Differences in O2 reduction by the iron complexes of adriamycin and daunomycin: the importance of the sidechain hydroxyl group

Role of Cerium Oxide Nanoparticles and Doxorubicin in Improving Cancer Management: A Mini Review

Cancer is one of the significant issues with public health and the second leading cause of death worldwide. The three most lethal cancers in the general population are stomach, lung, and liver cancers, in which lung and breast cancers cause the majority of cancer-associated deaths among men and women, respectively. CeO2 nanoparticles have a cytoprotectant effect in normal cells and a cytotoxic effect in cancer cells that enables them to induce the reactive oxygen species (ROS) production within cancer cells, which in turn develops reactive nitrogen species (RNS) that interfere with intracellular activities, and this property makes them an excellent anticancer agent. Because of its biofilm suppression, free radical scavenging ability, redox activity, and other unique properties, attention has been bestowed on cerium oxide nanoparticles as a potential alternative to solve many biomedical issues in the future. This review mainly focuses on the combinatorial effect of cerium dioxide nanoparticles and Doxorubicin in cancer management.

Influence of lipophilicity of anthracyclines on the interactions with cholesterol in the model cell membranes - Langmuir monolayer and SEIRAS studies

The interactions of anthracyclines with biological membranes strongly depend on the drug lipophilicity, which might also determine the specific affinity to cholesterol molecules. Therefore, in this work we show the studies concerning the effect of two selected anthracyclines, daunorubicin (DNR) and idarubicin (IDA) on simple models of healthy (DMPC:Chol 7:3) and cancer cells membranes with increased level of cholesterol (DMPC:Chol 3:7) as well as pure cholesterol monolayers prepared at the air-water interface and supported on gold surface. It has been shown that more lipophilic IDA is able to penetrate cholesterol monolayers more effectively than DNR due to the formation of IDA-cholesterol arrangements at the interface, as proved by the thermodynamic analysis of compression-expansion cycles. The increased interactions of IDA were also confirmed by the time measurements of pre-compressed monolayers exposed to drug solutions as well as grazing incidence X-ray diffraction studies demonstrating differences in the 2D organization of cholesterol monolayers. Langmuir studies of mixed DMPC:Chol membranes revealed the reorganization of molecules in the cancer cell models at the air-water interface at higher surface pressures due to the removal of DNR, while increased affinity of IDA towards cholesterol allowed this drug to penetrate the layer more efficiently without its removal. The SEIRAS spectra obtained for supported DMPC:Chol bilayers proved that IDA locates both in the ester group and in the acyl chain region of the bilayer, while DNR does not penetrate the membranes as deeply as IDA. The increased penetration of the mixed phospholipid layers by idarubicin might be attributed to the higher lipophilicity caused by the lack of methoxy group and resulting in a specific affinity towards cholesterol.

A comprehensive review on time-tested anticancer drug doxorubicin

Doxorubicin or Adriamycin, is one of the most widely used chemotherapeutic drug for treating a myriad of cancers. It induces cell death through multiple intracellular targets: reactive oxygen species generation, DNA-adduct formation, topoisomerase II inhibition, histone eviction, Ca²⁺ and iron hemostasis regulation, and ceramide overproduction. Moreover, doxorubicin-treated dying cells undergo cellular modifications that enable neighboring dendritic cell activation and enhanced presentation of tumor antigen. In addition, doxorubicin also aids in the immune-mediated clearance of tumor cells. However, the development of chemoresistance and cardiotoxicity side effect has undermined its widespread applicability. Several formulations of doxorubicin and co-treatments with inhibitors, miRNAs, natural compounds and other chemotherapeutic drugs have been essential in reducing its dosage-dependent toxicity and combating the development of resistance. Further, more advanced research into the molecular mechanism of chemoresistance development would be vital in improving the overall survivability of clinical patients and in preventing cancer relapse.

Inhibition of glycolysis disrupts cellular antioxidant defense and sensitizes HepG2 cells to doxorubicin treatment

Increased glucose consumption is a known hallmark of cancer cells. Increased glycolysis provides ATP, reducing agents and substrates for macromolecular synthesis in intensely dividing cells. Therefore, inhibition of glycolysis is one strategy in anticancer therapy as well as in improved efficacy of conventional anticancer chemotherapeutic agents. One such agent is doxorubicin (DOX), but the mechanism of sensitization of tumor cells to DOX by inhibition of glycolysis has not been fully elucidated. As oxidative stress is an important phenomenon accompanying DOX action and antioxidant defense is closely related to energy metabolism, the aim of the study was the evaluation of oxidative stress markers and antioxidant abilities of cancer cells treated with DOX while glycolysis is inhibited. HepG2 cells were treated with DOX and one of three glycolysis inhibitors: 2-deoxyglucose, dichloroacetate or 3-promopyruvate. To evaluate the possible interaction mechanisms, we assessed mRNA expression of selected genes related to energy metabolism and antioxidant defense; oxidative stress markers; and reduced glutathione (GSH) and NADPH levels. Additionally, glutamine consumption was measured. It was demonstrated that the chemotherapeutic agent and glycolysis inhibitors induced oxidative stress and associated damage in HepG2 cells. However, simultaneous treatment with both agents resulted in even greater lipid peroxidation and a significant reduction in GSH and NADPH levels. Moreover, in the presence of the drug and an inhibitor, HepG2 cells had a reduced ability to take up glutamine. These results indicated that cells treated with DOX while glycolysis was inhibited had significantly reduced ability to produce NADPH and antioxidant defenses.

