Natural products have made a crucial and unique contribution to human health, and this is especially true in the case of malaria, where the natural products quinine and artemisinin and their derivatives and analogues, have saved millions of lives. The need for new drugs to treat malaria is still urgent, since the most dangerous malaria parasite, Plasmodium falciparum, has become resistant to quinine and most of its derivatives and is becoming resistant to artemisinin and its derivatives. This volume begins with a short history of malaria and follows this with a summary of its biology. It then traces the fascinating history of the discovery of quinine for malaria treatment and then describes quinine’s biosynthesis, its mechanism of action, and its clinical use, concluding with a discussion of synthetic antimalarial agents based on quinine’s structure. The volume then covers the discovery of artemisinin and its development as the source of the most effective current antimalarial drug, including summaries of its synthesis and biosynthesis, its mechanism of action, and its clinical use and resistance. A short discussion of other clinically used antimalarial natural products leads to a detailed treatment of other natural products with significant antiplasmodial activity, classified by compound type. Although the search for new antimalarial natural products from Nature’s combinatorial library is challenging, it is very likely to yield new antimalarial drugs. The chapter thus ends by identifying over ten natural products with development potential as clinical antimalarial agents.
Kingston D.G.I., Cassera M.B. (2022) Antimalarial Natural Products. In: Kinghorn A.D., Falk H., Gibbons S., Asakawa Y., Liu JK., Dirsch V.M. (eds) Antimalarial Natural Products. Progress in the Chemistry of Organic Natural Products, vol 117. Springer, Cham. https://doi.org/10.1007/978-3-030-89873-1_1
Belen Cassera is leading a research team that will test two new drugs for the treatment of malaria. The team’s work will be funded by a $3.7 million grant from the National Institutes of Health. (Photo credit: Amy Ware)
Though malaria was eliminated from the U.S. 70 years ago, the mosquito-borne disease caused by the Plasmodium parasite is still rampant in many parts of the world – nearly 40% of the world’s population is at risk of contracting it, and nearly 450,000 people die each year from it. With the rise of drug resistance, the current medical treatments aren’t enough to end this disease.
“Every drug treatment currently in use for malaria is showing resistance or reduced efficacy,” said Belen Cassera, a member of the University of Georgia’s Center for Tropical and Emerging Global Diseases. “Furthermore, there are very limited treatments for the most vulnerable – children and pregnant women. Over 60% of deaths are children under the age of 5.”
Cassera is co-leading the research team that recently received a $3.7 million grant from the National Institutes of Health to test two new drug candidates.
“These compounds are really promising as they are easy to synthesize, cheap, reliable, have a low toxicity profile, and kill the parasites fast,” said Cassera, associate professor in the Department of Biochemistry and Molecular Biology, part of the Franklin College of Arts and Sciences.
What’s unique about these compounds is that they can kill the parasite in three development stages in humans. Current treatments only target the blood stage, which is when clinical symptoms appear.
The life cycle of the Plasmodium parasite is complex. When an infected mosquito bites a person, just a small number of parasites – usually less than a hundred – are injected into the bite site and then travel to the liver, where they multiply in number to thousands. Once their numbers are sufficient enough, they invade the bloodstream and infect red blood cells.
When the number of parasites reaches 100 million, symptoms occur and some of the parasites develop into a sexual form, also known as the gametocyte stage. This is when symptoms occur. The sexual form is then transmitted back to the mosquito when the person is bitten again.
This complex life cycle makes it difficult to find a treatment that will eradicate the disease. Breaking the cycle of transmission between humans and mosquitos is key to accomplishing that goal. That’s why the team is excited about discovering compounds that can attack the parasite on multiple fronts.
“We are really a powerhouse team,” said Cassera. “We have a leading medicinal chemistry expert in Paul Carlier, the robust parasitology resources of UGA, and Max Totrov brings the machine-learning expertise to tie it all together.”
Cassera is a UGA Innovation Fellow, and she also credits the knowledge gained at UGA’s 2019 Innovation Bootcamp with helping her prepare a grant proposal that would be of particular interest to drug manufacturers.
