Could you begin by outlining the focus and main objectives of your study into metal complexes?

The focus of my research is the development of small molecule drug delivery systems to improve the therapeutic profile of drugs and drug candidates. While many new compounds are highly successful during in vitro and preclinical studies, half of all drug candidates that enter clinical trials fail due to problems such as poor bioavailability, low efficacy and severe side effects. These issues also affect a number of drugs in current use.

We can overcome many of these limitations by reversibly coordinating a drug with a suitable metal complex. We can tune the properties of the metal-drug complexes so that they are very stable and non-toxic in normal cells, but can be selectively activated to release their drugs in diseased cells. This system also enables us to reversibly modify the properties of the drug to improve its stability, solubility and ability to cross cell membranes. With a wide range of potential ligands and diverse synthetic chemistry, metal complexes are versatile scaffolds that can be tailored to overcome the specific limitations of a broad spectrum of drugs.

How has your background prepared you for this project?

I began my research career in inorganic chemistry, developing molecular wires as catalysts to produce carbon monoxide with light or reduction. I later moved into the development of biologically active compounds for my PhD studies. During my postdoc and Australian Research Council (ARC) Discovery Early Career Researcher Award (DECRA) fellowships, I have had

opportunities to combine these areas, taking advantage of the properties that make metal complexes so interesting for catalysis, such as reactivity to light or reductive environments, to design responsive drug delivery vehicles.

By what means do you ensure that the metal complexes used in your research reach the target environment?

Metal complexes have a metal ion at their centre and several small molecules (ligands) bound to it. In our case, one of these small molecules is a drug or biologically active compound. The metal-ligand interaction is reversible, so we can temporarily bind a drug for transport, then release the drug in its original form at its target. We use the remaining ligands to tune the properties of the complex. For example, we can alter factors such as stability, solubility in the bloodstream, and ability to cross cell membranes. We can also incorporate targeting groups into these ligands, such as small peptides, that are recognised by receptors on the cell surface. By using targeting groups, where the receptor is overexpressed on the surface of the diseased cell, we can greatly increase the proportion of the metal-drug complex that is taken up by diseased cells relative to healthy cells.

How do you reduce any unwanted side effects in this area of drug delivery?

There are several approaches to reducing side effects with this approach. We are delivering the drugs in a stable, non-toxic form, so even if the drug is taken up by a healthy cell it should remain bound by the metal complex and therefore unable to act and cause side effects. By using a

targeting group, we ensure that the drug is preferentially taken up by the diseased cells to ensure that the proportion of the drug getting into healthy cells is very small.

Secondly, by having a controlled mechanism of drug release, the active form of the drug should not be released in non-target cells. In some cases, we use features of the diseased cells to induce drug release, such as low oxygen concentrations or acidic conditions. More recently, we have been exploring the application of visible light to release a drug from its carrier.

Could you outline your research group’s primary activities?

Researchers in my group are working on a number of different approaches to develop and improve the properties of these metal-drug complexes. This includes working with new classes of drugs and optimising the properties of existing complexes to be more selective and more biocompatible. Within my group, we carry out pre-clinical evaluation of the novel complexes in tumour cells to establish the toxicity, mechanism and fate of potential drug delivery systems. We have also recently established a collaboration with a group working on the development of new antituberculosis drugs.

Dr Anna Renfrew provides an insight into how her group is utilising metal complexes to create highly targeted drug delivery mechanisms, with the aim of boosting efficacy while reducing adverse side effects

Metal-based medicines

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BIOMEDICAL

REMARKABLE PROGRESS HAS been made in drug design over recent decades, with many highly targeted therapeutics now being designed at the molecular level. This progress is not always, however, reflected at the bedside. Indeed, approximately half of all potential drug candidates are unsuccessful in clinical trials, despite having previously shown promising activity in vitro.

Although the challenges that emerge at the clinical trial stage are often manifold, they frequently relate to high toxicity, often resulting in severe side effects, and low bioavailability and efficacy. Notably, these issues not only affect drug candidates, but also many pharmaceuticals currently in clinical use. Further research is therefore desperately needed to overcome these obstacles and develop safer, more effective drugs.

METAL COMPLEXES ARE THE SOLUTIONAt The University of Sydney, Dr Anna Renfrew is working with her research team to address this pressing need. The group’s focus is on the potential of metal complexes to facilitate

targeted anticancer and antibacterial drug delivery, as a means of reducing adverse side effects and dosing requirements while boosting overall efficacy. In this context, the drugs are coordinated to metal complexes as ligands. As metal-

ligand bonds are extremely responsive to their environment, often reacting to relatively subtle changes in factors from pH and oxidation states to temperature and light levels, metal complexes are ideal platforms for selective drug release. Moreover, the coordination of a drug to such complexes is a synthetically simple and reversible process, unlike the majority of other synthetic modifications.

Much of the team’s research is currently centred on tackling some of the major issues affecting contemporary drug development. “A drug must be effective at inhibiting or killing its target without affecting other areas,” Renfrew explains. In order to develop more targeted drug delivery systems, a series of selective drug release mechanisms are under investigation. Specifically, the scientists aim to create metal complexes that are inert and non-toxic under normal physiological conditions, and which are only activated to release the drug within their target environment. This would significantly limit distressing side effects and premature metabolism. At present, much of the group’s research in this area is focused on the development of metal-centred drug delivery systems that can be selectively activated in response to changes in oxygen concentration or by the localised application of visible light.

