The consequences of doxorubicin quinone reduction in vivo in tumour tissue

A clear role for quinone reduction in the mechanism of action of doxorubicin has still to be established. There are three possible outcomes of this form of doxorubicin metabolism: (1) drug free radical formation, redox cycling and generation of reactive oxygen species (ROS) resulting in lipid peroxidation and DNA damage; (2) covalent binding of reactive drug intermediates to DNA; and (3) formation of an inactive 7-deoxyaglycone metabolite. In this work, the occurrence of each of these pathways has been studied in vivo in a subcutaneously growing rat mammary carcinoma (Sp 107). Doxorubicin was administered by direct intratumoural injection either as the free drug or incorporated in albumin microspheres (10–40 μm diameter). There was no evidence of an increase in lipid peroxidation over background after either treatment at any time point studied. In fact, doxorubicin administration resulted in a statistically significant reduction in lipid peroxidation at the later time points studied compared to control (no drug treatment), e.g. 24 hr: control, 21.7 ± 2.8 SD nmol malondialdehyde/g tissue; free doxorubicin (70 μg drug), 14.5 ± 4.0 SD nmol/g (P < 0.01 Student's t-test) and doxorubicin microspheres (70 μg drug), 17.4 ± 1.1 nmol/g (P < 0.05). Covalent binding to DNA was measured by a 32P-post-labelling technique. Low levels of four putative drug-DNA adducts were detected; however, there were no qualitative or quantitative differences in profiles between free drug and microspheres. High 7-deoxyaglycone metabolite concentrations comparable to the parent drug itself were detected after administration of microspheres (3.0 μg/ g ± 1.7 SD at 24 hr and 3.1 μg/ g ± 1.1 SD at 48 hr). In contrast, these metabolites were present at levels close to the limit of detection of our HPLC assay after free drug (0.04 μg/ g ± 0.03 SD at 24 hr and 0.02 μg/ g ± 0.03 SD at 48 hr). Thus, 7-deoxyaglycone metabolite formation can occur in tumour tissue (indicating active drug quinone reduction) without concomitant increases in the level of lipid peroxidation or the levels of drug-DNA adducts. In conclusion, the main biological consequence of doxorubicin quinone reduction in vivo in tumour tissue would appear to be drug inactivation to a 7-deoxyaglycone metabolite rather than drug activation to DNA reactive species or ROS.