Metal-Mediated Ligand Affinity Chemistry (MLAC).

Metal-mediated ligand affinity chemistry (MLAC) enables site-specific protein modification and represents a powerful bioorthogonal strategy. Conventional bioorthogonal methods often involve two steps: (i) incorporation of the bioorthogonal handle (e.g., non-canonical amino acid, enzyme domain, peptide sequences) and (ii) the binding of functional molecules such as drugs, affinity tags, and fluorophores. This two-step protocol often involves genetic manipulation, which makes it impossible to chemically modify endogenous proteins in living systems. Thus, we propose the development of a transition metal-based chemical strategy that is ligand-directed to the endogenous protein of interest in a single step, which we refer to as metal-mediated ligand affinity chemistry (MLAC).

A Conformationally Restricted Gold(III) Complex Elicits Antiproliferative Activity in Cancer Cells.

Diamine ligands are effective structural scaffolds for tuning the reactivity of transition-metal complexes for catalytic, materials, and phosphorescent applications and have been leveraged for biological use. In this work, we report the synthesis and characterization of a novel class of cyclometalated [C^N] Au(III) complexes bearing secondary diamines including a norbornane backbone, (2R,3S)-N2,N3-dibenzylbicyclo[2.2.1]heptane-2,3-diamine, or a cyclohexane backbone, (1R,2R)-N1,N2-dibenzylcyclohexane-1,2-diamine. X-ray crystallography confirms the square-planar geometry and chirality at nitrogen. The electronic character of the conformationally restricted norbornane backbone influences the electrochemical behavior with redox potentials of -0.8 to -1.1 V, atypical for Au(III) complexes. These compounds demonstrate promising anticancer activity, particularly, complex 1, which bears a benzylpyridine organogold framework, and supported by the bicyclic conformationally restricted diaminonorbornane, shows good potency in A2780 cells. We further show that a cellular response to 1 evokes reactive oxygen species (ROS) production and does not induce mitochondrial dysfunction. This class of complexes provides significant stability and reactivity for different applications in protein modification, catalysis, and therapeutics.

Serum-Stable Gold(III) Bisphosphine Complex Induces Mild Mitochondrial Uncoupling and In Vivo Antitumor Potency in Triple Negative Breast Cancer.

The preparation of cyclometalated complexes offers a path to stable materials, catalysts, and therapeutic agents. Here, we explore the anticancer potential of novel biphenyl organogold(III) cationic complexes supported by diverse bisphosphine ligands, Au-1-Au-5, toward aggressive glioblastoma and triple negative breast cancer cells (TNBCs). The [C^C] gold(III) complex, Au-3, exhibits significant tumor growth inhibition in a metastatic TNBC mouse model. Remarkably, Au-3 displays promising blood serum stability over a relevant therapeutic window of 24 h and alteration in the presence of excess L-GSH. The mechanism-of-action studies show that Au-3 induces mitochondrial uncoupling, membrane depolarization, and G1 cell cycle arrest and prompts apoptosis. To the best of our knowledge, Au-3 is the first biphenyl gold-phosphine complex to uncouple mitochondria and inhibit TNBC growth in vivo.

Next Generation Gold Drugs and Probes: Chemistry and Biomedical Applications.

The gold drugs, gold sodium thiomalate (Myocrisin), aurothioglucose (Solganal), and the orally administered auranofin (Ridaura), are utilized in modern medicine for the treatment of inflammatory arthritis including rheumatoid and juvenile arthritis; however, new gold agents have been slow to enter the clinic. Repurposing of auranofin in different disease indications such as cancer, parasitic, and microbial infections in the clinic has provided impetus for the development of new gold complexes for biomedical applications based on unique mechanistic insights differentiated from auranofin. Various chemical methods for the preparation of physiologically stable gold complexes and associated mechanisms have been explored in biomedicine such as therapeutics or chemical probes. In this Review, we discuss the chemistry of next generation gold drugs, which encompasses oxidation states, geometry, ligands, coordination, and organometallic compounds for infectious diseases, cancer, inflammation, and as tools for chemical biology via gold-protein interactions. We will focus on the development of gold agents in biomedicine within the past decade. The Review provides readers with an accessible overview of the utility, development, and mechanism of action of gold-based small molecules to establish context and basis for the thriving resurgence of gold in medicine.

Gold(III)-P-chirogenic complex induces mitochondrial dysfunction in triple-negative breast cancer.

