Ruthenium complexes are at the forefront of developments in
metal-based anticancer drugs, but many questions remain open regarding their reactivity in
biological media, including the role of
transferrin (Tf) in their transport and cellular uptake. A well-known anticancer
drug,
KP1019 ((IndH)[RuIIICl4(Ind)2], where Ind =
indazole) and a reference complex, [RuIII(nta)2]3- (nta = nitrilotriacetato(3-)) interacted differently with human apoTf, monoFeTf, or Fe2Tf. These reactions were studied by biolayer interferometry (BLI) measurements of Ru-Fe-Tf binding to recombinant human
transferrin receptor 1 (TfR1) in conjunction with UV-vis spectroscopy and particle size analysis. Cellular Ru uptake in human
hepatoma (HepG2) cells was measured under the conditions of the BLI assays. The mode of Tf binding and cellular Ru uptake were critically dependent on the nature of Ru complex, availability of Fe(III) binding sites of Tf, and the presence of
proteins that competed for
metal binding, particularly
serum albumin. Cellular uptake of
KP1019 was not Tf-mediated and occurred mostly by passive diffusion, which may also be suitable for treatments of inoperable
cancers by intratumoral
injections. High cellular Ru uptake from a combination of [RuIII(nta)2]3- and Fe2Tf in the absence of significant Ru-Tf binding was likely to be due to trapping of Ru(III) species into the endosome during TfR1-mediated endocytosis of Fe2Tf.