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.

The synthesis and chemical characterization of three new transplatinum complexes of structural formula trans-[PtCl(2)(amine)(isopropylamine)] (amine = n,n-dimethylamine, propylamine, and butylamine), 1-3, are described. Cytotoxicity tests in tumor cell lines sensitive to cis-DDP (Jurkat, Hela, and Vero) and also in tumor cell lines overexpressing ras oncogenes and resistant to cis-DDP (HL-60 and Pam 212-ras) show that complexes 1 and 3 have higher cytotoxic activity than cisplatin. Moreover, these two trans-Pt(II) complexes kill Pam 212-ras cells through apoptosis induction. These results suggest that trans-PtCl(2) complexes with asymmetric aliphatic amines may be considered a new class of biologically active trans-platinum drugs.

Positively charged, water soluble cis/trans-[PtCl(2)(piperazine)(Am1)] (where Am1 = NH(3), n-butylamine, isopropylamine, 4-picoline, piperidine, and piperazine) has significant cytotoxic activity against cisplatin resistant ovarian cancer cells. The charged complexes are taken up by cancer cells much more rapidly than cisplatin and bind to cellular DNA and to calf thymus DNA much faster than cisplatin or transplatin. The platinum-piperazine complexes bind proteins (ubiquitin and myoglobin) very slowly as compared to cisplatin and to their neutral piperidine analogues. Altogether, the results reported here suggest that combination of positively charged ligands with a trans-Pt(II)Cl(2) center may lead to the discovery of platinum complexes that are able to circumvent cisplatin resistance.