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Department of Drug Sciences and Center of Excellence on Aging, G. d'Annunzio University School of Medicine, Chieti, Italy (G.M., P.M., E.S.); Institute of General Pathology, University of Milan School of Medicine, Milan, Italy (G.C.); and Unit of Medical Oncology, Istituto Nazionale Tumori, Milan, Italy (L.G.)
Abstract
Abstract I. Introduction II. Antitumor Activity of Anthracyclines A. General Considerations 1. Anthracyclines as Topoisomerase II Poisons. 2. Anthracyclines and Apoptosis: Role of DNA Damage and p53. B. Advances in DNA Damage by Anthracyclines 1. Role of the Proteasome. 2. Role of Free Radicals. 3. Lipid Peroxidation and DNA Damage: Malondialdehyde-DNA Adducts. 4. Oxidative Base Lesions as in Vivo Markers of Free Radical Formation and DNA Damage by Anthracyclines. 5. Anthracycline-Formaldehyde Conjugates and DNA Virtual Cross-Linking. 6. Anthracyclines and Telomeric DNA. III. Cardiotoxicity of Anthracyclines A. Morphology, Dose Dependence, Risk Factors B. Mechanisms 1. Advances in Apoptosis: in Vitro Studies. a. Doxorubicin, Iron, and Apoptosis: Role of Ferritin. b. Doxorubicin, Iron, and Apoptosis: Role of Cytoplasmic Aconitase/Iron Regulatory Protein-1. 2. Advances in Apoptosis: in Vivo Studies. 3. Multifactorial Processes in Chronic Cardiotoxicity. a. Pharmacokinetics of Secondary Alcohol Metabolites. b. Iron-Dependent and -Independent Mechanisms of Toxicity by Secondary Alcohol Metabolites. c. Unifying Mechanisms of Chronic Cardiomyopathy. C. Enhancement by Other Agents 1. Taxanes. 2. Trastuzumab. 3. Cyclooxygenase-2 Inhibitors. D. Prevention 1. Slow Infusion. 2. Antioxidants. 3. Iron Chelators (Dexrazoxane). E. Treatment IV. Toward a Better Anthracycline A. Tumor-Targeted Formulations 1. Liposomal Formulations. a. Polyethyleneglycol-Coated (''Pegylated'') Liposomal Doxorubicin. b. Uncoated Citrate-Containing Liposomal Doxorubicin. c. Liposomal Daunorubicin. d. Immunoliposomes. 2. Extracellularly Tumor-Activated Prodrugs 3. Polymer-Bound Doxorubicin B. Analogs 1. Nuclear-Targeted Anthracyclines. a. Morpholinyl Anthracyclines. b. Alkyl Anthracyclines. c. Disaccharide Anthracyclines. 2. Non-Nuclear-Targeted Anthracyclines: 14-O-Acyl-anthracyclines. V. Conclusions
The clinical use of anthracyclines like doxorubicin and daunorubicin can be viewed as a sort of double-edged sword. On the one hand, anthracyclines play an undisputed key role in the treatment of many neoplastic diseases; on the other hand, chronic administration of anthracyclines induces cardiomyopathy and congestive heart failure usually refractory to common medications. Second-generation analogs like epirubicin or idarubicin exhibit improvements in their therapeutic index, but the risk of inducing cardiomyopathy is not abated. It is because of their janus behavior (activity in tumors vis-à-vis toxicity in cardiomyocytes) that anthracyclines continue to attract the interest of preclinical and clinical investigations despite their longer-than-40-year record of longevity. Here we review recent progresses that may serve as a framework for reappraising the activity and toxicity of anthracyclines on basic and clinical pharmacology grounds. We review 1) new aspects of anthracycline-induced DNA damage in cancer cells; 2) the role of iron and free radicals as causative factors of apoptosis or other forms of cardiac damage; 3) molecular mechanisms of cardiotoxic synergism between anthracyclines and other anticancer agents; 4) the pharmacologic rationale and clinical recommendations for using cardioprotectants while not interfering with tumor response; 5) the development of tumor-targeted anthracycline formulations; and 6) the designing of third-generation analogs and their assessment in preclinical or clinical settings. An overview of these issues confirms that anthracyclines remain "evergreen" drugs with broad clinical indications but have still an improvable therapeutic index.
1Anthracyclines rank among the most effective anticancer drugs ever developed (Weiss, 1992
). The first anthracyclines were isolated from the pigment-producing Streptomyces peucetius early in the 1960s and were named doxorubicin (DOX1) and daunorubicin (DNR). As shown in Fig. 1, DOX and DNR share aglyconic and sugar moieties. The aglycone consists of a tetracyclic ring with adjacent quinone-hydroquinone groups in rings C-B, a methoxy substituent at C-4 in ring D, and a short side chain at C-9 with a carbonyl at C-13. The sugar, called daunosamine, is attached by a glycosidic bond to the C-7 of ring A and consists of a 3-amino-2,3,6-trideoxy-L-fucosyl moiety. The only difference between DOX and DNR is that the side chain of DOX terminates with a primary alcohol, whereas that of DNR terminates with a methyl. This minor difference has important consequences on the spectrum of activity of DOX and DNR. Whereas DOX is an essential component of treatment of breast cancer, childhood solid tumors, soft tissue sarcomas, and aggressive lymphomas, DNR shows activity in acute lymphoblastic or myeloblastic leukemias. As with any other anticancer agent, however, the clinical use of both DOX and DNR soon proved to be hampered by such serious problems as the development of resistance in tumor cells or toxicity in healthy tissues, most notably in the form of chronic cardiomyopathy and congestive heart failure (CHF). To avoid the latter, the maximum recommended cumulative doses of DNR and DOX were tentatively set at 500 or 450 to 600 mg/m2, respectively.
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The last 2 decades have witnessed numerous attempts to identifying novel anthracyclines that proved superior to DOX or DNR in terms of activity and/or cardiac tolerability (Weiss, 1992). The search for a "better anthracycline" has resulted in some 2000 analogs, a figure that should not sound like a surprise if one considers the number of chemical modifications or substitutions and/or conjugations that can be introduced in the tetracyclic ring, the side chain, or the aminosugar. Yet only few analogs have reached the stage of clinical development and approval; among them, epirubicin (EPI) and idarubicin (IDA) enjoy popularity as useful alternatives to DOX or DNR, respectively.
