Pharmacokinetic characterization of amrubicin cardiac safety in an ex vivo human myocardial strip model II. Amrubicin shows metabolic advantages over doxorubicin and epirubicin

Anthracycline-related cardiotoxicity correlates with cardiac anthracycline accumulation and bioactivation to secondary alcohol metabolites or Reactive Oxygen Species (ROS), like superoxide anion (O 2 ._ ) and hydrogen peroxide (H 2 O 2 ). We reported that in an ex vivo human myocardial strip model, 3 or 10 µM amrubicin accumulated to a lower level compared to equimolar doxorubicin or epirubicin (Salvatorelli et al., accompanying manuscript). Here, we characterized how amrubicin converted to ROS or secondary alcohol metabolite in comparison with doxorubicin (that formed both toxic species) or epirubicin (that lacked ROS formation and showed an impaired conversion to alcohol metabolite). Amrubicin and doxorubicin partitioned to mitochondria and caused similar elevations of H 2 O 2 , but the mechanisms of H 2 O 2 formation were different. Amrubicin produced H 2 O 2 by enzymatic reduction-oxidation of its quinone moiety, while doxorubicin acted by inducing mitochondrial uncoupling. Moreover, mitochondrial aconitase assays showed that 3 µM amrubicin caused an O 2 ._ dependent reversible inactivation, while doxorubicin always caused an irreversible inactivation. Low concentrations of amrubicin therefore proved similar to epirubicin in sparing mitochondrial aconitase from irreversible inactivation. The soluble fraction of human myocardial strips converted doxorubicin and epirubicin to secondary alcohol metabolites that irreversibly inactivated cytoplasmic aconitase; in contrast, strips exposed to amrubicin failed to generate its secondary alcohol metabolite, amrubicinol, and only occasionally exhibited an irreversible inactivation of cytoplasmic aconitase. This was caused by competing pathways that favoured formation and complete or near-to-complete elimination of 9-deaminoamrubicinol. These results characterize amrubicin metabolic advantages over doxorubicin and epirubicin, which may correlate with amrubicin cardiac safety in preclinical or clinical settings.