphenyl-4-y1)-4-oxo-2-methylbutanoic acid], a non-steroid antiinflammatory agent, were studied in vitro using the following biological models and species: rat and mouse liver homogenates and liver subcellular fractions (5 000 g and 100 000 g supernatant, mitochondria); rat, mouse, rabbit, guinea-pig and mini-pig liver microsomes; isolated rat hepatocytes; perfused rat liver and 5000 g rat muscle tissue supernatant. Reduced flobufen [4-(2',4'-difluorobiphenyl-4-yl)-4-hydroxy-2-methylbutanoic acid] is the major metabolite generated by the subcellular fractions (in the mild acidic extraction conditions during subsequent laboratory processing is converted to its lactone form). It was detected upon the incubation of flobufen with liver microsomes isolated from all the animals tested. Maximum yield of reduced flobufen in experiments with rat and mouse liver microsomes was found after anaerobic incubation with NADPH. This finding combined with the knowledge of subcellular distribution of enzymes suggest that metabolite formation depends on the activity of microsomal reductases and, probably, also on the activity of the important microsomal reductase, cytochrome P-450. Another flobufen metabolite, arylacetic acid [(2',4'-difluorobiphenyl-4-yl)ethanoic acid], is generated from the reduced metabolite by the cleavage of its side chain, and was detected in isolated hepatocytes - it was the only metabolite found in urine and faeces upon oral administration of the drug. All these metabolites were identified and quantified.

High-performance liquid chromatography study of stereospecific microsomal enzymes catalysing the reduction of a potential cytostatic drug, oracin. Interspecies comparison

One of the main metabolites of oracin (I) ¿6-[2-(2-hydroxyethyl)aminoethyl]-5,11-dioxo-5,6-dihydro-11H-indeno[1,2- c] isoquinoline¿, a potential cytostatic drug, is 11-dihydrooracin (II) ¿(+),(-)-6-[2-(2-hydroxyethyl)aminoethyl]-5-oxo-11-hydroxy-5,6-dihydro-1 1H- indeno[1,2-c]isoquinoline¿, a metabolite formed by the reduction of oracin's pro-chiral centre on C 11. This metabolite has been found in all laboratory species in vitro and in vivo and it constitutes the main metabolite in man. The stereospecificity of reducing enzymes participating in the oracin biotransformation pathway was investigated using microsomal preparations from standard laboratory animals. Enzyme stereospecificity has been defined as preferential formation by the enzyme of the (+) or (-) stereoisomer of II. Significant interspecies differences were observed in the stereospecificity of the respective biotransformation enzymes. HPLC quantitative determinations of both enantiomers were performed using a Chiralcel OD-R column as chiral stationary phase with excellent resolution and stability.

Separation of the stereoisomers of the main metabolite of a non-steroidal anti-inflammatory drug, flobufen, by chiral high-performance liquid chromatography

The major metabolite of a novel non-steroidal anti-inflammatory drug, DL-4-(2',4'-difluorobiphenyl-4-yl)-2-oxo-2-methylbutanoic acid (flobufen, I), namely 4-(2',4'-difluorobiphenyl-4-yl)-2-methyl-gamma-butyrolactone (4-dihydroflobufen lactone, III), has four stereoisomers consisting of two racemic pairs of enantiomers. Of three chiral stationary phases tested, Cyclobond I beta-RSP (Astec) (beta-cylodextrin derivatized with R,S-hydroxypropyl) was best able to separate the (+2)(--) racemate, with a liquid phase containing acetonitrile as modifier and triethylamine acetate as buffer. Using the Box-Wilson Central Composite Design for three factors, an optimum combination of pH and concentrations of the modifier and buffer was eventually obtained. A chromatographic response function based on a combination of the Kaiser peak separation function, Pi, and retention time of the second eluting enantiomer, tRL, served as a response criterion for the process of optimization. The optimum conditions developed for the (+2)(--) racemate were also found to be suitable for separating the (+-)(-+) racemate, for which earlier studies had shown the separation to be more facile. Separation of the four stereoisomers of III, for which the chiral chromatographic system optimized in this study is proposed as the second stage, is targeted at a biochemical study of the stereoisomeric metabolism of I.

