Daunorubicin-induced DNA lesions in isolated rat hepatocytes and mammary epithelial cells

Covalent modification of DNA by daunorubicin

Daunorubicin, a clinically useful antitumor agent, induces mammary adenocarcinoma in Sprague-Dawley rats. As part of an investigation of the mechanism of tumor induction by daunorubicin, the formation of daunorubicin-DNA adducts has been investigated by 32P-postlabeling assay. Rat-liver DNA incubated with either 0.05 or 0.1 mM daunorubicin, rat-liver microsomes, and 5 mM reduced nicotinamide adenine dinucleotide phosphate (NADPH) for 1 h contained covalent DNA adducts in addition to the endogenous adduct profile present in control DNA. With 1.5 mM cumene hydroperoxide serving as a cofactor, higher levels of these two adducts and two additional adducts were formed, all of which most likely were daunorubicin-DNA adducts. This latter treatment also resulted in an intensification of three endogenous DNA modifications over levels occurring in control DNA. Covalent DNA alterations in vivo were studied in rats treated with 20 mg/kg daunorubicin for 2 days and 200 mg/kg on the 3rd day. Daunorubicin-DNA adducts as observed in vitro could not be detected in DNA of liver or mammary epithelial cells. The levels of endogenous modifications in drug-treated rats were increased by 200% in mammary DNA and by 50% in hepatic DNA as compared with controls. It was concluded from these experiments that daunorubicin may be metabolically activated to a reactive metabolite that binds covalently to DNA. These daunorubicin-DNA adducts may not play a role in tumor induction because they were not detectable in vivo. However, the increase in levels of endogenous DNA modifications induced by daunorubicin both in vitro and in vivo is consistent with a role of this class of DNA modification in the carcinogenic process.

Techniques for pharmacological and toxicological studies with isolated hepatocyte suspensions

Since its introduction in 1969, the high-yield preparation of isolated hepatocytes has become a frequently used tool for the study of hepatic uptake, excretion, metabolism and toxicity of drugs and other xenobiotics. Basic preparative methods are now firmly established involving perfusion of the liver with a balanced-saline solution containing collagenase. Satisfactory procedures are available for determining cell yields, for expressing cellular activities and for establishing optimal incubation conditions. Gross cellular damage can be detected by means of trypan blue or by measuring enzyme leakage, and damaged cells can be removed from the preparation. Specialized techniques are available for preparing hepatocyte couplets and suspensions enriched with periportal or perivenous hepatocytes. The isolated hepatocyte preparation is particularly convenient for the study of the kinetics of hepatic drug uptake and excretion because the cells can be rapidly separated from the incubation medium. Isolated liver cells have also proved valuable for investigating drug metabolism since they show many of the features of the intact liver. However, they also show important differences such as losses of membrane specialization, some degree of cell polarity and the capacity to form bile. The many consequences of the hepatic toxicity of xenobiotics including lipid peroxidation, free radical formation, glutathione depletion, and covalent binding to macromolecules are also readily studied with the isolated liver cell preparation. A particular advantage is the ease with which morphological changes as a result of drug exposure can be observed in isolated hepatocytes. However, it must be remembered that the isolation procedure inevitably introduces changes that may make the cells more susceptible than the normal liver to damage by xenobiotic agents. Despite its limitations, the isolated hepatocyte preparation is now firmly established in the armamentarium of the investigator examining the interaction of the liver with xenobiotics.

Daunorubicin-induced mammary tumors in the rat

Eleven of 24 female Sprague-Dawley rats given a single i.v. injection of daunorubicin (10 mg/kg) developed mammary tumors within 8 months after the injection. Four of 12 rats given an intramammary injection of daunorubicin (4 or 8 micrograms) developed five mammary tumors in the injected area within 6.5 months of injection. Tissue distribution studies using tritiated daunorubicin revealed that the liver, kidney, lung, heart, and intestine had higher daunorubicin concentrations than mammary tissue during the first 24 h after i.v. injection. However, depletion of the drug from the internal organs was more rapid than from mammary tissue. Differences in ability to metabolize daunorubicin were compared in homogenates of isolated mammary epithelial cells and hepatocytes by high-performance liquid chromatography: after 90 min, hepatocytes metabolized about 70% of daunorubicin, whereas mammary epithelial cells did not metabolize the drug. Tritiated daunorubicin injected directly into rat mammary gland showed no metabolism in 24 h, and the drug did not get into the circulation. These results suggest that retention of daunorubicin because of the inability of mammary tissue to metabolize the drug is a cause of drug-induced mammary tumors in female Sprague-Dawley rats.