Ototoxicity is a typical dose-limiting side effect of cancer chemotherapy with cisplatin but much less so with carboplatin. To elucidate the underlying molecular pathological mechanisms, we have measured the formation and persistence of druginduced DNA adducts in the nuclei of inner ear cells of guinea pigs after short-term exposure to either cisplatin or carboplatin using immunofluorescence staining and quantitative image analysis. After application of carboplatin, all cells of the cochlea exhibited a similar burden of guanine-guanine intrastrand cross-links in DNA. In contrast, we observed a pronounced 3-to 5-fold accumulation of this cytotoxic adduct exclusively in the marginal cells of the stria vascularis between 8 and 48 h after treatment with cisplatin. In the kidney, the other critical target tissue of cisplatin toxicity, a similar high preferential formation of cytotoxic DNA adducts was measured in the tubular epithelial cells but not in other renal cell types. As for the ear, this excessive formation of DNA damage in a particular cell type was seen in animals treated with cisplatin but not those treated with carboplatin. Because cisplatin ototoxicity is often attributed to oxidative stress mediated by the generation of radical oxygen species (ROS), we have measured in parallel the levels of the lead DNA oxidation product 8-oxoguanine (8-oxoG) in cochlear cryosections. Compared with basal levels in untreated control cochleas, no additional formation of 8-oxoG was detectable up to 48 h after cisplatin treatment in the DNA of either inner-ear cell type. This suggests that the generation of ROS may be a secondary event in cisplatin ototoxicity.Platinum derivatives are frequently used in cancer chemotherapy for patients with malignancies of, for example, the urogenital tract, the lung, the colon, the stomach, or the head and neck region. It is widely accepted that the antineoplastic efficiency of cisplatin [cis-diamminedichloroplatinum (II)] as well as carboplatin (diammine-cyclobutane-dicarboxylatoplatinum) results from their interaction with the nuclear DNA of tumor cells (reviewed in Siddik, 2003). The drugs harboring two reactive groups initially induce monoadducts at nucleophilic sites (e.g., of guanine or adenine) and can lead subsequently to intra-and interstrand cross-links in the DNA. Once formed, these lesions can trigger apoptotic cascades (McKeage, 1995;Boulikas and Vougiouka, 2003) predominantly via the mitochondrial pathway (McDonald and Windebank, 2002;Lee et al., 2004). The vital role of DNA adducts in this process is confirmed by observations in human cells differing in their ability to repair drug-induced damage to their genome. Cells with impaired nucleotide excision repair functions are clearly more sensitive to cisplatin than their proficient counterparts (Furuta et al., 2002;Wu et al., 2003).On the other hand, the frequently dose limiting side effects of cisplatin on normal cells and tissues (e.g., in the inner ear and the kidney) were mainly ascribed until now to pathways not ...