High-performance liquid chromatographic assay for the separation and characterization of metabolites of the potential cytostatic drug oracine

Oracine (I), a potential cytostatic drug, is enzymically converted to a number of metabolites whose formation has been studied in vitro and in vivo. The metabolites were separated by reversed-phase HPLC and characterized by UV spectra. Preparative TLC served for the isolation of the individual metabolites to allow their identification. Two metabolites were identified by Fourier transform NMR as 11-dihydrooracine (II) and a phenolic product (III). Two further metabolites (IV,V) were characterized. Some minor, presumably 11-dihydro metabolites and an 11-oxo metabolite produced in vitro and in vivo were revealed.

Biotransformation of the sympathomimetic tetraminol in vitro

On incubation with the postmitochondrial fraction of the liver homogenate of rabbits, guinea-pigs, rats, and mice in the presence of NADPH and oxygen, the alpha-sympathomimetic trans-3-(2-hydroxyethylamino)-5,8-dimethoxy-1,2,3,4-tetrahydro-2-n aphthol (Tetraminol, 1) is preferentially O-demethylated in position 8, yielding metabolite 3. In male rats O-demethylation is stronger than in females.

Prolonged reduction of hepatocyte proliferative ability in rats after a single treatment with carbon tetrachloride

Female Wistar rats were pretreated with I ml of carbon tetrachloride/kg of body weight or with olive oil. All the rats were given this dose of CCl4 20 or 40 days later. Liver regeneration as evaluated by 3H-thymidine incorporation into liver DNA and by the number of mitotic hepatocytes was markedly impaired in CCl4-pretreated rats when compared with olive oil-pretreated controls. DNA labelling reached only 83 and 59% and mitotic index 35 and 58% of control values, respectively, at 20-day and 40-day time intervals. The variables characteristic of liver damage did not parallel the changes in cell division. About 20% of hepatocytes were necrotic both in the CCl4-pretreated and in the control rats. The activity of serum alanine aminotransferase was higher in the CCl4-pretreated rats. Only serum aspartate aminotransferase activities were somewhat lower when compared to controls. Similarly, serum aminotransferases were much less affected by the pretreatment than the markers of regeneration when two low doses of CCl4 (0.125 ml/kg) were given to rats 20 days apart. The activities of microsomal enzymes aniline hydroxylase and pethidine demethylase were equal in control and in experimental rats 20 days after CCl4 pretreatment which indicated that the effects of CCl4 were not mediated by an overall decrease in cytochrome P-450 enzymes. In summary, a single pretreatment of rats with CCl4 induced changes in liver that lasted for 40 days and impaired liver regeneration when another dose of CCl4 was applied.

Elimination of benfluron and its metabolites in the faeces and urine of rats

The study of the biotransformation of the potential cytostatic benfluron has been continued. The elimination of benfluron and of nine of its metabolites whose structure had been established, mainly on the basis of the comparison of their IR, MS and NMR spectra with those of standards, was studied. After oral administration of 500 mg.kg-1 to rats, the amounts of these substances in the faeces and urine were followed up by high-performance liquid chromatography for five days. Striking qualitative and quantitative differences were observed in the elimination of benfluron and its metabolites by both routes.

Evaluation of photometric high-performance liquid chromatographic data for the determination of benflurone metabolites in biological materials with and without concomitant use of a standard

The results produced by a new method of calculation based on the knowledge of the absorption coefficient and instrumental parameters without the concomitant use of a standard were compared with those calculated by the routine external standard method for a model system utilizing the quantification of benzofluorene derivatives. These were present in the incubation mixture of 5-[2-(dimethylamino)ethoxy]-7-oxo-7H-benzo[c]fluorene (benflurone) with the microsomal fraction of rat liver homogenate. Reasonable agreement between the two methods was observed. The potential utility of the new method of calculation is discussed.

Chromatographic characterization of in vitro metabolites of 5-[2-(N,N-dimethylamino)ethoxy]-7-oxo-7H-benzo[c]fluorene

Detection reactions and RF values in thin-layer chromatography on silica gel were studied for the antineoplastic drug Ih (benfluron) and related substances. On incubation of Ih with homogenate fractions of mammalian livers the N-oxide Ii and 5-[2-(N,N-dimethylamino)ethoxy]-7-hydroxy-7 H-benzo[c]fluorene (IIh) were established as products, and 5-[2-(N-methylamino)ethoxy]-7-oxo-7 H-benzo[c]fluorene (Ig), 5-[2-(N-methylamino)ethoxy]-7-hydroxy-7 H-benzo[c]fluorene (IIg) and a phenolic product of benfluron (IV) were tentatively identified.