Early and late sarcoplasmic reticulum changes in doxorubicin cardiomyopathy. An ultrastructural investigation with the zinc iodide-osmium tetroxide (ZIO) technique

Challenging the status quo: a framework for mechanistic and human-relevant cardiovascular safety screening

Traditional approaches to preclinical drug safety assessment have generally protected human patients from unintended adverse effects. However, these assessments typically occur too late to make changes in the formulation or in phase 1 and beyond, are highly dependent on animal studies and have the potential to lead to the termination of useful drugs due to liabilities in animals that are not applicable in patients. Collectively, these elements come at great detriment to both patients and the drug development sector. This phenomenon is particularly problematic in the area of cardiovascular safety assessment where preclinical attrition is high. We believe that a more efficient and translational approach can be defined. A multi-tiered assessment that leverages our understanding of human cardiovascular biology, applies human cell-based in vitro characterizations of cardiovascular responses to insult, and incorporates computational models of pharmacokinetic relationships would enable earlier and more translational identification of human-relevant liabilities. While this will take time to develop, the ultimate goal would be to implement such assays both in the lead selection phase as well as through regulatory phases.

Potential adverse interaction of human cardiac calsequestrin

Calsequestrin (CASQ) is a major Ca(2+) storage protein within the sarcoplasmic reticulum (SR) of both cardiac and skeletal muscles. CASQ reportedly acts as a Ca(2+) buffer and Ca(2+)-channel regulator through its unique Ca(2+)-dependent oligomerization, maintaining the free Ca(2+) concentration at a low level (0.5-1mM) and the stability of SR Ca(2+) releases. Our approach, employing isothermal titration calorimetry and light scattering in parallel, has provided valuable information about the affinity of human cardiac CASQ (hCASQ2) for a variety of drugs, which have been associated with heart- or muscle-related side effects. Those strongly binding drugs included phenothiazines, anthracyclines and Ca(2+) channel blockers, such as trifluoperazine, thioridazine, doxorubicin, daunorubicin, amlodipine and verapamil, having an average affinity of ~18 μM. They exhibit an inhibitory effect on in vitro Ca(2+)-dependent polymerization of hCASQ2 in a manner proportional to their binding affinity. Therefore accumulation of such drugs in the SR could significantly hinder the Ca(2+)-buffering capacity of the SR and/or the regulation of the Ca(2+) channel, RyR2. These effects could result in serious cardiac problems in people who have genetically impaired hCASQ2, defects in other E-C coupling components or problems with metabolism and clearance of those drugs.

Melatonin protects against epirubicin-induced cardiotoxicity

We investigated the cytoprotective effect of melatonin in epirubicin-induced cardiotoxicity using four experimental groups of male Wistar rats: untreated control rats, epirubicin-treated rats, epirubicin+melatonin-treated rats, and melatonin-treated rats. We examined the histopathological and biochemical effects of melatonin on the epirubicin-induced changes and measured the levels of the lipid peroxidation end-product (malondialdehyde, MDA), an indicator of nitric oxide (NO) synthesis (nitrite/nitrate production), and reduced glutathione (GSH) in the heart. We also studied the extracellular matrix components (fibronectin, laminin) in the heart. Vacuole formation, mitochondrial deformation and degeneration, and disordered myofibrillary structures were detected ultrastructurally in the epirubicin-treated group. The degeneration was reduced in the heart tissues of the epirubicin+melatonin group. Epirubicin increased the nitrite/nitrate production, but did not change the MDA and GSH levels significantly. Melatonin treatment lowered the nitrite/nitrate concentrations, while increasing the GSH levels, which exceeded the levels in epirubicin+melatonin-treated rats. We conclude that the epirubicin increased the nitrozative stress, not the oxidative stress, in heart tissue, and the cardioprotective effect of melatonin was partially attributed to the suppression of epirubicin-induced nitrozative stress. These results suggest that melatonin partially protects against epirubicin-induced cardiotoxicity.