Rethinking Drugs from Chemistry to Therapeutic Opportunities: Pixantrone beyond Anthracyclines

Pixantrone (6,9-bis[(2-aminoethyl)amino]benzo[g]isoquinoline-5,10-dione) has been approved by the European Medicines Agency for the treatment of refractory or relapsed non-Hodgkin's lymphoma (NHL). It is popularly referred to as a novel aza-anthracenedione, and as such it is grouped with anthracycline-like drugs. Preclinical development of pixantrone was in fact tailored to retain the same antitumor activity as that of anthracyclines or other anthracenediones while also avoiding cardiotoxicity that dose-limits clinical use of anthracycline-like drugs. Preliminary data in laboratory animals showed that pixantrone was active, primarily in hematologic malignancies, but caused significantly less cardiotoxicity than doxorubicin or mitoxantrone. Pixantrone was cardiac tolerable also in animals pretreated with doxorubicin, which anticipated a therapeutic niche for pixantrone to treat patients with a history of prior exposure to anthracyclines. This is the case for patients with refractory/relapsed NHL. Pixantrone clinical development, regulatory approval, and penetration in clinical practice were nonetheless laborious if not similar to a rocky road. Structural and nominal similarities with mitoxantrone and anthracyclines may have caused a negative influence, possibly leading to a general perception that pixantrone is a "me-too" anthracycline. Recent insights suggest this is not the case. Pixantrone shows pharmacological and toxicological mechanisms of action that are difficult to reconcile with anthracycline-like drugs. Pixantrone is a new drug with its own characteristics. For example, pixantrone causes mis-segregation of genomic material in cancer cells and inhibits formation of toxic anthracycline metabolites in cardiac cells. Understanding the differences between pixantrone and anthracyclines or mitoxantrone may help one to appreciate how it worked in the phase 3 study that led to its approval in Europe and how it might work in many more patients in everyday clinical practice, were it properly perceived as a drug with its own characteristics and therapeutic potential. The road is rocky but not a dead-end.

Mitochondrial catastrophe during doxorubicin-induced cardiotoxicity: a review of the protective role of melatonin

Anthracyclines, such as doxorubicin, are among the most valuable treatments for various cancers, but their clinical use is limited due to detrimental side effects such as cardiotoxicity. Doxorubicin-induced cardiotoxicity is emerging as a critical issue among cancer survivors and is an area of much significance to the field of cardio-oncology. Abnormalities in mitochondrial functions such as defects in the respiratory chain, decreased adenosine triphosphate production, mitochondrial DNA damage, modulation of mitochondrial sirtuin activity and free radical formation have all been suggested as the primary causative factors in the pathogenesis of doxorubicin-induced cardiotoxicity. Melatonin is a potent antioxidant, is nontoxic, and has been shown to influence mitochondrial homeostasis and function. Although a number of studies support the mitochondrial protective role of melatonin, the exact mechanisms by which melatonin confers mitochondrial protection in the context of doxorubicin-induced cardiotoxicity remain to be elucidated. This review focuses on the role of melatonin on doxorubicin-induced bioenergetic failure, free radical generation, and cell death. A further aim is to highlight other mitochondrial parameters such as mitophagy, autophagy, mitochondrial fission and fusion, and mitochondrial sirtuin activity, which lack evidence to support the role of melatonin in the context of cardiotoxicity.

Dexrazoxane for the prevention of cardiac toxicity and treatment of extravasation injury from the anthracycline antibiotics

The cumulative cardiac toxicity of the anthracycline antibiotics and their propensity to produce severe tissue injury following extravasation from a peripheral vein during intravenous administration remain significant problems in clinical oncologic practice. Understanding of the free radical metabolism of these drugs and their interactions with iron proteins led to the development of dexrazoxane, an analogue of EDTA with intrinsic antineoplastic activity as well as strong iron binding properties, as both a prospective cardioprotective therapy for patients receiving anthracyclines and as an effective treatment for anthracycline extravasations. In this review, the molecular mechanisms by which the anthracyclines generate reactive oxygen species and interact with intracellular iron are examined to understand the cardioprotective mechanism of action of dexrazoxane and its ability to protect the subcutaneous tissues from anthracycline-induced tissue necrosis.

Attenuation of doxorubicin-induced cardiac injury by mitochondrial glutaredoxin 2

While the cardiotoxicity of doxorubicin (DOX) is known to be partly mediated through the generation of reactive oxygen species (ROS), the biochemical mechanisms by which ROS damage cardiomyocytes remain to be determined. This study investigates whether S-glutathionylation of mitochondrial proteins plays a role in DOX-induced myocardial injury using a line of transgenic mice expressing the human mitochondrial glutaredoxin 2 (Glrx2), a thiotransferase catalyzing the reduction as well as formation of protein-glutathione mixed disulfides, in cardiomyocytes. The total glutaredoxin (Glrx) activity was increased by 76% and 53 fold in homogenates of whole heart and isolated heart mitochondria of Glrx2 transgenic mice, respectively, compared to those of nontransgenic mice. The expression of other antioxidant enzymes, with the exception of glutaredoxin 1, was unaltered. Overexpression of Glrx2 completely prevents DOX-induced decreases in NAD- and FAD-linked state 3 respiration and respiratory control ratio (RCR) in heart mitochondria at days 1 and 5 of treatment. The extent of DOX-induced decline in left ventricular function and release of creatine kinase into circulation at day 5 of treatment was also greatly attenuated in Glrx2 transgenic mice. Further studies revealed that heart mitochondria overexpressing Glrx2 released less cytochrome c than did controls in response to treatment with tBid or a peptide encompassing the BH3 domain of Bid. Development of tolerance to DOX toxicity in transgenic mice is also associated with an increase in protein S-glutathionylation in heart mitochondria. Taken together, these results imply that S-glutathionylation of heart mitochondrial proteins plays a role in preventing DOX-induced cardiac injury.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

