Only two anti-relapse medicines exist: primaquine, which has been available for more than 60 years, and tafenoquine, which is in development. Both medicines are associated with potentially severe haematological side effects in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency. New treatments for relapse prevention without the G6PD liability are urgently needed.
Identifying compounds active against the parasite in assays, however, has proved challenging, since these parasites are difficult to access and maintain in the laboratory. Owing to the clear, unmet medical and scientific needs, this area of research is a key focus for MMV and its partners. Today, the development and use of novel assays to identify next-generation anti-relapse compounds is ongoing at several sites.1
Dr Steve Maher, Assistant Research Scientist, Center for Tropical and Emerging Global Diseases, University of Georgia, USA, tells us about his work on a new high throughput assay.
1. How did you develp the assay platform?
We have been developing this and other assay platforms for the past 8–9 years. Several reports in the literature describe how liver cells prefer to be contained in a 3D space. However, we have been able to keep the liver cells viable for over 30 days by confining them in a commercial multi-titre plate, allowing fast analysis with standard equipment. This simple invention now allows us to screen over 300 compounds at a time against the dormant form of the parasite.
2. What challenges did you face and how did you overcome them?
There is a perception in scientific research that complexity is needed to overcome challenging problems, such as how to maintain liver cells in culture. We spent our first few years developing a 3D culture model, but found such a model difficult to reproduce and share. By going back and re-analysing the root cause of difficulty in culturing liver cells, we found simple, systematic solutions, while maintaining model simplicity. We were then able to develop a highly detailed protocol that would allow for easy implementation of the culture in nearly every lab worldwide.
Another challenge was the collection of P. vivax parasites to test drugs against. These parasites only exist in malaria-endemic countries where pharmaceutical companies, together with all the infrastructure necessary to perform high throughput screening, are typically not present.
To source the parasites, our team made numerous trips to partner labatories in endemic areas of Asia to collect blood donated by infected individuals. We are grateful to be working with fantastic collaborators, including Dr François Nosten from the Shoklo Malaria Research Unit in Thailand and Dr Benoit Witkowski from the Pasteur Institute in Cambodia.
3. What has been achieved through the platform in terms of screening?
The MMV portfolio contains compounds active against most stages and species of malaria parasites, but little was known about their anti-hypnozoite activity. This is the first platform capable of generating such data on a large scale. In the past 2 years, we have screened thousands of compounds and portfolio drugs, and have identified new compounds with activity against P. vivax hypnozoites. These hits represent the first anti-hypnozoite series discovered in over 70 years.
4. What are the next steps?
The next steps are to confirm that the activity of this series is specific to malaria parasites and will not adversely affect humans. We would also like to dramatically expand our screening capacity and are looking to work with partners in other malaria-endemic countries such as India and Brazil.
5. How does working with MMV help to further innovation in this exciting area?
MMV’s project directors have been extremely supportive, first by clearly laying out the goals of developing such a platform, then providing honest feedback as we developed and characterized the platform to meet those goals. I have really enjoyed working with the MMV team. They are objective and understanding as well as pragmatic when obstacles are encountered.
1. University of Georgia (USA), Shoklo Malaria Research Unit (Thailand), Pasteur Institute, Cambodia, Mahidol Vivax Research Unit (Thailand) and National Centre for Biological Sciences (India).