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.

Prevention of chronic anthracycline cardiotoxicity in the adult Fischer 344 rat by dexrazoxane and effects on iron metabolism.

PURPOSE: Anthracyclines, such as doxorubicin and daunorubicin, continue to be widely used in the treatment of cancer, although they share the adverse effect of chronic, cumulative dose-related cardiotoxicity. The only approved treatment in prevention of anthracycline cardiotoxicity is dexrazoxane, a putative iron chelator. Previous in vitro studies have shown that disorders of iron metabolism, including altered IRP1-IRE binding, may be an important mechanism of anthracycline cardiotoxicity. METHODS: This study examined the role of IRP1-IRE binding ex vivo in a chronic model of daunorubicin cardiotoxicity in the Fischer 344 rat and whether dexrazoxane could prevent any daunorubicin-induced changes in IRP1 binding. Young adult (5-6 months) Fischer 344 rats received daunorubicin (2.5 mg/kg iv once per week for 6 weeks) with and without pretreatment with dexrazoxane (50 mg/kg ip). Other groups received saline (controls) or dexrazoxane alone. Rats were killed either 4 h or 2 weeks after the last dose of daunorubicin to assess IRP1-IRE binding. RESULTS: Contractility (dF/dt) of atrial tissue, obtained from rats 2 weeks after the last dose of daunorubicin, was significantly reduced in daunorubicin-treated compared to control rats. Dexrazoxane pretreatment protected against the daunorubicin-induced decrease in atrial dF/dt. However, left ventricular IRP1/IRE binding was not affected by daunorubicin treatment either 4 h or 2 weeks after the last dose of daunorubicin. CONCLUSIONS: IRP1 binding may not be altered in the rat model of chronic anthracycline cardiotoxicity.

Additive effect of theophylline on the cardiac response to isoproterenol.

To ascertain the effect of theophylline on the cardiac chronotropic response to beta-adrenergic stimulation, isoproterenol dose-response curves in healthy young subjects were compared during saline and theophylline maintenance infusions. Each study was repeated 1 to 3 weeks later to evaluate reproducibility. Neither the dose of isoproterenol required to raise the heart rate by 25 bpm (2.32 +/- 0.81 vs. 1.55 +/- 0.46 micrograms on day 1 and 1.28 +/- 0.22 vs. 1.27 +/- 0.25 micrograms on day 2) nor the slopes of the dose-response curves were affected by theophylline. Higher heart rates were observed after isoproterenol bolus dosing during theophylline than during saline infusion because of additive chronotropic effects of theophylline and isoproterenol. Since theophylline does not interact in a synergistic fashion with isoproterenol, phosphodiesterase inhibition appears to be an unlikely mechanism of the chronotropic effect of methylxanthines at therapeutic concentrations.

Cigarette smoking and theophylline metabolism: effects of cimetidine.

The inhibition of theophylline metabolism by cimetidine was investigated in young male cigarette smokers (greater than 20 cigarettes/day) and nonsmokers by stable isotope methodology. Subjects received oral theophylline (510 mg/day) for 14 days and cimetidine (1200 mg/day) over days 1 to 7 or 8 to 14. On days 7 and 14, a tracer dose (10 mg) of stable isotope-labeled theophylline was injected intravenously with the oral dose of theophylline. Serial plasma samples were then obtained for 24 hours and both molecular forms of theophylline were assayed by mass spectrometry after purification by HPLC. Theophylline bioavailability, volume of distribution, and protein binding were of the same order in both groups and were not affected by cimetidine. Although the basal theophylline elimination rate constant was 46% greater and clearance was 54% greater in smokers than in nonsmokers, the proportionate changes in steady-state plasma concentrations, t1/2, and clearance due to cimetidine were much the same in both groups. Plasma thiocyanate concentrations were higher in smokers than in nonsmokers and were related to theophylline clearance. Our findings indicate that cimetidine inhibits theophylline metabolism to a similar extent in both smokers and nonsmokers. Determination of plasma thiocyanate levels may be valuable in the prediction of theophylline clearance.

