Carbon-phosphorus stapled Au(I) anticancer agents via bisphosphine induced reductive elimination

Abstract

Towards the goal of generating new stabilized gold complexes as potent anticancer agents, we report here a novel class of Au(I) agents from Au(III)-mediated C_aryl-P bond formation captured within the same complex by reacting a C^N cyclometalated Au(III) complex with bisphosphines. Cyclometalated Au(III) complexes of the type [Au(C^N)Cl₂], where C^N represent different aryl pyridine framework reacted with bis(2-diphenylphosphino)phenyl ether in refluxing methanol to access an unsymmetrical gold complex featuring C-P coupling and Au(I)-phosphine. The complexes were characterized by ¹H-NMR, ¹³C-NMR, and ³¹P-NMR and mass spectrometry. The structures of the complexes were characterized by X-ray crystallography and purity ascertained by HPLC and elemental analysis. The complexes demonstrate promising anticancer activity in a broad panel of cancer cell lines of different tumor origin. Mechanistically, the complexes induce apoptosis, generate mitochondrial ROS, depolarize mitochondrial membrane potential and modulate mitochondrial respiration in cancer cells. Overall, we developed a new structural class of Au(I) complexes with promising anticancer potential with potential utility in other applications.

Fig. 1. (A) Synthetic scheme to access novel C–P stapled Au(i) bisphosphine complexes studied in this work. (B) Structures of STG-1, STG-2, STG-3.

Fig. 2. X-ray crystal structures of (A) STG-1 and (B) STG-2. Ellipsoids are drawn at 50%. Hydrogens and solvent molecules have been omitted for clarity.

Fig. 3. Cyclic voltammograms of STG compounds (5 mM) in DMSO with [N(Bu)₄PF₆] as supporting electrolyte. Potentials are shown vs. Ag/AgCl. (A) Voltammogram of STG-1 showing two reduction events at −0.33 V and −0.99 V. (B) Voltammogram of STG-2 showing one reduction event at −1.57 V. Data collected with a glassy carbon working electrode and Pt counter electrode.

Fig. 4. Results from the FITC-Annexin V/PI assay. (A–E) Dot plots of representative populations of cells with different treatment of compounds. (F) Bar chart of percentage of apoptotic cells determined by the assay. Data analyzed via one-way ANOVA with Dunnett's multiple comparisons test (****p < 0.0001, **p < 0.01, *p < 0.05, ns – not significant).

Fig. 5. Results from TMRE assay to determine mitochondrial membrane potential. (A) Plotted mean fluorescence intensity (MFI) of cells treated with STG-1 at different concentrations. (B) Histograms of fluorescence intensity of TMRE amongst populations. Data analyzed via one-way ANOVA with Dunnett's multiple comparisons test (***p < 0.001, **p < 0.01, ns – not significant).

Fig. 6. Results from mitochondrial ROS assay with MitoSOX Red. (A) Plotted mean fluorescence activity (MFI) of MitoSOX Red in SUM-159 cells treated with STG-1. Significance plotted from one-way ANOVA with Dunnett's multiple comparisons test. (B) Histograms of fluorescence intensity of MitoSOX Red at different concentrations of STG-1. Data analyzed via one-way ANOVA with Dunnett's multiple comparisons test (**p < 0.01, ns – not significant).

Fig. 7. Results from the Seahorse XF Mito Cell Stress Test. (A) OCR graph of SUM-159 cells treated with STG-1. (B) Key mitochondrial parameters extrapolated from the OCR of SUM-159 cells after addition of ETC inhibitors. (C) OCR graph of OVCAR-3 cells treated with STG-1. (D) Key mitochondrial parameters extrapolated from the OCR of OVCAR-3 cells after addition of ETC inhibitors. Data plotted as the mean ± S.D. with GraphPad Prism 10.2.0. Data analyzed via one-way ANOVA with Dunnett's multiple comparisons test (****p < 0.0001, ***p < 0.001, **p < 0.01, *p < 0.05, ns – not significant).