Anthracyclines and their metabolism in human liver microsomes and the participation of the new microsomal carbonyl reductase.

Anthracyclines (ANTs) are widely used in the treatment of various forms of cancer. Although their usage contributes to an improvement in life expectancy, it is limited by severe adverse effects-acute and chronic cardiotoxicity. Several enzymes from both AKR and SDR superfamilies have been reported as participants in the reduction of ANTs. Nevertheless all of these are located in the cytosolic compartment. One microsomal reductase has been found to be involved in the metabolism of xenobiotics-11beta-HSD1, but no further information has been reported about its role in the metabolism of ANTs. The aim of this study is to bring new information about the biotransformation of doxorubicin (DOX), daunorubicin (DAUN) and idarubicin (IDA), not only in human liver microsomal fraction, but also by a novel human liver microsomal carbonyl reductase that has been purified by our group. The reduction of ANTs at C-13 position is regarded as the main pathway in the biotransformation of ANTs. However, our experiments with human liver microsomal fraction show different behaviour, especially when the concentration of ANTs in the incubation mixture is increased. Microsomal fraction was incubated with doxorubicin, daunorubicin and idarubicin. DOX was both reduced into doxorubicinol (DOXOL) and hydrolyzed into aglycone DOX and then subsequently reduced. The same behaviour was observed for the metabolism of DAUN and IDA. The activity of hydrolases definitely brings a new look to the entire metabolism of ANTs in microsomal fraction, as formed aglycones undergo reduction and compete for the binding site with the main ANTs. Moreover, as there are two competitive reducing reactions present for all three ANTs, kinetic values of direct reduction and the reduction of aglycone were calculated. These results were compared to previously published data for human liver cytosol. In addition, the participation of the newly determined human liver microsomal carbonyl reductase was studied. No reduction of DOX into DOXOL was detected. Nevertheless, the involvement in reduction of DAUN into DAUNOL as well as IDA into IDAOL was demonstrated. The kinetic values obtained were then compared with data which have already been reported for cytosolic ANTs reductases.