The oral iron chelator ICL670A (deferasirox) does not protect myocytes against doxorubicin

The oral iron chelating agent ICL670A (deferasirox) and the clinically approved cardioprotective agent dexrazoxane (ICRF-187) were compared for their ability to protect neonatal rat cardiac myocytes from doxorubicin-induced damage. Doxorubicin is thought to induce oxidative stress on the heart muscle through iron-mediated oxygen radical damage. While dexrazoxane was able to protect myocytes from doxorubicin-induced lactate dehydrogenase release, ICL670A, in contrast, depending upon the concentration, synergistically increased or did not affect the cytotoxicity of doxorubicin. This occurred in spite of the fact that ICL670A quickly and efficiently removed iron(III) from its complex with doxorubicin, and rapidly entered myocytes and displaced iron from a fluorescence-quenched trapped intracellular iron-calcein complex. Continuous exposure of ICL670A to either myocytes or Chinese hamster ovary (CHO) cells resulted in cytotoxicity while treatment of CHO cells with the ferric complex of ICL670A did not. These results suggest that ICL670A was cytotoxic either by removing or withholding iron from critical iron-containing proteins. Electron paramagnetic resonance spectroscopy was used to show that neither ICL670A nor its ferric complex were able to generate free radicals in either oxidizing or reducing systems suggesting that its cytotoxicity is not due to radical generation.

Prevention of doxorubicin-induced damage to rat heart myocytes by arginine analog nitric oxide synthase inhibitors and their enantiomers

The clinical use of the widely used anticancer drug doxorubicin is limited by a dose-dependent cardiotoxicity. Doxorubicin can be reduced to its semiquinone free radical form by nitric oxide synthases (NOS). The release of lactate dehydrogenase (LDH) from doxorubicin-treated neonatal cardiac rat myocytes was used as a model of doxorubicin-induced cardiotoxicity. The NOS inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-monomethyl-L-arginine (L-NMMA) protected myocytes from doxorubicin as did their non-inhibitory enantiomers D-NAME and D-NMMA. Thus, these agents did not protect by inhibiting NOS. L-NAME, which does not act at the reductase domain of NOS, also had no effect on the production of the doxorubicin semiquinone by myocytes. Nitric oxide (NO) EPR spin trapping experiments showed that L-NAME reacted with various biological reducing agents to produce NO. Ascorbic acid was highly effective in reacting with L-NAME to produce NO, while glutathione, NADPH, and NADH were much less effective. Thus, these guanadino-substituted analogs of L-arginine likely protected through their ability to slowly produce NO by reaction with intracellular ascorbic acid. Thus, some caution must be exercised in their use. NO may exert its protective effects either by directly acting as an antioxidant or through some other NO-dependent pathway.

Dexrazoxane in combination with anthracyclines lead to a synergistic cytotoxic response in acute myelogenous leukemia cell lines

In an attempt to improve current therapeutic strategies for acute myelogenous leukemia (AML), we studied the effects of a commercially available drug, dexrazoxane (DEX), which protects against anthracycline-induced cardiotoxicity. The rationale was that DEX would permit higher doses of cardiotoxic drugs to be given. The drug itself may have therapeutic potential as well. Finally, there are concerns that the drug may, as a protective agent, diminish the effectiveness of various chemotherapeutics. To help resolve the question about potential drug antagonism, we undertook a series of in vitro analyses of DEX and various combinations with anthracyclines and other agents. Colony-forming assays were used to evaluate stem-cell renewal of myeloid cells in vitro, and median-effect analysis was used to evaluate antagonism, synergism, and additivity. The anthracyclines doxorubicin, daunorubicin, and idarubicin were individually combined with DEX to study in vitro effects in leukemic myeloid cell lines. In the hope, we could extend the findings to non-anthracyclines, etoposide and cytosine arabinoside were also evaluated in combination with DEX using the same in vitro model and method. We found that the effects of DEX in combination with any of the anthracyclines were schedule dependent. The antitumor effect was greater for each combination than for any anthracycline alone except when DEX was administered 24h before doxorubicin or daunorubicin. These data were corroborated through median-effect analysis. Etoposide in combination with DEX was synergistic for all combinations and schedules, and the combination of cytosine arabinoside and DEX was effective depending on the schedule used. DEX appears to be a promising drug in the treatment of AML and warrants further clinical study involving novel drug combinations.