Cigarette smoking and theophylline metabolism: effects of phenytoin.

The induction of theophylline clearance by phenytoin was investigated in 12 young male subjects (six nonsmokers and six cigarette smokers). Each subject received intravenous theophylline to determine baseline pharmacokinetics. This was followed by an intravenous loading dose of phenytoin sodium and oral maintenance dosing for 2 weeks, after which the intravenous theophylline study was repeated. Phenytoin concentrations were similar in nonsmokers (10.8 +/- 2.0 micrograms/ml) and smokers (11.5 +/- 0.9 micrograms/ml). Control theophylline elimination half-life was 35% less and clearance 88% greater in smokers than in nonsmokers. The proportionate changes in half-life (26.8% +/- 5.6% in smokers and 25.8% +/- 3.5% in nonsmokers) and clearance (48.0% +/- 10.1% in smokers and 39.7% +/- 7.2% in nonsmokers) as the result of phenytoin induction were similar in both groups. These results demonstrate that the induction of theophylline clearance by phenytoin is additive to that caused by cigarette smoking and provide support for the suggestion that theophylline metabolism is influenced by multiple polymorphisms.

Hepatic drug metabolism and aging.

Although there is considerable variation in the effect of age on drug biotransformation, the metabolism of many drugs is impaired in the elderly. Age-related physiological changes, such as a reduction in liver mass, hepatic metabolising enzyme activity, liver blood flow and alterations in plasma drug binding may account for the decreased elimination of some metabolised drugs in the elderly. It is difficult, however, to separate an effect of aging from a background of marked variation in the rate of metabolism due to factors such as individual metabolic phenotype, environmental influences, concomitant disease states and drug intake. The prevailing data suggest that initial doses of metabolised drugs should be reduced in older patients and then modified according to the clinical response. In most studies the elderly appear as responsive as young individuals to the effects of compounds which induce or inhibit the activity of cytochrome P450 isozymes. Concurrent use of other agents, which induce or inhibit drug metabolism, mandates dose adjustment as in younger patients. Many questions remain unanswered. For instance, limitations of in vitro studies prevent any firm conclusion about changes in hepatic drug metabolising enzyme activity in the elderly. With aging, some pathways of drug metabolism may be selectively affected, but this has not been adequately scrutinised. The possibility that metabolism of stereoisomers may be altered in the elderly has not been adequately tested. The effect of aging on the distribution of polymorphic drug metabolism phenotypes is still not established, despite potential implications for disease susceptibility and survival advantage.

Time-related increases in cardiac concentrations of doxorubicinol could interact with doxorubicin to depress myocardial contractile function.

1. The present study evaluated the time-dependency of acute anthracycline cardiotoxicity by varying the duration of exposure of rabbit isolated atria to doxorubicin and determining changes (1) in contraction and relaxation and (2) in atrial concentrations of doxorubicin and its C-13 hydroxy metabolite, doxorubicinol. 2. Following addition of doxorubicin (175 microM) to atria, contractility (dF/dt), muscle stiffness (resting force, RF) and relaxation (90% relaxation time, 90% RT) were monitored for a 3.5 h period. 3. Doxorubicin (175 microM) progressively diminished mechanical function (decreased dF/dt, increased RF and prolonged 90% RT) over 3 h. Doxorubicinol (1.8 microM), however, failed to produce time-related cardiac dysfunction; it depressed contractile function and increased muscle stiffness during the first 30 min without causing additional cardiac dysfunction during the remaining 3 h of observation. Doxorubicinol had no effect on 90% RT. 4. During treatment with doxorubicin, atria contained considerably more doxorubicin than doxorubicinol (ratio of doxorubicin to doxorubicinol ranged from 778 to 74 at 0.5 and 3 h, respectively). Elevations of doxorubicin and doxorubicinol in atria paralleled the degree of dysfunction of both contraction and relaxation; increases in muscle stiffness, however, were more closely associated with increases of doxorubicinol than doxorubicin. 5. To probe the relation between cardiac doxorubicinol and myocardial dysfunction further, without confounding effects of cardiac doxorubicin, concentration-response experiments with doxorubicinol (0.9-7.2 microM) were conducted. 6. Plots of doxorubicinol concentrations in atria vs contractility indicated that the cardiac concentration of doxorubicinol, at which contractility is reduced by 50%, is five fold lower in doxorubicin-treated than in doxorubicinol-treated preparations. Thus, doxorubicin and doxorubicinol appear to interact to depress contractile function.7. Cardiac concentrations of both doxorubicin and doxorubicinol, as observed in these studies, were found to stimulate markedly Ca2+ release from isolated SR vesicles, but 3 microM doxorubicinol promoted a 15 fold greater release rate than 3 microM doxorubicin.8. Our observations coupled with the previously reported finding that doxorubicinol inhibits Ca2+loading of SR, suggests that doxorubicinol accumulation in heart contributes to the time-dependent component of doxorubicin cardiotoxicity, through a mechanism that could involve perturbations of Ca2+ homeostasis.

