Possible Incorporation of Free N7-Platinated Guanines in DNA by DNA Polymerases, Relevance for the Cisplatin Mechanism of Action

Abstract

Cisplatin, cis-diamminedichloroplatinum (II), is one of the most widely used anticancer drugs. The main cellular target of cisplatin is DNA, where the platinum atom is able to form covalent bonds with the N7 of purines. It is commonly accepted that there is a direct attack of cisplatin on DNA. But it should be noted that, inside cells, free purine bases, which can react with cisplatin, are also available. Free bases have many functional roles, not least the constitution of building blocks for the synthesis of new DNA and RNA molecules. For this reason, under physiological conditions, the erroneous insertion of platinated bases in the synthesized nucleic acids could compete with direct DNA/RNA platination. Moreover, due to the lower sterical hindrance offered by single nucleobases with respect to nucleic acids, platination is expected to be even easier for free purines with respect to DNA and RNA. We have recently shown, for the first time, that platinated DNA can be formed in vitro by Taq DNA polymerase promoted incorporation of platinated purines. Cytotoxicity tests with [Pt(dien)(N7-G)], dien = diethylenetriamine, G = 5′-dGTP, 5′-dGDP, 5′-GMP, 5′-dGMP, GUO, dGUO, complexes on HeLa cancer cells support this hypothesis of the relative cytotoxicity of [Pt(dien)(N7-G)] derivatives being clearly related to their bioavailability. In vivo platination of free purines before their incorporation in nucleic acids therefore opens new perspectives in platinum based antitumour drugs, for a better understanding of both the action mechanism and the new molecular design.