Ru super(II)( eta super(6)-p-cymene) Complexes of Bioactive 1,2-Benzothiazines: Protein Binding vs. Antitumor Activity

1,2-Benzothiazine-3-carboxamide 1,1-dioxide derivatives such as meloxicam are known to display numerous pharmacological activities. We prepared a series of 4-hydroxy-2-alkyl-2H-benzo[e][1,2 ]thiazine-3-carboxamide 1,1-dioxide ligands 1a-f and their Ru( eta super(6)-p-cymene) complexes 2a-f, inspired by synergistic effects observed with other bioactive ligands coordinated to metal centres. The molecular structures of 1a, 2a, and 2b were determined by X-ray diffraction analyses. The stability of the metal complexes was characterized in DMSO and DMSO/H sub(2)O on the basis of super(1)H NMR spectroscopy and their protein binding capabilities were studied using mass spectrometry. In vitro cytotoxicities of the Ru complexes were determined against human colorectal carcinoma (HCT116), non-small cell lung carcinoma (NCI-H460) and cervical carcinoma (SiHa) cell lines. The low levels of biological activity observed for these Ru complexes were put into context by considering their chemical reactivity with proteins. The binding of proteins resulted in cleavage of the benzothiazine backbone when the complex was present in concentrations equimolar with respect to protein. Newly synthesized Ru super(II)( eta super(6)-p-cymene)Cl complexes of 1,2-benzothiazines react with ubiquitin to yield 1:1 or 1:2 adducts. Importantly, the 1:2 complex retains a 1,2-benzothiazine ligand and the specific complexes formed rely critically on the specific protein-to-metal complex ratios contained within incubation mixtures. Moreover, a correlation of protein binding to Ru complex cytotoxicity is made.