Deferiprone protects against doxorubicin-induced myocyte cytotoxicity

The iron chelating hydroxypyridinone deferiprone (CP20, L1) and the clinically approved cardioprotective agent dexrazoxane (ICRF-187) were examined for their ability to protect neonatal rat cardiac myocytes from doxorubicin-induced damage. Doxorubicin is thought to induce oxidative stress on the heart muscle, both through reductive activation to its semiquinone form, and by the production of hydroxyl radicals mediated by its complex with iron. The results of this study showed that both deferiprone and dexrazoxane were able to protect myocytes from doxorubicin-induced lactate dehydrogenase release. Deferiprone quickly and efficiently removed iron(III) from its complex with doxorubicin. In addition, this study also showed that deferiprone rapidly entered myocytes and displaced iron from a fluorescence-quenched trapped intracellular iron-calcein complex, suggesting that in the myocyte, deferiprone should also be able to displace iron from its complex with doxorubicin. It was shown by electron paramagnetic resonance spectroscopy that under hypoxic conditions myocytes were able to reduce doxorubicin to its semiquinone free radical. Deferiprone also greatly reduced hydroxyl radical production by the iron(III)-doxorubicin complex in the xanthine oxidase/xanthine superoxide generating system. Together these results suggest that deferiprone may protect against doxorubicin-induced damage to myocytes by displacing iron bound to doxorubicin, or chelating free or loosely bound iron, thus preventing site-specific iron-based oxygen radical damage.

Anthracycline-induced cardiomyopathy: long-term effects on myocardial cell integrity, cardiac adrenergic innervation and fatty acid uptake

Cardiotoxicity of anthracyclines is a clinical challenge in cancer chemotherapy. Limited data is available on the physiological mechanisms responsible for anthracycline-induced heart failure or its recovery. We studied four patients with a history of severe anthracycline-induced heart failure manifested 2-116 months earlier by using radionuclide ventriculography for the measurement of left ventricular function, indium-111-antimyosin scintigraphy for the detection of myocardial cell injury and iodine-123-metaiodobenzylguanidine (MIBG) scintigraphy for the assessment of cardiac adrenergic innervation. Myocardial perfusion and fatty acid utilization were assessed with iodine-123-paraphenyl pentadecanoid acid (pPPA) and single photon emission computed tomography (SPECT). Symptoms of congestive heart failure (CHF) were still present in two patients whereas the others were asymptomatic at the time of the study. The patients who showed complete clinical recovery had normal or near normal left ventricular ejection fraction (LVEF) (47 and 52%), whereas the patients with symptoms of heart failure had low ejection fractions (21 and 31%). All patients presented with abnormal antimyosin uptake and decreased myocardial MIBG uptake. Patients with low ejection fraction tended to have higher antimyosin uptake suggesting more severe, persistent myocyte injury. All but one patient showed normal fatty acid utilization. These data suggest that patients with a history of severe anthracycline-induced cardiomyopathy have persistent myocardial cell injury and adrenergic dysfunction up to 10 years after the development of heart failure. These findings seem to be present regardless of recovery of left ventricular function.

Dexrazoxane (ICRF-187)

1. Dexrazoxane (ICRF-187) is the only clinically approved drug for use in cancer patients to prevent anthracycline mediated cardiotoxicity. 2. The mode of action appears to be mainly due to the potential of the drug to remove iron from iron/anthracycline complexes and thus reduce free radical formation by these complexes. 3. Dexrazoxane also influences cell biology by its ability to inhibit topoisomerase II and its effects on the regulation of cellular iron homeostasis. 4. Although the cardioprotective effect of dexrazoxane in cancer patients undergoing chemotherapy with anthracyclines is well documented, the potential of this drug to modulate topoisomerase II activity and cellular iron metabolism may hold the key for future applications of dexrazoxane in cancer therapy, immunology, or infectious diseases.

Modulation of transferrin receptor expression by dexrazoxane (ICRF-187) via activation of iron regulatory protein

Dexrazoxane (ICRF-187) has recently been demonstrated to reduce cardiac toxicity induced by chemotherapy with anthracyclines, although the reason for this phenomenon has remained obscure thus far. In order to investigate whether ICRF-187 might exert its effects by modulating iron metabolism, we studied the drug's potential to influence the maintenance of iron homeostasis in two human cell lines. We demonstrate that ICRF-187 enhanced the binding affinity of iron regulatory protein (IRP), the central regulatory factor for posttranscriptional iron regulation, to RNA stem loop structures, called iron responsive elements (IRE), in THP-1 myelomonocytic as well as K562 erythroleukemic cells. Increased IRE/IRP interaction was paralleled by an elevation of transferrin receptor (trf-rec) mRNA levels which, according to the well-established mechanism of posttranscriptional iron regulation, was likely due to stabilisation of trf-rec mRNA by IRP. Subsequently, ICRF-187 treatment of cells increased trf-rec surface expression and enhanced cellular iron uptake. All these events, i.e. IRP activation, stabilisation of trf-rec mRNA and increased surface expression of the protein in response to ICRF-187, follow a dose-response relationship. Increased cellular uptake and sequestration of iron in response to ICRF-187 may contribute to the protective activity of ICRF-187 by reducing the iron-anthracycline complex and iron-catalysed generation of hydroxyl radicals via the Haber-Weiss reaction.

The effects of adriamycin and adriamycin complexes with transitional metals on Ca(2+)-dependent K+ channels of human erythrocytes

The influence of adriamycin (ADR) and ADR complexes with transitional metals Fe2+, Cu2+ and Co2+ on Ca(2+)-dependent K+ channels of human erythrocytes was investigated. We show that the anthracycline moiety of ADR increases Ca(2+)-dependent K+ efflux from erythrocytes, induced by low concentrations of propranolol, while the whole molecule of ADR has not any effect on Ca(2+)-dependent K+ channels, induced by propranolol or A23187 and on Pb(2+)-dependent K+ efflux. Ethidium bromide, verapamil and trifluoroperazine inhibited Ca(2+)-dependent K+ efflux, induced by high doses of propranolol. The anthracycline moiety of ADR is able to abolish blocking effect of ethidium bromide and verapamil, but does not influence the blocking effect of trifluoroperazine. We further show that ADR complexes with Fe2+, Cu2+ and Co2+ are potent inhibitors of Ca(2+)-dependent K+ efflux, induced by propranolol, but not of Pb(2+)-dependent K+ efflux. On the contrary, ADR-Fe3+ complex activates K(+)-permeability of human red blood cell. It is suggested that opposite effects of anthracycline moiety of ADR and ADR complexes with transitional metals on Ca(2+)-dependent K+ channels, induced by propranolol is due to their influence on the pathways of Ca2+ transport into cells, rather than their action directly on K+ channels.

