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 N,N-bidentate ligands (R)- and (S)-2-(1-amino-2-methylpropyl)-3-benzyl-7-chloroquinazolin-4(3H)-one and studies on their chemical and biological properties. Using the enantiomerically pure (R)- and (S)-forms of the ligand, depending on the organometallic moiety, either the S_M,R or R_M,S diastereomers, respectively, were observed in the molecular structures of the Ru- and Os(cym) (cym = η⁶-p-cymene) compounds, whereas the R_M,R or S_M,S 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 (R)-enantiomer of the ligand being more potent than the (S)-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.

Chart 1. Structure of (R)-Ispinesib 1 and Synthetic Route to Complexes 3a–6b

Figure 1

Stereochemistry of complexes 3b (left) and 5a (right) in solution. Double arrows indicate key NOE contacts.

Figure 2

VT-¹H NMR spectra of 5b in CD₂Cl₂.

Figure 3

ORTEP representation of the molecular structures of (a) 3b and (b) 5a. Atomic displacement parameters are drawn at the 50% probability level. Hydrogen atoms are represented as fixed-size spheres and unlabeled for clarity. Counteranions and cocrystallized solvent have been removed for clarity.

Figure 4

¹H NMR spectra (aromatic region) of 6b in methanol-d₄ in the presence of various amounts of lithium chloride: (a) solution of 6b in CD₃OD, (b) 6b + 20 μL of LiCl, (c) 6b + 20 μL of LiCl after 24 h, (d) 6b + 40 μL of LiCl, (e) 6b + 90 μL of LiCl, and (f) 6b + 190 μL of LiCl.

Figure 5

KSP activity in the presence of increasing concentrations of the ligands (R)-2 and (S)-2 and of their organometallic complexes 3a–6a and 3b–6b. Data are presented as mean ± SEM, n = 3 or 4 (for controls).

Figure 6

Docked configurations of (A) 5a and (B) 5b into the binding site of KSP (PDB ID: 4AP0) using GoldScore. The hydrogen bond interaction is depicted as a green line, and lipophilic contacts are shown as purple dashed lines.

Figure 7

Ability to induce ROS generation in SW620 (left, A) and HepG2 (right, B) cells after 4-h exposure to complexes 3a−6b (1 μM). CTRL (expressed as 100%), cells in DMEM complete culture medium; DMSO, cells in DMEM complete culture medium with addition of DMSO (0.1%) as a solvent control; VER, cells in DMEM complete culture medium with addition of 10 μM verapamil to exclude potential impact of ABCB1 activity on the intracellular rhodamine 123 level. Results are expressed as mean ± SEM, n = 3. Asterisks denote statistical significance against respective solvent controls (P < 0.01, one-way ANOVA followed by post hoc Tukey test).

Figure 8

Total antioxidant capacity of the investigated compounds assayed by ABTS radical cation reduction. Data from a representative experiment performed in triplicate. Error bars were omitted to increase the legibility of the plot.

Figure 9

Accumulation of 3a–6b (10 μM) in SW620 cells treated for 1, 6, and 12 h. Data are presented as means ± SEM, n = 3.