Platinum complexes with imino ethers or cyclic ligands mimicking imino ethers: synthesis, in vitro antitumour activity, and DNA interaction properties

Both trans- and CiS-[PtCl2(NH3)(L)] compounds have been synthesized, L representing either the imino ether HN = C(OMe)Me having a Z or E configuration at the C = N double bond, or the cyclic ligands N = C(OMe)CH2CH2CH2 and C(Me)OCH2CH2 (compounds 1-4, for trans geometry and 5-8 for cis geometry, respectively). The cyclic ligands mimic the imino ether ligands but, differently from imino ethers, cannot undergo change of configuration. In a panel of human tumor cells, trans compounds inhibit growth much more than transplatin. Moreover, compound I in most cases is less active than 2, and 1 and 2 are less active than 3 and 4, respectively. For cis compounds with imino ethers, the activity is reduced (5) or unaffected (6) with respect to cisplatin. Moreover, unlike trans compounds, substitution of cyclic ligands (7, 8) for imino ethers (5, 6) generally decreases the activity. This determines, for compounds with cyclic ligands, an unusual inversion of the cis geometry requirement for activity of platinum(II) species. Importantly, 1-4 and 5-8 partially circumvent the multifocal cisplatin resistance of A2780cisR cells, and 1-4 also overcome resistance from reduced uptake of 41McisR cells. DNA interaction regioselectivity of 1-4 and 5-8 is not substantially modified with respect to transplatin and cis-platin. However, both imino ethers and cyclic ligands slow down the DNA interstrand, cross-link reaction, (E)-HN = C(OMe)Me and N = C(Me)OCH2CH2 decreasing also its extent. Therefore, DNA interaction of 1-4 and 5-8 appears to be characterized by persistent monoadducts (1-4), and by monoadducts and/or intrastrand cross-links structurally different from those of cisplatin (5-8). This study demonstrates that ligand configuration modulates the activity of both trans and cis compounds, and supports the development of platinum drugs based on their coordination chemistry to combat cisplatin resistance.