Medicines currently used to protect vulnerable populations are becoming less effective as the parasite develops resistance; they are also often not well tolerated. Moreover, as the burden of malaria declines, immunity will decline. These factors increase the need for new protective medicines.
For a compound to have prophylactic potential it ideally needs to act against the early lifecycle stages of the parasite – the sporozoites from the mosquito bite and the dividing form in the liver, known as schizonts. By targeting these forms we can stop asexual blood-stage parasites, which cause the clinical symptoms of malaria.
MMV is working with the University of California, San Diego (UCSD), to screen compounds for activity against these forms using a rodent parasite (P. berghei). The team has successfully optimized the assay and screened over half a million compounds in the last year, identifying hundreds of active compounds. They are now analyzing the data.
Dr Elizabeth Winzeler, University of California, San Diego, talks about the importance of continuing the hunt for new molecules and the collaboration with MMV.
1. Why do we need to identify new molecules active against the liver and other stages of malaria?
A lot of the medicines we have, like quinine and artemisinin, were developed from natural products. They were never really designed to get rid of all the stages of the parasite in the body; they were designed to make you feel better. Today, we have a much deeper understanding of the role of various stages of the parasite’s lifecycle leading to transmission and relapse, which means we can really design ‘so called’ sterilizing medicines that could eliminate all the stages in the lifecycle. It’s a high hurdle, but I think we can do it.
2. How does the assay work?
We receive mosquitoes carrying rodent malaria. The parasites are treated with a specialized luciferase marker, which means they output light. We dissect the mosquitoes and put them onto a lawn of liver cells treated with compounds. If the compound has no effect they invade and develop a productive infection in the cells. The parasites will then convert luciferin to light. We can detect compounds that prevent this from happening based on the light output from our screening wells. We can screen large numbers of compounds in this way.
3. What have you found so far?
We are currently analyzing the data from the set of 550,000 compounds provided by MMV that we recently screened. Previously, we screened about 100,000 compounds from different libraries from MMV’s partners. Of these, we performed a dose–response analysis of 10,000 compounds. We have at least 2,000 compounds with promising activity at appropriate concentrations that we will followup. Seven hundred are very potent and might be interesting for further development.
4.What are the next steps?
The same compounds have also been screened in a blood-stage assay. The data sets will be analyzed together and the compounds prioritized for testing in specialized assays. We’re also interested in testing the most promising compounds for activity against gametocytes for transmission blocking and the hypnozoite for radical cure. I expect at least a subset of these compounds would have what I like to call ‘sterilizing activity’ so they would eliminate hypnozoites as well as the transmission and blood stages. While we’re not 100% clear how to identify this magical compound, we are taking a pragmatic approach to screening compounds in a sequence of assays to identify which could serve as scaffolds for drug discovery efforts.