SW480 colon carcinoma cells were treated with complexes 1–4 for 48 h with concentrations between 5 and 40 μM, and cells were then collected for annexin V–FITC and propidium iodide staining. Exemplarily, dot plots of cell populations treated PLX4032 with 5 μM of each compound from one representative experiment are shown in Fig. 6. Complex 1 shows the strongest impact on cell viability, only 15% cells remain viable, whereas cells in early and late apoptosis amount to 72% in total.

Complex 2 shows a much more moderate impact on cell viability, indicated by 63% viable cells and only 31% apoptotic cells. The same applies for complex 3, yielding a slightly lower amount EPZ-6438 nmr of viable

cells (56%) and a slightly higher amount of apoptotic cells (35%). Complex 4 is the least potent compound and has hardly any impact on the cells at a concentration of 5 μM. Percentages of necrotic cells remain generally low (with a maximum of 14% in the case of 1). The concentration dependence of apoptosis/necrosis induction is illustrated in Fig. 7, and the corresponding values are listed in Table 2. They provide further evidence for the differences in cytotoxic potencies of the compounds, correlating with those observed in the MTT assay. Whereas 5 μM of compound 1 is sufficient for near-maximum effect, even 40 μM of compound 4 is insufficient for comparable effects. Compounds 2 and 3 require concentrations Parvulin of 20 μM to induce 57% and 61% apoptosis, respectively, taking intermediate positions. Furthermore, compounds 2–4 induce higher proportions

of necrotic cells relative to those undergoing apoptosis, making compound 1 the one with the most favorable properties. Binding paullone ligands to ruthenium(II) and osmium(II) arene moieties led to a considerable improvement of solubility compared to the uncomplexed compounds, enabling biological studies. A comparison with previous results for Sadler’s ruthenium complex with ethylenediamine (instead of the paullone ligand), [(η6-p-cymene)RuII(en)Cl](PF6), (IC50 values of 7.1, 3.5 and 4.4 μM in A549, SW480 and CH1 cells, respectively) under the same experimental conditions  [17] reveals that the presence of the paullone ligand causes a 2.3- to 6.6-fold (complex 1) and a 1.2- to 2.9-fold (complex 3) increase in cytotoxicity, depending on the cell line. In general, complexes with L1 show stronger cytotoxic effects than those with L2 in all human carcinoma cell lines tested. In the most sensitive cell lines SW480 and CH1, IC50 values of complex 1 are in the nanomolar range, whereas in the least sensitive cell lines A549 and LNCaP IC50 values are in the low micromolar range.