Platinum(II) oxalato complexes with adenine-based carrier ligands showing significant in vitro antitumor activity

[Pt(L)(2)(ox)] (1), [Pt(2-OMeL)(2)(ox)] (2), [Pt(3-OMeL)(2)(ox)] (3), [Pt(2,3-diOMeL)(2)(ox)] (4), [Pt(2,4-diOMeL)(2)(ox)] (5), [Pt(3,4-diOMeL)(2)(ox)] (6) and [Pt(3,5-diOMeL)(2)(ox)].4H(2)O (7) platinum(II) oxalato (ox) complexes were synthesized using the reaction of potassium bis(oxalato)platinate(II) dihydrate with 2-chloro-N6-(benzyl)-9-isopropyladenine or its benzyl-substituted analogues (nL). The complexes 1-7, which represent the first platinum(II) oxalato complexes involving adenine-based ligands, were fully characterized by various physical methods including multinuclear and two dimensional NMR spectroscopy. A single-crystal X-ray analysis of [Pt(2,4-diOMeL)(2)(ox)].2DMF (5.2DMF; DMF=N,N'-dimethylformamide), proved the slightly distorted square-planar geometry in the vicinity of the Pt(II) ion with one bidentate-coordinated oxalate dianion and two adenine derivatives (nL) coordinated to the Pt(II) centre through the N7 atom of an adenine moiety, thereby giving a PtN(2)O(2) donor set. In vitro cytotoxicity of the prepared complexes was tested by an MTT assay against osteosarcoma (HOS) and breast adenocarcinoma (MCF7) human cancer cell lines. The best results were achieved for the complexes 2 and 5 in the case of both cell lines, whose IC(50) values equalled 3.6+/-1.0, and 4.3+/-2.1microM (for 2), and 5.4+/-3.8, and 3.6+/-2.1microM (for 5), respectively. The IC(50) equals 9.2+/-1.5microM against MCF7 cells in the case of 1. The in vitro cytotoxicity of the mentioned complexes significantly exceeded commercially used platinum-based anticancer drugs cisplatin (34.2+/-6.4microM and 19.6+/-4.3microM) and oxaliplatin (>50.0microM for both cancer cell lines).