Redox cycling of anthracyclines by cardiac mitochondria. I. Anthracycline radical formation by NADH dehydrogenase

Subcellular distribution of two spin trapping agents in rat heart: possible explanation for their different protective effects against doxorubicin-induced cardiotoxicity

Previous investigations, performed on isolated rat atria, showed that the lipophylic spin-trapping agent N-tert-butyl- alpha-phenylnitrone (PBN) is able to prevent the acute cardiotoxic effects produced by doxorubicin (DXR), whereas the hydrophylic compound 5,5-dimethyl-pyrroline-N-oxide (DMPO) is inactive. The present study was designed to ascertain whether differences in the pharmacological effects of the two spin traps are related to their different subcellular distribution. Langendorff rat hearts were perfused for 60 minutes with [14C]-DXR and either PBN or DMPO. The subcellular mapping of the three compounds was performed by measuring DXR by liquid scintillation counting, PBN by GC/MS, and DMPO by HPLC in the following isolated fractions: nuclei, mitochondria, sarcoplasmic reticulum, sarcolemma, cytosol. DMPO was shown to accumulate in the cytosolic compartment; both PBM and DXR are taken up by nuclei and mitochondria, while only trace amounts of DXR were detected in the sarcoplasmic reticulum. These results suggest that mitochondrial (and not sarcoplasmic) enzymes are mainly involved in DXR-induced free radical production, which is thought to cause the acute cardiotoxic effects of DXR. An involvement of DXR-induced free radical generation in the nuclear compartment seems unlikely in the short-term "in vitro" effects observed with the experimental model adopted for these studies, although it may play a role in the delayed pathology.

Paraquat damage of rat liver mitochondria by superoxide production depends on extramitochondrial NADH

Pure rat liver heavy mitochondrial fractions, in which the absence of significant microsomal contamination was confirmed by electron microscopy and by the lack of glucose-6-phosphatase activity, were used to demonstrate the effect of paraquat on mitochondrial ultrastructure in the presence of external NADH. Starved mitochondria (orthodox conformation) did not show O2 uptake or structural injury from either paraquat alone or NADH alone. Marked O2 uptake and structural breakage occurred only when paraquat and NADH were added in combination. These alterations were resistant to rotenone and malate plus glutamate or NADPH could not substitute for NADH. Paraquat was reduced anaerobically by the mitochondria in the presence of NADH, but not of NADPH. The addition of superoxide dismutase, ferricytochrome c or p-benzoquinone protected against the breakage of mitochondria caused by paraquat plus NADH. These results demonstrate that mitochondria may produce paraquat radicals in the presence of extramitochondrial NADH and thus generate superoxide anion radicals, resulting in structural injury to the mitochondria, by mechanisms that may involve the mitochondrial outer membrane rather than the electron transfer chain. These mitochondrial mechanisms in paraquat toxicity seemed to be more probable in vivo than are microsomal mechanisms; the latter are postulated to function in detoxication because phenobarbital diminished paraquat toxicity and SKF 525-A or cobaltous ions enhanced the toxicity.

Protection by salvianolic acid A against adriamycin toxicity on rat heart mitochondria

It was found that salvianolic acid A (Sai A) has potent antioxidant activity. The effects of Sai A on adriamycin-induced heart mitochondrial toxicity of rats in vitro and on adriamycin antitumor activity are investigated in this article. Malondialdehyde (MDA) formation and membrane rigidification of rat heart mitochondria intoxicated with adriamycin were significantly reduced by Sai A. In the electron spin resonance (ESR) studies, Sai A has no significant effect on the formation of adriamycin semiquinone radicals (AQ.), while hydroxyl radicals generated by electron transfer from AQ. to H2O2 were scavenged by Sai A dose-dependently. On the other hand, Sai A was shown to have no effects on the antitumor activity of adriamycin in cultured L1210 ascitic tumor cells and in mice with P388 ascite tumor. These results indicate that Sai A protects against adriamycin induced heart mitochondrial toxicity of rats, while Sai A has no antagonizing effect on the antitumor activity of adriamycin.

