Cytotoxicity of Platinum(II) Dinuclear Complexes with 1-Alkylthymine Ligands against Mouse Sarcoma 180 Cells

We synthesized five platinum(II) dinuclear complexes containing 1-alkylated thymines. Two of the 1-alkylated thymine, 1-MeThy and 1-EtThy, complexes afforded good crystals. The X-ray structures of these complexes were determined. The 1-MeThy complex has a head-to-head (H-H) arrangement, while the 1-EtThy complex has a head-to-tail (H-T) arrangement. The 1-MeThy complex (H-H) shows high electrophilicity against chloride anion (Cl^-) and high cytotoxicity against mouse sarcoma 180 (S-180) cells in vitro. The 1-EtThy complex (H-T) is inactive. The other 1-MeThy complex does not produce CDDP in the reaction with chloride ion and is inactive against the S-180 cell line. This complex is assumed to have an H-T arrangement. Similarly, two different 1-PrⁿThy complexes, one with high electrophilicity and cytotoxicity and the other without, must have an H-H and H-T arrangement, respectively. For comparison, we investigated six complexes, 1-methyluracil (1-MeUra) (H-H) dimer, α-pyridone (H-T) dimer, α-pyridone blue tetramer (PPB), 1-methylcytosine (1-MeCyt) (H-T) dimer, acetate dimer, and 1-MeUra monomer complexes. The α-pyridone (H-T), PPB, 1-MeCyt (H-T) dimer, and 1-MeUra monomer complexes are inert to chloride ion and inactive against mouse sarcoma S-180. The 1-MeUra (H-H) dimer and acetate dimer complexes show high electrophilicity and high cytotoxicity. Cellular accumulation of the platinum complexes phenomenally shows that all are incorporated to cancer cells to a lesser extent than CDDP. The relationships between the accumulation, the electrophilicity, and the interaction of these complexes with proteins are discussed.