EPI is a semisynthetic derivative of DOX obtained by an axial-to-equatorial epimerization of the hydroxyl group at C-4' in daunosamine (see also Fig. 1). This positional change has little effect on the mode of action and spectrum of activity of EPI compared with DOX, but it introduces pharmacokinetic and metabolic changes like increased volume of distribution (Vd), 4-O-glucuronidation, and consequent enhanced total body clearance (CL) or shorter terminal half-life (Robert and Gianni, 1993; Danesi et al., 2002). It is because of these kinetic and metabolic changes that EPI was soon used at cumulative doses almost double those of DOX, resulting in equal activity but not in increased cardiotoxicity (Robert, 1993). In practice, early studies of patients with advanced breast cancer demonstrated that the median doses to the development of laboratory indices of cardiotoxicity were 935 mg/m2 EPI compared with 468 mg/m2 DOX, and the median cumulative dose to the development of symptomatic CHF was 1134 mg/m2 EPI compared with 492 mg/m2 DOX (Jain et al., 1985). These figures were refined in subsequent studies, since a significantly increasing risk of CHF was documented in patients who received cumulative doses greater than 950 mg/m2, and the recommended maximum cumulative dose of EPI was cautiously adjusted to 900 mg/m2 (Ryberg et al., 1998). Thus, replacing DOX with EPI does not eliminate the risk of developing chronic cardiotoxicity. It should also be noted that the mechanisms underlying the reduced cardiotoxicity of EPI versus DOX might not be confined to glucuronidation and increased elimination (see Section III.B.3.a.). The actual mechanisms and dose dependence of the improved cardiac tolerability of EPI may therefore require further assessment in both preclinical and clinical settings.
IDA, an analog obtained from DNR after removal of the 4-methoxy group in ring D, is active in acute myelogenous leukemia, multiple myeloma, non-Hodgkin's lymphoma, and breast cancer (Borchmann et al., 1997). The broader spectrum of activity of IDA compared with DNR may be attributed to increased lipophilicity and cellular uptake and improved stabilization of a ternary drug-topoisomerase II-DNA complex [a major mechanism of anthracycline activity (see Section II.A.1.) (Binaschi et al. (2001)]. In addition, IDA may be administered orally [with 
10 to 30% bioavailability (Toffoli et al., 2000)], and in vitro studies have indicated that it might be more effective than DNR in tumor cell lines displaying the multidrug resistance (MDR) phenotype (Toffoli et al., 1994; Jonsson-Videsater et al., 2003). There is some controversy about whether IDA offers advantages over DOX or DNR also in regard to cardiac toxicity. Some authors conclude that oral IDA does not induce cardiotoxicity (Borchmann et al., 1997; Lipp and Bokemeyer, 1999; Toffoli et al., 2000), not even in patients previously exposed to DOX or EPI (Toffoli et al., 1997); in contrast, others have shown that IDA decreases left ventricular ejection fraction (LVEF) in anthracycline-naïve patients, and causes CHF in patients with pre-existing cardiovascular disease or previous anthracycline treatment (Anderlini et al., 1995). Thus, the cardiac safety of IDA awaits further assessment in patients properly randomized in terms of cumulative dose and individual risk factors.
Only a few more anthracyclines have attained clinical approval; these include pirarubicin, aclacinomycin A (aclarubicin), and mitoxantrone (a substituted aglyconic anthraquinone) (Fig. 2). Both pirarubicin and aclarubicin demonstrate only modest improvements over DOX and DNR in terms of drug resistance (Lothstein et al., 2001). Pirarubicin, a 4-tetrahydropyranyl doxorubicin, has been reported to induce much less cardiotoxicity than DOX in animal models (Koh et al., 2002), but studies in women with metastatic breast cancer have indicated that it may cause significant decrease in LVEF or full-blown CHF at cumulative doses of 460 mg/m2 or >500 mg/m2, respectively (Dhingra et al., 1995). In elderly patients with non-Hodgkin's lymphoma, pirarubicin may cause severe cardiac dysfunction at cumulative doses as low as 360 mg/m2 (Niitsu et al., 1998). Aclarubicin, a trisaccharide anthracycline, was shown to be active and cardiac-tolerable in adult patients with acute myeloblastic leukemia (Case et al., 1987; Wojnar et al., 1989). However, aclarubicin induced late cardiac events in a phase II study of adult patients with refractory acute myelogenous or lymphoblastic leukemia (Dabich et al., 1986) and proved to be inactive in women with metastatic breast cancer (Natale et al., 1993).
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Mitoxantrone is active in breast cancer, acute promyelocytic or myelogenous leukemias, and androgen-independent prostate cancer. Early reports indicated that mitoxantrone was less cardiotoxic than other anthracyclines (Estorch et al., 1993), but this conclusion has been challenged in more recent studies (Thomas et al., 2002). Moreover, mitoxantrone causes chronic cardiotoxicity in patients with worsening relapsing-remitting or secondary progressive multiple sclerosis, a disease in which it showed activity worth of approval by the Food and Drug Administration (Gonsette, 2003).
These introductory remarks on the activity and toxicity of most commonly used anthracyclines are meant to indicate that a better anthracycline has yet to come. It is therefore not surprising that relatively old drugs like DOX and DNR remain the focus of clinical and preclinical research aimed at improving our appraisal of their mechanisms of activity or toxicity and at identifying new strategies for better use in cancer patients. Likewise, the search for new analogs or formulations continues unabated. In this review we will describe and discuss some recent advances in the fields of anthracycline activity and cardiotoxicity as well as recent developments in the pharmaceutical designing and pharmacological or clinical assessment of new analogs or formulations. In preparing for this, we considered that several seminal reviews have appeared over the last 2 or 3 years and focused on the same or closely related subjects. Authoritative analyses of the mechanisms of action of anthracyclines have been provided by Binaschi et al. (2001), Laurent and Jaffrezou (2001), Perego et al. (2001), and Kim et al. (2002b), among others. Molecular mechanisms and pharmacological or clinical correlates of anthracycline-induced cardiotoxicity have been reviewed by Myers (1998), Singal et al. (2000), Kalyanaraman et al. (2002), and Zucchi and Danesi (2003), among others. The pharmacokinetic-pharmacodynamic relationships of anthracycline activity or toxicity have been reviewed by Danesi et al. (2002), and progress in the pharmaceutical designing of new anthracyclines has been reviewed by Monneret (2001), among others. Finally, mechanisms of tumor resistance and possible methods for overcoming them have been reviewed by Benjamin et al. (2000), Tan et al. (2000), Lothstein et al. (2001), and Ejendal and Hrycyna (2002), among others. In apologizing to colleagues whose reviews are not cited because of the size restrictions of our present review or because of our personal ignorance, we will focus on selected advances that, to the best of our knowledge, were not addressed in previously published commentaries or have only recently surfaced to the literature. Moreover, we will concentrate on issues that have remained a matter of unsettled controversy and that we believe are important to accommodate in a unifying picture [e.g., the role of iron and oxidative damage in antitumor activity or cardiotoxicity, or the role of apoptosis in the settings of transient/benign versus chronic/life-threatening cardiotoxicity (Sections II. and III.)]. Wherever possible, advances in analogs or new formulations (Section IV.) will be discussed within the framework of accepted or controversial mechanisms described for antitumor activity or cardiotoxicity.