The rotenone-insensitive reduction of quinones and nitrocompounds by mitochondrial NADH:ubiquinone reductase

The rotenone-insensitive reduction of quinones and aromatic nitrocompounds by mitochondrial NADH: ubiquinone reductase (complex I, EC 1.6.99.3) has been studied. It was found that these reactions proceed via a mixed one- and two-electron transfer. The logarithms of the bimolecular rate constants of oxidation (TN/Km) are proportional to the one-electron-reduction potentials of oxidizers. The reactivities of nitrocompounds are close to those of quinones. Unlike the reduction of ferricyanide, these reactions are not inhibited by NADH. However, they are inhibited by NAD+ and ADP-ribose, which also act as the mixed-type inhibitors for ferricyanide. TN/Km of quinones and nitrocompounds depend on the NAD+/NADH ratio, but not on NAD+ concentration. They are diminished by the limiting factors of 2.5-3.5 at NAD+/NADH greater than 200. It seems that rotenone-insensitive reduction of quinones and nitrocompounds takes place near the NAD+/NADH and ferricyanide binding site, and the inhibition is caused by induced conformational changes after the binding of NAD+ or ADP-ribose.

Protection by schisanhenol against adriamycin toxicity in rat heart mitochondria

The effects of schisanhenol (Sal) on Adriamycin (ADM)-induced rat heart mitochondrial toxicity in vitro were investigated. Malondialdehyde formation, lysis, disintegration and membrane rigidification in mitochondria treated with ADM were reduced significantly by Sal. In the electron spin resonance studies, Sal did not affect significantly the formation of ADM semiquinone radicals (AQ.), whereas hydroxyl radicals generated by electron transfer from AQ.to H2O2 were scavenged by Sal dose dependently. These results indicate that Sal could protect against ADM-induced rat heart mitochondrial toxicity.

On the mechanism of rotenone-insensitive reduction of quinones by mitochondrial NADH:ubiquinone reductase. The high affinity binding of NAD+ and NADH to the reduced enzyme form

NADH acts as an incomplete competitive inhibitor for 5,8-dioxy-1,4-naphtoquinone during its rotenone-insensitive reduction by mitochondrial NADH:ubiquinone reductase. NAD+ and ADP-ribose act as incomplete mixed-type inhibitors. Ki of NAD+ and NADH towards quinone are about one order less than towards ferricyanide. The bimolecular rate constant of the reduction of the enzyme by NADH in the quinone reductase reaction is about 2 times less than that of ferricyanide reductase reaction. These data indicate that the reduction site of 5,8-dioxy-1,4-naphtoquinone is close to NAD+/NADH and ferricyanide binding site. It seems that during the steady-state reduction of ferricyanide and 5,8-dioxy-1,4-naphtoquinone these oxidizers react with NADH:ubiquinone reductase reduced to different extents.

Inactivation of cytochrome c oxidase activity in mitochondrial membranes during redox cycling of doxorubicin

Interactions of doxorubicin (DX) with the cardiolipin-dependent cytochrome c oxidase have been examined by using pig heart submitochondrial particles (SMP). A progressive and irreversible loss of oxidase activity is demonstrated in 2 hr incubations of the SMP with 10-100 microM DX in air-equilibrated medium with excess NADH to support redox-cycling of the drug. This oxidative mechanism for oxidase inactivation occurs in connection with a peroxidation process in the bulk membrane lipid, and is independent on turnover of the enzyme. It is related in a complex manner to the electron flux in the respiratory chain with antioxidant properties, and is maximal at the high reduction level of respiratory chain Complex I obtained in the presence of rotenone. Reduction of DX per se plays a minor role, and trace concentrations of chelatable metal ions (iron) are required to catalyse the reaction. Iron in the iron storage protein ferritin is released by DX, and at physiological low O2 concentrations ([O2] less than 20 microM), this iron is a better promoter of oxidase inactivation than is endogenous iron in the SMP. Kinetic analysis of inactivation data indicates the interaction of DX with low affinity (Km 35-55 microM) binding sites in the SMP membranes. Overall, the results point to the possible role of ferritin-iron in the mechanism of DX mitochondrial toxicity and argue against site specific effects of the DX-reduction/oxidation cycle on the cytochrome c oxidase or on its essential phospholipid (cardiolipin) environment.

