Mass spectrometry analysis reveals the distinct reaction pathways of d(Cp^oxG) with a photoactivatable Pt(<scp>iv</scp>) anticancer prodrug

The interactions between d(Cp^oxG) (^oxG = 8-oxo-guanine), a major form of the oxidatively damaged CpG island motif, and a photoactivatable anticancer Pt(IV) prodrug, <i>trans</i>,<i>trans</i>,<i>trans</i>-[Pt(N₃)₂(OH)₂(pyridine)₂] (1), were investigated using electrospray ionization mass spectrometry (ESI-MS). Surprisingly, the primary MS analysis showed that the major photooxidative products were the platinum-free dinucleotides d(CpGh) (2a)/d(CpIa) (2b) (possibly a mixture of the two isomers) and d(CpDGh) (3), in which the guanine was oxidized to 5-guanidino-hydantoin (Gh) or iminoallantoin (Ia) and 5-guanidino-dehydrohydantoin (DGh), respectively. Moreover, two mono-platinated adducts, {[CpGh] + 1'}⁺ (4) and {[CpDGh] + 1'}⁺ (5) (1' = [Pt^II(N₃)(py)₂]⁺), and three Pt-crosslinked dinucleotide adducts, {[CpGh]₂ + 1''}²⁺ (6), {[CpGh] + [CpDGh] + 1''}²⁺ (7) and {[CpDGh]₂ + 1''}²⁺ (8) (1'' = [Pt^II(py)₂]²⁺), were observed as the main platinated adducts. Tandem mass spectrometry with collision induced dissociation (CID) demonstrated that 1' bound at Gh or DGh in 4 and 5, while the inter-dinucleotide crosslinks by 1'' between Ghs, Gh and DGh, or DGhs in 6, 7 and 8 were implicated. Unexpectedly, the proposed platinated d(Cp^oxG) adducts were not observed, indicating that ^oxG preferentially undergoes further oxidation by the reactive oxygen species released during the photodecomposition of complex 1 rather than coordination with the reduced Pt(II). These results revealed the greater complexity of the photo-interaction of complex 1 with d(Cp^oxG) than with d(CpG), with the implication that ^oxG-containing DNA, in particular, the oxidative CpG island, might play a vital role in the mechanism of action of photoactivatable Pt(IV) prodrugs, which merits further exploration.