Cassera has been working for several years to identify new drug candidates, along with Carlier, a professor in the Virginia Tech College of Science’s Department of Chemistry and director of the Virginia Tech Center for Drug Discovery, and Max Totrov, a computational chemist at Molsoft.
“We started working with the Malaria Box from Medicines for Malaria Venture, and the discoveries we made in basic malaria biochemistry and medicinal chemistry really springboarded us to a new level and led us in this new direction,” Cassera said.
Cassera is leading the testing of the new chemical variations of the antimalarial compounds prepared by Carlier for effectiveness in cellular and animal models.
“My lab will be looking at levels of toxicity, the potential for resistance, and how well they work both directly on the parasite and in infected mice,” she said. “We’ll be performing the studies for making the go/no-go decision for these compounds.”
A joint patent application for both drug candidates was recently filed, and the team is optimistic that their research will yield fast-acting candidates for advanced pre-clinical evaluation.
Associate professor Belen Cassera is one step closer to introducing her research to the marketplace. Having spent the summer as UGA’s newest Innovation Fellow, Cassera has learned a lot about how to bring parasitic disease therapeutics arising from her research to market.
“In fall 2019, I was among the 18 chosen women from UGA who participated in the inaugural Innovation Bootcamp, where we learned about the Innovation Fellow program, among several other opportunities designed to guide faculty seeking to commercialize their discoveries,” said Cassera, an associate professor in biochemistry and molecular biology and member of the Center for Tropical and Emerging Global Diseases. “The bootcamp was the ‘switch on’ I needed to refocus my research, and being chosen as an Innovation Fellow is the ‘takeoff’ of this new journey for me.”
Cassera’s research focuses primarily on the discovery and development of novel anti-parasitic drugs, aiming to understand how therapeutics work at the biochemical and cellular levels. A month into her fellowship, Cassera is already gaining new insight into the commercialization process and how it can inform her approach to research.
“I have experienced a great transformation in my research goals,” she said. “In every aspect that we have addressed, I see a translation back to my lab—everything is connected. For instance, I now understand how to utilize knowledge and resources that we already have to expand and grow into other areas that will bring in more funding, new knowledge and potentially new products.”
Launched in 2019 as part of UGA’s Innovation District initiative, the Innovation Fellows program encourages faculty and staff to pursue commercialization and development of their research through Innovation Gateway. Fellows are trained in how to successfully translate their research projects into a marketable products, receive mentorship from a fellow faculty and/or industry partner, and receive up $10,000 to support their activities.
“Belen is a very technical person with a very precise end goal in mind,” said Ian Biggs, director of programming for the Innovation District and director of Innovation Gateway’s startup program. “The goal of the Gateway team is to provide her with the tools, expertise and guidance she needs to turn her vision into a commercialized reality.”
Thanks to the Innovation Fellows program, the future is not only bright for Cassera’s research, but also for the rest of her academic career as well.
“The insights and knowledge I’ve gained from this fellowship will help me substantially improve my teaching, training and mentoring of students pursuing their careers in the biotech and pharmaceutical industries,” she said.
Applications for the 2021 fall cohort are now open. The deadline to apply is Aug. 15.
Last summer, biochemistry associate professor and Center for Tropical and Emerging Global Diseases member Belen Cassera was named the parasitology section editor for Current Clinical Microbiology Reports and has produced her first issue of reviews.
Before former Editor-in-Chief Alan Hudson from Wayne State University School of Medicine stepped down, he recruited Cassera based on a recommendation from Robert Cramer at the Geisel School of Medicine. She met Cramer during an NIH study section.
“Networking is really important if you want to get involved in the editorial side of academic journals,” said Cassera.
In addition to participating in study sections and attending conferences, participating in the peer-review process at journals can also help get your name out there, particularly for senior trainees. She believes that having senior trainees review scientific articles is an important teaching tool.