Oxidation of mitochondrial pyridine nucleotides by aglycone derivatives of adriamycin

Adriamycin (AdM) and related anthracyclines are potent antineoplastic agents, the clinical utility of which is limited by severe cardiotoxicity. Aglycone derivatives of AdM have recently been reported to trigger the release of Ca2+ from isolated, preloaded rat heart mitochondria and to modify mitochondrial sulfhydryl (-SH) groups. Both mitochondrial Ca2+ retention and -SH status are sensitive to mitochondrial NAD(P)+/NAD(P)H ratios. This investigation examined the effects of AdM and its aglycone derivatives on the pyridine nucleotide redox status of isolated, intact heart mitochondria with the following results. (i) AdM aglycones induced the slow, Ca2(+)-independent oxidation of mitochondrial NAD(P)H. Oxidation was proportional to aglycone concentration between 5 and 60 microM. (ii) In terms of potency, 7-deoxy AdM aglycone greater than or equal to 7-hydroxy AdM aglycone much greater than AdM. (iii) Inhibitor data suggested that NAD(P)H oxidation reflects the rotenone-insensitive reduction of AdM aglycone and subsequent electron transfer to O2 generating superoxide. (iv) NAD(P)H oxidation mediated by AdM aglycone could be distinguished from the Ca2(+)-dependent NAD(P)H oxidation associated with mitochondrial Ca2+ release. This communication is the first to describe redox interactions of AdM with intact mitochondria.

On the detection of paramagnetic species in the adriamycin-perfused rat heart: a reappraisal

Recently, L. Costa et al. reported the direct detection of the superoxide anion and other paramagnetic species in the isolated, adriamycin-perfused rat heart [L. Costa et al. (1988) Biochem. Biophys. Res. Commun. 153, 275-280]. We have reevaluated the results of their study and concluded that the ESR parameters of the spectrum obtained from the adriamycin-perfused heart are consistent with that of the peroxyl radical and not with that of the superoxide anion. In addition, the ESR spectrum of the peroxyl radical is very likely produced as an artifact caused by the grinding of myocardial tissue. This artifact may mask the ESR spectra of the adriamycin-derived semiquinone radical and the iron-sulfur protein components of myocardium.

Evidence of the selective alteration of anthracycline activity due to modulation by ICRF-187 (ADR-529)

Anthracyclines are powerful anticancer drugs whose use is limited by the development of chronic cardiotoxicity. The bisdioxopiperazine compound ICRF-187 (ADR-529) specifically abrogates this toxicity both in preclinical animal models and in humans. It does this without effecting either the acute toxicities or the anticancer activity. Therefore, with a specific antagonist, the mechanism of activity of the anthracyclines can be explored. This review discusses recent clinical trials and animal models addressing this issue and concludes by hypothesizing a mechanism of action.

Superoxide-dependent and superoxide-independent pathways for reduction of nitroblue tetrazolium in isolated rat cardiac myocytes

Spectroscopic studies indicated that nitroblue tetrazolium (NBT) could be reduced to blue formazan by several distinct reactions in suspensions of isolated rat cardiac myocytes. Both NADPH- and NADH-linked pathways for reduction of NBT were observed. NADPH-linked NBT reduction showed little activity in the absence of digitonin, but could be stimulated an average of 9.5-fold by digitonin permeabilization of the plasma membrane. NADH-linked NBT reduction occurred in the absence of digitonin, and could be increased an average of 3.5-fold by digitonin treatment. Analysis of the effects of cell viability on the extent of digitonin stimulation with these substrates suggested that the NADPH-linked reaction involved a cytosolic component, while the NADH-linked reaction involved an intracellular membrane enzyme system. With either NADPH or NADH, NBT reduction was completely inhibited by dicoumarol (100 microM). Dicoumarol-insensitive NBT reduction could subsequently be observed following the addition of 2 mM cyanide, a level of cyanide known to inhibit cytosolic superoxide dismutase. Cyanide-stimulated, dicoumarol-insensitive NBT reduction was augmented by the presence of either antimycin or doxorubicin, two agents which enhance superoxide formation by different mechanisms. The results indicate the existence of multiple pathways for both superoxide-independent and superoxide-dependent reduction of NBT. Dicoumarol-insensitive, cyanide-stimulated NBT reduction may be useful as a spectroscopic probe for intracellular superoxide formation.

Effect of glutathione and N-acetylcysteine on in vitro and in vivo cardiac toxicity of doxorubicin

The effects of two sulfhydryl compounds, glutathione (GSH) and N-acetylcysteine (NAC), on the cardiotoxicity of doxorubicin (DXR) were tested on in vitro and in vivo models. DXR was administered to rats as 4 weekly i.v. doses of 3 mg/kg. GSH (1.5 mmoles/kg), given i.v. 10 min before and 1 hr after DXR, was found to prevent the development of the delayed cardiotoxic effects of DXR, as assessed by electrocardiographic and mechanical parameters, as well as by histological examination of left ventricular preparations. In contrast, equimolar oral doses of NAC (1 hr before and 2 hrs after DXR) were found to be ineffective. Both GSH and NAC prevented the negative inotropic effect produced by DXR on isolated rat atria. A good correlation exists between the cardioprotective effects of the two agents and their ability to enhance the non-protein sulfhydryl group content of the myocardium. Differences observed in vivo between GSH and NAC might be accounted for by pharmacokinetic factors.

