Water-soluble Ru(II)- and Ru(III)-halide-PTA complexes (PTA=1,3,5-triaza-7-phosphaadamantane): Chemical and biological properties

Four structurally related Ru(II)-halide-PTA complexes, of general formula trans- or cis-[Ru(PTA)4X2] (PTA = 1,3,5-triaza-7-phosphaadamantane, X = Cl (1, 2), Br (3, 4), were prepared and characterized. Whereas compounds 1 and 2 are known, the corresponding bromo derivatives 3 and 4 are new. The Ru(III)-PTA compound trans-[RuCl4(PTAH)2]Cl (5, PTAH = PTA protonated at one N atom), structurally similar to the well-known Ru(III) anticancer drug candidates (Na)trans-[RuCl4(ind)2] (NKP-1339, ind = indazole) and (Him)trans-[RuCl4(dmso-S)(im)] (NAMI-A, im = imidazole), was also prepared and similarly investigated. Notably, the presence of PTA confers to all complexes an appreciable solubility in aqueous solutions at physiological pH. The chemical behavior of compounds 1–5 in water and in physiological buffer, their interactions with two model proteins – cytochrome c and ribonuclease A – as well as with a single strand oligonucleotide (5′-CGCGCG-3′), and their in vitro cytotoxicity against a human colon cancer cell line (HCT-116) and a myeloid leukemia (FLG 29.1) were investigated. Upon dissolution in the buffer, sequential halide replacement by water molecules was observed for complexes 1–4, with relatively slow kinetics, whereas the Ru(III) complex 5 is more inert. All tested compounds manifested moderate antiproliferative properties, the cis compounds 2 and 4 being slightly more active than the trans ones (1 and 3). Mass spectrometry experiments evidenced that all complexes exhibit a far higher reactivity towards the reference oligonucleotide than towards model proteins. The chemical and biological profiles of compounds 1–5 are compared to those of established ruthenium drug candidates in clinical development.