“Reviewing papers can enhance your own critical thinking,” said Cassera. “Can you ask questions to improve the study or see something missing from the research – reviewing other’s reports helps you to think out the box.”
As section editor, Cassera is responsible for determining the content for the yearly parasitology issue. And she has lofty goals for the parasitology section. She is particularly interested in topics other journals are not covering and that expand the reader’s thoughts on the subject.
“I want readers to have more questions than answers after reading the review,” said Cassera. “I want the articles to spark new ideas.”
For her first issue, she came up with a list of topics and approached researchers that could write on the subjects. However, she is open to unsolicited submissions.
“I would like to include papers for different parasites, but what I’m really looking for are papers that bring new questions to the topic,” said Cassera. “If the scientific community would benefit – if it will lead to new questions or shift our thinking, then I’m interested in it.”
If you have an idea for a possible white paper or review that you think would be a good fit for the parasitology section of Current Clinical Microbiology Reports, then contact Cassera.
Also, be sure to check out the latest reviews, two of which come from former CTEGD members:
Purpose: Variants in NUS1 are associated with a congenital disorder of glycosylation, developmental and epileptic encephalopathies, and are possible contributors to Parkinson disease pathogenesis. How the diverse functions of the NUS1-encoded Nogo B receptor (NgBR) relate to these different phenotypes is largely unknown. We present three patients with de novo heterozygous variants in NUS1 that cause a complex movement disorder, define pathogenic mechanisms in cells and zebrafish, and identify possible therapy.
Methods: Comprehensive functional studies were performed using patient fibroblasts, and a zebrafish model mimicking NUS1 haploinsufficiency.
Results: We show that de novo NUS1 variants reduce NgBR and Niemann-Pick type C2 (NPC2) protein amount, impair dolichol biosynthesis, and cause lysosomal cholesterol accumulation. Reducing nus1 expression 50% in zebrafish embryos causes abnormal swim behaviors, cholesterol accumulation in the nervous system, and impaired turnover of lysosomal membrane proteins. Reduction of cholesterol buildup with 2-hydroxypropyl-ß-cyclodextrin significantly alleviates lysosomal proteolysis and motility defects.
Conclusion: Our results demonstrate that these NUS1 variants cause multiple lysosomal phenotypes in cells. We show that the movement deficits associated with nus1 reduction in zebrafish arise in part from defective efflux of cholesterol from lysosomes, suggesting that treatments targeting cholesterol accumulation could be therapeutic.
Seok-Ho Yu, Tong Wang, Kali Wiggins, Raymond J. Louie, Emilio F. Merino, Cindy Skinner, Maria B. Cassera, Kirsten Meagher, Paul Goldberg, Neggy Rismanchi, Dillon Chen, Michael J. Lyons, Heather Flanagan-Steet & Richard Steet. Genet Med. 2021 Mar 17. doi: 10.1038/s41436-021-01137-6.
Unique lindenane sesquiterpenoid dimers from Chloranthecae spp. were recently identified with promising in vitro antiplasmodial activity and potentially novel mechanisms of action. To gain mechanistic insights to this new class of natural products, in vitro selection of Plasmodium falciparum resistance to the most active antiplasmodial compound, chlorajaponilide C, was explored. In all selected resistant clones, the half-maximal effective concentration (EC50) of chlorajaponilide C increased >250-fold, and whole genome sequencing revealed mutations in the recently discovered P. falciparum prodrug activation and resistance esterase (PfPARE). Chlorajaponilide C was highly potent (mean EC50 = 1.6 nM, n=34) against fresh Ugandan P. falciparum isolates. Analysis of the structure-resistance relationships revealed that in vitro potency of a subset of lindenane sesquiterpenoid dimers was not mediated by PfPARE mutations. Thus, chlorajaponilide C, but not some related compounds, required parasite esterase activity for in vitro potency, and those compounds serve as the foundation for development of potent and selective antimalarials.
