Novel tetranuclear orthometalated complexes of Pd(II) and Pt(II) derived from p-isopropylbenzaldehyde thiosemicarbazone with cytotoxic activity in cis-DDP resistant tumor cell lines. Interaction of these complexes with DNA.

The reaction of p-isopropylbenzaldehyde thiosemicarbazone [p-is.TSCN], 1, with palladium(II) acetate and potassium tetrachloroplatinate yielded two tetrameric orthopalladated isomers, [Pd(p-is.TSCN)]4 (complexes 2 and 3), and the platinum analogue [Pt(p-is.TSCN)]4 (complex 4), respectively. All of these complexes contain the thiosemicarbazone bonded as a terdentate ligand to the metallic atom, through the thiol sulfur, the azomethinic nitrogen and the ortho carbon of the p-isopropylphenyl ring to which the imine group is attached to as deduced from the study of the IR, NMR, and XRD spectra of complexes 2 and 4. Complexes 2 and 4 crystallize in the centrosymmetric monoclinic space group C2/c, with Z = 8. Unit cell parameters for complex 2 are as follows: a = 25.742(5) A, b = 19.560(4) A, c = 24.199(5) A, beta = 101.70(3)o. Unit cell parameters for complex 4 are as follows: a = 25.8728(19) A, b = 19. 5053(14) A, c = 24.0899(16) A, beta = 101.305(2)o. As can be deduced from the NMR study, the palladated isomers 2 and 3 interconvert in DMSO which may be a consequence of the existence in both complexes of a flexible eight-membered ring with alternating Pd-S atoms. The testing of the cytotoxic activity of these compounds against several human and murine cell lines sensitive and resistant to cisplatin (cis-DDP) suggests that compounds 2, 3, and 4 may be endowed with important anticancer properties since they elicit IC50 values in the microM range as does the clinically used drug cis-DDP, and, moreover, they display cytotoxic activity in tumor lines resistant to cis-DDP. The analysis of the interaction of these novel tetrameric cyclometalated compounds with DNA suggests that they form DNA interhelical cross-links.