Inhibition of anthracycline semiquinone formation by ICRF-187 (dexrazoxane) in cells

The formation of semiquinone free radicals of doxorubicin, epirubicin, daunorubicin, and idarubicin was measured by electron paramagnetic resonance (EPR) spectroscopy in hypoxic suspensions of chinese hamster ovary (CHO) cells. The amount of semiquinone produced was in the order idarubicin >> doxorubicin > daunorubicin > epirubicin. The idarubicin semiquinone signal was both the fastest to be formed and to decay. Idarubicin, which was the most lipophilic of the anthracyclines studied, also displayed the fastest fluorescence-measured cellular uptake of drug. Thus, it was concluded that semiquinone formation was dependent upon the rate of cellular uptake. Lysed cell suspensions were also shown to be capable of producing the doxorubicin semiquinone in the presence of added NADPH. The cardioprotective agent dexrazoxane (ICRF-187) was observed to decrease the amount of doxorubicin semiquinone observed in cell suspensions. Dexrazoxane also decreased the amount of doxorubicin semiquinone observed in the NADPH-lysed cell suspension mixture. Neither bipyridine nor deferoxamine decreased NADPH-dependent doxorubicin semiquinone formation. These results suggest that dexrazoxane does not decrease doxorubicin semiquinone formation through an iron complex formed from hydrolyzed dexrazoxane. Dexrazoxane may be inhibiting an NADPH-dependent enzyme.

Semiquinone free radical formation by daunorubicin aglycone incorporated into the cellular membranes of intact Chinese hamster ovary cells

The production of semiquinone free radicals has been measured by electron paramagnetic resonance spectroscopy (EPR) in Chinese hamster ovary cells in which 7-hydroxy daunorubicin aglycone had been incorporated. The highly lipophilic daunorubicin aglycone was incorporated into the cellular membrane by swirling a cell suspension over a thin layer of daunorubicin aglycone. Thus, the observed semiquinone free radical was likely formed directly in the lipophilic environment of the cellular membrane. The linewidth of the observed EPR signal suggested that a neutral protonated semiquinone species was formed. In the presence of the cell-impermeant paramagnetic line broadening agent chromium(III) oxalate, no detectable signal was observed. This result indicates that even though the semiquinone is embedded in the membrane, it is still partly accessible to the external chromium(III) oxalate. Analysis of chloroform extracts of the cells after EPR experiments indicated that daunorubicin aglycone was extensively metabolized. The results of a growth inhibition assay carried out on cells into which daunorubicin aglycone had been incorporated showed almost no effect on cell growth. This result indicates that in spite of significant daunorubicin aglycone-induced radical formation taking place directly in the cell membrane, little cell damage results.

Secondary alcohol metabolites mediate iron delocalization in cytosolic fractions of myocardial biopsies exposed to anticancer anthracyclines. Novel linkage between anthracycline metabolism and iron-induced cardiotoxicity

The cardiotoxicity of doxorubicin (DOX) and other quinone-containing antitumor anthracyclines has been tentatively attributed to the formation of drug semiquinones which generate superoxide anion and reduce ferritin-bound Fe(III), favoring the release of Fe(II) and its subsequent involvement in free radical reactions. In the present study NADPH- and DOX-supplemented cytosolic fractions from human myocardial biopsies are shown to support a two-step reaction favoring an alternative mechanism of Fe(II) mobilization. The first step is an enzymatic two-electron reduction of the C-13 carbonyl group in the side chain of DOX, yielding a secondary alcohol metabolite which is called doxorubicinol (3.9 +/- 0.4 nmoles/mg protein per 4 h, mean +/- SEM). The second step is a nonenzymatic and superoxide anion-independent redox coupling of a large fraction of doxorubicinol (3.2 +/- 0.4 nmol/mg protein per 4 h) with Fe(III)-binding proteins distinct from ferritin, regenerating stoichiometric amounts of DOX, and mobilizing a twofold excess of Fe(II) ions (6.1 +/- 0.7 nmol/mg protein per 4 h). The formation of secondary alcohol metabolites decreases significantly (Pi < 0.01) when DOX is replaced by less cardiotoxic anthracyclines such as daunorubicin, 4'-epi DOX, and 4-demethoxy daunorubicin (2.1 +/- 0.1, 1.2 +/- 0.2, and 0.6 +/- 0.2 nmol/mg protein per 4 h, respectively). Therefore, daunorubicin, 4'-epi DOX, and 4-demethoxy daunorubicin are significantly (P < 0.01) less effective than DOX in mobilizing Fe(II) (3.5 +/- 0.1, 1.8 +/- 0.2, and 0.9 +/- 0.3 nmol/mg protein per 4 h, respectively). These results highlight the formation of secondary alcohol metabolites and the availability of nonferritin sources of Fe(III) as novel and critical determinants of Fe(II) delocalization and cardiac damage by structurally distinct anthracyclines, thus providing alternative routes to the design of cardioprotectants for anthracycline-treated patients.

