Ruthenium nitrosyl complexes of the general formulas (cation)+[cis-RuCl4(NO)(Hazole)]−, where (cation)+ = (H2ind)+, Hazole = 1H-indazole (Hind) (1c), (cation)+ = (H2pz)+, Hazole = 1H-pyrazole (Hpz) (2c), (cation)+ = (H2bzim)+, Hazole = 1H-benzimidazole (Hbzim) (3c), (cation)+ = (H2im)+, Hazole = 1H-imidazole (Him) (4c) and (cation)+[trans-RuCl4(NO)(Hazole)]−, where (cation)+ = (H2ind)+, Hazole = 1H-indazole (1t), (cation)+ = (H2pz)+, Hazole = 1H-pyrazole (2t), as well as osmium analogues of the general formulas (cation)+[cis-OsCl4(NO)(Hazole)]−, where (cation)+ = (n-Bu4N)+, Hazole =1H-indazole (5c), 1H-pyrazole (6c), 1H-benzimidazole (7c), 1H-imidazole (8c), (cation)+ = Na+; Hazole =1H-indazole (9c), 1H-benzimidazole (10c), (cation)+ = (H2ind)+, Hazole = 1H-indazole (11c), (cation)+ = H2pz+, Hazole = 1H-pyrazole (12c), (cation)+ = (H2im)+, Hazole = 1H-imidazole (13c), and (cation)+[trans-OsCl4(NO)(Hazole)]−, where (cation)+ = n-Bu4N+, Hazole = 1H-indazole (5t), 1H-pyrazole (6t), (cation)+ = Na+, Hazole = 1H-indazole (9t), (cation)+ = (H2ind)+, Hazole = 1H-indazole (11t), (cation)+ = (H2pz)+, Hazole = 1H-pyrazole (12t), have been synthesized. The compounds have been comprehensively characterized by elemental analysis, ESI mass spectrometry, spectroscopic techniques (IR, UV–vis, 1D and 2D NMR) and X-ray crystallography (1c·CHCl3, 1t·CHCl3, 2t, 3c, 6c, 6t, 8c). The antiproliferative activity of water-soluble compounds (1c, 1t, 3c, 4c and 9c, 9t, 10c, 11c, 11t, 12c, 12t, 13c) in the human cancer cell lines A549 (nonsmall cell lung carcinoma), CH1 (ovarian carcinoma), and SW480 (colon adenocarcinoma) has been assayed. The effects of metal (Ru vs Os), cis/trans isomerism, and azole heterocycle identity on cytotoxic potency and cell line selectivity have been elucidated. Ruthenium complexes (1c, 1t, 3c, and 4c) yielded IC50 values in the low micromolar concentration range. In contrast to most pairs of analogous ruthenium and osmium complexes known, they turned out to be considerably more cytotoxic than chemically related osmium complexes (9c, 9t, 10c, 11c, 11t, 12c, 12t, 13c). The IC50 values of Os/Ru homologs differ by factors (Os/Ru) of up to ∼110 and ∼410 in CH1 and SW480 cells, respectively. ESI-MS studies revealed that ascorbic acid may activate the ruthenium complexes leading to hydrolysis of one M–Cl bond, whereas the osmium analogues tend to be inert. The interaction with myoglobin suggests nonselective adduct formation; i.e., proteins may act as carriers for these compounds.

Indazolium trans-[tetrachlorobis(1H-indazole)ruthenate(III)] (1, KP1019) and its analogue sodium trans-[tetrachlorobis(1H-indazole)ruthenate(III)] (2, KP1339) are promising redox-active anticancer drug candidates that were investigated with X-ray absorption near edge structure spectroscopy. The analysis was based on the concept of the coordination charge and ruthenium model compounds representing possible coordinations and oxidation states in vivo. 1 was investigated in citrate saline buffer (pH 3.5) and in carbonate buffer (pH 7.4) at 37 °C for different time intervals. Interaction studies on 1 with glutathione in saline buffer and apo-transferrin in carbonate buffer were undertaken, and the coordination of 1 and 2 in tumor tissues was studied too. The most likely coordinations and oxidation states of the compound under the above mentioned conditions were assigned. Microprobe X-ray fluorescence of tumor thin sections showed the strong penetration of ruthenium into the tumor tissue, with the highest concentrations near blood vessels and in the edge regions of the tissue samples.

A one-pot synthesis of osmium(IV) complexes with two different tautomers of indazole, 1H-indazole and 2H-indazole, namely (H2ind)[OsIVCl5(2H-ind)] (1) and (H2ind)[OsIVCl5(1H-ind)] (2) is reported. Both compounds have been comprehensively characterized by NMR spectroscopy, ESI (electrospray ionization) mass spectrometry, electronic absorption spectroscopy, IR spectroscopy, cyclic voltammetry and tested for antiproliferative activity in vitro in three human cancer cell lines, CH1 (ovarian carcinoma), A549 (non-small cell lung cancer) and SW480 (colon carcinoma), as well as in vivo in a Hep3B SCID mouse xeno-transplantation model. 2H-Indazole tautomer stabilization in 1 has been confirmed by X-ray diffraction.