II. Antitumor Activity of Anthracyclines
Despite extensive clinical utilization, the mechanisms of action of anthracyclines in cancer cells remain a matter of controversy. In a seminal commentary the following mechanisms were considered: 1) intercalation into DNA, leading to inhibited synthesis of macromolecules; 2) generation of free radicals, leading to DNA damage or lipid peroxidation; 3) DNA binding and alkylation; 4) DNA cross-linking; 5) interference with DNA unwinding or DNA strand separation and helicase activity; 6) direct membrane effects; 7) initiation of DNA damage via inhibition of topoisomerase II; and 8) induction of apoptosis in response to topoisomerase II inhibition (Gewirtz, 1999). An important issue that was raised in that commentary pertained to the concentrations at which DOX and other anthracyclines exhibited a mode of action or another. In particular, it was pointed out that several in vitro experiments reported in the literature had been performed at concentrations of DOX which were considered too high compared with peak or steady-state plasma concentrations (Cmax, Css) observed in patients after standard bolus infusions (5 µM and 25-250 nM, respectively). It was therefore concluded that any study involving intact cells exposed to >1 to 2 µM DOX needed a cautionary re-evaluation. The same cautionary issue was considered in examining studies with subcellular fractions, often performed with submillimolar concentrations of anthracyclines (Gewirtz, 1999). When examined in this context, topoisomerase II remains an attractive and persuasive mechanism to explain the antitumor activity of DOX and other approved anthracyclines at clinically relevant concentrations.
1. Anthracyclines as Topoisomerase II Poisons.
Topoisomerases modify the topology of DNA without altering deoxynucleotide structure and sequence. They can cause transient single-strand (topoisomerase I) or double-strand (topoisomerase II) DNA breaks that are resealed after changing the twisting status of the double helix. This activity confers an important role on topoisomerases as the supercoiling of the DNA double helix is modulated according to the cell cycle phase and transcriptional activity (Binaschi et al., 2001).
Anthracyclines act by stabilizing a reaction intermediate in which DNA strands are cut and covalently linked to tyrosine residues of topoisomerase II, eventually impeding DNA resealing. The formation and stability of an anthracycline-DNA-topoisomerase II ternary complex rely on defined structural determinants. The planar ring system is important for intercalation into DNA, as rings B and C overlap with adjacent base pairs and ring D passes through the intercalation site. The external (nonintercalating) moieties of the anthracycline molecule (i.e., the sugar residue and the cyclohexane ring A) seem to play an important role in the formation and stabilization of the ternary complex. In particular, the sugar moiety, located in the minor groove, is a critical determinant of the activity of anthracyclines as topoisomerase II poisons. Topoisomerase II inhibition increases after removal of aminosubstituents at C-3' in the sugar or of the methoxy group at C-4 in ring D (as already mentioned for IDA); moreover, the nature of 3'-substituents greatly influences the sequence selectivity of anthracycline-stimulated DNA cleavage (Binaschi et al., 2000, 2001). Doxorubicin has been reported to also inhibit topoisomerase I, an effect shared by IDA and investigational IDA analogs bearing a disaccharide moiety in which the second sugar retains an axial orientation relative to the first one (Guano et al., 1999). The cell-killing activity of anthracyclines is weakly but significantly dependent on cellular topoisomerase I content, suggesting that inhibition of topoisomerase I may represent an ancillary mode of action of anthracyclines (Guano et al., 1999). Topoisomerase II-mediated DNA damage is followed by growth arrest in G1 and G2 and programmed cell death (Perego et al., 2001; Zunino et al., 2001). It follows that tumor cells may become resistant to anthracyclines because of altered topoisomerase II gene expression or activity (Lage et al., 2000). In clinical settings, the degree of apoptosis induction correlates with tumor response and patient's outcome (Buchholz et al., 2003).
2. Anthracyclines and Apoptosis: Role of DNA Damage and p53.
Doxorubicin, as many other genotoxic agents, activates p53-DNA binding. On the basis of the crucial role of p53 in the execution of some forms of apoptosis, it has been proposed that p53 could play an important function in anthracycline cytotoxicity. Preclinical and clinical studies support this concept (Penault-Llorca et al., 2003; Ruiz-Ruiz et al., 2003; Stearns et al., 2003), but negative reports have also appeared (Inoue et al., 2000; Perego et al., 2001; Bertheau et al., 2002; Gariboldi et al., 2003). Uncertainties about the role of p53 in anthracycline-induced apoptosis may be attributed to such various factors as heterogeneity of the tumors examined or of the methods used for assessing p53 status and tumor response (Bertheau et al., 2002).
An additional factor of consideration pertains to the role of p53 in regulating cell cycle transition in DOX-treated cells. In fact, DOX-dependent p53 activation contributes to the induction of the WAF1/CIP1 p21 gene product, a strong inhibitor of cyclin-dependent kinases involved in G1 to S transition. Whereas this mechanism has been proposed to contribute to G1 arrest of p53-proficient cells, it has also been suggested that WAF1 expression might protect cells from DOX because the G1 block facilitates DNA repair before the cells undergo DNA replication. It is in keeping with this concept that constitutively high levels of WAF1/CIP1 protein were shown to associate with chemoresistance in acute myelogenous leukemia (Zhang et al., 1995). On the other hand, the ability of p53-deficient cells to progress through the S phase may be a favorable event, since the expression of the 
-isoform of topoisomerase II is increased during DNA synthesis (Perego et al., 2001). Further complexity is introduced by recent data showing that p53 might be important not only in connecting DNA damage to downstream execution of apoptosis but also in determining the net levels of DNA strand breaks induced by DOX (Dunkern et al., 2003). How precisely this occurs cannot be said at this time. Studies of p53-proficient versus -deficient cells showed comparable levels of expression and activity of topoisomerase II in the two cell types, yet p53-proficient cells exhibited more DNA damage (Dunkern et al., 2003). One possibility is that p53 interacts with topoisomerase II and inhibits its ligase function, eventually amplifying the net levels of formation of irreversible strand breaks (Cowell et al., 2000; Dunkern et al., 2003). All such issues clearly require refinements, since many human tumors show p53 mutations that bear important implications for chemotherapy.
Uncertainties about the complex interplay between p53 and anthracycline-induced apoptosis are also due to the presence of alternative networks that are not bound to an inhibition of topoisomerase II nor do they always require functional p53, and therefore extend beyond the tentative list of mechanisms provided by Gewirtz (1999) in his thoughtful analysis. For example, present knowledge suggests that clinically relevant concentrations of anthracyclines trigger a cyclical cascade of sphingomyelin hydrolysis and formation of ceramide, which in turn activates downstream cell death effector-mediated pathways not always involving the p53 checkpoint [e.g., c-Jun N-terminal kinase (JNK) stimulation and activation of c-Jun/AP-1 (Laurent and Jaffrezou, 2001); serinethreonine Akt degradation and down-regulation of the Akt/protein kinase B survival pathway (Martin et al., 2002)]. Moreover, it is becoming increasingly evident that anthracyclines can directly release cytochrome c from mitochondria, thereby inducing apoptosis regardless of DNA damage or signaling pathways or p53 status (see also Section III.B.1.) (Green and Leeuwenburgh, 2002; Clementi et al., 2003). Needless to say, these are just a few of the plethora of mechanisms that have been characterized in recent years in relation to the mode of action of anthracyclines. Because authoritative commentaries of these mechanisms have already appeared (Laurent and Jaffrezou, 2001; Perego et al., 2001; Kim et al., 2002b), we will focus on the most recent advances in DNA damage by anthracyclines, with particular reference to the discovery of novel mechanisms for the nuclear import of anthracyclines; the role of oxidative damage to DNA; and the identification of telomeric DNA as a potential new target of anthracyclines.