Melatonin as an effective protector against doxorubicin-induced cardiotoxicity

The present study was designed to explore the protective effects of melatonin and its analogs, 6-hydroxymelatonin and 8-methoxy-2-propionamidotetralin, on the survival of doxorubicin-treated mice and on doxorubicin-induced cardiac dysfunction, ultrastructural alterations, and apoptosis in mouse hearts. Whereas 60% of the mice treated with doxorubicin (25 mg/kg ip) died in 5 days, almost all the doxorubicin-treated mice survived when melatonin or 6-hydroxymelatonin (10 mg/l) was administered in their drinking water. Perfusion of mouse hearts with 5 microM doxorubicin for 60 min led to a 50% suppression of heart rate x left ventricular developed pressure and a 50% reduction of coronary flow. Exposure of hearts to 1 microM melatonin or 6-hydroxymelatonin reversed doxorubicin-induced cardiac dysfunction. 8-Methoxy-2-propionamidotetralin had no protective effects on animal survival and on in vitro cardiac function. Infusion of melatonin or 6-hydroxymelatonin (2.5 microg/h) significantly attenuated doxorubicin-induced cardiac dysfunction, ultrastructural alterations, and apoptosis in mouse hearts. Neither melatonin nor 6-hydroxymelatonin compromised the antitumor activity of doxorubicin in cultured PC-3 cells. These results suggest that melatonin protect against doxorubicin-induced cardiotoxicity without interfering with its antitumor effect.

Doxorubicin cardiotoxicity: diastolic cardiac myocyte dysfunction as a result of impaired calcium handling in isolated cardiac myocytes

We examined intracellular calcium transients of isolated single cardiac myocytes from rats with doxorubicin (DOX)-induced cardiomyopathy with simultaneous measurement of cell motion. DOX was administered i.p. to Sprague-Dawley rats at 2.5 mg/kg once a week for 10 weeks. Field-stimulated calcium transients and simultaneous cell motion in single myocytes were measured in the presence or absence of isoproterenol using fura-2/AM. Histopathologic examination revealed slight changes. The time courses of both calcium transients and cell motion were significantly prolonged by DOX. There was a slight but not significant reduction in parameters of contractility in both calcium transients and cell motion. The beta-adrenoceptor responsiveness of both calcium transients and cell motion was not significantly impaired compared with the controls. Our data indicated that, despite the slight histologic changes in the heart in DOX-induced cardiomyopathy, impaired sequestration of intracellular free calcium ions in individual myocytes may be one factor leading to diastolic dysfunction. Monitoring of diastolic function is important to detect early cardiotoxicity caused by DOX.

From the pigments of the actinomycetes to third generation antitumor anthracyclines

After the assessment of the antitumor activity of the anthracycline pigments, the S peucetius group of metabolites was discovered and eventually doxorubicin, a major anticancer agent of established clinical usefulness was developed in the early seventies. A second generation of compounds followed, represented mainly by the better tolerated epirubicin and by the highly potent antileukemic drug, idarubicin. This was the result of a wide program of analog development that provided the basis for further investigations concerning both the study of structure-activity relationships and the synthesis of novel promising derivatives including the 8- and 10-fluoro compounds and the disaccharides. A member of the latter group, namely 7-O-(4'-O-alpha-L-daunosaminyl-2'-deoxy- alpha-L-fucosyl)-4-demethoxyadriamycinone, is undergoing clinical trials as a third generation antitumor anthracycline.

Chronic administration of the mutagenic heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine induces cardiac damage with characteristic mitochondrial changes in Fischer rats

Fischer-344 rats of both sexes were administered the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) for 52 wk at the dietary level of 400 ppm. Light microscopic and ultrastructural investigation of the myocardium revealed prominent changes in all PhIP-treated male and female animals investigated. These were characterized by a diffuse proliferation of markedly enlarged mitochondria, with abundant cristae and often containing lamellar bodies as inclusions. PhIP is well known to cause DNA adducts in the rat heart, and there are numerous reports of mutations in mitochondrial DNA in both humans and experimental animals being associated with very similar lesions to those observed in the present study. The results thus suggest that this heterocyclic amine induces cardiac damage by the same mechanism.

