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.