Detection of free radicals during the cellular metabolism of adriamycin

Experiments were conducted to determine which free radicals are generated during the metabolism of adriamycin (ADM) by canine tracheal epithelial (CTE) cells, guinea pig enterocytes, and rat hepatocytes. The technique employed in this study was spin trapping; the spin trap utilized was 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). The spin adduct 2-hydroxy-5,5-dimethyl-1-pyrrolidinyloxyl (DMPO-OH) was observed during the metabolism of ADM by CTE cells. However, the addition of dimethyl sulfoxide to the in vitro system suggested that superoxide is initially spin trapped by the nitrone, and that the adduct 2-hydroperoxy-5,5-dimethyl-1-pyrrolidinyloxyl (DMPO-OOH) is rapidly bioreduced to afford DMPO-OH. The addition of superoxide dismutase to the system indicated that superoxide generation was primarily intracellular. The adriamycin semiquinone free radical (ADM-SQ) was produced during the metabolism by enterocytes and hepatocytes. The rate of the production of ADM-SQ was enhanced under anaerobic conditions, suggesting that molecular oxygen was responsible for the degradation of this carbon-centered free radical. However, spin trapping of oxygen radicals was not observed; this observation suggests that these reactive intermediates are not produced at concentrations sufficient for detection by spin-trapping experiments.

One-electron reduction of daunorubicin intercalated in DNA or in a protein: a gamma radiolysis study

The one-electron reduction of daunorubicin, an anthracycline antibiotic, intercalated in DNA or in the apoprotein of the riboflavin binding protein, was studied by gamma radiolysis. The two reduction mechanisms appear very similar to the one found for the non-intercalated drug. Hydrogen peroxide, which oxidizes non-intercalated hydroquinone daunorubicin with two electrons in one step (C. Houée-Levin, M. Gardès-Albert and C. Ferradin, FEBS lett., 173, 27-30, (1984)), reacts with daunorubicin hydroquinone in DNA but not in the protein. It appears thus that the site accessibility to hydrogen peroxide in DNA is better than in the protein. Biological consequences are discussed.

One-electron reduction of daunorubicin intercalated in DNA or in a protein: a gamma radiolysis study

The one-electron reduction of daunorubicin, an anthracycline antibiotic, intercalated in DNA or in the apoprotein of the riboflavin binding protein, was studied by gamma radiolysis. The two reduction mechanisms appear very similar to the one found for the non-intercalated drug. Hydrogen peroxide, which oxidizes non-intercalated hydroquinone daunorubicin with two electrons in one step (C. Houée-Levin, M. Gardès-Albert and C. Ferradin, FEBS lett., 173, 27-30, (1984], reacts with daunorubicin hydroquinone in DNA but not in the protein. It appears thus that the site accessibility to hydrogen peroxide in DNA is better than in the protein. Biological consequences are discussed.

Lack of competition between cytochrome c and anthraquinone type drugs for the reductive sites of NADH dehydrogenase

We have shown that (i) the cytochrome c reductase activity of the commercial NADH dehydrogenase does not perturb its ability to catalyse the reduction of various antitumor compounds of the anthracycline class, (ii) the reduction of these compounds by NADH, catalysed by commercial NADH dehydrogenase, correlates with their reduction by NADH catalysed by microsomes. Moreover, our data strongly suggest that two catalytic sites are present, one for cytochrome c and one for quinone type compounds.

Increasing therapeutic effect and reducing toxicity of doxorubicin by N-acyl dehydroalanines

Doxorubicin toxicity is generally accepted to be free radical-mediated. N-Substituted dehydroalanines (indexed as AD compounds) are capto-dative olefins which react and scavenge free radicals, especially the superoxide anion (O2-) and hydroxyl radical (HO). AD-20, an orthomethoxyphenylacetyl dehydroalanine derivative, decreases the mortality of mice when administered before an acute single dose or multiple non-toxic doses of doxorubicin. Doxorubicin administered to mice induces elevated serum transaminase levels, and the pretreatment of mice with AD-20 decreases significantly these serum enzymatic activities. Preliminary histological examinations suggest that these serum transaminase elevations reflect most likely liver injury. In addition to its cardiotoxicity, doxorubicin induces a severe bone marrow depletion. Although this initial decrease in the peripheral leukocytes induced by doxorubicin is not prevented by the administration of AD-20, it produces a fast recuperation in the white blood cells levels after 1 week, supporting a protective effect at this level. Moreover, the antitumor effect of doxorubicin in L1210 tumor-bearing mice was enhanced when AD-20 was injected before doxorubicin. We postulate that these effects may be related to the free radical scavenging ability of AD-20.