In-vivo and in-vitro mitochondrial membrane damages induced in mice by adriamycin and derivatives

A major limitation to a prolonged use of adriamycin (ADM) during a clinical treatment is its dose-dependent cardiotoxicity. This toxicity has been related to a general disturbance of the inner mitochondrial membrane structure and its essential biological functions, associated to the production of free radicals by the anthracyclines. 4'-Epiadriamycin (4'-epiADM), 4'-deoxyadriamycin (4'-deoxyADM), 4'-deoxy-4'-iodoadriamycin (4'-deoxy-4'-iodoADM) and 4'-demethoxydaunorubicin (4-demethoxyDNR) are ADM and daunorubicin (DNR) derivatives differing from their parent compounds by minor structural modifications. They are nevertheless documented as less cardiotoxic. Our purpose was to establish whether mitochondrial membrane damages induced in vivo in mice heart by those compounds are correlated with the free radical formation. Heart mitochondria of treated mice were isolated 48 h after a single drug injection in order to measure the acute mitochondrial toxicity. Enzymatic activities of complex I-III and complex IV of the mitochondrial respiratory chain, mitochondrial membrane fluidity and lipid peroxidation were measured. None of the ADM and DNR derivatives displayed a significant acute mitochondrial toxicity. A mitochondrial toxicity was however detected for 4-deoxyADM and 4-demethoxyDNR when drugs were given chronically, but it was strongly reduced as compared with ADM and DNR. Electron transfer between NADH and cytochrome c, formation of superoxide radicals and lipid peroxidation were measured in vitro for the various drugs. Comparison of the in-vivo and in-vitro results provides evidence that free radical production explains only partly the in-vivo toxicities.

Doxorubicin cardiomyopathy is associated with a decrease in calcium release channel of the sarcoplasmic reticulum in a chronic rabbit model

Doxorubicin is a highly effective cancer chemotherapeutic agent that produces a dose-dependent cardiomyopathy that limits its clinical usefulness. Clinical and animal studies of morphological changes during the early stages of doxorubicin-induced cardiomyopathy have suggested that the sarcoplasmic reticulum, the intracellular membrane system responsible for myoplasmic calcium regulation in adult mammalian heart, may be the early target of doxorubicin. To detect changes in the calcium pump protein or the calcium release channel (ryanodine receptor) of the sarcoplasmic reticulum during chronic doxorubicin treatment, rabbits were treated with intravenous doxorubicin (1 mg/kg) twice weekly for 12 to 18 doses. Pair-fed controls received intravenous normal saline. The severity of cardiomyopathy was scored by light and electron microscopy of left ventricular papillary muscles. Developed tension was measured in isolated atrial strips. In subcellular fractions from heart, [3H]ryanodine binding was decreased in doxorubicin-treated rabbits (0.33 +/- 0.03 pmol/mg) compared with control rabbits (0.66 +/- 0.02 pmol/mg; P < 0.0001). The magnitude of the decrease in [3H]ryanodine binding correlated with both the severity of the cardiomyopathy graded by pathology score (light and electron microscopy) and the decrease in developed tension in isolated atrial strips. Bmax for [3H]ryanodine binding and the amount of immunoreactive ryanodine receptor by Western blot analysis using sequence-specific antibody were both decreased, consistent with a decrease in the amount of calcium release channel of sarcoplasmic reticulum in doxorubicin-treated rabbits. In contrast, there was no decrease in the amount or the activity of the calcium pump protein of the sarcoplasmic reticulum in doxorubicin-treated rabbits. Doxorubicin treatment did not decrease [3H]ryanodine binding or the amount of immunoreactive calcium release channel of sarcoplasmic reticulum in skeletal muscle. Since the sarcoplasmic reticulum regulates muscle contraction by the cyclic uptake and release of a large internal calcium pool, altered function of the calcium release channel could lead to the abnormalities of contraction and relaxation observed in the doxorubicin cardiomyopathy.

