The pharmaceutical scientist

Professor Patrick Sexton, head of the Drug Discovery Biology theme at the Monash Institute of Pharmaceutical Science, reflects on a research career in pharmacology and how working within a large research program that encompasses translational drug discovery, drug delivery and drug development has enriched his team’s research.

Lab+Life Scientist: What drew you to study science?

Professor Patrick Sexton: I was always interested in maths and biology at school and that drove me to do a science degree at the University of Melbourne, where I also did my postgraduate degree.

I majored in pharmacology in my undergraduate degree and after that took up a PhD position at the Department of Medicine at the Austin Hospital. My principal supervisor was Fred Mendelson, a medically trained physician who had trained overseas and come back to Australia to set up his own lab, and Jack Martin, based at the Repatriation General Hospital, was my co-supervisor.

L+LS: Was it a good choice to do a PhD in a medical environment?

PS: The Austin was a vibrant environment to do a PhD in. The seminars, journal clubs and other activities were done in the context of a large number of physicians together with basic scientists - it was a very good environment to start a research career in.

L+LS: How did you come to study G protein-coupled receptors?

PS: It began with my PhD project. I investigated G protein-coupled receptors (GPCRs), mainly the calcitonin receptor, and looked at their basic pharmacological characterisation. I was trying to understand more about what they did and where they were distributed.

When I began research in this area our understanding of these receptors was relatively limited. We knew they were coupled to G proteins but we didn’t know anything about their structure and there were no sequences available — looking up protein sequences in a database is something we take for granted nowadays.

I looked at the distribution of the calcitonin receptor in the brain and other tissues, including the kidney, using what was a relatively new imaging technique at the time — in vitro autoradiography. We cut tissue slices, incubated them with ligands (calcitonin gene-related peptide and calcitonin) and then imaged where the ligand had bound to receptors using X-ray film.

Part of that PhD work led to the discovery of a new subtype of GPCR that was eventually recognised as a receptor for the peptide amylin.

The calcitonin, amylin and related receptors form part of a subfamily of GPCRs, the family B receptors, which are mostly receptors that are important for circulating peptide hormones.

We also mapped where the calcitonin receptor was located in the brain and that work has been important in understanding the function of calcitonin and amylin, such as the control of appetite and where peptides from the peripheral circulation are likely to bind in the brain.

My research interest then moved more into the molecular understanding of the receptors as I moved on in my career.

I did my postdoc initially at the Repatriation General Hospital Heidelberg and then moved with Jack Martin’s group to St Vincent’s Institute where he took up the directorship. I established my own lab and worked there until the end of 1997 — I was there for almost 10 years.

Some of the early work I was involved with included purifying and cloning the receptors to determine their sequences. Some of this was successful, some not.

L+LS: Can you tell us about some of your career highlights?

PS: One career highlight was a consequence of my PhD work on the novel GPCR subtype.

Shortly after I finished my PhD, US pharmaceutical company Amylin Pharmaceuticals made contact. It turned out that the receptor subtype I had identified was a receptor for a small peptide called amylin, which was the potential therapeutic this company was pursuing.

Amylin is co-secreted with insulin in response to meal ingestion and helps control glucose levels and prevent hypoglycaemic episodes.

The company invited myself and colleagues to visit and funded a range of our work. It was a collaborative program and there were three groups involved in the research — one at what was Glaxo at the time, one at Amylin and our group. We also had funding from Glaxo. This was very exciting for a young researcher, and very helpful for me as an early-career scientist, to have a pot of pharmaceutical money to fund research.

We spent quite a lot of time trying to clone the amylin receptor.

The bottom line of that early work was that we didn’t identify a separate protein that could be classified as an amylin receptor.

Later on — and this was another exciting point in my career — we did work out why this was the case. We showed that unlike most GPCRs the amylin receptor was composed of a G protein-coupled receptor protomer and a separate protomer that was a single transmembrane spanning protein that changed the pharmacology of the calcitonin receptor.

L+LS: And the area of allostery — the binding of molecules at sites other than the endogenous ligand binding site of a receptor — has become a focus for your work?

PS: A lot of my more recent work has been in understanding how family B GPCR receptors work, which extends to what we call allosteric ligands — ligands that can bind to distinct sites to those used by the peptides that bind and activate the receptor.

