While the parasite, Plasmodium falciparum, is responsible for the majority of the annual 610,000–971,000 global malaria deaths,1Plasmodium vivax results in 70–80 million cases each year.2 In addition, severe complications are increasingly being associated with P. vivax malaria; the long-held perception that this is a benign form of malaria is changing.3
The only approved anti-relapse medicine able to eliminate the dormant liver-stage form (the hypnozoite) is primaquine, which has a 14-day treatment regimen, making compliance difficult to achieve. It is also associated with potentially fatal haemolytic side effects in patients deficient in the enzyme glucose-6-phosphate dehydrogenase (G6PD).4,5 Additionally, it has been in use for approximately 60 years and so the risk of resistance is ever present. Our goal to eradicate malaria cannot be achieved without new anti-relapse medicines.
1. Why have so few medicines been discovered and developed to treat relapsing malaria?
For a long time, the species P. vivax was considered benign and so focus was placed on the more lethal P. falciparum. We now know P. vivax is far from benign and so it is starting to get the attention it deserves.
Up until 5 years ago, there were a lack of suitable assays in which to test potential molecules: there was no cell assay6 and the only biological assay7 relied on a substitute primate model of infection. Significant progress has been made; we now also have a substitute rodent cell assay that can be used to prioritize testing. However, we are not yet using human parasites, so some molecules active solely against P. vivax could be missed.
2. What is MMV’s discovery strategy to identify anti-relapse molecules?
To be pragmatic and until we have a P. vivax cell assay, we plan to use currently available cell assays to screen each of the blood-stage active series in our portfolio.
In parallel, MMV and the Bill & Melinda Gates Foundation are working with different groups to develop the optimal assay: a cost-effective P. vivax cell assay able to screen large numbers of compounds at the same time. One of the biggest challenges is gaining access to sporozoites (which are used to infect liver cells and generate hypnozoites).
We are working with partners in disease endemic countries such as India, Peru and Thailand, which have laboratory facilities to dissect sporozoites from mosquitoes that have fed on infected blood. In addition, we have partners in the USA who are working to culture P. vivax parasites in the lab, which simplifies the sporozoite supply issue. Because this is such a challenging area of research it’s important to integrate our activities and share knowledge between the groups. MMV is taking a key role in this integration process.
All being well, in the next 5 years we plan to screen as many compounds as possible for activity against the hypnozoite of P. vivax.8 It’s an ambitious goal, as no one has been able to do this in a low- let alone a high-throughput fashion before. If we are successful with these approaches we will have made a huge advance towards identifying the nextgeneration of anti-relapse medicines.
3. What progress has been made so far?
We’ve found that some of our compounds already in development have activity against the hypnozoite as well as the blood stages. One promising chemical series has recently transitioned from discovery to preclinical development, and several others show some signs of activity.
As for the assays, the cell assay based on rodent parasites has been improved and is now ready to be used for the screening of a large library of 500,000 compounds. This will help us prescreen to see which compounds really can work in liver tissue. We will then progress interesting liver-stage hits into a specific hypnozoite assay.
The P. vivax cell assays are also progressing well. With the support of the Gates Foundation, Prof. Sangeeta Bhatia of Massachusetts Institute of Technology has developed a new culture system that may provide the assay we need to identify the next-generation anti-relapse medicine.9 We hope to be able to demonstrate feasibility of such an assay and begin high-throughput screening in the coming years.
1. World Health Organization. World Malaria Report 2012.
2. Mendis K at al. “The neglected burden of Plasmodium vivax malaria.” Am J Trop Med Hyg. 64(1-2 Suppl):97-106 (2001).
3. Price RN et al. “Vivax malaria: neglected and not benign.” Am J Trop Med Hyg 77:79–87 (2007).
4. Glucose-6-phosphate dehydrogenase (G6PD) is a key enzyme involved in protecting all human cells from oxidative stress. Deficiency in this enzyme is thought to have co-evolved with malaria as it offers a degree of protection against severe malaria.
5. Wells TN, Burrows JN, Baird JK. “Targeting the hypnozoite reservoir of Plasmodium vivax: the hidden obstacle to malaria elimination.” Trends Parasitol. 2010 26(3):145-51 (2010).
6. Cell or in vitro assay: using components of an organism isolated from their usual biological surroundings to test, in this case, the efficacy of molecules to kill the dormant liver stage of P. vivax malaria; also known as a ‘test tube model’.
7. Biological in vivo assay: using a living organism, in this case to test the efficacy of a molecule to kill the dormant liver stage of malaria; also known as an ‘animal model’.
8. Between 2008–2012, MMV and partners screened more than six million molecules for activity against the blood stage of malaria, leading to the identification of 25,000 chemical compounds. Given the priority and unmet medical need for P. vivax malaria, we would plan a similar screening campaign against this parasite.
9. Khetani SR, Bhatia SN. “Microscale culture of human liver cells for drug development.” Nat Biotechnol. (1):120-6 (2008).