Laser time-resolved fluorescence study of the interaction between anthracyclines and cardiolipin

The molecular interaction between cardiolipin vesicles and two representative anthracyclines, daunomycin and 5-iminodaunomycin, has been studied at pH 7.1 by laser time-resolved fluorescence, for a cardiolipin-to-anthracycline ratio r ranging from 0.02 to 5. The fluorescence lifetime of daunomycin is 1.03 ns. For r = 0.3 - 5 a longer-lived transient (1.91 - 1.49 ns) is present and originates from the excitation of daunomycin bound on a single phosphate group of cardiolipin. At r = 0.3 two lifetimes are observed, the second one being due, partially, to free daunomycin and bound drug molecules embedded in the lipid bilayer. The fastest-decaying species is present for r = 0.5 - 2.0 and identified as two adjacent, stacked-up daunomycin molecules bound onto the two phosphate groups of the cardiolipin. In the case of 5-iminodaunomycin, a less cardiotoxic analogue, three-exponential decay is never observed and a fast-decaying component, pi approximately 0.2 ns, is already present at low r and vanishes for r greater than 0.5. The constancy of the lifetimes of the longer-lived species may originate from the reorientation of the bound drug from the hydrophilic to the lipid domain.

One- and two-electron reduction of quinones by glutathione reductase

Yeast glutathione reductase (E.C. 1.6.4.2) catalyzes the oxidation of NADPH by p-quinones and ferricyanide with a maximal turnover number (TNmax) of 4-5 s-1.NADP+ stimulates the reaction and the TNmax/Km value of acceptors is reached at NADP+/NADPH greater than or equal to 100. TNmax is increased up to 30-33 s-1. The stimulatory effect of NADP+ may be associated with its complexation with the NADPH-binding site in the reduced enzyme (Kd = 40-60 microM). It is suggested that NADP+ shifts the electron density towards FAD in the two-electron-reduced enzyme and, evidently, changes its one-electron-reduction potentials, while quinones oxidize an equilibrium form of glutathione reductase containing reduced FAD. In the absence of NADP+ the reduction of quinones by glutathione reductase proceeds mainly in a two-electron manner. At NADP+/NADPH = 100 a one-electron reduction makes up 44% of the total process. At pH 6.0-7.0 the reduced forms of naphthoquinones undergo cyclic redox conversions. A hyperbolic dependence exists of the log TN/Km of quinones on their one-electron-reduction potentials.

Triplet state characteristics and singlet oxygen generation properties of anthracyclines

The triplet states of adriamycin (Ad), daunomycin (D) and two daunomycin analogues, daunomycinone (Dc) and daunomycin N-trifluoroacetamide (DAc), have been studied using laser flash photolysis and pulse radiolysis techniques. Triplet lifetimes, molar absorption coefficients, energy levels and quantum yields have been obtained for Dc and DAc, and estimated for D and Ad. Time-resolved near-infrared singlet oxygen luminescence measurements have been carried out on D, Ad and 5-iminodaunomycin (5-ID) in 2H2O solution and Dc in benzene solution at room temperature. Singlet oxygen quenching by the water-soluble anthracyclines was observed and a second-order rate constant of approx. 10(8) M-1.s-1 obtained. Electron spin resonance experiments have demonstrated that D photoexcited at lambda less than or 365 nm gives rise to singlet oxygen as shown by its reaction with 2,2,6,6-tetramethyl-4-piperidone to give the corresponding nitroxyl radical. Although all the anthracyclines studied have the ability to photosensitize the formation of singlet oxygen, the quantum yields are very low (phi delta approximately 0.02-0.03), suggesting that these anthracyclines would be poor photodynamic sensitisers.