Almost all small molecule therapeutics for this class of receptor bind to these topographically distinct sites. So understanding this interaction is key to developing therapeutics and this has been a recent focus.

L+LS: You are seeking out a mechanistic model of these receptors?

PS: Yes. A common component to GPCRs is a seven transmembrane section or core that spans the cell membrane. It is through this section of the protein that transmission of the signal from the extracellular to intracellular environment occurs.

The family B receptors have a rather large extracellular domain as well. The endogenous peptides or related mimetics tend to be between 27 and 50 amino acids in length and they bind to a great extent to this large extracellular component. This binding or interaction with the receptor appears to orientate the end of the peptide to the transmembrane section causing a conformational change in the receptor that drives signalling.

In general small molecules don’t bind the large extracellular domain of the receptor, they bind into pockets in the seven transmembrane core and some actually bind to intracellular sites and change the receptor response.

In this way we are gaining mechanistic understanding of the receptor ligand interaction.

L+LS: So the idea is to develop more specific or cleaner drugs?

PS: Yes that’s exactly the point.

In addition to allosteric drugs, there are drugs that can bind to the same receptor but engender different signalling; these can be either classical orthosteric drugs or allosteric drugs. These ‘biased’ ligands have the potential to sculpt biological response to maintain therapeutically relevant signalling but reduce or eliminate detrimental signalling.

It’s relatively new — the concept of bias has only been studied for a little over 10 years but recently has attracted a lot of interest as a therapeutically exploitable area for drug development.

L+LS: Does your work extend to drug discovery?

PS: Some of our work includes a large collaborative project with the French pharmaceutical company Servier. This is a structured program and the company funds 11 people in our lab — for basic research on a range of projects that are very much about drug discovery.

One project with Servier was trying to understand the differences between some of the drugs they had already discovered, and another two projects have been very early phase in terms of target validation in drug discovery research.

Some of the projects have already moved into high-throughput screening to identify compounds that can then be processed into lead development and hopefully move into phase 1 clinical trials.

We have an extensive amount of preclinical work going on but we haven’t gone into the clinical setting yet. It would be at least three years — assuming everything went well — before any of these lead compounds would be ready to go into clinical trials.

L+LS: Do you collaborate with any Australian companies?

PS: We have a collaboration that is just finishing with Queensland-based biotech company Alchemia.

This work is in the drug discovery area. We were looking at their internal proprietary library to identify whether they had good hits for the family B group of receptors that we work on.

We received funding from Alchemia, along with an ARC linkage program. The disease targets for that work were metabolic or lung disease, particularly chronic obstructive pulmonary disease.

We’ve had less direct involvement with local companies at this point in time and that’s at least partly because we have established relationships with large pharmaceutical companies.

There are opportunities with biotech in Australia — certainly a range of people at our institute interact with different Australian companies.

L+LS: Is the small size of the Australian sector one of the reasons for lack of involvement?

PS: I think investment is more problematic in Australia than overseas.

Investors in Australia often desire short-term gains rather than medium- or long-term gains, and it is the latter that is required for drug discovery and development.

There are venture capital groups that have made investments into the sector, but there are not many and, to be honest, I don’t know how well they’ve fared with their choices.

To have a vibrant industry a degree of private sector investment is needed, in addition to what can be done with government support. The government does fund a range of schemes that support drug development programs — the linkage program with ARC, NHMRC development grants — but this funding is available to progress early-phase discoveries to a point where they might have some commercial value or to push a partnership looking at the basic science aspects of something that might have commercial value.

L+LS: How did you come to join the Monash Institute of Pharmaceutical Science?

PS: After St Vincent’s Institute I moved to the Department of Pharmacology at the University of Melbourne.

I was there for about three years, after which I moved my research group across to the Florey — my PhD mentor Fred Mendelson was then director of the Florey Institute. Our primary focus was neurochemistry, although I’d established strong collaborations with Arthur Christopoulos when I was at the University of Melbourne and we continued to collaborate.

In 2006 the opportunity arose for Arthur and me to move our groups to Monash University and join another researcher, Roger Summers, who was also active in the GPCR space. We moved to the Department of Pharmacology at the Clayton campus.

