An international collaborative research group, including National Institutes of Health (NIH; MD, USA) funded researchers from Stanford University (CA, USA), has developed a therapeutic compound that effectively inhibits Plasmodium falciparum, the malaria parasite responsible for the highest number of fatalities. The compound acts by targeting and inactivating an enzyme complex necessary for all stages of the parasite’s life cycle, a target which is also important for drug resistance to a current front-line antimalarial, artemisinin.
"The impact of malaria is widespread and devastating, and there is an urgent need for approaches to maximize clinical efficacy while minimizing side effects and drug resistance," explained Richard Conroy, director of the Division of Applied Science and Technology at the National Institute of Biomedical Imaging and Bioengineering, part of NIH. "This research presents a novel drug discovery approach and a new drug candidate that is selective in its target and could be used to enhance the efficacy of existing antimalarial drugs."
Malaria poses the greatest risk in equatorial regions, particularly in sub-Saharan Africa and South Asia, but there is a growing threat of malaria’s reintroduction to countries such as the US, where malaria was a significant public health problem until the 1950s.
The team used electron microscopy to study the structure of the proteasome, a protein complex necessary in human cells for cellular reproduction and protein degradation, and which is relied upon by the malaria parasite for rapid division and replication in host cells. Senior author Matthew Bogyo elaborated: "I've always had an interest in the proteasome as a target. Potentially, if you could inhibit this enzyme you could inhibit the parasites at all stages of their different life-cycle stages to prevent their transmission. So it would be both curative and potentially transmission blocking."
Unfortunately, the first studied that demonstrated proteasome inhibitors’ effectiveness in killing malaria were also toxic to the test animals. "We decided we needed to take a systematic approach and really screen for differences between the human and the parasite proteasome,” Bogyo continued. His team at Stanford University collaborated with researchers at the University of California, San Francisco (CA, USA), and used mass spectrometry to profile human and malaria parasite proteasomes, looking for protein segments each proteasome preferred to act upon. Bogyo added: "That tells you how the human and malaria enzymes differ from one another.”
The team was able to identify amino acid sequences preferred by the malaria parasite proteasome, but not the human, and they utilized this information to design and test three therapeutic compounds, one of which was able to inhibit the parasite’s proteasome activity without disrupting the human proteasome. In mouse models, malaria was almost completely eliminated without toxicity to the host.
The researchers then collaborated with the Laboratory of Molecular Biology (Cambridge, UK) to generate a high resolution image of the malaria proteasome, utilizing cryo-electron microscopy. "You previously weren't able to get such high-resolution images by cryo-EM, but recent advances in both instrumentation and the methods used to refine the images have dramatically changed the field of structural biology" Bogyo described.
Recent research has shown that P. falciparum has been acquiring mutations rendering it resistant to front-line antimalarial artemisinin, strengthening a pathway the parasites use to handle stress from the drug. This pathway requires the proteasome, and therefore this study has the potential to prevent the emergence of artemisinin-resistant malaria.
"That's what we're really excited about," Bogyo enthused. "Even at a low dose of the drug you could potentially put enough pressure on the parasite so that resistance to artimisinin would be much harder to attain for the parasite. There is also the possibility that different types of drugs may also work by inducing stress in the parasite. If you add on top of that a proteasome inhibitor, you're going to get a synergistic effect."
"The paper was a proof of concept that it's a viable strategy," Bogyo concluded. "It was hard to sell the viability of the proteasome as a drug target because it was hard to convince people that it is possible to get around the toxicity issues of proteasome inhibitors. It's finally becoming clear that it is a feasible approach."
Li H, O’Donoghue AJ, van der Linder WA et al. Structure- and function-based design of Plasmodium-selective proteasome inhibitors. Nature 530, 233–236 (2016); www.nibib.nih.gov/news-events/newsroom/nih-funded-researchers-develop-promising-candidate-next-generation-anti