Synthesis, X-ray diffraction and anti-proliferative biological activity of hispolon derivatives and their (η6-p-cymene)(Hispolonato)Ruthenium[II] chloride complexes

Hispolon is a natural product extracted from Phellinus igniarius and Phellinus linteus fungi that has previously shown antitumor activity. In this study we present the synthesis, chemical characterization and in vitro anti-proliferative activity of three [(η6-p-cymene)Ru(L)Cl] neutral complexes, where L = hispolon derivatives. The single crystal X-ray structures of the three Ru complexes all have the expected piano stool geometry with the p-cymene ligand at the apex of the piano stool and occupying three of the sites in a distorted-octahedral arrangement. Completing the octahedral coordination sphere are two β-diketone oxygen atoms of the deprotonated hispolon derivatives which are coordinated in a bidentate fashion and a chlorine atom. The cytotoxicity of the three hispolon derivatives along with their corresponding complexes was studied for A549 lung, HCT116 colon and U87 glioblastoma cell lines. In the glioblastoma cell line, increase of biological activity (lower IC50) is seen for all three hispolons after arene-ruthenium complexation: 5.1 times higher for 2,3,4-trimethoxy-hispolon (Hisp8), and 1.5 times for both 3-methoxy,4-hydroxy-hispolon (Hisp1) and 3,4-dimethoxy-hispolon (Hisp4). This effect was less pronounced in the colon HCT116 cell line with complexation having 1.2–1.4 times higher biological activity. For the lung cell line A549, there was no such effect and complex formation slightly decreased the biological activity. Regardless, in the glioblastoma cell line, the presence of Hisp8, the most liposoluble agent, either as the pure compound or as ligand in the η6-p-cymene-Ru complex, proved effective as an antitumor agent. More specifically, the [(η6-p-cymene)Ru(Hisp8)Cl] complex showed a decrease in IC50 value for all 3 cell lines, when compared to Hisp8. We also performed molecular mechanics, DFT and docking calculations to determine the inhibitory ability of hispolon analogs towards aldehyde dehydrogenase, which targets glioblastoma stem cells. All three [(η6-p-cymene)Ru(Hisp)Cl] coordination complexes of hispolon derivatives are more effective than the related neutral hispolons in the U87 MG human glioblastoma cell line. Our computational studies clearly indicate an important role for aldehyde dehydrogenase Arg139 amino acid. In addition, computational results indicate hispolonato anions are much better inhibitors of aldehyde dehydrogenase and improve the anti-glioblastoma biological activity, consistent with IC50 data.