Received for publication May 13, 2008.
Revised July 14, 2008.
Accepted for publication July 15, 2008.

CBR1 is a predominant doxorubicin reductase in the human liver

Abstract

A first step in the enzymatic disposition of the antineoplastic drug doxorubicin (DOX) is the reduction to doxorubicinol (DOX-OL). Since DOX-OL is less antineoplastic but more cardiotoxic than the parent compound, the individual rate of this reaction may affect the antitumor effect and the risk of DOX-induced heart failure. Using purified enzymes and human tissues we determined enzymes generating DOX-OL and inter-individual differences in their activities. Human tissues express at least two DOX-reducing enzymes. High-clearance organs (kidney, liver, and the gastro-intestinal tract) express an enzyme with an apparent K_m of 140 µM. Out of 6 enzymes found to reduce DOX, K_m values in this range are exhibited by CBR1 and AKR1C3. CBR1 is expressed in these three organs at higher levels than AKR1C3, while AKR1C3 has higher catalytic efficiency. However, inhibition constants for DOX reduction with 4-amino-1-tert-butyl-3-(2-hydroxyphenyl) pyrazolo[3,4-d]pyrimidine (an inhibitor that can discriminate between CBR1 and AKR1C3) were identical for CBR1 and for human liver cytosol, but not for AKR1C3. These results suggest that CBR1 is a predominant hepatic DOX reductase. In cytosols from 80 human livers, the expression level of CBR1 and the activity of DOX reduction varied >70-fold and 22-fold, respectively, but showed no association with CBR1 gene variants found in these samples. Instead, the inter-individual differences in CBR1 expression and activity may be mediated by environmental factors acting via recently identified xenobiotic response elements in the CBR1 promoter. The variability in the CBR1 expression may affect outcomes of therapies with DOX, as well as with other CBR1 substrates.

Key words: anticancer agents, enzyme kinetics, HPLC, metabolite kinetics, polymorphisms, reductases