Induction of transferrin aggregation by indazolium [tetrachlorobis(1H-indazole)ruthenate(iii)] (KP1019) and its biological function

Abstract

The ruthenium complex indazolium [trans-RuCl₄(1H-indazole)₂] (KP1019) shows significant anticancer activity, having successfully completed phase I clinical trials. It was proposed that an essential component of this success is associated with the targeted delivery of the complex to cancer cells by serum proteins, particularly by transferrin (Tf). Our studies proved that direct interaction of KP1019 with transferrin leads to the formation of ruthenium complex concentration-dependent aggregates. QCM (quartz microbalance) measurements of the KP1019 interaction with immobilized transferrin confirmed aggregate adsorption rather than stoichiometric binding, due to a high molar excess of the complex per transferrin molecule. The HaCat cell culture studies indicated that even a short pre-incubation of KP1019, with both apo and holoTf and serum albumin, resulted in inhibition of the cytotoxic activity of KP1019 against the investigated cells. In contrast, simultaneous administration of KP1019 with either apo or holoTf did not greatly diminish the cytotoxic activity of KP1019; however, it was lower than the Ru complex alone. Model studies revealed that when the reaction of transferrin and KP1019 was performed by dialysis, protecting the protein against insoluble Ru species, monomeric [Ru–Tf] adducts can be obtained.