The researchers are also working hard to address another important hurdle in contemporary drug design. “A major challenge is designing a drug that is both sufficiently polar to be soluble in the

bloodstream and also sufficiently non-polar to cross cell membranes,” Renfrew elaborates. By coordinating a drug to a metal complex, its physiochemical properties can be reversibly modified without affecting its activity. The scientists are investigating ways to tune the pharmacokinetic profile of a drug by finding the optimum balance of lipophilic and charged ligands. In this way, coordination can greatly enhance properties such as solubility and ability to cross cell membranes, as well as protecting the drug from hydrolysis and protein binding.

DELIVERING CURCUMIN TO SOLID TUMOURSIn addition to these endeavours, the team is exploring the use of bioreductive complexes as drug chaperones. The goal is to target the hypoxic environment of solid tumours (regions where the cells have limited access to oxygen and nutrients), which are often resistant to conventional treatment strategies such as chemotherapy and radiotherapy. During her postdoctoral fellowship under the mentorship of Professor Trevor Hambley, Renfrew and her colleagues explored cobalt(III) complexes, which remain inert and stable under healthy physiological conditions but are reduced to a more reactive cobalt(II) state in a reducing, low-oxygen environment, quickly followed by release of the drug.

Capitalising on Hambley’s previous work on cobalt complexes, Renfrew later developed a cobalt(III) prodrug for the delivery and release

At The University of Sydney, research is underway to formulate novel ways of delivering drugs, combining improved stability and bioavailability with mechanisms for selective drug release in diseased cells

Selective drug delivery

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RESPONSIVE METAL COMPLEXES FOR SELECTIVE DRUG DELIVERY

OBJECTIVETo develop novel drug delivery systems based on photoactive metal complexes to effectively target cancer and bacterial cells.

KEY COLLABORATORSAssociate Professor Jamie Triccas, The University of Sydney, Australia

Associate Professor Richard Hartshorn, University of Canterbury, New Zealand

FUNDINGAustralian Research Council (ARC) Discovery Early Career Researcher Award (DECRA)

CONTACTDr Anna Renfrew ARC DECRA Fellow

School of Chemistry Room 412a, Building F11 The University of Sydney Sydney NSW 2006 Australia

T +61 293 512 830 E anna.renfrew@sydney.edu.au

http://bit.ly/Anna_Renfrew

DR ANNA RENFREW was born in Edinburgh, UK, and received her Masters in Chemistry from the University of York in 2006. She obtained her PhD in 2010 from

the École Polytechnique Fédérale de Lausanne, Switzerland, under the supervision of Professor Paul Dyson, followed by a Swiss National Science Foundation postdoctoral fellowship at The University of Sydney, Australia, under the mentorship of Professor Trevor Hambley. In 2013, she established her own research group at The University of Sydney with the support of a DECRA grant. Her research interests are metal-based anticancer agents and light-activated prodrugs.

of curcumin, a natural product and anticancer agent that can be extracted from the spice turmeric. By using a range of cutting-edge fluorescence and X-ray imaging techniques, the researchers have been able to demonstrate in spheroid solid tumour models that reduction of this novel prodrug leads to the successful release of curcumin in hypoxic tumour cells.

LOCALISED LIGHT ACTIVATIONNow as an independent researcher leading her own group, Renfrew is exploring the use of light-activated ruthenium complexes for use in photodynamic therapy – a treatment where light from a lamp or laser is applied to a tumour in combination with a photosensitiser. Photodynamic therapy is currently used to treat many topical cancers and other diseases. As the light source can be directed specifically at the tumour or diseased cells, it allows highly selective treatment with very little damage to healthy cells, however current photodynamic therapy agents require high levels of oxygen and are ineffective in hypoxic solid tumours. The Renfrew group is approaching this problem from a new angle, by using light to initiate metal-ligand bond cleavage in a process that does not depend on oxygen. This could result in novel drug release mechanisms for the treatment of malignant hypoxic tumours, with broader applications across a range of diseases, from tuberculosis to bacterial and fungal infections. “Ruthenium(II) complexes are a particularly interesting platform for this application as they can form light-sensitive bonds to many different types of ligands,” Renfrew reveals.

Already, Renfrew and her collaborators have developed a range of ruthenium complexes for the purpose of selectively releasing imidazole- and pyridine-containing drugs in light-treated cancer and bacterial cells. While the ruthenium drug complexes show very low toxicity in the dark, they are up to 50 times

more effective at killing cells in combination with light. The luminescence properties of these complexes also mean that they can be observed in live cells using fluorescence microscopy and release of the drugs can be monitored in real time. At present, work is underway to develop complexes that can be activated by red and infrared light. “This is much less damaging to surrounding tissue than blue or UV light,” expands Renfrew. “Additionally, it is able to penetrate more deeply through tissue so that it can be used to treat internal tumours.”

A SYNTHETICALLY SIMPLE SOLUTIONRenfrew is confident that her group’s work in this area will produce clinical benefits. “This approach can greatly improve the therapeutic profile of many drugs and drug candidates, and has the potential to be applied to a diverse library of compounds,” she enthuses. “What’s more, we can easily tune the properties of our complexes to optimise their selectivity and biological activity, making them suitable for the treatment of a number of different diseases.” For the time being, however, the research team will continue working to drive forward development in this area.

Fluorescence lifetime images of tumour cell spheroids treated with d) curcumin e) a cobalt-curcumin prodrug complex f) a ruthenium-curcumin prodrug complex. Fluorescence lifetime measurement allow evaluation of the ability of the prodrug to penetrate through solid tumour models and to determine when and where the drug is released. Image reproduced with permission from Renfrew, et al. Chemical Science, 2013, 4, 3731-9.

Luminescent ruthenium complexes can be observed in live tumour cells using confocal fluorescence microscopy.

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