B. Advances in DNA Damage by Anthracyclines
1. Role of the Proteasome.
Proteasomes are cytoplasmic and nuclear proteinase complexes involved in nonlysosomal mechanisms of protein degradation. The 26S proteasome (composed of a 20S core particle and two 19S cap structures) plays a crucial role in the normal turnover of cytosolic and nuclear proteins and also plays a role in the processing and degradation of regulatory proteins that control cell growth and metabolism (Adams, 2003; Cusack, 2003). The last few years have witnessed an emerging role of the proteasome in modulating anthracycline activity. Proteasomes are present both in the nucleus and in the cytoplasm, but transformed cells and proliferating tissues usually exhibit a preferential accumulation of the proteasome in the nucleus (Kiyomiya et al., 2001b). Conditions typical of solid tumors (like glucose starvation or hypoxia) may accentuate nuclear localization of the proteasome, probably through an increased expression of nuclear localization signals in the -type subunits of the 20S proteasome (Ogiso et al., 2002). In xenografted human tumors, this is accompanied by development of the resistance phenotype, mediated by proteasome degradation of topoisomerase II and reverted by administration of proteasome inhibitors (Ogiso et al., 2000).
An important recent advance pertains to the definition of a multistep mechanism by which the proteasome transports DOX into the nucleus. In step 1, DOX enters cancer cells by simple diffusion and binds with high affinity to the proteasome in cytoplasm. In step 2, DOX binds to the 20S proteasomal subunit, forming a DOX-proteasome complex that translocates into the nucleus via nuclear pores (an ATP-dependent process facilitated by nuclear localization signals). Finally, in step 3, DOX dissociates from the proteasome and binds to DNA due to its higher affinity for DNA than for proteasome (Kiyomiya et al., 2001b). Elucidation of these mechanisms offers one more clue to explaining the reduced activity of anthracyclines in cells with increased nuclear sequestration of the proteasome, since accumulation of the proteasome within the nucleus would diminish the net levels of proteasome available for complexation of DOX in cytosol and its transport toward DNA.
Of particular note is the fact that anthracyclines bind to an allosteric site of the chymotrypsin-like protease activity of 20S proteasome, acting as reversible noncompetitive inhibitors of the protease (Figueireido-Pereira et al., 1996). The biochemical consequences and potential therapeutic advantages of DOX-proteasome interactions may therefore be 2-fold: increased targeting of the anthracycline at the nucleus and accumulation of undegraded proteins that signal apoptosis. The occurrence of both mechanisms was confirmed by studies in which 1) the nuclear uptake and activity of structurally different anthracyclines correlated with their binding affinity to the proteasome (Kiyomiya et al., 2002b); and 2) DOX-treated cells accumulated proteasome-committed ubiquinated proteins and underwent apoptosis to an extent similar to that induced by inhibitors targeted at the catalytic site of the proteasome (Kiyomiya et al., 2002a). Mechanisms and consequences of DOX-proteasome interactions are sketched in Fig. 3.
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Proteasome inhibitors are used as novel therapeutic agents for inducing apoptosis through reduced degradation of the inhibitory subunit (I
B) and consequent reduced activation of an important tumor survival factor like Rel/nuclear factor B (NF-B), for example (Adams, 2003; Cusack, 2003). The fact that inhibitors and anthracyclines bind to distinct catalytic or allosteric sites of the proteasome offered a rationale to design schedules in which the two drugs were given in combination and showed additive or synergic effects compared with single agent treatments. The potential value of such strategy was confirmed by studies in which subtoxic levels of the proteasome inhibitor PS-341 sensitized multiple myeloma cell lines and patient cells to DOX, including cells resistant to either drug or cells isolated from a patient who had relapsed after proteasome inhibitor monotherapy (Mitsiades et al., 2003).
2. Role of Free Radicals.
One-electron addition to the quinone moiety in ring C of DOX and other anthracyclines has long been known to result in formation of a semiquinone that quickly regenerates its parent quinone by reducing oxygen to reactive oxygen species (ROS) like superoxide anion (O2·-) and hydrogen peroxide (H2O2). This futile cycle is supported by a number of NAD(P)H-oxidoreductases [cytochrome P450 or -b5 reductases, mitochondrial NADH dehydrogenase, xanthine dehydrogenase, endothelial nitric oxide synthase (reductase domain)] (Vasquez-Vivar et al., 1997; Minotti et al., 1999). During this cycle the semiquinone can also oxidize with the bond between ring A and daunosamine, resulting in reductive deglycosidation and formation of 7-deoxyaglycone (Fig. 4). Due to their increased lipid solubility, aglycones intercalate into biologic membranes and form ROS in the closest proximity to sensitive targets (Gille and Nohl, 1997; Licata et al., 2000). One-electron redox cycling of DOX is also accompanied by a release of iron from intracellular stores (see Sections III.B.1.a. and III.B.1.b.); ligand binding interactions of DOX with released iron then result in formation of 3:1 drug-iron complexes that convert O2·- and H2O2 into more potent hydroxyl radicals (·OH) (Myers, 1998; Minotti et al., 1999). Oxidative damage has therefore been considered an important mechanism of anthracycline activity in tumor cells.
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Although no doubt exists about whether DOX and other anthracyclines possess the chemical requisites to generate free radicals in cancer cells, too often this is seen at supraclinical drug concentrations. In those cases when cancer cells were exposed to clinically relevant concentrations of DOX, there was a long lag phase between drug administration and detection, e.g., of H2O2. This raised the possibility that free radicals were formed in response to delayed perturbation of cell metabolism and function rather than in response to the activation of the primary drug (Gewirtz, 1999). An alternative explanation may be that available methods lack sufficient sensitivity to probe discrete amounts of free radicals in cells exposed to clinically relevant concentrations of anthracyclines. Another important concept to be kept in mind when considering the role of free radicals in anthracycline activity pertains to the function of ROS as signaling molecules rather than as mediators of oxidative post-translational modifications of cell constituents. Thus, ceramide formation occurs after ROS activation of neutral sphingomyelinase, and ROS can also modulate the activity of several kinases or transcription factors that control cell cycle and pro- or anti-apoptotic networks (Laurent and Jaffrezou, 2001; Bezombes et al., 2002; Kim et al., 2002b; Martin et al., 2002). In scrutinizing the importance of ROS, one should therefore distinguish their role in signaling events (probably mediated by minute amounts of ROS that escape detection by available techniques) and the role of ROS as direct oxidizing agents (probably requiring higher levels of ROS formation by supraclinical concentrations of anthracyclines).
The patterns of DNA damage in anthracycline-treated cancer cells seem to support the notion that direct oxidative lesions only occurred if cancer cells were exposed to supraclinical concentrations of anthracyclines. Concentrations of anthracyclines below 5 µM, and hence of potential clinical significance, caused formation of protein-associated DNA single- and double-strand breaks, which reflected anthracycline inhibition of topoisomerase II; in contrast, the formation of nonprotein-associated strand breaks, i.e., DNA lesions caused by free radical formation and reactivity on the DNA backbone, only occurred when the cells were treated with supraclinical concentrations of DOX (reviewed by Gewirtz, 1999).