Prevention by dexrazoxane of down-regulation of ryanodine receptor gene expression in anthracycline cardiomyopathy in the rat.

Anthracyclines can cause cumulative dose-related cardiotoxicity characterized by changes in Ca(2+) metabolism, including dysfunction of the sacroplasmic reticulum (SR) and decreased expression of Ca(2+)-handling proteins, such as the ryanodine receptor (RyR2). In this study, we examined the effect of dexrazoxane (ICRF-187), an iron chelator which prevents anthracycline cardiotoxicity, on RyR2 gene expression in rats treated chronically with daunorubicin. Daunorubicin (2.5 mg kg(-1) i.v. weekly for 6 weeks) produced cardiotoxicity as demonstrated by histopathologic changes. The ryanodine receptor/glyceraldehyde phosphate dehydrogenase (GAPDH) mRNA ratio was decreased by 38+/-3% (P<0.02) compared to values in control rats. Dexrazoxane pre-treatment (50 mg kg(-1); 1 h prior to each daunorubicin injection) prevented the decrease in RyR2/GAPDH mRNA ratio and histopathologic lesions in daunorubicin-treated rats. This is the first report that a protective agent such as dexrazoxane can ameliorate the decreased expression of a specific gene involved in anthracycline-induced cardiotoxicity.

Problems in the use of anticancer drugs in the elderly.

The elderly represent a special challenge to the physician in providing effective cancer chemotherapy. Though they represent the majority of the patients who eventually will need such therapy, until recently little information was available on its use in this population. There are variable age changes in pharmacokinetics, particularly in renal elimination of drug and metabolites, which may necessitate dosage amendment. Concomitant renal impairment or hepatic disease may further alter drug disposition. Other common pre-existing conditions in the elderly also may increase susceptibility to adverse drug effects. For example, the risk of toxicity from doxorubicin and vincristine can be increased in the presence of pre-existing cardiac disease or peripheral neuropathy, respectively. Because of the variability of the ageing process and the effects of concomitant disease, each patient must be assessed on an individual basis. Furthermore, in treatment planning, not only age and health status but also the patient's attitude and the tumour type are important considerations. Chemotherapy for most malignancies appears beneficial and well tolerated in the elderly, and there is little evidence that age per se is a determinant of chemotherapy regimen selection and dosing. The exceptions may be the curable haematological malignancies for which chemotherapy seems less efficacious and more toxic in geriatric than younger patients. The complications of chemotherapy such as vomiting, mucositis and bone marrow depression must be anticipated, diagnosed early and managed aggressively in aged patients. Guidelines are provided to help manage these problems. Chemotherapy in the elderly is still at a relatively early stage of development. Further research is required to establish optimal regimens for use in this population, in particular for curable haematological neoplasms.

Aging and the response to inhibition and induction of theophylline metabolism.

