Metal-Dependent Cytotoxic and Kinesin Spindle Protein Inhibitory Activity of Ru, Os, Rh, and Ir Half-Sandwich Complexes of Ispinesib-Derived Ligands

Abstract

Ispinesib is a potent inhibitor of kinesin spindle protein (KSP), which has been identified as a promising target for antimitotic anticancer drugs. Herein, we report the synthesis of half-sandwich complexes of Ru, Os, Rh, and Ir bearing the ispinesib-derived <i>N</i>,<i>N</i>-bidentate ligands (<i>R</i>)- and (<i>S</i>)-2-(1-amino-2-methylpropyl)-3-benzyl-7-chloroquinazolin-4(3H)-one and studies on their chemical and biological properties. Using the enantiomerically pure (<i>R</i>)- and (<i>S</i>)-forms of the ligand, depending on the organometallic moiety, either the <i>S</i>_M,<i>R</i> or <i>R</i>_M,<i>S</i> diastereomers, respectively, were observed in the molecular structures of the Ru- and Os(cym) (cym = η⁶-<i>p</i>-cymene) compounds, whereas the <i>R</i>_M,<i>R</i> or <i>S</i>_M,<i>S</i> diastereomers were found for the Rh- and Ir(Cp*) (Cp* = η⁵-pentamethylcyclopentadienyl) derivatives. However, density functional theory (DFT) calculations suggest that the energy difference between the diastereomers is very small, and therefore a mixture of both will be present in solution. The organometallics exhibited varying antiproliferative activity in a series of human cancer cell lines, with the complexes featuring the (<i>R</i>)-enantiomer of the ligand being more potent than the (<i>S</i>)-configured counterparts. Notably, the Rh and Ir complexes demonstrated high KSP inhibitory activity, even at 1 nM concentration, which was independent of the chirality of the ligand, whereas the Ru and especially the Os derivatives were much less active.