Modulation of the in vitro cardiotoxicity of doxorubicin by flavonoids

Cancer therapy with the anthracycline doxorubicin (Dox) is limited by cardiomyopathy, which develops in animals and patients after cumulative dosing. Generation of free radicals by Dox may be involved in this cardiotoxicity. Dox binds strongly to metal ions, especially iron(III). This Dox-metal complex stimulates the generation of free radicals through self-reduction of the complex. We investigated the possibility of inhibiting Dox-induced cardiotoxicity by scavenging of free radicals and/or chelating metal ions. The effects of Dox, both alone and in combination with iron-chelating agents, were studied on inotropy of the isolated mouse left atrium, lipid peroxidation (LPO) in cardiac microsomal membranes, ferricytochrome c (cyt.c3+) reduction, and oxygen consumption. The flavonoids 7-monohydroxyethylrutoside (mono-HER) and 7,3',4'-trihydroxyethylrutoside (tri-HER) and the ethylenediaminetetraacetic acid (EDTA) analogue ICRF-198 and its precursor ICRF-187 were used as iron-chelating agents. The latter were used for comparison since ICRF-187 has been reported to inhibit the cardiotoxic effects of Dox both in vitro and in vivo. Only the flavonoids could inhibit the negative inotropic effect of Dox (35 microM) on the mouse left atrium; in the presence of tri-HER (500 microM) the beating force decreased by 18% instead of 50%, whereas mono-HER completely prevented the Dox-induced negative inotropic effect. ICRF-198 and both flavonoids (500 microM) completely inhibited Dox (35 microM)-induced LPO, whereas ICRF-187 provided 65% inhibition. The observation that both cyt.c3+ reduction and oxygen consumption induced by the Dox-iron(III) complex (50/16.6 microM Dox3Fe3+) could be inhibited by superoxide dismutase proved the involvement of superoxide anions (O2-.). The iron-chelating agents (50 microM) inhibited cyt.c3+ reduction by 91% (mono-HER), 43% (tri-HER), and 100% (ICRF-198). Only mono-HER and ICRF-198 (50 microM) were capable of inhibiting the oxygen consumption by 70% and 43%, respectively. It is concluded that flavonoids offer a good perspective for further studies on the prevention of Dox-induced cardiomyopathy.

Enhancement of daunomycin toxicity by the differentiation inducer hexamethylene bisacetamide in erythroleukemia cells

Cytotoxic effects of daunomycin were investigated upon differentiation of Friend erythroleukemia cells induced with hexamethylene bisacetamide, a process during which a 20-fold increase in the hemoglobin content occurred. Daunomycin proved to be more toxic to differentiated Friend cells than to their undifferentiated counterparts. No changes in the daunomycin uptake rates of the two cell types were detectable. Externally added catalase and desferrioxamine mesylate protected against the additional cytotoxicity of daunomycin in differentiated cells, pointing to hydrogen peroxide and iron ions as mediators of the toxic effect. Daunomycin-dependent, cyanide-insensitive oxygen consumption of control and induced cells did not differ significantly, and the rate of formation of the daunomycin semiquinone radical electron paramagnetic resonance signal was similar in both cell types, indicating that the difference in toxicity was not due to increased drug activation by plasma membrane enzymes. Differentiated cells had a lowered catalase content; the cellular iron content was shown to increase by 2.8-fold upon cell differentiation, with hemoglobin-bound iron being about 50% of the total. Altogether, the results suggest increased intracellular hydrogen peroxide generation mediated by hemoglobin, combined with a decrease in catalase activity and an increase in accessible iron, as responsible for the higher sensitivity to daunomycin shown by differentiated Friend cells. This represents the first experimental system where the increase in anthracycline cytotoxicity upon cell differentiation can be attributed to redox activation and the formation of reactive oxygen species.

The MTT cell viability assay for cytotoxicity testing in multidrug-resistant human leukemic cells

The MTT cell viability assay is widely used in determining drug sensitivity profiles for patients with hematological malignancies and in primary screening of potential chemotherapeutic drugs. Because the multidrug resistance (MDR) phenotype is associated with these malignancies, and since many vital dyes are effluxed from MDR expressing cells, we have investigated whether the MDR phenotype interferes with the MTT assay. In CCRF-CEM and K562 human leukemic cell lines and drug-resistant sub-lines developed from them, comparison of the MTT assay with other cell viability assays showed significant variation in IC50 concentrations, although the resistance relative to the sensitive parent cell was correlated. Inclusion of verapamil, an inhibitor of drug efflux activity, had no effect on the MTT assay.

An enhanced ability for transforming adriamycin into a noncytotoxic form in a multidrug-resistant cell line (LZ-8)

Multidrug-resistant LZ-8 cells are 9000-fold more resistant to Adriamycin (ADRM) exposure than wild-type V79 cells. To understand more about the mechanisms producing such high level resistance, we tested whether LZ-8 cells inactivate ADRM toxicity to a greater extent than wild-type V79 cells. ADRM was recovered from (1) culture media of wild-type V79 and ADRM-resistant LZ-8 cells; (2) V79 and LZ-8 cells; and (3) LZ-8 cell plasma membrane, and the cytotoxicity was determined by treating V79 cells for 1 hr with a known concentration of the recovered ADRM. ADRM obtained from LZ-8 cells or its culture medium exhibited less cytotoxicity than that recovered from V79 cells or its culture medium. ADRM extracted from LZ-8 cell plasma membrane was noncytotoxic. HPLC analysis revealed that the extracted ADRM was structurally changed compared to stock ADRM. The retention time in the column was 7 min for stock ADRM, and 23 min for the recovered ADRM. Thus, LZ-8 cells have an increased ability to transform ADRM into a noncytotoxic form compared to wild-type V79 cells. This transformation involves structural conversion into a previously unidentified ADRM metabolite. The greatly increased survival of LZ-8 cells compared to V79 cells after ADRM treatment is due to at least two mechanisms: (1) an enhanced ability to inactivate the cytotoxicity of ADRM, and (2) increased drug efflux resulting from the amplification and overexpression of the pgp 1 gene in these cells. Our results suggest the possibility that P-glycoprotein participates in drug binding/inactivation in addition to serving as a drug efflux pump.

