Anti-cancer N-heterocyclic carbene complexes of platinum(II) and gold(III) : luminescence probes for mismatched DNA, target(s) identification and anti-angiogenesis agents

Abstract

Cancer remains a major threat to human. This can be explained by the difficulties for diagnosis of cancer in early stage and the lack of effective anti-cancer treatment agents with low toxicity. Since DNA mismatches are strongly associated with cancer transformation, recognition of mismatched DNAs should be of great importance for cancer diagnosis. This, together with the fact that high sensitivity and high temporal and spatial resolution can be achieved by non-invasive fluorescence microscopy, initiated my study on luminescent platinum(II) complexes that selectively probed DNA mismatches. An introduction of strong σ-donors such as N-heterocyclic carbene (NHC) ligands rendered the platinum(II) complexes strongly emissive in solution, owing to the increase in energy level of non-radiative d-d excited state. The high versatility of the NHC ligands allowed fine-tuning of the binding of the complexes to mismatched DNAs and matched DNAs. With judicious choice of the steric bulkiness of the NHC ligands, the platinum(II) complexes showed significantly higher luminescence enhancement upon binding to mismatched DNA (e.g., CC mismatch), as compared to that of matched DNA (up to 15-fold). Therefore, the complexes could be utilized for differentiation of cancer cells and human tissue samples with different levels of mismatch repair by emission spectroscopy and fluorescence microscopy.

Identification of molecular target(s) of anti-cancer drugs can provide valuable information on the anti-cancer mechanism of the drugs and the design of combination therapy. Recently, cyclometalated gold(III) NHC complexes, [Au(C^N^C)(NHC)]+, have been reported to show promising in vitro and in vivo anti-cancer properties. Nonetheless, the primary molecular target(s) of the gold(III) complexes has not been identified. Based on chemical biology approach, novel cyclometalated gold(III) NHC complexes with photo-affinity and “clickable” moieties were synthesized and characterized. Photo-activation of the complexes in vitro resulted in covalent binding of the complexes onto their molecular target(s). Subsequent labelling of the protein-complex adducts by fluorescent dyes allowed visualization of the adducts by two-dimensional gel electrophoresis (2DE) and in-gel fluorescence scanning. The proteins were then identified by Peptide Mass Fingerprinting (PMF) using MALDI-TOF MS analysis, and heat shock protein 60, vimentin, nucleophosmin, Y box binding protein, nucleoside diphosphate kinase A and peroxiredoxin 1 were determined to be potential protein target(s) of the gold(III) complexes.

Angiogenesis, which is the process of forming new blood vessels from pre-existing ones, plays important roles on supplying oxygen and nutrient to tumor cells for their growth and progression. It is anticipated that metal complexes with potent cytotoxicity and anti-angiogenic properties should be excellent candidates for anti- cancer treatment. A series of novel platinum(II) complexes containing pincer type NHC ligands were identified as angiogenesis inhibitors. They slowed down the extracellular matrix proteolysis process by inhibiting MMP expression (e.g. urinary-type plasminogen activator, uPA), and this could inhibit the downstream signaling pathway of uPAR and protect native VEGF from cleavage, thereby inhibiting the proliferation of endothelial cells, resulting in promising anti-angiogenic properties. Together with the high cytotoxicity to cancer cells, the platinum(II) complexes demonstrated significant inhibition of tumor growth in vivo.