Evaluation of the JT and corrected JT intervals as a new ECG method for monitoring doxorubicin cardiotoxicity in the dog

A comparison was made of the sensitivity of ECG, ultrastructural heart pathology, and plasma enzymes CK-MB and alpha-HBDH as methods to assess doxorubicin cardiotoxicity in adult beagle dogs given doxorubicin 30 mg/m2 i.v. once a week for three times. A progressive increase in JT and QT intervals, in corrected JT (JTc) and QT (QTc) intervals as well as a reduction in both T wave amplitude and RR duration, were observed in doxorubicin-treated dogs; the electrocardiogram (ECG) abnormalities were associated with doxorubicin-induced ultrastructural changes in cardiac tissue, consisting of dilation of the sarcoplasmic reticulum, multiform, flasklike invaginations of T-tubules containing electrondense material, and interruption of the junctional sarcoplasmic reticulum, which became more severe as the observation period progressed. On the contrary, doxorubicin treatment was associated with transient changes in plasma CK-MB and alpha-HBDH, which were unrelated to the severity of chronic cardiotoxicity. Overall results suggest that the monitoring of the ECG parameters related to the repolarization of the cardiac muscle, and particularly JT and JTc, might be regarded as a noninvasive method for the study of doxorubicin cardiotoxocity in the dog.

Cardiac mitochondrial calcium content during fatal doxorubicin toxicity

The purpose of this study was to determine whether abnormalities of mitochondrial divalent cation metabolism are early, causative events in doxorubicin (DXR, Adriamycin) cardiotoxicity. We used electron probe microanalysis (EPMA) to examine the calcium (Ca) and magensium (Mg) content of in situ mitochondria in cryosections of rat hearts, rapidly frozen at 6 hr and 1, 3, and 5 days after a single iv injection of 20 mg/kg DXR. This dose produced 100% mortality in 7 days, with a mean survival of 5.8 days. Mean control mitochondrial Ca and Mg was 0.7 and 28 mmol/kg dry wt, respectively (+/- SEM), and did not change in the DXR-injected animals, even in severely symptomatic rats 5 days after DXR. This suggests that an alteration in mitochondrial divalent cation metabolism is unlikely to be a primary event in the pathogenesis of DXR-induced cardiotoxicity, and that the mitochondrial Ca accumulation demonstrated in previous studies represents a secondary event in cells damaged by another mechanism.

Mitigation of an anthracycline-induced cardiomyopathy by pretreatment with razoxane: a quantitative morphological assessment

A quantitative evaluation of structural modifications was undertaken in the myocardium of daunorubicin (DNR)-treated and razoxane (RZ)-protected mice. BDF1 mice were injected with DNR, 15 mg/kg; a second group of mice was subjected to the same conditions but, in addition, received a pretreatment of RZ, 200 mg/kg. Representative cubes of myocardial tissue were processed for viewing with the electron microscope. Five hundred myocardial cells in each group were examined for the presence of lesions which had been categorized as early, moderate, or advanced. Contrasting the total number of demonstrable lesions in each group revealed a statistically significant reduction of 38% in abnormalities present in RZ-protected mice. By category, RZ-pretreated mice showed a mitigation in the appearance of early and moderate alterations and a striking reduction in the incidence of advanced, irreversible lesions. These results indicate that the cardiomyopathy associated with DNR administration can be ameliorated by pretreatment with RZ; this protective effect is markedly exerted by preventing the development of severe, irreversible lesions in the murine myocardium; the initial, non-transient structural alteration subsequent to DNR-exposure appears to affect the myocardial sarcoplasmic reticulum.