Combining a Ru(II) "Building Block" and Rapid Screening Approach to Identify DNA Structure-Selective "Light Switch" Compounds

A Semi-Automated, High-Throughput Approach for the Synthesis and Identification of Highly Photo-Cytotoxic Iridium Complexes

The discovery of new compounds with pharmacological properties is usually a lengthy, laborious and expensive process. Thus, there is increasing interest in developing workflows that allow for the rapid synthesis and evaluation of libraries of compounds with the aim of identifying leads for further drug development. Herein, we apply combinatorial synthesis to build a library of 90 iridium(III) complexes (81 of which are new) over two synthesise-and-test cycles, with the aim of identifying potential agents for photodynamic therapy. We demonstrate the power of this approach by identifying highly active complexes that are well-tolerated in the dark but display very low nM phototoxicity against cancer cells. To build a detailed structure-activity relationship for this class of compounds we have used density functional theory (DFT) calculations to determine some key electronic parameters and study correlations with the experimental data. Finally, we present an optimised semi-automated synthesise-and-test protocol to obtain multiplex data within 72 hours.

Three's a crowd - stabilisation, structure, and applications of DNA triplexes

DNA is a strikingly flexible molecule and can form a variety of secondary structures, including the triple helix, which is the subject of this review. The DNA triplex may be formed naturally, during homologous recombination, or can be formed by the introduction of a synthetic triplex forming oligonucleotide (TFO) to a DNA duplex. As the TFO will bind to the duplex with sequence specificity, there is significant interest in developing TFOs with potential therapeutic applications, including using TFOs as a delivery mechanism for compounds able to modify or damage DNA. However, to combine triplexes with functionalised compounds, a full understanding of triplex structure and chemical modification strategies, which may increase triplex stability or in vivo degradation, is essential - these areas will be discussed in this review. Ruthenium polypyridyl complexes, which are able to photooxidise DNA and act as luminescent DNA probes, may serve as a suitable photophysical payload for a TFO system and the developments in this area in the context of DNA triplexes will also be reviewed.

Ruthenium(II) Polypyridyl Complexes and Their Use as Probes and Photoreactive Agents for G-quadruplexes Labelling

Due to their optical and electrochemical properties, ruthenium(II) polypyridyl complexes have been used in a wide array of applications. Since the discovery of the light-switch ON effect of [Ru(bpy)2dppz]2+ when interacting with DNA, the design of new Ru(II) complexes as light-up probes for specific regions of DNA has been intensively explored. Amongst them, G-quadruplexes (G4s) are of particular interest. These structures formed by guanine-rich parts of DNA and RNA may be associated with a wide range of biological events. However, locating them and understanding their implications in biological pathways has proven challenging. Elegant approaches to tackle this challenge relies on the use of photoprobes capable of marking, reversibly or irreversibly, these G4s. Indeed, Ru(II) complexes containing ancillary π-deficient TAP ligands can create a covalently linked adduct with G4s after a photoinduced electron transfer from a guanine residue to the excited complex. Through careful design of the ligands, high selectivity of interaction with G4 structures can be achieved. This allows the creation of specific Ru(II) light-up probes and photoreactive agents for G4 labelling, which is at the core of this review composed of an introduction dedicated to a brief description of G-quadruplex structures and two main sections. The first one will provide a general picture of ligands and metal complexes interacting with G4s. The second one will focus on an exhaustive and comprehensive overview of the interactions and (photo)reactions of Ru(II) complexes with G4s.

Combinatorial Synthesis to Identify a Potent, Necrosis-Inducing Rhenium Anticancer Agent

Combinatorial synthesis can be applied for developing a library of compounds that can be rapidly screened for biological activity. Here, we report the application of microwave-assisted combinatorial chemistry for the synthesis of 80 rhenium(I) tricarbonyl complexes bearing diimine ligands. This library was evaluated for anticancer activity in three different cancer cell lines, enabling the identification of three lead compounds with cancer cell growth-inhibitory activities of less than 10 μM. These three lead structures, Re-9B, Re-9C, and Re-9D, were synthesized independently and fully characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray crystallography. The most potent of these three complexes, Re-9D, was further explored to understand its mechanism of action. Complex Re-9D is equally effective in both wild-type and cisplatin-resistant A2780 ovarian cancer cells, indicating that it circumvents cisplatin resistance. This compound was also shown to possess promising activity against ovarian cancer tumor spheroids. Additionally, flow cytometry showed that Re-9D does not induce cell cycle arrest or flipping of phosphatidylserine to the outer cell membrane. Analysis of the morphological changes of cancer cells treated with Re-9D revealed that this compound gives rise to rapid plasma membrane rupture. Collectively, these data suggest that Re-9D induces necrosis in cancer cells. To assess the in vivo biodistribution and stability of this compound, a radioactive ^99mTc analogue of Re-9D, ^99mTc-9D(H₂O), was synthesized and administered to naı̈ve BALB/c mice. Results of these studies indicate that ^99mTc-9D(H₂O) exhibits high metabolic stability and a distinct biodistribution profile. This research demonstrates that combinatorial synthesis is an effective approach for the development of new rhenium anticancer agents with advantageous biological properties.

Mechanistic Investigation into DNA Modification by a RuII ,RhIII Bimetallic Complex

Despite significant progress in the treatment of cancer, there remains an urgent need for more effective therapies that also have less impact on patient wellbeing. Photodynamic therapy employs targeted light activation of a photosensitizer in selected tissues, thereby reducing off-target toxicity. Our group previously reported a Ru^II ,Rh^III bimetallic architecture that displays multifunctional covalent photomodification of DNA in the therapeutic window in an oxygen-independent manner, features that are essential for treating deep and hypoxic tumors. Herein, we explore the mechanism by which a new analogue, [(phen)₂ Ru(dpp)Rh(phen)Cl₂ ]³⁺ , or Ru^II -Rh^III , interacts with DNA. We established that Ru^II -Rh^III exhibits "light switch" behavior in the presence of DNA, undergoing strong electrostatic interactions that might involve groove binding. Furthermore, these noncovalent interactions play a major role in the covalent photobinding and photocleavage of DNA, which occur according to an oxygen-independent mechanism. Polymerase chain reaction (PCR) revealed that covalent modification of DNA by Ru^II -Rh^III , especially photobinding, is critical to inhibiting amplification, thus suggesting that the complex could exert its toxic activity by interfering with DNA replication in cells. This new structural motif, with phenanthroline at all three terminal ligand positions, has a number of properties that are promising for the continued refinement of photodynamic-therapy strategies.