Free radical formation by antitumor quinones

Quinones are among the most frequently used drugs to treat human cancer. All of the antitumor quinones can undergo reversible enzymatic reduction and oxidation, and form semiquinone and oxygen radicals. For several antitumor quinones enzymatic reduction also leads to formation of alkylating species but whether this involves reduction to the semiquinone or the hydroquinone is not always clear. The antitumor activity of quinones is frequently linked to DNA damage caused by alkylating species or oxygen radicals. Some other effects of the antitumor quinones, such as cardiotoxicity and skin toxicity, may also be related to oxygen radical formation. The evidence for a relationship between radical formation and the biological activity of the antitumor quinones is evaluated.

Self-reduction of the iron(III)-doxorubicin complex

The Fe3(+)-doxorubicin complex undergoes reactions that suggest that the complex self-reduces to a ferrous oxidized-doxorubicin free radical species. The Fe3(+)-doxorubicin system is observed to reduce ferricytochrome c, consume O2 and react with 2,2'-bipyridine. Bipyridine acts as a "ferrous ion scavenger" as it reacts with the ferrous ion produced by Fe3(+)-doxorubicin self-reduction. In the absence of O2, a ferrous doxorubicin complex accumulates. In the presence of oxygen, Fe2+ recycles back to Fe3+. The rates of these reactions were measured and the Fe3(+)-doxorubicin self-reduction was determined to be the rate-determining step. The Fe3(+)-doxorubicin induced inactivation of cytochrome c oxidase and NADH cytochrome c reductase on beef heart submitochondrial particles occurs at a rate similar to Fe3(+)-doxorubicin self-reduction. Thus the rate at which damage to these mitochondrial enzymes occurs may be controlled by a nonenzymatic Fe3(+)-doxorubicin self-reduction.

gamma-radiolysis study of the reduction by COO- free radicals of daunorubicin intercalated in DNA

The reduction of daunorubicin intercalated in DNA was studied using COO- free radicals produced by gamma-radiolysis as reductants. The reduction process of the drug intercalated in DNA was found to be very similar to the one of daunorubicin in aqueous solution without DNA. (a) the final product is the same (7-deoxy daunomycinone); (b) the reduction yield is the same [2.6 +/- 0.2) x 10(-7) mol.J-1); (c) H2O2 reacts with hydroquinone daunorubicin giving back the drug in a one-step reaction. However 7-deoxy daunomycinone precipitation was so slow that this aglycone could be reduced by COO- free radicals giving its hydroquinone form, which cannot be observed without DNA. This shows that the whole 4-electron reduction process takes place in DNA. The implications of these findings are discussed.

A new class of free radical scavengers reducing adriamycin mitochondrial toxicity

Beef heart mitochondria were incubated with ADM and NADH. An adriamycin semiquinone radical was detected using ESR spectroscopy. The semiquinone radical production rate is decreased upon addition of a scavenger (AD 20) in the reaction medium. NMRI mice were treated with AD 20 (70 mg/kg, i.p.) 15 min prior ADM injection (20 mg/kg, i.p.) or with ADM alone. Heart mitochondria were isolated 48 hr later. The enzymatic activities of complex I-III and complex IV of the mitochondrial respiratory chain were strongly depressed in animals receiving ADM alone, whereas these activities were almost completely restored in animals receiving AD 20 and ADM. Fluorescence depolarization measurements indicated that only mice treated with ADM alone presented a decreased fluidity of their cardiac mitochondrial membrane.

Different cytotoxicity and metabolism of doxorubicin, daunorubicin, epirubicin, esorubicin and idarubicin in cultured human and rat hepatocytes

Both cytotoxicity and metabolism of five anthracyclines, namely doxorubicin, daunorubicin, epirubicin, esorubicin and idarubicin, were investigated in primary cultures of both rat and human adult hepatocytes and, for comparison, in a rat liver epithelial cell line. Toxicity was assessed by morphological examination and measurement of lactate dehydrogenase leakage after 24 hr of treatment. The rank order of toxicity for both rat and human hepatocytes was esorubicin greater than doxorubicin = epirubicin greater than or equal to idarubicin greater than daunorubicin, and for rat epithelial cells: esorubicin greater than or equal to epirubicin greater than idarubicin = daunorubicin = doxorubicin. Human cells were around 2-fold less sensitive than rat hepatocytes to all anthracyclines. Anthracyclines and their metabolites were analyzed by HPLC. Differences in both the percentages and routes of metabolism were demonstrated between rat and human hepatocytes. The main metabolite was the 13-dihydro-derivative (-ol derivative) in both species from daunorubicin, idarubicin and esorubicin. Glucuronides of epirubicin and epirubicinol were found only in human hepatocytes. In addition, several unidentified metabolites were detected of esorubicin, idarubicin and daunorubicin in rat hepatocytes. In human hepatocytes, only one unknown metabolite from daunorubicin and doxorubicin was found to be formed by cells from a different donor. In spite of variations between individuals, human hepatocytes generally metabolized anthracyclines more actively than did rat hepatocytes. Rat liver epithelial cells were only able to convert daunorubicin and idarubicin, the two molecules which have the best affinity for the non-specific NADPH-dependent aldoketoreductase system. Three compounds (doxorubicin, epirubicin and esorubicin) were present in large amounts in the cells as the parent drug, another (idarubicin) as the 13-dihydro-derivative. This comparative study on cytotoxicity and metabolism of five anthracyclines in rat and human hepatocyte cultures emphasises species differences and the importance of this in vitro model system for further analysis of the metabolism and effect of anthracyclines.

