Although novel anticancer drugs are being developed intensively, anthracyclines remain the gold standard in the treatment of acute myeloid leukaemia (AML). The reductive conversion of daunorubicin (Dau) to less active daunorubicinol (Dau-ol) is an important mechanism that contributes to the development of pharmacokinetic anthracycline resistance. Dau is a key component in many AML regimes, in which it is combined with many drugs, including all-trans-retinoic acid (ATRA), cytarabine, cladribine and prednisolone. In the present study, we investigated the influence of these anticancer drugs on the reductive Dau metabolism mediated by the aldo-keto reductases AKR1A1, 1B10, 1C3, and 7A2 and carbonyl reductase 1 (CBR1). In incubation experiments with recombinant enzymes, cladribine and cytarabine did not significantly inhibit the activity of the tested enzymes. Prednisolone inhibited AKR1C3 with an IC50 of 41.73 µM, while ATRA decreased the activity of AKR1B10 (IC50 = 78.33 µM) and AKR1C3 (IC50 = 1.17 µM). Subsequent studies showed that AKR1C3 inhibition mediated by ATRA exhibited tight binding (Kiapp = 0.54 µM). Further, the combination of 1 µM ATRA with different concentrations of Dau demonstrated synergistic effects in HCT116 and KG1a human cells expressing AKR1C3. Our results suggest that ATRA-mediated inhibition of AKR1C3 can contribute to the mechanisms that are hidden beyond the beneficial clinical outcome of the ATRA-Dau combination.

Department of Biochemical Sciences Faculty of Pharmacy Charles University Akademika Heyrovskeho 1203 Hradec Kralove 50005 Czech Republic

Department of Pharmacology and Toxicology Faculty of Pharmacy Charles University Akademika Heyrovskeho 1203 Hradec Kralove 50005 Czech Republic

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