Fluorescent NHC Gold(I) Complexes as Tools for Target Identification and Metallomic Studies in Cancer Cells

Abstract

In recent years, gold(I) complexes have attracted increasing interest as an alternative to conventional metallo-chemotherapeutic agents. Their mode of action differs significantly from that of traditional platinum-based drugs. However, the identification of their intracellular targets and the elucidation of their mechanisms of action remain major challenges, largely due to the lack of suitable molecular tools for tracking these compounds in biological systems. To address these issues, three novel gold(I) complexes bearing two different fluorescent labels were synthesized and characterized. These complexes include an N-heterocyclic monocarbene gold(I) complex 3 and the corresponding biscarbene 4, both of which are functionalized with an anthracenyl moiety, as well as an NHC gold(I) monocarbene complex bearing a BODIPY label 5. The specific aim of this study was to generate trackable metallodrug models for mechanistic and metallomics-oriented research. These compounds were characterized using 1H and 13C NMR spectroscopy and elemental analysis. The crystallographic structure of the anthracenyl gold monocarbene was also determined. Interaction studies with human serum albumin (HSA) using UV–vis absorption spectroscopy and ESI-MS provided clear evidence of metallodrug–protein interactions. The uptake of the complexes in A2780 ovarian cancer cells was measured using fluorescence-activated cell sorting (FACS) cytofluorimetry, which indicated strong cellular uptake for the anthracenyl-labeled compounds 3 and 4. A cell fractionation approach was used for the quantitative analysis of subcellular drug disposition. These approaches collectively enabled a direct visualization of the intracellular fate of the gold complexes, providing a framework for correlating cellular distribution with biological activity. Finally, the gold complexes bearing the anthracenyl probe were evaluated for their cytotoxic activity in cisplatin-sensitive and -resistant A2780 cancer cells; the BODIPY complex could not be evaluated due to its insolubility. Overall, this study provides solid evidence that gold complexes bearing the anthracenyl probe retain significant cytotoxic properties and enable monitoring the uptake and fate of these gold species inside cancer cells, thus providing a proof-of-concept that fluorescently labeled gold complexes can serve as powerful tools for mechanistic investigations, including metallomics and proteomics-oriented studies aimed at identifying molecular targets and affected pathways.