Activation of clinically used anthracyclines by the formaldehyde-releasing prodrug pivaloyloxymethyl butyrate

The anthracycline group of compounds is extensively used in current cancer chemotherapy regimens and is classified as topoisomerase II inhibitor. However, previous work has shown that doxorubicin can be activated to form DNA adducts in the presence of formaldehyde-releasing prodrugs and that this leads to apoptosis independently of topoisomerase II-mediated damage. To determine which anthracyclines would be useful in combination with formaldehyde-releasing prodrugs, a series of clinically relevant anthracyclines (doxorubicin, daunorubicin, idarubicin, and epirubicin) were examined for their capacity to form DNA adducts in MCF7 and MCF7/Dx (P-glycoprotein overexpressing) cells in the presence of the formaldehyde-releasing drug pivaloyloxymethyl butyrate (AN-9). All anthracyclines, with the exception of epirubicin, efficiently yielded adducts in both sensitive and resistant cell lines, and levels of adducts were similar in mitochondrial and nuclear genomes. Idarubicin was the most active compound in both sensitive and resistant cell lines, whereas adducts formed by doxorubicin and daunorubicin were consistently lower in the resistant compared with sensitive cells. The adducts formed by doxorubicin, daunorubicin, and idarubicin showed the same DNA sequence specificity in sensitive and resistant cells as assessed by lambda-exonuclease-based sequencing of alpha-satellite DNA extracted from drug-treated cells. Growth inhibition assays were used to show that doxorubicin, daunorubicin, and idarubicin were all synergistic in combination with AN-9, whereas the combination of epirubicin with AN-9 was additive. Although apoptosis assays indicated a greater than additive effect for epirubicin/AN-9 combinations, this effect was much more pronounced for doxorubicin/AN-9 combinations.