Similar concerns hold true when considering lipid peroxidation as a possible mechanism of antitumor activity induced by DOX. With one noticeable exception, regarding a selective induction of lipid peroxidation by 1 µM DOX in mouse lymphocytic leukemia cell line L1210 but not in pig kidney proximal tubular epithelial cell line LLC-PK1 (Kiyomiya et al., 2001a), there is apparent evidence to conclude that anthracyclines do not induce lipid peroxidation in cancer cells at clinically relevant concentrations. Under defined conditions, a dissociation actually exists between anthracycline cytotoxicity and lipid peroxidation. This was the case when noncytotoxic amounts of docosahexaenoic acid (22:6 N-3) synergized the cytotoxicity of DOX in glioblastoma cell lines A-172 and U-87 MG and bronchial carcinoma cell lines A-427 and SK-LU-1, whereas lipid peroxidation showed no or very small increases over background levels (Rudra and Krokan, 2001). Supportive or disproving evidence for the formation of free radicals in cancer cells and a role for oxidative damage in anthracycline activity are reported in Table 1.
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Recent advances in the field of lipid peroxidation introduce some cautionary issues about how lipid peroxidation was measured and evaluated in relation to the action of DOX and ROS in cancer cells. In most studies, lipid peroxidation was assayed as the formation of thiobarbituric acid (TBA)-reactive materials; although of practical value for experiments with isolated subcellular fractions, this popular assay lacks sufficient sensitivity and specificity for in vivo experiments or studies with intact cells (Minotti, 1993). Moreover, the TBA assay is popularly believed to measure malondialdehyde (MDA), but it is now clear that it actually detects a broad array of aldehydes and alkenals or peroxides. The possibility therefore exists that anthracyclines did induce lipid peroxidation in cancer cells, but the low sensitivity (and specificity) of the TBA assay may have failed to produce unambiguous evidence that such process had indeed occurred. Perhaps more importantly, the lack of specificity of the TBA assay tells us nothing about the most important pathologic consequence of MDA formation, which is that of linking lipid peroxidation to DNA damage.
3. Lipid Peroxidation and DNA Damage: Malondialdehyde-DNA Adducts.
Mass spectroscopy techniques now show that MDA, like other enals, can react at the exocyclic amino groups of deoxyguanosine (dG), deoxyadenosine (dA), and deoxycytidine (dC) to form alkylated products such as etheno adducts from dA, dG, and deoxycytidine; 8-hydroxypropanodeoxyguanosine adducts from dG; a pyrimidopurinone (M1dG) adduct from dG (Fig. 5) (Marnett et al., 2003). MDA is therefore mutagenic in human cells, with the majority of MDA-induced mutations occurring at GC base pairs and consisting of large insertions and deletions (Niedernhofer et al., 2003). These mutations probably are preceded by premutagenic lesions like DNA interstrand cross-links recognized by the nucleotide excision repair system (Niedernhofer et al., 2003).
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In proliferating cells the formation of M1dG is accompanied by cell cycle arrest and inhibition of cyclin E- and cyclin B-associated kinase activities in both wild-type p53 and p53-null cell lines (Ji et al., 1998). MDA-DNA adducts therefore seem to be closely connected to cell cycle checkpoints possibly relevant to the cytostatic properties of anthracyclines. Importantly, anthracyclines can form M1dG not only by increasing the levels of formation of MDA but also by favoring oxopropenyl transfer from preformed MDA to DNA; in fact, very low concentrations of DOX and DNR increase MDA-dependent DNA oxopropenylation severalfold, an effect due to the DNA-intercalating and minor groove-binding properties of the anthraquinone and daunosamine moieties (Plastaras et al., 2002). Thus, both oxidative stress-MDA formation and DNA intercalation-oxopropenylation may enable anthracyclines to increase the cellular levels of M1dG (Plastaras et al., 2002). These reactions establish potential new links between anthracycline-dependent generation of ROS, induction of lipid peroxidation, and DNA intercalation and damage; they also highlight the importance of replacing the TBA assay with appropriate mass spectral analyses in detecting cellular levels of MDA and related DNA adducts (Otteneder et al., 2003).
4. Oxidative Base Lesions as in Vivo Markers of Free Radical Formation and DNA Damage by Anthracyclines.
Studies by Doroshow et al. introduce novel information on DNA oxidative damage induced by DOX under pharmacokinetic conditions. Using gas chromatography/mass spectrometry with selected ion monitoring these investigators examined oxidative modifications of DNA in peripheral blood mononuclear cells (PBMC) from breast cancer patients receiving DOX as slow intravenous infusion. Under these conditions, the steady-state plasma level of DOX was as low as 0.1 µM when the drug was infused for 96 h at a total dose of 165 mg/m2. Before DOX infusion, all PBMC contained 13 different DNA oxidized bases; after DOX infusion, at least nine of these bases increased 4-fold over baseline, with the most remarkable increases regarding thymine glycol (ThyGly), 5-hydroxyhydantoin (5-OH-Hyd), 5-(hydroxymethyl)uracil (5-OH-MeUra), 4,6-diamino-5-formamido-pyrimidine (FapyAde), and, to a lesser extent, 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) (Doroshow et al., 2001b).
The spectrum of oxidative DNA base damage induced by DOX reproduced that caused by ionizing radiation (Gajewski et al., 1990), suggesting that bases were damaged by ROS (presumably ·OH radicals) generated after redox cycling of DOX. Other typical fingerprints of ·OH-dependent DNA oxidation like 8-oxo-dG and 8-oxo-dA were not detected in PBMC after DOX infusion; however, the observed increases of FapyAde and FapyGua clearly demonstrated that both 8-oxo-dG and 8-oxo-dA had been formed to some significant extent after DOX infusion, as FapyAde and FapyGua derive from imidazole ring opening of 8-oxo-dA and 8-oxo-dG (Douki et al., 1997).
In addition to providing unequivocal evidence for an oxidative stress induced by DOX under clinical conditions, the studies by Doroshow and associates offer important insights into the mechanisms of DOX activity and mutagenicity. Among the bases that were shown to increase in PBMC, ThyGly and FapyGua and 5-OH-Ura or 5-OH-MeUra have an established mutagenic potential (Jaruga and Dizdaroglu, 1996), mediated by GC
CG transversions (FapyGua) or GCAT transitions and GCCG transversions (5-OH-Ura), for example. Moreover, ring-fragmented FapyGua and ThyGly block DNA replication or increase reading error frequency by DNA polymerase, resulting in cytotoxic or mutagenic DNA lesions (Doroshow et al., 2001b; Marnett et al., 2003). DNA polymerase dysfunction may also occur as a result of conformational changes in DNA induced by the presence of oxidized DNA.