Chemical agents that specifically exploit metabolic vulnerabilities of cancer cells will be beneficial but are rare. The role of oxidative phosphorylation (OXPHOS) in promoting and maintaining triple-negative breast cancer (TNBC) growth provides new treatment opportunity. In this work, we describe AuPhos-19, a small-molecule gold(III)-based agent bearing a chiral phosphine ligand that selectively disrupts mitochondrial metabolism in murine and human TNBC cells but not normal epithelial cells. AuPhos-19 induces potent cytotoxic effect with half maximal inhibitory concentration (IC50) in the nanomolar range (220-650 nM) across different TNBC cell lines. The lipophilic cationic character of AuPhos-19 facilitates interaction with mitochondrial OXPHOS. AuPhos-19 inhibits mitochondria respiration and induces significant AMPK activation. Depolarization of the mitochondria membrane, mitochondria ROS accumulation, and mitochondria DNA depletion provided further indication that AuPhos-19 perturbs mitochondria function. AuPhos-19 inhibits tumor growth in tumor-bearing mice. This study highlights the development of gold-based compounds targeting mitochondrial pathways for efficacious cancer treatment.

Tuning Cyclometalated Gold(III) for Cysteine Arylation and Ligand-Directed Bioconjugation.

Transition-metal-based approaches to selectively modify proteins hold promise in addressing challenges in chemical biology. Unique bioorthogonal chemistry can be achieved with preformed metal-based compounds; however, their utility in native protein sites within cells remain underdeveloped. Here, we tune the ancillary ligands of cyclometalated gold(III) as a reactive group, and the gold scaffold allows for rapid modification of a desired cysteine residue proximal to the ligand binding site of a target protein. Moreover, evidence for a ligand association mechanism toward C-S bond formation by X-crystallography is established. The observed reactivity of cyclometalated gold(III) enables the rational design of a cysteine-targeted covalent inhibitor of mutant KRAS. This work illustrates the potential of structure-activity relationship studies to tune kinetics of cysteine arylation and rational design of metal-mediated ligand affinity chemistry (MLAC) of native proteins.

Gold-Based Pharmacophore Inhibits Intracellular MYC Protein.

Direct targeting of intrinsically disordered proteins, including MYC, by small molecules for biomedical applications would resolve a longstanding issue in chemical biology and medicine. Thus, we developed gold-based small-molecule MYC reagents that engage MYC inside cells and modulate MYC transcriptional activity. Lead compounds comprise an affinity ligand and a gold(I) or gold(III) warhead capable of protein chemical modification. Cell-based MYC target engagement studies via CETSA and co-immunoprecipitation reveal specific interaction of compounds with MYC in cells. The lead gold(I) reagent, 1, demonstrates superior cell-killing potential (up to 35-fold) in a MYC-dependent manner when compared to 10058-F4 in cells including the TNBC, MDA-MB-231. Subsequently, 1 suppresses MYC transcription factor activity via functional colorimetric assays, and gene-profiling using whole-cell transcriptomics reveals significant modulation of MYC target genes by 1. These findings point to metal-mediated ligand affinity chemistry (MLAC) based on gold as a promising strategy to develop chemical probes and anticancer therapeutics targeting MYC.

Cyclometalated Gold(III) Complexes Bearing DACH Ligands.

The synthesis of a novel class of cyclometalated gold(III) complexes supported by benzoylpyridine, benzylpyridine, and (1R,2R)-(+)-1,2-diaminocyclohexane (DACH) ligands, along with their crystal structures, is reported. These compounds provide a new scaffold to investigate biological properties of gold(III) complexes. The six complexes were prepared and characterized, following reactions of (C,N) cyclometalated gold(III) scaffolds, [Au(C^N)Cl2] with DACH, which yielded a new series of cyclometaled gold(III), 3-5, of the type [Au(C^NH)(DACH)2]+ and the nitrogen-substituted cyclometalated Au(III), 6-8, of the type [Au(C^N)(DACH)]2+. Antiproliferative activity of these complexes in a panel of cancer cells showed promising results with IC50 in the micromolar range and selectivity over normal epithelial cells, MRC5. Whereas 8 shows minimal interaction with superhelical DNA except at high gold concentrations of 500 µM, complex 5 does not show interaction even at 1000 µM. The complexes display significant uptake in OVCAR8 cancer cells within 200-1200 pmol/million cells with the exception of complex 4. Differential cellular uptake was observed for the complexes; for example, while 3 and 8 display significant uptake, 4 showed minimal uptake. The compounds proved to be stable under physiological conditions and were minimally affected by either glutathione or sodium ascorbate. Cell cycle studies reveal a G1 arrest induced by representative complexes. The results reveal that enhanced Au(III) stabilization promoted by combined cyclometalated and DACH ligands may offer ligand tuning insights for novel anticancer drug design.