The Abell Group

Biologically important small molecules, such as metal ions, reactive oxygen species (ROS) and reactive nitrogen species (RNS) play crucial roles in most physiological processes, and hence are promising biomarkers for disease diagnosis.

In particular, the ability to sense the above-mentioned molecules in vitro and in vivo provides important basis for early diagnosis of developmental diseases, chronic pain and cardiovascular diseases.

The Abell group focuses on the design and development of biocompatible fluorescent chemosensors that allow sensitive and selective detection of ions, ROS and RNS. These sensors enable the biologists to further investigate the connection between the analyte and the biological process of interest.

Furthermore, we incorporate the sensors with solid platforms including optical fibres, nanoparticles and liposomes to create portable sensing devices that facilitate non-toxic, minimally invasive real-time monitoring of disease progression.

A common limitation of fluorescence measurements is photobleaching of the sensor molecule.

One approach to circumvent this is to attach the fluorophore to a nitrogen-vacancy nanodiamond. The nanodiamonds produce a red fluorescence which does not photobleach under intense illumination, thus providing a versatile platform for tracking and imaging in microscopy. The surface bound fluorophore is freed from any pre-measurement illumination that is typically required, thus significantly reducing any photobleaching that may occur. This system has been demonstrated in M1 macrophages with a surface bound H2O2 sensor, with current research efforts to expand this pilot system to other novel sensors for an array of biomolecules.

Key publications

• Biological hydrogen peroxide detection with aryl boronate and benzil BODIPY-based fluorescent probes
• Photoswitchable calcium sensor: ‘On’–‘Off’ sensing in cells or with microstructured optical fibers
• An organic fluorophore-nanodiamond hybrid sensor for photostable imaging and orthogonal, on-demand biosensing

Light is often used as an experimental control of biological systems as it can be fine-tuned with regard to timing, location and intensity, is generally non-invasive, and does not contaminate the sample.

Thus, the ability to control the bioactivity of specific biomolecules using light would be highly desirable.

One of the main ways of introducing light-sensitivity into biological molecules is through the incorporation of a photoswitch, making it possible to control its biological activity by reversibly switching between the two isomeric forms, with ideally, one configuration having high affinity for the biological target and the other with low affinity.

These photoswitching molecules can be used as research tools for biologists to gain insight into cellular events by being able to spatiotemporally control biological processes in vivo using light to turn "on" and "off" a biomolecule.

Additionally, new drugs, termed photopharmaceuticals, can be developed by incorporating photoswitches into bioactive compounds to control the drug’s bioactivity to select locations, such as tumours, resulting in lower toxicity and side-effects.

Current biological targets include proteases, proteasome, GPCRs and GRKs. Our work is focused on the design, synthesis and activity of a library of spiropyran and azobenzene-based photoswitchable inhibitors for these targets.

For example, Gramicidin S is a potent naturally occurring antimicrobial cyclic peptide, though it exhibits toxicity against human erythrocytes. Photopharmacology presents as an ideal approach to regulate the activity of Gramicidin S to allow further exploitation of its desirable antimicrobial properties.

In this project, cyclic peptidomimetics of Gramicidin S are synthesised incorporating an azobenzene photoswitch to reversibly control secondary structure and hence antimicrobial activity and potentially toxicity. Antimicrobial activity can then be switched ‘on’ or ‘off’ upon irradiation with light of a specific wavelength, with spatiotemporal precision. This important strategy provides an opportunity to turn antimicrobial activity ‘on’ and ‘off’ to allow potential future point-of-care applications.

We are also exploring the use of optical fibres as a tool to help us to deliver light precisely, deep inside the body, hence opening up the potential to photoswitch and hence activate drugs in vivo.

Key publications

• Azobenzene-containing photoswitchable proteasome inhibitors with selective activity and cellular toxicity
• Photoregulation of α‐chymotrypsin activity by spiropyran‐based inhibitors in solution and attached to an optical fiber

Many therapeutics based on protease inhibitors are currently in late clinical trials or are already available as drugs. However, inhibitors of the cysteine protease family are very much under represented, primarily because of flaws in their design: existing inhibitors are conformationally flexible and biologically unstable structures with a 'reactive warhead' that makes them non-drug-like.

We have recently computationally designed and prepared (using ring closing metathesis and click chemistry) potent cyclic inhibitors of cysteine proteases that overcome these basic problems.

The constituent cycle constrains the inhibitor into a beta-strand geometry that is known to favour binding to the enzyme, resulting in improved biostability, an entropic advantage to inhibitor binding, and increased potency without the need for a 'reactive warhead'.

We have shown that these inhibitors stop the progression of cataract in lens culture (see image) and also in animal trials by inhibiting a cysteine protease and the study is entering a commercial phase.

This project involves the stereoselective synthesis of examples of these macrocyclic inhibitors and their assay against a range of proteases (including the proteasome) and an investigation into their potential to stop the growth of various cancer cell lines.

Key publications

• Macrocyclic Protease Inhibitors with Reduced Peptide Character
• A Macrocyclic Calpain Inhibitor Slows the Development of Inherited Cortical Cataracts in a Sheep Model
• Molecular Modelling, Synthesis, and Biological Evaluation of Macrocyclic Calpain Inhibitors

Antibiotic resistance is a significant threat to human health world-wide and new antibiotics are desperately required to control bacterial infections that are currently untreatable.

Two million patients contract infections each year at a cost of $5 billion annually in extended stays and expensive drug regimens. Around 90,000 of these will die from hospital-acquired infections untreatable with current antibiotics.

Biotin protein ligase (BPL) is an attractive target for the development of a new class of antibiotics. We have designed inhibitors of BPL from the pathogenic bacterium Staphylococcus aureus based on our solved x-ray structures of this enzyme.

These structures have been prepared and shown to be specific inhibitors of the Staphylococcus aureus BPL. We have recently reported the synthesis and evaluation of 5-halogenated-1,2,3-triazoles as BPL inhibitors.

The halogenated compounds exhibit significantly improved antibacterial activity over their nonhalogenated counterparts. The project involves all aspects of this study.

Key publications

• Halogenation of Biotin Protein Ligase Inhibitors Improves Whole Cell Activity against Staphylococcus aureus
• Optimising in situ click chemistry: the screening and identification of biotin protein ligase inhibitors