Evidence for polynuclear aggregates of ferric daunomycin. A Mössbauer, EPR, X-ray absorption spectroscopy and magnetic susceptibility study

The interaction of the antitumor agent daunomycin (DN) with ferric iron has been analysed by Mössbauer spectroscopy, EPR, extended X-ray absorption fine structure (EXAFS), and magnetic susceptibility measurements. In contrast to literature data, at millimolar iron and anthracycline concentrations no solitary Fe(DN)3 complexes are formed in appreciable amounts. The Mössbauer spectroscopic analysis revealed severe dependencies on temperature, on the preparation procedure, the time allowed for equilibration, and on the metal/ligand ratio. The Mössbauer spectra exhibit two components: a broad magnetic sextet and a quadrupole doublet at an Fe/DN molar ratio of 1:3 and exclusively a doublet at a molar ratio of 1:20, indicating an equilibrium of these two spectral components. The EPR spectra are dominated by a signal at g(eff) = 2. Double integration of the EPR signals enabled the determination of their spin density and a correlation between EPR and Mössbauer spectra. The Mössbauer sextet species is EPR invisible and corresponds to magnetically ordered polynuclear aggregates with high magnetic anisotropy. EXAFS and susceptibility measurements provide additional evidence for the formation of polynuclear aggregates of ferric daunomycin. The quadrupole doublet species in the Mössbauer spectra correlates with the g = 2 signal in EPR. This species is also related to a magnetically ordered system, exhibiting, however, superparamagnetic behavior due to less magnetic anisotropy. Since daunomycin forms dimers in aqueous solution at millimolar concentrations, we conclude that the cooperative phenomena observed in EPR and Mössbauer spectra are a consequence of its stacking effects.

Novel, quinone-thiosemicarbazone hybrid (QTSCHY) non-platinum antitumor agents: inhibition of DNA biosynthesis in P388 lymphocytic cells by coordinatively unsaturated copper(II) and iron(III) complexes of naphthoquinone thiosemicarbazones

Coordinately unsaturated Cu(II) and Fe(III) complexes of the stoichiometry [Cu(L)Cl] and [Fe(L)Cl2], where L = tridentate anion of 2-hydroxy-1,4-naphthoquinone 1-thiosemicarbazone (2HNQTSC) and its 3-methyl derivative (3M2HNQTSC), were screened in vitro against P388 lymphocytic leukemia cells. Copper complexes were found to be more effective inhibitors of DNA synthesis than analogous Fe(III) compounds. The inhibitory activities are suggested to be related to Cu(II)-Cu(I) redox couple or nitrogen adduct formation.

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.

Detection of oxygen-derived free radical generation in the canine postischemic heart during late phase of reperfusion

To define the relation between oxygen-derived free radical (oxy-radical) generation in the reperfused ischemic myocardium and the progression of myocardial damage, we measured oxy-radical generation in the ischemic myocardium and the propagating infarct size in a model of canine coronary occlusion (90 minutes) and reperfusion. We used electron paramagnetic resonance spin-trapping techniques (5,5-dimethyl-1-pyrroline N-oxide [DMPO]) to detect oxy-radicals in the rapidly frozen myocardial samples taken by needle biopsy. There was no detectable generation of DMPO adducts in the normal myocardium before or after reperfusion. In the reperfused ischemic myocardium, electron paramagnetic resonance signals of DMPO-OOH (superoxide anion) and DMPO-OH (hydroxyl radical) were detected, with peak concentrations at 1 hour after reperfusion for DMPO-OOH and at 3 hours after reperfusion for DMPO-OH, respectively. These DMPO adducts were also detected during the early phase (15 seconds) of reperfusion, but the concentrations of these signals were much less than those during the late phase of reperfusion. Treatment with human recombinant superoxide dismutase (2.5 mg/kg/hr) and catalase (2.5 mg/kg/hr) during the course of experiments abolished DMPO-OOH formation but had little effect on DMPO-OH formation. Infarct size (percent of risk area infarcted), quantified by a dual staining method with Evans blue dye and triphenyltetrazolium chloride, was 18.3 +/- 4.8% (mean +/- SEM) at 90 minutes of occlusion. After 5 hours of reperfusion, infarct size increased to 43.6 +/- 7.2%. These results indicate that a greater magnitude of oxy-radical generation was sustained in the ischemic myocardial tissue during the late phase (1-3 hours) of reperfusion, associated with the progression of myocardial infarction. The concurrent appearance of oxy-radicals and progressive infarction may support the view that a chain reaction of oxy-radicals contributes to the propagation of myocardial cell damage in the postischemic heart.

