Mass spectrometry analysis revealed distinct reaction pathways of d(CpoxG) with a photoactivatable Pt(IV) anticancer prodrug

The interactions between d(Cp^oxG) (^oxG = 8-oxo-guanine), a major form of oxidative damaged motif of CpG island, and a photoactivatable anticancer Pt(IV) prodrug, trans,trans,trans-[Pt(N₃)₂(OH)₂(pyridine)₂] (1) was investigated by electrospray ionization mass spectrometry (ESI-MS). Surprisingly, the primary MS analysis showed the major photooxidative products being platinum-free dinucleotides, d(CpGh) (2a)/d(CpIa) (2b) (possibly a mixture of the two isomers) and d(CpDGh) (3), of which the guanine was oxidized to guanidinohydantoin (Gh) or iminoallantoin (Ia), and hydroguanidinohydantoin (DGh), respectively. Moreover, two mono-platinated adducts, {[CpGh] + 1′}+ (4) and {[CpDGh] + 1′}+ (5) (1′ = [Pt^II(N₃)(py)₂]²⁺), and three Pt-crosslinked dinucleotide adducts, {[CpDGh]₂ + 1″}²⁺ (6), {[CpGh] + [CpDGh] + 1″}²⁺ (7) and {[CpGh]₂ + 1″}²⁺ (8) (1″ = [Pt^II(py)₂]²⁺), were observed as 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 DGhs, DGh and Gh, or Ghs in 6, 7 and 8 were implicated. Unexpectedly, the supposed platinated d(Cp^oxG) adducts were not observed, indicating that ^oxG is preferential to be further oxidated by the reactive oxygen species released during the photodecomposition of complex 1 rather than to coordination with the reduced Pt(II). These results revealed a more complexity of photo-interaction of complex 1 with d(Cp^oxG) than with d(CpG), implicating that ^oxG-containing DNA, in particular oxidative CpG island, might play a vital role in the action of mechanism of photoactivatable Pt(IV) prodrugs, deserving further exploration.