Adriamycin and its iron(III) and copper(II) complexes. Glutathione-induced dissociation; cytochrome c oxidase inactivation and protection; binding to cardiolipin

Some reactions of adriamycin (doxorubicin) and its Fe3+ and Cu2+ complexes were investigated with a view to understanding the mechanisms by which metal ion-adriamycin complexes damage cellular components. The ability of adriamycin in the presence of Cu2+ to inactivate the mitochondrial enzyme cytochrome c oxidase was effectively prevented by physiologic levels of glutathione. This result is explained by the observation that glutathione reacts with the Cu2+-adriamycin complex to produce free adriamycin. As sulfhydryl compounds are, in contrast, known to promote Fe3+-adriamycin-induced damage to cellular components, these results suggest that the response of a metal ion-adriamycin system to the presence of sulfhydryl compounds may be indicative of whether or not Cu2+-adriamycin is the damaging species. The partition of adriamycin into the octanol phase of an octanol-water two-phase system was greatly enhanced by the presence of cardiolipin. This result can be explained by the formation of a strong adriamycin-cardiolipin complex in the octanol phase which is one-half formed at an adriamycin concentration of 6 microM.

Adriamycin-induced lipid peroxidation in mitochondria and microsomes

The effect of the anti-neoplastic agent adriamycin on the peroxidation of lipids from rat liver and heart mitochondria and rat liver microsomes was investigated. The extent of total lipid peroxidation was determined by assaying for malondialdehyde (MDA), while the degradation of unsaturated fatty acids was monitored using gas chromatography. For liver mitochondria and microsomes, the formation of MDA was dependent on the concentrations of adriamycin, Fe3+, and protein, as well as time. In the presence of 50 microM adriamycin and saturating amounts of NADH, 1.5 +/- 0.2 nmol MDA/mg protein/60 min was produced with liver mitochondria. Upon addition of 25 microM Fe3+, the amount of MDA generated was increased to 6.5 +/- 0.1 nmol/mg protein/60 min. Liver microsomes produced amounts which were approximately 2-fold higher under all conditions. No MDA formation could be detected in rat heart mitochondria. The addition of 50 microM chlorpromazine completely inhibited peroxidation, whereas 0.5 to 1.0 mM p-bromophenacyl bromide blocked MDA formation by 50%. Analysis of fatty acids by gas chromatography showed that there was about a 50% decrease in arachidonic and docosahexaenoic acids in liver mitochondria and microsomes, but no change in the fatty acid content of heart mitochondria when incubated with both 50 microM adriamycin and 25 microM Fe3+ for 1 hr. These results suggest that (1) therapeutic concentrations of adriamycin enhance the peroxidation of lipids in liver mitochondria and microsomes through an enzymatic mechanism, especially in the presence of Fe3+; and (2) toxicity of this drug may be related to the degradation of membrane lipids.

DT-diaphorase activity and the cytotoxicity of quinones in C3H/10T1/2 mouse embryo cells

A permanent mouse fibroblast cell line derived from C3H mouse embryos, C3H/10T1/2 C18, was used to study the cytotoxicity of some model quinones under conditions in which DT-diaphorase (EC 1.6.99.2) activity was induced or inhibited. Sudan III [1-[[4-(phenylazo)phenyl]azo]-2-naphthalenol] and 3-methylcholanthrene (MCA), but not butylated hydroxyanisole (BHA), induced DT-diaphorase in a concentration-dependent manner. Induction of DT-diaphorase activity was dependent upon new RNA and protein synthesis, as shown by experiments employing actinomycin D and cycloheximide respectively. Induction of DT-diaphorase by Sudan III or MCA was associated with protection against the cytotoxicity of quinones as measured by a colony survival assay. When control and induced cells were also exposed to dicoumarol, a specific and potent inhibitor of DT-diaphorase, the cytotoxicity of the quinones in both control and induced cells was enhanced markedly. The results support the hypothesis that DT-diaphorase competes with one-electron quinone-reducing enzymes (such as cytochrome P-450 reductase) which generate auto-oxidizable semiquinones and forms more stable hydroquinones as an initial step in the detoxification of quinones in 10T1/2 cells.

