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February 11, 2016

Malaria Proteasome Inhibitors Could Reverse Parasite Drug Resistance

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    This structure (bottom left) of the malaria parasite's proteasome, obtained using the revolutionary Cryo-Electron Microscopy technique, enabled the design of a specific inhibitor (front) against the mosquito-borne malaria parasite (pictured at back). [University of Melbourne]

    With media attention recently focused on the spread of the Zika virus, it’s easy to forget about the mosquito-borne disease that has been credited with killing one out of every two people who have ever lived—malaria. Currently, close to 50 percent of the world’s population live in malaria-endemic areas, leading to between 200–500 million new cases and close to 500,000 deaths annually (mostly children under the age of five).        

    Adding to the complexities of trying to control this disease is that resistance to the most effective antimalarial drug, artemisinin, has developed in Southeast Asia, with fears it will soon reach Africa. Artemisinin-resistant species have spread to six countries in five years.

    A collaborative team of scientists from Stanford University, University of California, San Francisco, University of Melbourne, and the MRC in Cambridge have used cutting-edge technology to design a smarter drug to combat the resistant strain.

    "Artemisinin causes damage to the proteins in the malaria parasite that kill the human cell, but the parasite has developed a way to deal with that damage. So new drugs that work against resistant parasites are desperately needed," explained coauthor Leann Tilley, Ph.D., professor and deputy head of biochemistry and molecular biology in the Bio21 Molecular Science and Biotechnology Institute at The University of Melbourne.

    Malaria is caused by the protozoan parasite from the genus Plasmodium. Five different species are known to cause malaria in humans, with P. falciparum infection leading to the most deaths. The parasite is transmitted through the bite of the female mosquito and ultimately ends up residing within the host's red blood cells (RBCs)—replicating and then bursting forth to invade more RBCs in a recurrently timed cycle.     

    “This penetration/replication/breakout cycle is rapid—every 48 hours—providing the opportunity for large numbers of mutations that can produce drug resistance,” said senior study author Matthew Bogyo, Ph.D., professor in the department of pathology at Stanford Medical School. “Consequently, several generations of antimalarial drugs have long since been rendered useless.”

    The compound that investigators developed targets the parasites proteasome—a protein degradation pathway that removes surplus or damaged proteins through a cascade of proteolytic reactions.

    "The parasite's proteasome is like a shredder that chews up damaged or used-up proteins. Malaria parasites generate a lot of damaged proteins as they switch from one life stage to another and are very reliant on their proteasome, making it an excellent drug target," Dr. Tilley noted.

    The scientists purified the proteasome from the malaria parasite and examined its activity against hundreds of different peptide sequences. From this, they were able to design inhibitors that selectively targeted the parasite proteasome while sparing the human host enzymes.

    The findings from this study were published recently in Nature through an article titled “Structure- and function-based design of Plasmodium-selective proteasome inhibitors.”

    Additionally, scientists at the MRC used a new technique called Single-Particle Cryo-Electron Microscopy to generate a three-dimensional, high-resolution structure of a protein, based on thousands composite images.

    The researchers tested the new drug in red blood cells infected with parasites and found that it was as effective at killing the artemisinin resistant parasites as it was for the sensitive parasites.

    “The compounds we’ve derived can kill artemisinin-resistant parasites because those parasites have an increased need for highly efficient proteasomes,” Dr. Bogyo commented. “So, combining the proteasome inhibitor with artemisinin should make it possible to block the onset of resistance. That will, in turn, allow the continued use of that front-line malaria treatment, which has been so effective up until now.”

    "The new proteasome inhibitors actually complement artemisinin drugs," Dr. Tilley added. "Artemisinins cause protein damage and proteasome inhibitors prevent the repair of protein damage. A combination of the two provides a double whammy and could rescue the artemisinins as antimalarials, restoring their activity against resistant parasites."

    The scientists were excited by their results, as they may provide a much-needed strategy to combat the growing levels of resistance for this deadly pathogen. However, the researchers tempered their exuberance by noting that many more drug libraries needed to be screened before clinical trials can begin.   

    “The current drug is a good start, but it's not yet suitable for humans. It needs to be able to be administered orally and needs to last a long time in the blood stream," Dr. Tilley concluded.

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