The twofold to threefold higher incidence of adverse drug reactions in elderly as opposed to younger patients is due mainly to more severe disease and the requirement for more complex drug treatment regimens. The incidence of adverse drug reactions increases with the number of prescribed drugs. Because of multiple drug use by the elderly, the potential for drug interactions is greater in this patient group. Surprisingly, the effect of age on the clinical pharmacology of drug interactions has not been thoroughly investigated. Our studies have shown that cimetidine inhibits and phenytoin induces the metabolism of theophylline to a similar extent in healthy male nonsmokers and smokers. Preliminary analysis of the results of a study to investigate the inhibition of theophylline metabolism by cimetidine and ciprofloxacin administered in combination to healthy male and female nonsmokers also does not show an age difference in response. Additional careful studies are needed to evaluate further the pharmacology and clinical importance of pharmacokinetic and pharmacodynamic drug interactions in the elderly.

Taurine deficiency and doxorubicin: interaction with the cardiac sarcolemmal calcium pump.

An anticancer drug, doxorubicin, and a naturally occurring beta-amino acid, taurine, exert opposing actions on myocardial calcium content and lipid peroxidation. Thus, we tested the hypothesis that the two agents may interact to modify cardiac calcium metabolism and indices of lipid peroxidation. Cardiac taurine levels were reduced by half in rats given tap water containing a beta-amino transport inhibitor, beta-alanine. Taurine deficiency was associated with an increased susceptibility of the heart to doxorubicin-mediated calcium accumulation, a phenomenon commonly associated with doxorubicin cardiotoxicity. Taurine deficiency also predisposed the heart to enhanced formation of malondialdehyde caused by doxorubicin administration. While increases in malondialdehyde levels are often associated with lipid peroxidation, the failure of doxorubicin to cause changes in oxidized glutathione content makes peroxidative mechanisms a less likely explanation for the potentiation of doxorubicin-mediated myocardial calcium accumulation in taurine-deficient rats. A more likely possibility is the interaction between taurine deficiency and doxorubicin to inhibit the sarcolemmal calcium pump. The data also suggest that the interaction between doxorubicin and taurine deficiency does not involve alterations in the pharmacokinetics of doxorubicin or the cardiotoxic metabolite, doxorubicinol. It is concluded that reduction in sarcolemmal calcium pump activity by taurine deficiency may contribute to myocardial calcium accumulation in hearts whose calcium homeostasis has been compromised by doxorubicin.

Daunorubicin cardiotoxicity: evidence for the importance of the quinone moiety in a free-radical-independent mechanism.

Anthracyclines, such as daunorubicin (Daun), and other quinone-containing compounds can stimulate the formation of toxic free radicals. The present study tests the hypothesis that the quinone moiety of Daun, by increasing free-radical production, disrupts sarcoplasmic reticulum (SR) function and thereby inhibits myocardial contractility in vitro. We compared Daun with its quinone-deficient analogue, 5-iminodaunorubicin (5-ID), using experimental interventions to produce various contractile states that depend on SR function. At concentrations of Daun or 5-ID that did not alter contractility (dF/dt) of steady-state contractions (1 Hz) in electrically paced atria isolated from adult rabbits, only Daun significantly attenuated the positive inotropic effects on dF/dt of increased rest intervals (PRP; post-rest potentiation) or increased stimulation frequencies. Attenuation was to 98+/-6% at 1 Hz, and 73+/-8 and 67+/-8% for 30 and 60 sec PRP, respectively, and 73+/-3 and 63 +/-3% at 2 and 3 Hz, respectively, for 88 microM Daun (P<0.05, vs pre-drug baseline values, mean +/- SEM). These effects of Daun were similar to those of caffeine (2 mM), an agent well known to deplete cardiac SR calcium. We also examined the effect of Daun in isolated neonatal rabbit atria, which lack mature, functional SR; Daun did not alter the force-frequency relationship or PRP contractions. Additional studies in Ca(2+)-loaded SR microsomes indicated that both Daun and 5-ID opened Ca(2+) release channels, with Daun being 20-fold more potent than 5-ID in this respect. Neither anthracycline, however, induced free-radical formation in SR preparations (assayed via nicking of supercoiled DNA) prior to stimulating Ca(2+) release. Thus, our results indicate that Daun impairs myocardial contractility in vitro by selectively interfering with SR function; the quinone moiety of Daun appears to mediate this cardiotoxic effect, acting through a mechanism that does not involve free radicals.

