Chronic cardiotoxicity of anticancer anthracyclines in the rat: role of secondary metabolites and reduced toxicity by a novel anthracycline with impaired metabolite formation and reactivity

(1) The anticancer anthracycline doxorubicin (DOX) causes cardiomyopathy upon chronic administration. There is controversy about whether DOX acts directly or after conversion to its secondary alcohol metabolite DOXol. Here, the role of secondary alcohol metabolites was evaluated by treating rats with cumulative doses of DOX or analogues--like epirubicin (EPI) and the novel disaccharide anthracycline MEN 10755--which were previously shown to form less alcohol metabolites than DOX when assessed in vitro. (2) DOX induced electrocardiographic and haemodynamic alterations, like elongation of QalphaT or SalphaT intervals and suppression of isoprenaline-induced dP/dt increases, which developed in a time-dependent manner and were accompanied by cardiomegaly, histologic lesions and mortality. EPI caused less progressive or severe effects, whereas MEN 10755 caused essentially no effect. (3) DOX and EPI exhibited comparable levels of cardiac uptake, but EPI formed approximately 60% lower amounts of its alcohol metabolite EPIol at 4 and 13 weeks after treatment suspension (P<0.001 vs DOX). MEN 10755 exhibited the lowest levels of cardiac uptake; hence, it converted to its alcohol metabolite MEN 10755ol approximately 40% less efficiently than did EPI to EPIol at either 4 or 13 weeks. Cardiotoxicity did not correlate with myocardial levels of DOX or EPI or MEN 10755, but correlated with those of DOXol or EPIol or MEN 10755ol (P=0.008, 0.029 and 0.017, respectively). (4) DOX and EPI inactivated cytoplasmic aconitase, an enzyme containing an Fe-S cluster liable to disassembly induced by anthracycline secondary alcohol metabolites. DOX caused greater inactivation of aconitase than EPI, a finding consistent with the higher formation of DOXol vs EPIol. MEN 10755 did not inactivate aconitase, which was because of both reduced formation and impaired reactivity of MEN 10755ol toward the Fe-S cluster. Aconitase inactivation correlated (P<0.01) with the different levels of cardiotoxicity induced by DOX or EPI or MEN 10755. (5) These results show that (i) secondary alcohol metabolites are important determinants of anthracycline-induced cardiotoxicity, and (ii) MEN 10755 is less cardiotoxic than DOX or EPI, a behaviour attributable to impaired formation and reactivity of its alcohol metabolite.

Impairment of myocardial contractility by anticancer anthracyclines: role of secondary alcohol metabolites and evidence of reduced toxicity by a novel disaccharide analogue

1. The anticancer anthracycline doxorubicin (DOX) causes cardiotoxicity. Enzymatic reduction of a side chain carbonyl group converts DOX to a secondary alcohol metabolite that has been implicated in cardiotoxicity. We therefore monitored negative inotropism, assessed as inhibition of post-rest contractions, in rat right ventricle strips exposed to DOX or to analogues forming fewer amounts of their alcohol metabolites (epirubicin, EPI, and the novel disaccharide anthracycline MEN 10755). 2. Thirty microM EPI exhibited higher uptake than equimolar DOX, but formed comparable amounts of alcohol metabolite due to its resistance to carbonyl reduction. MEN 10755 exhibited also an impaired uptake, and consequently formed the lowest levels of alcohol metabolite. Accordingly, DOX and EPI inhibited post-rest contractions by approximately 40-50%, whereas MEN 10755 inhibited by approximately 6%. 3. One hundred microM EPI exhibited the same uptake as equimolar DOX, but formed approximately 50% less alcohol metabolite. One hundred microM MEN 10755 still exhibited the lowest uptake, forming approximately 60% less alcohol metabolite than EPI. Under these conditions DOX inhibited post-rest contractions by 88%. EPI and MEN 10755 were approximately 18% (P<0.05) or approximately 80% (P<0.001) less inhibitory than DOX, respectively. 4. The negative inotropism of 30-100 microM DOX, EPI, or MEN 10755 correlated with cellular levels of both alcohol metabolites (r=0.88, P<0.0001) and carbonyl anthracyclines (r=0.79, P<0.0001). Nonetheless, multiple comparisons showed that alcohol metabolites were approximately 20-40 times more effective than carbonyl anthracyclines in inhibiting contractility. The negative inotropism of MEN 10755 was therefore increased by chemical procedures, like side chain valeryl esterification, that facilitated its uptake and conversion to alcohol metabolite but not its retention in a carbonyl form. 5. These results demonstrate that secondary alcohol metabolites are important mediators of cardiotoxicity. A combination of reduced uptake and limited conversion to alcohol metabolite formation might therefore render MEN 10755 more cardiac tolerable than DOX and EPI.