X‑ray Structure Analysis of Indazolium trans-[Tetrachlorobis(1H‑indazole)ruthenate(III)] (KP1019) Bound to Human Serum Albumin Reveals Two Ruthenium Binding Sites and Provides Insights into the Drug Binding Mechanism

Ruthenium­(III) complexes are promising candidates for anticancer drugs, especially the clinically studied indazolium trans-[tetrachlorobis­(1H-indazole)­ruthenate­(III)] (KP1019) and its analogue sodium trans-[tetrachlorobis­(1H-indazole)­ruthenate­(III)] (NKP-1339). Several studies have emphasized the likely role of human serum proteins in the transportation and accumulation of ruthenium­(III) complexes in tumors. Therefore, the interaction between KP1019 and human serum albumin was investigated by means of X-ray crystallography and inductively coupled plasma mass spectrometry (ICP-MS). The structural data unambiguously reveal the binding of two ruthenium atoms to histidine residues 146 and 242, which are both located within well-known hydrophobic binding pockets of albumin. The ruthenium centers are octahedrally coordinated by solvent molecules revealing the dissociation of both indazole ligands from the ruthenium-based drug. However, a binding mechanism is proposed indicating the importance of the indazole ligands for binding site recognition and thus their indispensable role for the binding of KP1019.