Aging increases the cardiotoxicity of daunorubicin and daunorubicinol in the rat.

This study examined effects of aging on the cardiac response in vitro to daunorubicin, a cancer chemotherapeutic agent that causes cardiotoxicity. Left ventricular trabeculae carneae from adult (aged 6-9 months) and old (aged 24-28 months) Fischer 344 rats were placed in oxygenated, physiological buffer. Preparations were treated with daunorubicin (175 microM) or saline (controls) over a 210-minute study period. Daunorubicin-induced decline in contractility (DS and dS/dt) was greater in old compared to adult myocardium (p < .02). Similarly, cardiac relaxation (90% relaxation time) was more impaired by daunorubicin in older preparations (p < 01). Although daunorubicin concentrations were unaffected by age, daunorubicinol concentrations in ventricular strips increased with time to a greater extent in the older group (p < .05). This study suggests that senescence increases the acute in vitro cardiotoxicity of daunorubicin and that the metabolite, daunorubicinol, may contribute to this toxicity.

Daunorubicin and daunorubicinol pharmacokinetics in plasma and tissues in the rat.

Recent evidence suggests that 13-hydroxy metabolites of anthracyclines may contribute to cardiotoxicity. This study was designed to determine the pharmacokinetics of daunorubicin and the 13-hydroxy metabolite daunorubicinol in plasma and tissues, including the heart. Fisher 344 rats received 5 mg kg-1 daunorubicin i.v. by bolus injection. Rats were killed at selected intervals for up to 1 week after daunorubicin administration for determination of concentrations of daunorubicin and daunorubicinol in the plasma, heart, liver, kidney, lung, and skeletal muscle. Peak concentrations of daunorubicin were higher than those of daunorubicinol in the plasma (133 +/- 7 versus 36 +/- 2 ng ml-1; P < 0.05), heart (15.2 +/- 1.4 versus 3.4 +/- 0.4 micrograms g-1; P < 0.05), and other tissues. However, the apparent elimination half-life of daunorubicinol was longer than that of daunorubicin in most tissues, including the plasma (23.1 versus 14.5 h) and heart (38.5 versus 19.3 h). In addition, areas under the concentration/time curves (AUC infinity) obtained for daunorubicinol exceeded those found for daunorubicin in almost all tissues, with the ratios being 1.9 in plasma and 1.7 in the heart. The ratio of daunorubicinol to daunorubicin concentrations increased dramatically with time from < 1 at up to 1 h to 87 at 168 h in cardiac tissue. Thus, following daunorubicin injection, cumulative exposure (AUC infinity) to daunorubicinol was greater than that to daunorubicin in the plasma and heart. If daunorubicinol has equivalent or greater potency than daunorubicin in causing impairment of myocardial function, it may make an important contribution to the pathogenesis of cardiotoxicity.

Doxorubicin and doxorubicinol pharmacokinetics and tissue concentrations following bolus injection and continuous infusion of doxorubicin in the rabbit.

