Doxorubicinolone Formation and Efflux: A Salvage Pathway against Epirubicin Accumulation in Human Heart

Abstract

Secondary alcohol metabolites and reactive oxygen species mediate cardiomyopathy induced by cumulative doses of antitumor anthracyclines, such as doxorubicin and epirubicin. Epirubicin exhibits a defective conversion to both toxic species, thereby inducing cardiotoxicity at doses higher than equiactive to doxorubicin; however, the gain in cardiac tolerability seems to be marginal compared with the magnitude of the metabolic defects of epirubicin. Cardiomyopathy may occur independent of toxic metabolites if a given anthracycline tends to accumulate in the heart; therefore, we characterized whether epirubicin showed an unusual accumulation in human myocardial strips incubated in plasma. Epirubicin exhibited a higher uptake and reached myocardial levels 2 times higher than those of doxorubicin. Epirubicin also showed a unique metabolization to doxorubicinolone, the product of epirubicin deglycosidation and carbonyl reduction. In diffusing from the strips to plasma, doxorubicinolone caused membrane permeation effects that augmented epirubicin elimination. Experiments with purified doxorubicinolone showed that the efflux of 1 mol doxorubicinolone promoted the concomitant elimination of as many as ϳ40 mol epirubicin. Doxorubicinolone could also diffuse from plasma back to the strips, causing a permeation effect that promoted epirubicin reuptake; however, this reverse process was slower and less potent. On balance, doxorubicinolone efflux diminished the epirubicin to doxorubicin accumulation ratio to ϳ1.5. These results suggest that the cardiac tolerability of epirubicin is limited by its accumulation in the heart and that such accumulation would be even higher in the absence of doxorubicinolone formation and efflux. These results may also serve guidelines for developing noncardiotoxic anthracyclines.

Pharmacokinetic Characterization of Amrubicin Cardiac Safety in an Ex Vivo Human Myocardial Strip Model. I. Amrubicin Accumulates to a Lower Level than Doxorubicin or Epirubicin

“…2; Table 1). This discrepancy is explained by the fact that in standard 4-h incubations, longer equilibration, of anthracycline uptake and clearance with anthracycline metabolization allowed epirubicin to form minute amounts of doxorubicinolone (Ͻ0.5 M), a potent membrane-permeabilizing agent that favored the release of excess epirubicin (Salvatorelli et al, 2009).…”

“…During the experiment, plasma samples (200 l) were taken at regular times and assayed for parent anthracyclines or metabolites. The values determined at each time point were added with values measured at the preceding time point: this provided the time course of the cumulative efflux of anthracyclines or their metabolites, without confounding effects caused by their competing reuptake in the strips (Salvatorelli et al, 2009). At the end of the experiments, the strips were assayed for anthracycline that had not been released during washout (anthracycline retention).…”

“…By adding anthracycline retention with cumulative anthracycline efflux, the amount of anthracycline that had been taken up during the 30-min loading could be calculated. Anthracycline clearance was calculated by the formula [100 ϫ (cumulative efflux/uptake)] (Salvatorelli et al, 2009).…”

“…Limited amrubicin accumulation was demonstrated under conditions of standard 4-h incubations or multiple washout experiments. Standard 4-h incubations were adopted to allow maximal formation and optimize identification of metabolites that could influence myocardial pharmacokinetics of one anthracycline analog in comparison with another; however, standard 4-h incubations do not allow us to distinguish and quantify anthracycline uptake versus clearance (Salvatorelli et al, 2009). To determine whether the low steady-state accumulation of amrubicin could be caused by its reduced uptake and/or its enhanced clearance, we assessed amrubicin and doxorubicin or epirubicin under conditions of loading and multiple washouts that were more suitable for defining the two processes separately (Salvatorelli et al, 2009).…”

“…Standard 4-h incubations were adopted to allow maximal formation and optimize identification of metabolites that could influence myocardial pharmacokinetics of one anthracycline analog in comparison with another; however, standard 4-h incubations do not allow us to distinguish and quantify anthracycline uptake versus clearance (Salvatorelli et al, 2009). To determine whether the low steady-state accumulation of amrubicin could be caused by its reduced uptake and/or its enhanced clearance, we assessed amrubicin and doxorubicin or epirubicin under conditions of loading and multiple washouts that were more suitable for defining the two processes separately (Salvatorelli et al, 2009). Our results suggest that a pattern of increased clearance contributed to limiting amrubicin accumulation in experiments with anthracycline at 3 and 10 M; reduced amrubicin uptake was shown in experiments with anthracycline at 10 M, but this was relatively marginal compared with the near-to-complete clearance that occurred in the experiments.…”