As a third-row transition metal ion, OsII might be expected to be relatively inert compared to the second-row ion RuII. While fast exchange of the chlorido ligand and partial loss of the arene ligand was observed learn more for all four complexes, a different number of cymene and cymene-free paullone

species was detected for ruthenium and osmium complexes, but remarkably metal-paullone bonds remained intact in water/DMSO mixtures. The previous observation that the ruthenium complexes form N7 adducts with 5′-GMP, whereas osmium analogues do not under the same conditions, suggests a higher reactivity of the former to biological target molecules and may provide an explanation for the different cytotoxic potencies, which were not so evident in our previous studies [13]. In this context, covalent DNA binding cannot be excluded as a mode of action of this type of compounds, similar to simple ruthenium(II)-arene complexes lacking a biologically active co-ligand [18], but it seems unlikely that the above-mentioned increased potency mediated by the presence of a (sterically demanding) paullone ligand (see first paragraph GDC-0199 nmr of

Discussion) is related to the formation of DNA adducts. A certain extent of DNA intercalation might be conceivable (compare the results with a related indolobenzazepine complex [19]), but the compounds are structurally not particularly predestined for this kind of interaction, leaving protein interactions as a more likely cause of the high antiproliferative activities of paullone-based ruthenium(II) and osmium(II) complexes. Activity of Cdk2/cyclin E, envisaged as a potential protein target, is concentration-dependently inhibited by all four compounds, again strongest by complex 1, which shows at 10 μM about 50% of the inhibitory activity Molecular motor of the well-known Cdk inhibitor flavopiridol. Inhibitory potency on Cdk1/cyclin B, which

is responsible for the G2/M transition, was not tested because previous studies with a related osmium–paullone complex showed a much lower inhibition of Cdk1/cyclin B than of Cdk2/cyclin E [19]. Furthermore, the lack of strong cell cycle effects, in particular the absence of a distinct G2 arrest, argued against further studies in that direction. Overall, the results presented here suggest that Cdks are not the crucial target of the complexes. Probably, the derivatization at the lactam unit of the paullones is the reason for the decrease in inhibitory potency, in accordance with the structure–activity relationships described by Kunick and coworkers [11]. Complex 1 is also most potent in the inhibition of DNA synthesis, as indicated by reduced BrdU incorporation into newly synthesized DNA. Overall, the reduction of DNA synthesis, requiring concentrations considerably higher than 5 μM, can hardly be interpreted as a direct interference with processes of the S phase.