Cumulative dose-related, chronic cardiotoxicity is a serious clinical complication of anthracycline therapy. Clinical and animal studies have demonstrated that continuous infusion, compared to bolus injection of doxorubicin, decreases the risk of cardiotoxicity. Continuous infusion of doxorubicin may result in decreased cardiac tissue concentrations of anthracyclines, including the primary metabolite doxorubicinol, which may also be an important contributor to cardiotoxicity. In this study, doxorubicin and doxorubicinol plasma pharmacokinetics and tissue concentrations were compared in New Zealand white rabbits following intravenous administration of doxorubicin (5 mg.kg-1) by bolus and continuous infusion. Blood samples were obtained over a 72-h period after doxorubicin administration to determine plasma doxorubicin and doxorubicinol concentrations. Rabbits were killed 7 days after the completion of doxorubicin administration and tissue concentrations of doxorubicin and doxorubicinol in heart, kidney, liver, and skeletal muscle were measured. In further experiments, rabbits were killed 1 h after bolus injection of doxorubicin and at the completion of a 24-h doxorubicin infusion (anticipated times of maximum heart anthracycline concentrations) to compare cardiac concentrations of doxorubicin and doxorubicinol following both methods of administration. Peak plasma concentrations of doxorubicin (1739 +/- 265 vs 100 +/- 10 ng.ml-1) and doxorubicinol (78 +/- 3 vs 16 +/- 3 ng.ml-1) were significantly higher following bolus than infusion dosing. In addition, elimination half-life of doxorubicinol was increased following infusion. However, other plasma pharmacokinetic parameters for doxorubicin and doxorubicinol, including AUC infinity, were similar following both methods of doxorubicin administration. Peak left ventricular tissue concentrations of doxorubicin (16.92 +/- 0.9 vs 3.59 +/- 0.72 micrograms.g-1 tissue; P < 0.001) and doxorubicinol (0.24 +/- 0.02 vs 0.09 +/- 0.01 micrograms.g-1 tissue; P < 0.01) following bolus injection of doxorubicin were significantly higher than those following infusion administration. Tissue concentrations of parent drug and metabolite in bolus and infusion groups were similar 7 days after dosing. The results suggest that cardioprotection following doxorubicin infusion may be related to attenuation of the peak plasma or cardiac concentrations of doxorubicin and/or doxorubicinol.

Age-related pharmacokinetics of daunorubicin and daunorubicinol following intravenous bolus daunorubicin administration in the rat.

Age-related differences in pharmacokinetics may be important in determining altered anthracycline cardiotoxicity in the senescent rat and also in older humans. This study examined the effect of aging on daunorubicin pharmacokinetics in the Fischer 344 rat. Daunorubicin 7.5 mg/kg was administered i.v. to 6- and 24-month-old male Fischer 344 rats and plasma and tissue sampling was performed over 168 h for assay of daunorubicin and daunorubicinol concentrations by high-performance liquid chromatography. Systemic clearance of daunorubicin was decreased in older compared to younger animals (56 +/- 4 versus 202 +/- 17 ml min-1 kg-1; P < 0.05). In addition, the area under the plasma daunorubicinol concentration/time curve was significantly increased in older rats. In the heart, the area under the concentration/time curve was significantly increased in senescence both in the case of daunorubicin (201 +/- 12 versus 86 +/- 4 micrograms h g-1; P < 0.05) and daunorubicinol (1347 +/- 118 versus 182 +/- 4 micrograms h g-1; P < 0.05). Furthermore, the peak mean concentrations of daunorubicin were increased in older compared to younger rats both in plasma (1078 +/- 82 versus 663 +/- 66 ng ml-1; P < 0.05) and in heart (27 +/- 1 versus 10 +/- 1 micrograms g-1; P < 0.05). This also was true for daunorubicinol in plasma (284 +/- 39 versus 168 +/- 27 ng ml-1; P < 0.05) and in myocardium (8.6 +/- 0.6 versus 2.4 +/- 0.2 micrograms g-1; P < 0.05). Following daunorubicin injection, the ratio of daunorubicinol to daunorubicin concentrations in tissues increased with time, particularly in plasma and heart in senescent rats. Thus, there are significant age-related changes in daunorubicin and daunorubicinol kinetics in the rat that could alter susceptibility to acute systemic toxicity and to chronic cardiotoxicity.

Effect of phenytoin on the pharmacokinetics of doxorubicin and doxorubicinol in the rabbit.

