Antitumoral effects of mitochondria-targeting neutral and cationic -[bis(1,3-dibenzylimidazol-2-ylidene)Cl(L)]Pt() complexes

Recently, we opened a synthetic access to antitumoral platinum complexes of the type cis -[(NHC) 1 (NHC) 2 Pt II Cl(L)] which interact with DNA in a way correlated to the complex charge and to the sterical accessibility of the leaving chlorido ligand. We now identified mitochondria rather than nuclei as the cellular target of the neutral dichlorido complex 1 (L = Cl) and the delocalized lipophilic cationic phosphine complex 2 (L = PPh 3 ), both carrying the same cis -bis(1,3-dibenzylimidazol-2-ylidene) ligands. Their uptake into 518A2 melanoma cells was concentration-dependent and distinctly greater for complex 2 which was also more cytotoxic against sensitive cancer cell lines with submicromolar IC 50 values. Both complexes interfered strongly with various forms of DNA in vitro , but only complex 2 caused a melanoma cell cycle arrest in G1-phase, setting both apart from the S-phase arresting drug cisplatin. Studies of the intracellular localisation of 1 and 2 were carried out with their alkyne-tagged analogues 6 and 7 , which showed identical patterns of cancer cell cytotoxicity, cell cycle interference and effects on mitochondria. Click reactions with 7-hydroxycoumarin azide, colocalisation with Mitotracker™ and confocal microscopy, proved complexes 6 and 7 to accumulate mainly in the mitochondria rather than the nuclei of melanoma cells. Complex 1 and even more so complex 2 reduced the mitochondrial membrane potential and also increased the cellular ROS levels. As a consequence, both complexes caused stress fibre formation in the F-actin cytoskeleton of melanoma cells, most distinctly so complex 2 which also activated the apoptotic cascade mediated by capases-3 and -7. cis -[Bis(1,3-dibenzylimidazol-2-ylidene)Pt II Cl(L)] complexes target mitochondria regardless of charge and size of ligands L, yet show different anticancer effects.