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 microns 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 micrograms drug), 14.5 +/- 4.0 SD nmol/g (P < 0.01 Student's t-test) and doxorubicin microspheres (70 micrograms 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 micrograms/g +/- 1.7 SD at 24 hr and 3.1 micrograms/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 microgram/g +/- 0.03 SD at 24 hr and 0.02 microgram/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.
Cummings
J
J
Imperial Cancer Research Fund, Western General Hospital, Edinburgh, U.K.
Willmott
N
N
Hoey
B M
BM
Marley
E S
ES
Smyth
J F
JF
D000418:Albumins / Q000008:administration & dosage; D000818:Animals; D004273:DNA, Neoplasm / Q000378:metabolism; D004317:Doxorubicin / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology; D005260:Female; D015552:Injections, Intralesional; D007553:Isotope Labeling; D015227:Lipid Peroxidation; D008325:Mammary Neoplasms, Experimental / Q000188:drug therapy / Q000378:metabolism*; D008863:Microspheres; D008954:Models, Biological; D009279:Naphthacenes / Q000378:metabolism; D009368:Neoplasm Transplantation; D010084:Oxidation-Reduction; D010761:Phosphorus Radioisotopes; D011809:Quinones / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D014018:Tissue Distribution
D000418:Albumins; D004273:DNA, Neoplasm; D009279:Naphthacenes; D010761:Phosphorus Radioisotopes; D011809:Quinones; D004317:Doxorubicin; C049326:7-deoxyadriamycin aglycone