title: Increased DNA-binding activity of cis-1,1-cyclobutanedicarboxylatodiammineplatinum(II) (carboplatin) in the presence of nucleophiles and human breast cancer MCF-7 cell cytoplasmic extracts: activation theory revisited.
abstract: The molecular mechanism of carboplatin [cis-1,1-cyclobutanedicarboxylatodiammineplatinum(II)] activation is still unresolved. We studied the binding of carboplatin to calf thymus DNA in the presence of thiourea, glutathione, and human breast cancer MCF-7 cell cytoplasmic extracts by measurement of DNA-dependent ethidium bromide fluorescence and atomic absorption spectroscopy. After a 96-hr period of reaction, the decrease in the DNA-dependent fluorescence yield of ethidium bromide due to the formation of platinum (Pt)-DNA adducts increased significantly in the presence of thiourea (6-fold) and glutathione (3- to 4-fold) as compared to the controls in the absence of the nucleophiles. There was also a marked elevation in the levels of platinum incorporated into DNA, measured by atomic absorption spectroscopy (2- to 3-fold and 5- to 7-fold for thiourea and glutathione, respectively). More remarkably, the Pt-DNA adducts formed in the presence of cytoplasmic extracts of MCF-7 human breast cancer cells also showed similar results in a dose-related fashion. Carboplatin, therefore, displayed a characteristic increase in DNA binding/damaging in the presence of the very same S-containing nucleophiles that showed the expected quenching effects in the case of cisplatin [cis-diamminedichloroplatinum (II)]. We propose a nucleophile-facilitated release of the active species of carboplatin prior to binding with DNA.
authors.affiliation: Department of Genetics, Dr. Alm Post Graduate Institute of Basic Medical Sciences, University of Madras, India.
authors.forename: G
authors.initials: G
authors.lastname: Natarajan
authors.forename: R
authors.initials: R
authors.lastname: Malathi
authors.forename: E
authors.initials: E
authors.lastname: Holler
mesh_terms: D000970:Antineoplastic Agents / Q000378:metabolism / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000473:pathology; D016190:Carboplatin / Q000378:metabolism / Q000494:pharmacology*; D002457:Cell Extracts / Q000494:pharmacology; D004247:DNA / Q000187:drug effects / Q000378:metabolism*; D004996:Ethidium / Q000378:metabolism; D005453:Fluorescence; D006801:Humans; D008715:Methionine / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D002457:Cell Extracts; D004247:DNA; D008715:Methionine; D016190:Carboplatin; D004996:Ethidium