These results are important in many respects. As mentioned earlier, popularly accepted mechanisms of anthracycline activity have been characterized in model systems requiring supraclinical concentrations of anthracyclines. Inhibition of DNA and RNA synthesis or of specific DNA polymerases does not escape this bias, which may explain the lack of correlation between DOX-induced inhibition of DNA or RNA synthesis and tumor cell killing in experimental settings (Gewirtz, 1999). Likewise, the formation and disappearance of topoisomerase II-mediated DNA breaks do not always correlate with tumor cell killing or seem to be of too modest an extent to explain the antitumor potency of anthracyclines, unless one assumes that double-strand breaks occur at genomic loci unusually prone to be converted into irreversible lesions (Binaschi et al., 1997; Gewirtz, 1999). The observation that DOX infusions induce detectable levels of potentially cytotoxic, oxidized DNA bases therefore unravels an alternative mechanism to explain the action of DOX under clinically relevant conditions. In addition, it has long been known that combining DOX with cyclophosphamide causes a dramatic increase of the risk of secondary malignancies, most often acute myelomonocytic leukemia (Hoffmann et al., 1995). Thus, if DOX-induced DNA base oxidation occurs in hematopoietic precursors the same way it occurs in PBMC, this may represent an important mechanism to explain the development of secondary hematologic malignancies.
There are other aspects in Doroshow's work that call for consideration. One of particular note is that the spectrum and net levels of oxidized DNA bases detected in PBMC after 96-h DOX infusions were appreciably broader and higher than those characterized in lymphocytes from patients treated with a short intravenous bolus of 70 mg of EPI/m2 (Olinski et al., 1997). This has been attributed to a greater systemic exposure to DOX, leading to a depletion of intracellular antioxidants and/or an overruling of repair systems [e.g., glycosylases for Fapy adducts (Hazra et al., 2001)]. In considering that the slow infusion schedule was adopted for reducing the risk of cardiotoxicity while also maintaining good antitumor activity (Synold and Doroshow, 1996; Doroshow et al., 2001b), one cannot escape the conclusion that the free radical-generating activity of DOX correlates with not only Cmax but also with the total AUC. This issue will be re-examined when addressing the correlates between cardiotoxicity and slow infusion versus bolus anthracyclines (see Section III.D.1.).
5. Anthracycline-Formaldehyde Conjugates and DNA Virtual Cross-Linking.
Anthracyclines have long been known to form unstable covalent bonds to DNA when redox-activated in chemical systems with NAD(P)H oxidoreductases and transition metals. Two types of covalent bonding have been described: more stable drug-DNA cross-links and less stable drug-DNA adducts. Again, the concentrations required to promote the formation and/or to allow the detection of either cross-links or adducts often exceeded those achievable in patients, making the pathophysiologic relevance of such findings uncertain (Gewirtz, 1999). Seminal work by Taatjes, Koch, and associates has led to an-in-depth reappraisal of this picture. They have shown that iron-mediated free radical reactions enable anthracyclines to produce formaldehyde (HCHO, FORM) from carbon cellular sources like spermine and lipids (Taatjes et al., 1997, 1998, 1999; Taatjes and Koch, 2001). Elevated levels of HCHO have been detected in DOX-sensitive cancer cells but not in DOX-resistant cancer cells equipped with higher levels of ROS-detoxifying enzymes (Kato et al., 2001). Doxorubicin and HCHO then react to give a conjugate (DOXFORM) in which two anthracycline molecules bind together with three methylene groups, two forming oxazolidine rings and one binding the oxazolidines together at their 3'-amino nitrogens. DOXFORM eventually hydrolyzes to give an active monomeric metabolite in which the carbon of HCHO is recovered in the form of a Schiff's base at the aminogroup of daunosamine. Similar reactions occur with EPI and DNR but not with anthracyclines lacking a 3'-amino group (Cutts et al., 2003).
Anthracycline-FORM conjugates have attracted interest because of their unique ability to intercalate into DNA by covalent bonding of the Schiff's base with the 2-amino group of a G-base in the minor groove of DNA. If the interaction with DNA occurs at the trinucleotide 5'-NGC-3', then the drug intercalates between N and G and covalently bonds to the G-base on one strand using HCHO, and to the G-base on the opposing strand using hydrogen bonds. Such an unusual combination of intercalation, covalent bonding, and hydrogen bonding is referred to as the virtual cross-linking of DNA by anthracyclines (Taatjes and Koch, 2001) (Fig. 6). In the case of DOXFORM (and, presumably, EPIFORM and DNRFORM) the virtual cross-link slows DNA strand exchange by 640-fold relative to anthracycline-free DNA, and by 160-fold relative to DNA bearing intercalated unchanged anthracycline. Such a 160-fold difference in strand exchange rate clearly denotes the importance of the covalent linkage in the drug-DNA interaction (Zeman et al., 1998; Taatjes and Koch, 2001).
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The discovery of the virtual cross-linking mechanism provided a rationale for assessing anthracycline-FORM conjugates as novel drugs with improved activity in both sensitive cells and cells that had developed resistance to anthracyclines due to overexpression of P glycoprotein (Pgp) (Gottesmann and Pastan, 1993) or reduced expression of the enzyme-mediating redox activation of anthracyclines (e.g., NADPH cytochrome P450 reductase), or increased expression of ROS detoxifying enzymes [e.g., superoxide dismutase (SOD), catalase, GSH peroxidase (Mimnaugh et al., 1991; Gariboldi et al., 2003)]. Activity in Pgp-overexpressing tumors was anticipated based on two factors: HCHO-dependent reduction of the pKa of the protonated amino residue of anthracyclines, making this residue unprotonated at physiological pH and decreasing anthracycline affinity for Pgp (Lampidis et al., 1997); and rapid binding of anthracycline-FORM conjugates to DNA in competition with Pgp (Taatjes et al., 1998). Activity in cells with reduced levels of redox-activating enzymes or increased levels of ROS scavengers was anticipated based on the fact that preconjugation of anthracyclines with HCHO would obviate the need for a redox cycling of the anthracycline and consequent generation of HCHO from cellular carbon sources (Taatjes and Koch, 2001). In agreement with such expectations of improved activity, both DOXFORM and EPIFORM or DNRFORM were shown to exhibit enhanced toxicity to anthracycline-sensitive and -resistant tumor cells. This correlated with increased nuclear targeting of the conjugates, accumulation in DNA, prolonged cellular retention, and reduced cellular release of anthracyclines (Taatjes et al., 1999). DNA lesions attributed to the action anthracycline-FORM conjugates were shown to be unstable and to hydrolyze at rates that were reflected in a biexponential pattern of drug efflux. The faster rate of drug release was assigned to hydrolysis of more labile lesions at isolated G-bases, and the slower rate was assigned to hydrolysis of relatively fewer labile lesions at NGC sites, which is the site more directly linked to the formation of a virtual cross-linking (Taatjes and Koch, 2001).
EPIFORM, the lead compound in a program of development of anthracycline-FORM conjugates, has been evaluated in the National Cancer Institute human tumor cell screen and shown to be more active than EPI in all but one cell line. Of note, EPIFORM significantly exceeded the toxicity of DOX or EPI to the most resistant breast cancer cell line, MCF-7/Adr, and to the most resistant prostate cancer cell line, DU-145 (Taatjes and Koch, 2001). EPIFORM also proved more active than EPI in efficacy trials conducted in a mouse mammary carcinoma model (Dernell et al., 2002). In regard to comparisons between DOX and DOXFORM, studies in HeLa S3 cells showed that both drugs induced apoptosis, but DOXFORM was effective at concentrations 1 order of magnitude lower than DOX and well in the range of clinically achievable concentrations (86 nM anthracycline equivalents) (Burke and Koch, 2001).