Doxorubicin is metabolized extensively to doxorubicinol by the ubiquitous aldoketoreductase enzymes. The extent of conversion to this alcohol metabolite is important since doxorubicinol may be the major contributor to cardiotoxicity. Aldoketoreductases are inhibited in vitro by phenytoin. The present study was conducted to examine the effect of phenytoin on doxorubicin pharmacokinetics. Doxorubicin single-dose pharmacokinetic studies were performed in 10 New Zealand White rabbits after pretreatment with phenytoin or phenytoin vehicle (control) infusions in crossover fashion with 4-6 weeks between studies. Infusions were commenced 16 h before and during the course of the doxorubicin pharmacokinetic studies. Phenytoin infusion was guided by plasma phenytoin estimation to maintain total plasma concentrations between 20 and 30 micrograms/ml. Following doxorubicin 5 mg/kg by i.v. bolus, blood samples were obtained at intervals over 32 h. Plasma doxorubicin and doxorubicinol concentrations were measured by HPLC. The mean plasma phenytoin concentrations ranged from 17.4 to 33.9 micrograms/ml. Phenytoin infusion did not alter doxorubicin pharmacokinetics. The elimination half-life and volume of distribution were almost identical to control. Clearance of doxorubicin during phenytoin administration (60.9 +/- 5.8 ml/min per kg, mean +/- SE) was similar to that during vehicle infusion (67.5 +/- 5.4 ml/min per kg). Phenytoin administration was associated with a significant decrease in doxorubicinol elimination half-life from 41.0 +/- 4.8 to 25.6 +/- 2.8 h. The area under the plasma concentration/time curve (AUC) for doxorubicinol decreased significantly from 666.8 +/- 100.4 to 491.5 +/- 65.7 n.h.ml-1. These data suggest that phenytoin at clinically relevant concentrations does not alter the conversion of doxorubicin to doxorubicinol in the rabbit. The reduction in the AUC for doxorubicinol caused by phenytoin appears to be due to an increased rate of doxorubicinol elimination. Phenytoin or similar agents may have the effect of modifying doxorubicinol plasma concentrations by induction of doxorubicinol metabolism rather than by inhibition of aldoketoreductase enzymes.

Effect of a low-protein diet on doxorubicin pharmacokinetics in the rabbit.

Malnutrition involving protein deficiency, which commonly occurs in cancer patients receiving anthracycline treatment, is considered to be a risk factor for the development of cardiotoxicity. Protein deficiency has been shown to impair the metabolism of drugs such as theophylline and acetaminophen. If protein deficiency also impairs anthracycline metabolism, it could explain at least in part the enhanced anthracycline toxicity associated with malnutrition. We tested this idea by determining the effect of a low-protein, isocaloric diet on doxorubicin pharmacokinetics in rabbits. The animals were randomized into two groups for 8-12 weeks. Rabbits in group 1 received a low-protein (5%), isocaloric diet, whereas those in group 2 received a normal-protein (15%) diet. Both groups (group 1, n = 15; group 2, n = 14) were given 5 mg/kg doxorubicin by i.v. bolus. After doxorubicin injection, blood samples were obtained over the next 52 h for the measurement of doxorubicin and doxorubicinol plasma concentrations by high-performance liquid chromatography (HPLC) with fluorometric detection. The low-protein diet significantly decreased doxorubicin clearance (48 +/- 3 vs 59 +/- 4 ml min-1 kg-1; P less than 0.05), prolonged the terminal elimination half-life (28 +/- 2 vs 22 +/- 2 h; P less than 0.05), and increased the area under the plasma concentration/time curve extrapolated to infinity (1722 +/- 122 vs 1405 +/- 71 ng h ml-1; P less than 0.05) as compared with the values determined for rabbits fed the standard rabbit chow (15% protein). The volume of distribution for doxorubicin was not altered by the low-protein diet. In addition, in rabbits fed the the low-protein diet, the terminal elimination half-life of the alcohol metabolite, doxorubicinol was prolonged (52 +/- 5 vs 40 +/- 2 h; P less than 0.05). Thus, a low-protein diet causes a reduction in the ability of rabbits to eliminate doxorubicin and possibly its alcohol metabolite doxorubicinol. If a similar alteration in anthracycline pharmacokinetics occurs in malnourished cancer patients, this phenomenon may contribute to their increased risk of developing cardiotoxicity associated with anthracycline therapy.