Effects of adriamycin on respiratory chain activities in mitochondria from rat liver, rat heart and bovine heart. Evidence for a preferential inhibition of complex III and IV

The inhibition of respiratory chain activities in rat liver, rat heart and bovine heart mitochondria by the anthracycline antibiotic adriamycin was measured in order to determine the adriamycin-sensitive sites. It appeared that complex III and IV are efficiently affected such that their activities were reduced to 50% of control values at 175 +/- 25 microM adriamycin. Complex I displayed a minor sensitivity to the drug. Of the complex-I-related activities tested, only duroquinone oxidation appeared sensitive (50% inhibition at approx. 450 microM adriamycin). Electron-transfer activities catalyzed by complex II remained essentially unaltered up to high drug concentrations. Of the activities measured for this complex, only duroquinone oxidation was significantly affected. However, the adriamycin concentration required to reduce this activity to 50% exceeded 1 mM. Mitochondria isolated from rat liver, rat heart and bovine heart behaved essentially identical in their response to adriamycin. These data support the conclusion that, in these three mitochondrial systems, the major drug-sensitive sites lie in complex III and IV. Cytochrome c oxidase and succinate oxidase activity in whole mitochondria exhibited a similar sensitivity towards adriamycin, as inner membrane ghosts, suggesting that the drug has direct access to its inner membrane target sites irrespective of the presence of the outer membrane. By measuring NADH and succinate oxidase activities in the presence of exogenously added cytochrome c, it appeared that adriamycin was less inhibitory under these conditions. This suggests that adriamycin competes with cytochrome c for binding to the same site on the inner membrane, presumably cardiolipin.

Intracellular proteolytic systems may function as secondary antioxidant defenses: an hypothesis

In recent years it has become clear that various free radicals and related oxidants can cause serious damage to intracellular enzymes and other proteins. Several investigators have shown that in extreme cases this can result in an accumulation of oxidatively damaged proteins as useless cellular debris. In other instances, proteins may undergo scission reactions with certain radicals/oxidants, resulting in the direct formation of potentially toxic peptide fragments. Data has also been gathered (recently) demonstrating that various intracellular proteolytic enzymes or systems can recognize, and preferentially degrade, oxidatively damaged proteins (to amino acids). In this hypothesis paper I present evidence to suggest that proteolytic systems (of proteinases, proteases, and peptidases) may function to prevent the formation or accumulation of oxidatively damaged protein aggregates. Proteolytic systems can also preferentially degrade peptide fragments and may thus prevent a wide variety of potentially toxic consequences. I propose that many proteolytic enzymes may be important components of overall antioxidant defenses because they can act to ameliorate the consequences of oxidative damage. A modified terminology is suggested in which the primary antioxidants are such agents as vitamin E, beta-carotene, and uric acid and such enzymes as superoxide dismutase, glutathione peroxidase, and DT-diaphorase. In this classification scheme, proteolytic systems, DNA repair systems, and certain lipolytic enzymes would be considered as secondary antioxidant defenses. As secondary antioxidant defenses, proteolytic systems may be particularly important in times of high oxidative stress, during periods of (primary) antioxidant insufficiency, or with advancing age.

Pulse radiolysis study of daunorubicin redox reactions: redox cycles or glycosidic cleavage?

Two aspects of daunorubicin reactivity were investigated by pulse radiolysis. The reactions of O2 and O2- with the semiquinone and the hydroquinone transients of daunorubicin were determined and their rate constants measured. Although O2- can reduce the drug and its semiquinone form, it is a more powerful oxidant towards the two reduced transients. The hydroquinone daunorubicin glycosidic cleavage in aqueous solution was studied. Three intermediates were seen and characterized by their absorption spectra, their formation and decay kinetics. The competition between these two main processes was evaluated in the conditions of pulse radiolysis. Even under low O2 partial pressures the redox cycles are much more rapid than the glycosidic cleavage and a relatively high O2- steady state is settled. Biological implications are discussed.