The antimalarial candidate MMV008138 (1a) is of particular interest because its target enzyme (IspD) is absent in human. To achieve higher potency, and to probe for steric demand, a series of analogs of 1a were prepared that featured methyl-substitution of the B- and C-rings, as well as ring-chain transformations. X-ray crystallography, NMR spectroscopy and calculation were used to study the effects of these modifications on the conformation of the C-ring and orientation of the D-ring. Unfortunately, all the B- and C-ring analogs explored lost in vitro antimalarial activity. The possible role of steric effects and conformational changes on target engagement are discussed.
Sha Ding, Maryam Ghavami, Joshua H.Butler, Emilio F. Merino, Carla Slebodnick, Maria B. Cassera, Paul R. Carlier. Bioorg Med Chem Lett. 2020 Sep 5;127520. doi: 10.1016/j.bmcl.2020.127520
The cis-polyisoprenoid lipids namely polyprenols, dolichols and their derivatives are linear polymers of several isoprene units. In eukaryotes, polyprenols and dolichols are synthesized as a mixture of four or more homologues of different length with one or two predominant species with sizes varying among organisms. Interestingly, co-occurrence of polyprenols and dolichols, i.e. detection of a dolichol along with significant levels of its precursor polyprenol, are unusual in eukaryotic cells. Our metabolomics studies revealed that cis-polyisoprenoids are more diverse in the malaria parasite Plasmodium falciparum than previously postulated as we uncovered active de novo biosynthesis and substantial levels of accumulation of polyprenols and dolichols of 15 to 19 isoprene units. A distinctive polyprenol and dolichol profile both within the intraerythrocytic asexual cycle and between asexual and gametocyte stages was observed suggesting that cis-polyisoprenoid biosynthesis changes throughout parasite’s development. Moreover, we confirmed the presence of an active cis-prenyltransferase (PfCPT) and that dolichol biosynthesis occurs via reduction of the polyprenol to dolichol by an active polyprenol reductase (PfPPRD) in the malaria parasite.
Flavia M Zimbres, Ana Lisa Valenciano, Emilio F Merino, Anat Florentin, Nicole R Holderman, Guijuan He, Katarzyna Gawarecka, Karolina Skorupinska-Tudek, Maria L Fernández-Murga, Ewa Swiezewska, Xiaofeng Wang, Vasant Muralidharan, Maria Belen Cassera. Sci Rep. 2020 Aug 6;10(1):13264. doi: 10.1038/s41598-020-70246-0.
Chromatographic separation of the acetone extracts from the twigs and barks of Artocarpus lakoocha led to the isolation of the one new flavanone, lakoochanone (1), together with eleven known compounds (2-12). Lakoochanone (1) and moracin C (4) exhibited weak antiplasmodial activity against Plasmodium falciparum Dd2 with IC50 values of 36.7 and 33.9 µM, respectively. Moreover, moracin C (4) and sanggenofuran B (5) showed cytotoxic activity against A2780 cell line with the respective IC50 values of 15.0 and 57.1 µM. In addition, cyclocommunin (7) displayed strong antimycobacterial activity against Mycobacterium tuberculosis H37Ra with the minimum inhibitory concentration (MIC) value of 12.3 µM.
Sirada Boonyaketgoson, Yongle Du, Ana L. Valenciano Murillo, Maria B. Cassera, David G. I. Kingston, Kongkiat Trisuwan. Chem Pharm Bull (Tokyo). 2020;68(7):671-674. doi: 10.1248/cpb.c20-00080.
Eighteen new limonoids, including eight methyl angolensates (1–8) and 10 cipadesins (9–18), were isolated from the leaves of Cipadessa baccifera. Their structures were characterized by means of spectroscopic data analyses, single-crystal X-ray diffraction, and quantum chemistry computational methods. The C-6 configurations in those compounds possessing a C-6 hydroxy group were all assigned as S regardless of the magnitude of J5,6, and the C-2′ configuration in those bearing a 2-methylbutyryl residue was defined by single-crystal X-ray diffraction and NMR data. Compounds 1, 5, 6, 7, 11, and 12 showed moderate antimalarial activities with IC50 values ranging from 12 to 28 μM.