Aging alters the force-frequency relationship and toxicity of oxidative stress in rabbit heart.

Adult (6 months) and senescent (greater than 5 years) rabbit atria were studied under conditions known to increase cytoplasmic calcium (increased frequency of contraction and oxidative stress). At a contraction frequency of 1/sec, cardiac relaxation (90% relaxation time) was similar in senescent and adult atria but at a frequency of 2 or 3/sec, relaxation was significantly slower in senescent preparations (P less than 0.05). Additional experiments indicated that H2O2 (500 microM), a powerful oxidant, increased resting force and decreased developed force (DF) much more rapidly in senescent than adult atria; the maximum decrease in DF, however, was less in senescent preparations (adult = 81 +/- 6% and senescent = 42 +/- 27% of pre-H2O2 values; P less than 0.05). Age-related differences in effects of H2O2 did not result simply from a decreased ability of senescent hearts to detoxify an oxidative stress by the glutathione pathway. Both basal glutathione (GSH) concentrations and the H2O2-mediated decreases in GSH were similar in adult and senescent ventricular preparations, as were activities of glutathione peroxidase and glutathione reductase. These observations suggest that interventions known to increase cytoplasmic calcium can amplify age-related impairments of cardiac relaxation through mechanisms that may be independent of the glutathione pathway.

Salicylate intoxication in the elderly. Recognition and recommendations on how to prevent it.

Aspirin (acetylsalicylic acid) and its salicylate derivatives are effective antipyretic, analgesic, and anti-inflammatory agents that are still very widely used by the elderly despite the advent of newer, potentially safer nonsteroidal anti-inflammatory drugs (NSAIDs). However, none of the new NSAIDs have been proven to be more effective than aspirin or salicylic acid. Chronic salicylate intoxication which is most common in the elderly, may occur with therapeutic doses. Increased toxicity in older patients often appears due to inadvertent overdosage. Dual prescribing or additional use of nonprescription salicylates are some causes of unwitting long term toxicity. According to some studies, systemic clearance of salicylate (mainly by hepatic metabolism) is reduced with age, as is renal elimination. These changes are of increased importance in the elderly using high therapeutic doses of salicylates when metabolism is saturated and more unchanged drug is available for renal excretion. In the face of renal impairment, the risk of toxicity is increased. The diagnosis of acute salicylate intoxication generally does not pose diagnostic problems. Patients often present with a history of intentional overdose, with hyperventilation, fever, and nausea. The diagnosis can be confirmed by measuring serum salicylate concentrations. Chronic intoxication often poses a diagnostic dilemma with atypical presentations mimicking other disease states such as diabetic ketoacidosis, delirium, cerebrovascular accident, myocardial infarction or cardiac failure. The diagnosis of salicylate intoxication should be borne in mind when an older patient presents with recent deterioration in activities of daily living with no known cause. Plasma salicylate concentrations should be measured if salicylate intoxication is suspected, even if there is no documented history of salicylate ingestion. The risk of salicylate nephrotoxicity is also increased with age, and upper gastrointestinal haemorrhage is associated with increased mortality in older age groups. Treatment of acute toxicity consists of prompt recognition of salicylate intoxication, use of activated charcoal, correction of acid-base abnormalities, general supportive measures, and if concentrations are extremely high, dialysis can be effectively used. Chronic toxicity, which can occur even with marginally high salicylate concentrations, is treated with drug withdrawal and supportive therapy. Chronic salicylate toxicity can be averted by prescription of conservative doses of drug, avoidance of concomitant use of different salicylate preparations, and therapeutic monitoring to guide dosage. Renal function should be monitored to detect nephrotoxicity from chronic salicylate therapy. Patients should be regularly screened for evidence of gastrointestinal bleeding.(ABSTRACT TRUNCATED AT 400 WORDS)