The adduct formation between the thioguanine-polyamine ligands and DNA with the AP site under UVA irradiated and non-irradiated conditions.

The AP sites are representative of DNA damage and known as an intermediate in the base excision repair (BER) pathway which is involved in the repair of damaged nucleobases by reactive oxygen species, UVA irradiation, and DNA alkylating agents. Therefore, it is expected that the inhibition or modulation of the AP site repair pathway may be a new type of anticancer drug. In this study, we investigated the effects of the thioguanine-polyamine ligands (SG-ligands) on the affinity and the reactivity for the AP site under UVA irradiated and non-irradiated conditions. The SG-ligands have a photo-reactivity with the A-F-C sequence where F represents a tetrahydrofuran AP site analogue. Interestingly, the SG-ligands promoted the ß-elimination of the AP site followed by the formation of a covalent bond with the ß-eliminated fragment without UVA irradiation.

DNA cleavage at the AP site via ß-elimination mediated by the AP site-binding ligands.

DNA is continuously damaged by endogenous and exogenous factors such as oxidation and alkylation. In the base excision repair pathway, the damaged nucleobases are removed by DNA N-glycosylase to form the abasic sites (AP sites). The alkylating antitumor agent exhibits cytotoxicity through the formation of the AP site. Therefore blockage or modulation of the AP site repair pathway may enhance the antitumor efficacy of DNA alkylating agents. In this study, we have examined the effects of the nucleobase-polyamine conjugated ligands (G-, A-, C- and T-ligands) on the cleavage of the AP site. The G- and A-ligands cleaved DNA at the AP site by promoting ß-elimination in a non-selective manner by the G-ligand, and in a selective manner for the opposing dT by the A-ligand. These results suggest that the nucleobase-polyamine conjugate ligands may have the potential for enhancement of the cytotoxicities of the AP site.