Further evidence for the improved activity of anthracycline-FORM conjugates comes from experiments in which DOX was administered in combination with drugs that released HCHO in the cell, like AN-9 or HMTA. Pivaloyloxymethyl butyrate (AN-9) was developed as a butyric acid-releasing prodrug. Butyric acid is known to induce cell differentiation via inhibition of histone deacetylase, but its clinical use would be limited by rapid clearance. The advantage of AN-9 is that it undergoes hydrolysis within the cell, releasing butyric acid, pivalic acid, and HCHO. Doxorubicin and AN-9 proved to be synergistic when administered simultaneously to neuroblastoma or breast adenocarcinoma cells or when the administration of DOX preceded that of AN-9; however, the reverse sequence (AN-9 DOX) resulted in antagonism (Cutts et al., 2001). These studies demonstrated that the levels of DOX-DNA adducts increased when the drugs were administered in the synergistic sequence but decreased when the antagonistic schedule was used.
Hexamethylenetetramine (HMTA) hydrolyzes intracellularly to release six molecules of HCHO. In neuroblastoma cells, HMTA increased the levels of formation of cytotoxic DOX-DNA adducts, and the IC50 of DOX + HMTA proved to be 3-fold lower compared with DOX single agent. Of note, DOX-DNA adducts were formed in a pH-dependent manner, with 4-fold more detected at pH 6.5 compared with pH 7.4 (Swift et al., 2002). While in agreement with the known acid lability of HMTA, the pH dependence of anthracycline-FORM-DNA interactions offers further advantages in light of the low pH of solid tumors.
Possible mechanisms of resistance to the formation of anthracycline-FORM conjugates have to be considered nonetheless. Once formed inside the cell upon DOX-induced oxidative stress or hydrolysis of AN-9 or HMTA, HCHO would be trapped by GSH as hydroxymethyl-GSH, which in turn would become a substrate for GSH-dependent HCHO-dehydrogenase, resulting in generation of formate and recycling of GSH (Deltour et al., 1999). Overexpression of HCHO-dehydrogenase has therefore been considered a potential mechanism of resistance to the formation of anthracycline-FORM conjugates. Careful investigation by Brazzolotto et al. (2003) seems to refute such possibility, as the expression levels of HCHO-dehydrogenase in DOX-resistant human small-cell lung carcinoma cells were actually lower than in sensitive parental cells. Moreover, DOX treatment was shown to decrease the expression of HCHO-dehydrogenase in both sensitive and resistant cells (Brazzolotto et al., 2003). The apparent ineffectiveness of HCHO-dehydrogenase in mediating resistance to DOX may relate to the fact that mammalian isoenzymes, in contrast to microbial homologs, are not induced by substrate (Edenberg, 2000). An alternative explanation is that the reaction rate between HCHO and the aminoresidue of DOX, or endogenous cellular moieties, is fast enough to allow HCHO to escape metabolization by HCHO-dehydrogenase. Not surprisingly, recent studies have led to the proposal that GSH-dependent HCHO-dehydrogenase may be involved in controlling the levels of nitroso-thiols rather than HCHO (Liu et al., 2001).
6. Anthracyclines and Telomeric DNA.
Telomeres are the ends of linear chromosomes of eukaryotic cells. In most eukaryotes, telomeres consist of as many as 500 to 3000 5'-TTAGGG-3' repeats and serve to protect the end of chromosomes from degradation and ligation. The length of telomeres is primarily controlled by telomerase, a ribonucleoprotein composed of a catalytic protein subunit (telomerase reverse transcriptase, TERT), a telomerase-associated protein, and a stably associated RNA moiety, which serves the function of an intrinsic template for the elongation of telomeres. It has been well established that telomerase is not active in most somatic tissues; therefore, telomeres shorten gradually with age, both in vitro and in vivo, and such erosion is sensed by the cells as a clock to switch toward a p53-mediated senescence program (Chin et al., 1999). Conversely, telomerase is activated in the majority of cancer-derived cell lines and malignant tumors, a finding suggesting that telomerase is pivotal to cell immortalization and tumorigenesis or tumor aggressiveness (Hahn, 2003). Supportive evidence is offered by, among several other reports, increased development of breast cancer in transgenic mice overexpressing mTERT (Artandi et al., 2002); induction of massive apoptosis in human acute leukemia cells transfected with a dominant-negative human TERT (Nakajima et al., 2003); inverse correlation between telomerase activity and overall or disease-free survival in non-small-cell lung carcinoma patients (Marchetti et al., 2002); positive correlation between attenuation of telomerase activity and inhibition of cellular growth or induction of apoptosis in immortal breast cancer cell lines transiently transfected with hammerhead ribozyme cleaving human TERT mRNA (Ludwig et al., 2001).
Interestingly, telomeres do not always shorten after telomerase inhibition and consequent induction of apoptosis. This suggests the existence of telomerase-independent mechanisms of telomere elongation (Kim et al., 2002a); it also suggests that telomerase may extend the lifespan of the cell by alternative mechanisms such as capping of free G-rich single-stranded telomeric DNA, which otherwise would become exposed to the nucleoplasm and could trigger cell cycle arrest or apoptosis, depending on the cellular context (Ludwig et al., 2001).
In a significant number of experimental systems, there was a long delay between inhibition of telomerase and cessation of cell growth or induction of apoptosis, and this lag was especially evident in cells exhibiting long telomeres (Herbert et al., 1999). Because telomeres accumulate single-strand breaks and shorten more rapidly after exposure to agents inducing oxidative stress and DNA damage (von Zglinicki et al., 2000), combining telomerase inhibition with anthracycline treatment has been considered a new option for improved cancer treatment. The therapeutic benefit of combining an anthracycline regimen with telomerase inhibition was also anticipated by experimental evidence for multiple crosstalks between DOX activity and changes in telomerase activity or regulation of telomerase by pro- or anti-apoptotic factors (e.g., p53 and ceramide or Bcl-2, respectively). Breast cancer cells acutely exposed to DOX exhibited an increase in p53 activity, a decline in telomerase activity, and replicative senescence characterized by G0G1 arrest (Elmore et al., 2002). Similarly, DOX-sensitive gastric carcinoma cells responded to anthracycline administration with a decline of both telomerase activity/hTERT mRNA and Bcl-2 protein levels, whereas DOX-resistant cells exhibited no such change (Yoon et al., 2003) or exhibited telomere-elongating mechanisms that were not mediated by telomerase (Kim et al., 2002a). Finally, elevation of endogenous ceramide inhibited telomerase and contributed to G0G1 arrest of human lung adenocarcinoma cells exposed to nontoxic concentrations of DNR (Ogretmen et al., 2001). It was therefore expected that concomitant administration of DOX and telomerase inhibitor(s) resulted in additive or synergic effects in telomerase-positive/anthracycline-sensitive cells. In agreement with such expectations, DOX induced more apoptosis in breast cancer cells in which telomerase had been muted with the ribozyme technology (Ludwig et al., 2001) and formed more DNA double-strand breaks in neoplastic cells derived from telomerase RNA-null mice (Lee et al., 2001). These results clearly illustrate telomerase inhibition as a novel therapeutic approach in combination with DOX and other anthracyclines. It is hoped that clinically impractical strategies like ribozyme cleavage of telomerase mRNA or vector transfection of dominant-negative telomerase subunits will soon be replaced by more doable measures like administration of natural telomerase inhibitors, which are proving promising in preclinical screens [e.g., telomestatin (a natural product isolated from Streptomyces annulatus) or epigallocatechin gallate (a major tea polyphenol)] (Kim et al., 2003a; Naasani et al., 2003).
