Inorganic Chemistry Figure 3. Concentration-dependent impact of 2c, 4c, and 5c on the cell cycle distribution of CH1 cells after exposure for 24 h. The DNA content of cells stained with propidium iodide was analyzed by flow cytometry. superseded by an S phase arrest at 160 μM (52 ± 0.3% of cells in the S phase). In addition, the appearance of a pronounced sub-G1/G0 fraction (excluded from analysis) and the tremendous decrease of the G2/M fraction (27 ± 6%) at this highest concentration suggest that apoptotic cell death is preferentially induced in G2/M cells. In accordance with the slightly lower cytotoxicity in the MTT assay, 4c is also somewhat less effective on the cell cycle. Neither an S phase arrest nor a comparable sub-G1/G0 fraction could be observed at the highest concentration, but the compound as well induces a G2/M arrest reaching 68 ± 1% at 160 μM. In conclusion, the differences in the position of the chelating moiety in 4c and 5c (whether on the lactam ring or not) seem to merely modulate the antiproliferative potency of the compounds rather than fundamentally change the capacity of inhibiting cell cycle progression. Final Remarks. Herein we describe the synthesis and characterization of a new series of organometallic complexes of the general formula [RuCl(η6-arene)(L)]Cl [where L = 3-(1H- benzimidazol-2-yl)-1H-pyrazolo[3,4-b]pyridines and indolo- [3,2-d]benzazepines (L1−L5), which are potential kinase Article inhibitors]. Complexation of L1−L5 to the ruthenium(II) arene unit yielded compounds with increased solubility in biological media, yet lower, but more selective antiproliferative activity in human cancer cell lines. In order to improve the mild cytotoxic effects of the ruthenium derivatives, we coupled the compounds to serum albumin, which is known to accumulate in tumors. HSA has previously been used to deliver various anticancer drugs such as chlorambucil, doxorubicin, paclitaxel, and cisplatin to cancer cells.33 Chlorambucil− and paclitaxel− HSA conjugates exhibit cytotoxicity comparable to that of the parent drugs in vitro but are less toxic in vivo,26,27 and a doxorubicin prodrug using endogenous serum albumin as a drug carrier displays excellent in vivo properties.34,35 Thus, the five organometallic complexes were conjugated to rHSA, tethering them to the protein via pH-triggered linkers, as previously described for the organometallic RAPTA com- pounds that are not cytotoxic but active as antimetastatic agents in vivo.36−38 MALDI-TOF-MS analysis of the rHSA−Ru adducts showed that typically two ruthenium-containing moieties were bound to the protein. The rHSA conjugates were found to be more cytotoxic than the “free” complexes on human ovarian cancer A2780 cell lines sensitive and resistant to cisplatin. These results are encouraging, and the further development of macromolecular organometallic ruthenium complexes that should selectively target tumor tissue appears to be worthwhile. ■ ASSOCIATED CONTENT *S Supporting Information Synthetic routes to free ligands and [RuCl(μ-Cl)(η6-arene)]2 (Schemes S1−S3), analytical data of [RuCl(μ-Cl)(η6-arene)]2 and [RuCl2(η6-arene)(DMSO)], assigned NMR (1H, 13C, and 15N) signals for 1c−5c and 2c−HCl compared to [RuCl(μ- Cl)(η6-arene)]2 and free ligands (Tables S1−S6), sections of the 1H,1H ROESY NMR spectra of 1c, 4c, and 5c (Figures S7− S13), ORTEP views of [RuCl2(η6-arene)(DMSO)] and L2·DMSO (Figures S14 and S15), stability of complexes in solutions, time-dependent UV−vis spectra of complexes in MeOH (1c−5c) and water (4c and 5c) (Figures S16−S18), concentration−effect curves of 1c−5c and [RuCl(μ-Cl)(η6- arene)]2 in CH1 cells (Figure S19), and crystallographic data for [RuCl2(η6-arene)(DMSO)]·0.5H2O, L2·DMSO, cis,cis- [RuIICl2(DMSO)2(L1)]·H2O, and mer-[RuIICl(DMSO)3(L2− H)]·H2O in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: vladimir.arion@univie.ac.at (V.B.A.), paul.dyson@epfl. ch (P.J.D.), bernhard.keppler@univie.ac.at (B.K.K.). Present Address ⊥Research Institute of Pharmacy, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Nether- lands. ■ ACKNOWLEDGMENTS We thank Prof. Markus Galanski for the 2D NMR measure- ments, Alex Roller for collection of the X-ray data, and the Hertha Firnberg Programme of Austrian Science Fund (Project T 393-N19) for financial support. A.C. thanks the Swiss National Science Foundation (AMBIZIONE Project PZ00P2- 136908/1) for funding. Furthermore, we are indebted to Prof. 12678 dx.doi.org/10.1021/ic201801e | Inorg. Chem. 2011, 50, 12669−12679