The Monash Institute of Pharmaceutical Science (MIPS) was beginning to be established around this time and there was an initial realignment of groups into thematic research areas from what used to be departments. There was a group around drug delivery and dynamics, one on medicinal chemistry and another focused on drug candidate optimisation, but what they didn’t have was strong biology groups.

We were collaborating with people at the Faculty of Pharmacy, particularly in medicinal chemistry, and it was partly as a consequence of that that we ended up joining MIPS.

After discussions with Professor Bill Charman, director of MIPS, about the opportunities that would exist if we moved, they fitted out new floors for us and we moved into new labs and offices in 2009.

L+LS: Was it a productive move?

PS: Yes, of course. Working at MIPS has provided an enriched environment for us to work in and has facilitated strong collaborations across medicinal chemistry, monitoring drug levels in animal models, and drug delivery, for example, nanoparticle delivery.

It has also given us the capacity to recruit both young and senior research scientists and develop a critical mass for the GPCR biology space that we work in. There are now about 90 people who work in this space within drug discovery biology, which is the theme that I head within the institute.

One of the advantages we have here at MIPS is that we can go from the molecular, for example, with high-resolution crystal structures, all the way through to late-stage preclinical testing in animal models of disease. The integration with other groupings here at MIPS is very important for that. We can integrate with medicinal chemistry and get new compounds made, we can integrate with the drug delivery people about the best way to deliver drugs, we can integrate with the drug candidate optimisation group and measure the level of drugs getting into different compartments — this puts us in quite a powerful position with respect to developing a preclinical understanding of drugs as well as mechanism of action.

L+LS: How do you maintain a competitive edge in your research?

PS: We are always looking at the next challenges to ensure we remain competitive and at the leading edge of the field.

For example, as noted above, one of the things the broader group has been involved with here is the phenomenon of ligand-directed signal bias. At a conceptual level it gives you an ability to separate on-target therapeutic effects from on-target side effects. We have molecules that appear to do just that.

One target class that we are interested in as a bigger group, for example, is adenosine receptors, in work that is led by Lauren May, Paul White and Arthur Christopoulos.

The activation of adenosine receptors is highly cardio-protective; however, this also causes profound bradycardia (slow heart rate) and basically people can die from cardiac arrest at cardioprotective doses. So a dose that is high enough to have a strong therapeutic effect is limited by the side effects.

We published a paper in Proceedings of the National Academy of Sciences at the end of last year in which we identified a molecule that could give equivalent cardioprotection but didn’t have the bradycardia side effect.

That work was a proof in principle that on-target therapeutic effects could be separated from on-target side effects by sculpting conformations that the receptor could sample and thus send signals down a subset of pathways and not to others. This is a major area of work that we are pursuing.

We are also generating animal models in which receptor proteins have been changed to alter signalling, based on our molecular studies, to work out what are physiological or therapeutically important signalling pathways. Through these animal models we can ask questions about the physiology of signal transduction — what it means in terms of physiological response for a particular signalling pathway of a particular receptor and whether there are benefits to developing drugs with different profiles.

L+LS: Does collaboration play a big role in your work?

PS: We have a lot of collaborations with researchers around the world and others at Monash and across Australia.

For example, people working in electrophysiology, animal models of metabolic disease, chemistry, GPCR structural studies, NMR studies — I could keep going for a while on collaborations.

In order to do enriched science you need to be able to access the expertise and the technologies that sit across broad areas — there’s no way we can set up and maintain everything so we concentrate on our core areas of expertise. Sometimes it makes more sense for us to develop new areas of expertise internally, but in many cases it makes more sense to establish collaborations to get those studies done.

L+LS: Do you still get your hands dirty in the lab?

PS: I don’t get out a pipette and squirt drugs into tubes any more — even though I quite like doing benchwork. I don’t know where anything is in the lab so it can take me a long time to find things and then because I have meetings on a regular basis it is very difficult to manage an experiment. So what could take somebody who is constantly in the laboratory a couple of hours would take me a day. I did try probably four or five years ago but it was just wasn’t practical.

I am involved in the research direction, decision-making around projects, reviewing data, writing manuscripts and that side of the research.

Much of my time is directing and reviewing research, writing papers, writing grants — too much time writing grants.

Image caption: Professor Patrick Sexton about to deliver the keynote lecture at the 2014 Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists annual scientific meeting.