There are several important factors to be taken in account when considering the biological aspects and clinical perspectives of pharmacological interventions targeted at telomeres. One factor pertains to p53, whose mutations or absence attenuate or abrogate the therapeutic benefit of combining an anthracycline with anti-telomerase measures [as one would expect if p53 served to relay the senescence program signaled by telomeres shortening (Lee et al., 2001; Elmore et al., 2002)]. Thus, the presence or absence of a functional p53 will dictate the appropriateness of combining DOX or other anthracyclines with antitelomerase treatments. Another important factor pertains to the role of telomere length versus telomere dysfunction. In some studies the overexpression of hTERT in tumor cells was able to compensate for DOX-induced down-regulation of telomerase and prevented telomere shortening; however, all such changes did not preclude DOX from inducing proliferative senescence (Elmore et al., 2002). Such an apparent dissociation between telomere length and cellular senescence is reconciled based upon the appearance of telomerase-independent cytogenetic changes, which are induced by the anthracycline and are referred to as telomere dysfunction (chromosomal ends with no detectable telomere signals or signal-free ends, aneuploidy, and end-to-end chromosome fusions). In cellular systems there are cases when anthracycline sensitivity and formation of double-strand breaks correlate with telomere dysfunction rather than telomerase activity (Lee et al., 2001; Elmore et al., 2002). Thus, cytogenetic assessment of telomere dysfunction will soon become as important as evaluation of telomerase activity in predicting tumor chemosensitivity. One last factor of consideration pertains to the impact of combined antitelomerase chemotherapy regimens on normal cells. Telomere shortening decreases the capacity to cope with stresses such as wound healing and blood cell depletion, especially in aged animals; thus, a potential elevation of the hematotoxic side effects of telomerase inhibitors should be a prominent consideration as clinical trials move forward, especially in view of reports demonstrating that anthracycline-based chemotherapy is per se capable of reducing telomerase activity and telomere length in leukocytes of patients (Schroder et al., 2001). The risk of developing secondary malignancies in response to telomere shortening and genetic instability should also be considered and weighed against the actual benefit of combining anthracyclines with antitelomerase therapy.
III. Cardiotoxicity of Anthracyclines
A. Morphology, Dose Dependence, Risk Factors
Dilative cardiomyopathy and CHF develop after completion of cumulative anthracycline regimens, usually within a year, but very late forms of cardiac dysfunction have been described (Steinherz et al., 1991).2 The ultrastructural features of anthracycline-induced cardiomyopathy, characterized in patients' endomyocardial biopsies, include the loss of myofibrils, dilation of the sarcoplasmic reticulum, cytoplasmic vacuolization, swelling of mitochondria, and increased number of lysosomes. This morphologic pattern is seen also in mice and rats or rabbits treated with adequate doses of anthracycline, indicating the existence of a species-independent final pathway of morphologic damage (Singal et al., 2000). The severity of morphologic damage is inversely correlated to the levels of Pgp in the endothelium of both arterioles and capillaries of heart samples, showing that a close link exists among the administered dose of DOX, its accumulation in the heart, and the development of cardiomyopathy (Meissner et al., 2002).
In a seminal retrospective study of 399 patient records, DOX-induced cardiomyopathy and CHF proved to be dose-dependent, and their incidence rose to unacceptably high levels when the cumulative dose of the anthracycline exceeded 500 mg/m2 (Lefrak et al., 1973). Thus, CHF developed in >4, >18, or 36% of patients who had received cumulative doses of 500 to 550, 551 to 600, or 
601 mg/m2, respectively (Lefrak et al., 1973). In another retrospective review of several thousand patients receiving DOX-containing chemotherapy, the risk of CHF correlated with patient age, total anthracycline dose, and dose schedule (Von Hoff et al., 1979). Valvular, coronary, or myocardial heart disease and a long-standing history of hypertension were recognized as independent risk factors of developing cardiomyopathy at cumulative doses of DOX below 500 to 550 mg/m2. Previous mediastinal irradiation or concurrent administration of other chemotherapeutics (e.g., cyclophosphamide) was also considered to increase the risk of developing cardiomyopathy, but neither factor turned out to influence the incidence of CHF once the effects of age, schedule, and cumulative dose were taken into account (Von Hoff et al., 1979).
Compared with adults, children were shown to have a reduced risk of cardiomyopathy at any given cumulative dose of anthracycline (Von Hoff et al., 1979), but other reports suggested that the risk of cardiomyopathy may actually be increased in children, particularly in those who received mediastinal irradiation or irradiation modalities that included the lower part of the heart (Pinkel et al., 1982). Moreover, the incidence of cardiac toxicity was shown to increase in female patients (Lipshultz et al., 1995) or in those who had longer follow-up, consistent with the fact that children may develop cardiotoxicity at longer intervals after treatment completion (sometime as late as 15 years after anthracycline regimens). The current thinking is that children present a pattern of development of cardiotoxicity distinct from that observed in adults (Lipshultz et al., 2002b). This is also indicated by the fact that anthracycline-treated children sometime develop restrictive rather than dilative cardiomyopathy (Zucchi and Danesi, 2003).
The sharp increase in the incidence of cardiomyopathy at cumulative doses above 550 to 600 mg of DOX/m2 has formed the basis to set an empirical dose limit of 500 mg of DOX/m2 as a strategy to minimize the risk of cardiomyopathy. However, ultrastructural changes in endomyocardial biopsies and/or impaired contractility after provocative tests (e.g., exercise echocardiography) have been documented in patients treated with reportedly safe cumulative doses. Concerns about whether non-symptomatic cardiac dysfunction eventually surfaces in the form of late cardiac events, diminishing or even abrogating the benefit of cumulative doses below 500 mg/m2, have never been satisfied. This issue is of particular importance when DOX is used as adjuvant therapy for women with early breast cancer. The problem of late cardiac morbidity caused by subthreshold dose levels of DOX in this defined patients population has been reappraised in a retrospective analysis of women who had received DOX (median total dose of 300 mg/m2) or cyclophosphamide/methotrexate/fluorouracil as adjuvant therapy. Results of the analyses showed that non-symptomatic cardiac dysfunction (defined as pathologic or borderline decrease of LVEF) was higher in women treated with DOX (8%) than in women receiving cyclophosphamide/methotre
