Eliminating dormant malaria parasites in the liver: A missing piece of the malaria eradication puzzle

One of the sub-projects of the MMV-PDP Consortium recently received funding from the Bill and Melinda Gates Foundation, to further develop an innovative in vitro assay to find a drug to eliminate dormant malaria parasites in the liver – an area of unmet medical need that is critical for the malaria eradication agenda. To find out more about the significance of this research, we spoke with Clemens Kocken, Chairman of the Department of Parasitology at the Biomedical Primate Research Centre (BPRC) – the team leading the assay development.

Can you introduce yourself and tell us a bit about your background?

I’m Clemens Kocken and I’m part of a team of researchers at BPRC, working on malaria vaccines and malaria drug development, using appropriate primate models where necessary. I’ve been working at BPRC for 25 years and in addition to malaria drug and vaccine development, we’re also generally interested in developing new technologies to study malaria parasite biology so as to better understand parasite host interactions. Ultimately, we hope to be able to contribute to the development of better drugs and vaccines.

Can you tell us about your role within the MMV-PDP project and the progress you’ve made to date?

I’ve been working on drug discovery for dormant malaria parasites for more than 10 years and first got involved when we received a collaborative grant from the Wellcome Trust to develop drugs against falciparum and vivax malaria. Our role was to find drugs for dormant stages in the liver caused by vivax malaria, that can reactivate and cause relapsing malaria, as there are currently no suitable drugs for this. We started by developing an in vitro model, culturing the parasite liver stages to screen chemical compounds at a larger scale to see if we could find any activity in the dormant stages. The Medicines for Malaria Venture (MMV) was involved in the project from the beginning and in 2016 BPRC joined, funded by the Dutch Product Development Partnership Fund. We are developing new methodologies to improve the scale at which we can screen. In short, we have developed a transgenic parasite that expresses two enzymes (luciferase and nanoluc) to be able to perform an enzymatic assay as a read-out rather than a parasite staining and counting assay. By implementing this new system and screening on a much larger scale, the chances of finding new drugs will be much greater. We are currently working to finalise the assay and have set up a collaboration with the Pivot Park Screening Centre (PPSC) with whom we will develop and implement an automated medium- to high throughput screening.

Why is it so important to develop drugs against relapsing malaria?

When a patient is infected with relapsing malaria, they experience no symptoms once the clinical malaria has been cured. The vivax parasite simply sits in the liver and for some reason, it can reactivate and trigger malaria onset. The problem with this particular parasite is that before a patient experiences malaria symptoms, they can already transmit the infection to mosquitoes. So, before they are ill, they are already infectious to mosquitoes. This means that people that have been infected with vivax malaria are a hidden risk for parasites that can easily be transmitted. To eradicate malaria, you need to start at the source. We need to get rid of the relapsing parasites – this is one of the missing pieces in the puzzle when it comes to eradication.

There are two related drugs currently available that have shown to have an effect on hypnozoites (dormant forms in the life cycles of certain parasitic protozoa), but they have several severe side effects. In malaria endemic regions in particular, a genetic trait has been found in red blood cells that provides slight protection from the disease. People who have this genetic trait can’t take the drugs as they will lyse (rupture) their red blood cells. This is a real complication. Another complication for one of the drugs is that patients have to take it every day for two weeks. Since the patients are no longer sick by the time they take the drug, there is a big adherence problem. The bottom line is that there is a definite and real need for safe drugs to treat relapsing malaria.

Why do we have to look into primate malaria parasites?

For one, if you work in Europe, it’s very difficult to get hold of the human vivax parasite and it cannot be maintained in culture long-term. The only malaria parasites that form hypnozoites in the liver infect humans and non-human primates and there are no rodent animal models for these relapsing malaria parasites. The primate malaria parasite we use is a sister of the human vivax malaria parasite, so the biology is almost identical. Primate parasites also offer great consistency – you can work with the same parasite line for years, which is not possible when working with vivax malaria parasites from patients, that differ from patient to patient. Downstream, we can genetically modify the primate parasite and make the ones needed for the assay. This would not be possible with human vivax malaria parasites.

What are some of the gaps in our current knowledge of malaria research?

If one focuses only on hypnozoites, the main gap is that we don’t understand the parasite-host relationship and how the parasite survives. It can survive in humans for years and we don’t know how this is possible – why doesn’t the body attack the infected hepatocyte (liver cell tissue)? What does the parasite do to prevent this and how does the parasite reactivate after long periods of dormancy? The biology of dormant parasites is still a major black box.

What makes your approach innovative and how could it help fill the current gaps?

We are helping to shed some light, having developed transcriptomes of hypnozoites and developing parasites in the liver. While this hasn’t brought us closer to finding any specific drug targets, we do at least know what is expressed in the dormant parasite. We’re now using this information to make our transgenic parasite so that we can specifically identify whether drugs target hypnozoites in the assay that we run with an enzymatic readout.

You’ve recently received funding from the Bill & Melinda Gates Foundation to progress your research – can you tell us more about the significance of this? 

This is very exciting and essential; it took some time to develop the grant and the COVID-19 pandemic did not help. Luckily, we now have the funding to finalise the assay and get it running at the PPSC. We will then be able to screen chemical compounds in a quantity we have never been able to do before and hopefully this will result in new hits.

The progress we have made to date is a result of teamwork. It’s the result of the BPCR malaria liver stage team that has been working around the clock on this assay, but also of MMV, who provided a lot of input in guiding us and giving us the confidence that we would make it. This is teamwork that has been well promoted by Lygature and the MMV-PDP group as a whole and is proof of how partnerships like this should work.

Bron: Lyguture, 1 July 2020