Increased DNA binding activity of cis-1,1-cyclobutanedicarboxylatodiammineplatinum(II) (Carboplatin) in the presence of nucleophiles and human breastcancer MCF-7 cell cytoplasmic extracts: Activation theory revisited

The molecular mechanism of carboplatin [cis-1,1-cyclobutanedicarboxylatodia mmineplatinum(II)] activation is still unresolved. We studied the binding o f carboplatin to calf thymus DNA in the presence of thiourea, glutathione, and human breast cancer MCF-7 cell cytoplasmic extracts by measurement of D NA-dependent, ethidium bromide fluorescence and atomic absorption spectrosc opy. After a 96-hr period of reaction, the decrease in the DNA-dependent fl uorescence yield of ethidium bromide due to the formation of platinum (Pt)- DNA adducts increased significantly in the presence of thiourea (6-fold) an d 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 platin um 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 extrac ts of MCF-7 human breast cancer cells also showed similar results in a dose -related fashion. Carboplatin, therefore, displayed a characteristic increa se in DNA binding/damaging in the presence of the very same S-containing nu cleophiles that showed the expected quenching effects in the case of cispla tin [cis-diamminedichloroplatinum (II)]. We propose a nucleophile-facilitat ed release of the active species of carboplatin prior to binding with DNA. (C) 1999 Elsevier Science Inc.