Inhibition and inactivation of NADH-cytochrome c reductase activity of bovine heart submitochondrial particles by the iron(III)-adriamycin complex

The NADH-cytochrome c reductase activity of bovine heart submitochondrial particles was found to be slowly (half-time of 16 min) and progressively lost upon incubation with the Fe2(+)-adriamycin complex. In addition to this slow progressive inactivation seen on incubation, a reversible fast phase of inhibition was also seen. However, if EDTA was added to the incubation mixture within 15 s, the slow progressive loss in activity was largely preventable. Separate experiments indicated that EDTA removed about one-half of the iron from the Fe2(+)-adriamycin complex in about 40 s. These results indicated the requirement for iron for the inactivation process. Since the Vmax. for the fast phase of inhibition was decreased by the inhibitor, the inhibition pattern was similar to that seen for uncompetitive or mixed-type inhibition. The direct binding of both Fe3(+)-adriamycin and adriamycin to submitochondrial particles was also demonstrated, with the Fe3(+)-adriamycin complex binding 8 times more strongly than adriamycin. Thus binding of Fe3(+)-adriamycin to the enzyme or to the inner mitochondrial membrane with subsequent generation of oxy radicals in situ is a possible mechanism for the Fe3(+)-adriamycin-induced inactivation of respiratory enzyme activity.

Role of the glutathione-glutathione peroxidase cycle in the cytotoxicity of the anticancer quinones

Recent studies have suggested that the selenoenzyme glutathione peroxidase, in the presence of reducing equivalents from the tripeptide glutathione, is responsible for detoxifying hydrogen peroxide and lipid hydroperoxides generated as a consequence of the cyclic reduction and oxidation of quinone-containing anticancer agents including doxorubicin, daunorubicin, mitomycin C, diaziquone, and menadione. Alterations in the intracellular levels of glutathione peroxidase or glutathione can significantly affect the activity of these drugs against human tumor cells and the expression of their normal tissue toxicity, especially with respect to the heart. Furthermore, augmentation of the glutathione peroxidase pathway appears to render certain human tumor cells relatively resistant to the anticancer quinones; therefore, the glutathione peroxidase system may, at least in part, modulate certain forms of acquired drug resistance in man. Thus, the glutathione peroxidase cycle appears to play a central role in maintaining intracellular peroxide homeostasis during quinone-induced oxidative stress.

Doxorubicin (adriamycin): a critical review of free radical-dependent mechanisms of cytotoxicity

The antineoplastic drug doxorubicin is capable of generating a variety of free radical species in subcellular systems and this capacity has been considered critical for its antitumor action. However, for most tumor cell lines this mechanism of cytotoxicity does not appear to play a major role. Free radical-independent cytotoxicity mechanisms, taking place in the nuclear compartment of the cell, may more likely be involved in the antitumor effect of doxorubicin.

A novel mechanism for the generation of superoxide anions in hematoporphyrin derivative-mediated cutaneous photosensitization. Activation of the xanthine oxidase pathway

Prior studies, both in vitro and in vivo, have suggested that cutaneous porphyrin photosensitization requires the generation of superoxide anion (.O2-) and various other reactive oxygen metabolites. No unifying concept has emerged, however, that unequivocally demonstrates the source of generation of these species. Since xanthine oxidase is known to generate .O2- in reperfused ischemic tissue and in certain inflammatory disorders, we attempted to assess its role in porphyrin photosensitization. C3H mice were rendered photosensitive by the intraperitoneal administration of dihematoporphyrin ether (DHE) (5 mg/kg) followed by irradiation with visible light. Murine ear swelling was used as a marker of the acute photosensitization response and involvement of oxygen radicals was evaluated using electron spin resonance (ESR) spectroscopy. The administration of allopurinol, a potent inhibitor of xanthine oxidase, afforded 90% protection against DHE-mediated acute photosensitivity in vivo. Furthermore, xanthine oxidase activity was twofold higher in the skin of photosensitized mice than in unirradiated animals. ESR spectra of 5,5-dimethyl-1-pyrroline N-oxide-trapped radicals from the skin of photosensitized mice verified the presence of .O2- and .OH, while neither of these species was detected in the skin of control mice or mice receiving allopurinol. The administration of a soybean trypsin inhibitor or verapamil before irradiation also partially blocked the photosensitivity response, suggesting that calcium-dependent proteases play a role in the activation of xanthine oxidase in this photodynamic process. These data provide in vivo evidence for the involvement of .O2- in DHE-mediated cutaneous photosensitization and suggest that these radicals are generated through the activation of the xanthine oxidase pathway. The administration of allopurinol and calcium channel blockers may thus offer new approaches for the treatment of cutaneous porphyrin photosensitization.

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.

Reperfusion injury

Several lines of evidence point to a major role of oxygen free radicals in the pathogenesis of cell death or dysfunction in a variety of disease processes. Recent studies from this as well as other laboratories have demonstrated that oxygen free radicals play a major role in the pathogenesis of post-ischemic reperfusion injury in the heart. We have recently developed methods for direct measurement of radical species and/or specific byproducts of radical injury. Timely administration of oxygen radical scavengers reduced the quantity of free radicals generated following reperfusion and in addition improved recovery of post-ischemic ventricular function and metabolism. In a regionally ischemic model the free radical scavenger recombinant human superoxide dismutase also administered at the time of reflow was shown to limit infarct size. In this article we review the biophysical and molecular mechanisms of oxygen free radical generation that are viewed as contributing to post-ischemic reperfusion injury. We also discuss the mechanisms by which the body defends against free radical attack and the interrelationships of free radical injury to other mechanisms of tissue injury.