The development of parasite resistance to both artemisinin derivatives and their partner drugs jeopardizes the effectiveness of the artemisinin combination therapy. Thus, the discovery of new antimalarial drugs, with new mechanisms of action, is urgently needed. We recently disclosed that β-carboline 1a was orally efficacious in Plasmodium berghei-infected mice and that it showed low cross-resistance between susceptible Plasmodium falciparum and four different drug-resistant strains. In this report, we describe the synthesis and in vitro antimalarial evaluation of 91 new derivatives of 1a. The asexual blood stage growth inhibition data show a clear preference for a 3,4-dihalogenated, 3,5-dihalogenated, 3,4,5-trichloro-, or 4-trifluoromethyphenyl ring at the C1-position. The most potent compound, 3,4,5-trichlorophenyl-substituted 42a, is twice as potent as 1a. Six potent analogues were assessed for their drug-like properties, and four of these were subjected to in vitro barcoded cross-resistance profiling. Compounds 1a, 1m, 42a, and 42m showed no cross-resistance to 32 resistance mutations on the Dd2 genetic background and 10 resistance mutations on the 3D7 genetic background. These data suggest that compounds in this scaffold possess a novel mechanism of antimalarial action.
Jopaul Mathew, Bo Zhou, Reagan S Haney, Kevin A Kunz, Leticia S Do Amaral, Rudraneel Roy Chowdhury, Joshua H Butler, Haibo Li, Amarraj J Chakraborty, Anika Tabassum, Emily K Bremers, Emilio F Merino, Rachael Coyle, Marcus C S Lee, Delphine Baud, Stephen Brand, Maxim Totrov, Maria Belen Cassera, Paul R Carlier. ACS Infect Dis. 2024 Oct 28. doi: 10.1021/acsinfecdis.4c00653.
Calophyllum tomentosum belonging to Clusiaceae family is an Indian medicinal plant used as folklore medicine to cure various kinds of diseases reported in Ayurveda, and the leaves of the plant are also used as an active ingredient for the preparation of a botanical medicine known as ‘Punnaga’, ‘Surapunnaga’ and ‘Tamoil’ among other common names. Chemical profiling of the methanol extract of the defatted leaf revealed the presence of amentoflavone as one of the constituents along with coumarins, terpenoids, steroids, and apetalic acids. Structural determination of these amentoflavone has been conducted by chemical, spectral, and spectrometric methods in comparison with spectral values available in the literature and confirmed by a single crystal X-ray diffraction study. Amentoflavone (1) and its derivative (2-5) tested to check the efficacy of anti-malarial activity against Plasmodium falciparum. Amongst them, only tetra methoxy amentoflavone, (2) exhibited moderate anti-malarial activity with IC50 value 1.99 ± 0.42 µM against Plasmodium falciparum in comparison with artemisinin as control, whereas the other products possessed almost negligible activity although their structural skeletons are identical with little variation of number and nature of substituents. The structure activity relationship (SAR) of the active constituent and its derivatives is reported herein.
Ajoy Kumar Bauri, Joshua H Butler, Maria B Cassera, Sabine Foro. Chem Biodivers. 2024 Oct 14:e202401576. doi: 10.1002/cbdv.202401576.
Figure 1. Schemes of the positive and negative selection cycles are illustrated.
Dolichols are isoprenoid end-products of the mevalonate and 2C-methyl-D-erythritol-4-phosphate pathways. The synthesis of dolichols is initiated with the addition of several molecules of isopentenyl diphosphate to farnesyl diphosphate. This reaction is catalyzed by a cis-prenyltransferase and leads to the formation of polyprenyl diphosphate. Subsequent steps involve the dephosphorylation and reduction of the α-isoprene unit by a polyprenol reductase, resulting in the generation of dolichol. The size of the dolichol varies, depending on the number of isoprene units incorporated. In eukaryotes, dolichols are synthesized as a mixture of four or more different lengths. Their biosynthesis is predicted to occur in the endoplasmic reticulum, where dolichols play an essential role in protein glycosylation. In this study, we have developed a selection of aptamers targeting dolichols and enhanced their specificity by incorporating fatty acids for negative selection. One aptamer showed high enrichment and specificity for linear polyisoprenoids containing at least one oxygen atom, such as an alcohol or aldehyde, in the α-isoprene unit. The selected aptamer proved to be a valuable tool for the subcellular localization of polyisoprenoids in the malaria parasite. To the best of our knowledge, this is the first time that polyisoprenoids have been localized within a cell using aptamer-based imaging techniques.
Flavia M Zimbres, Emilio F Merino, Grant J Butschek, Joshua H Butler, Frédéric Ducongé, Maria B Cassera. Molecules. 2023 Dec 28;29(1):178. doi: 10.3390/molecules29010178.
Super-resolution microscopy showing malaria parasites infecting human red blood cells. credit: Muthugapatti Kandasamy, Biomedical Microscopy Core
They say what doesn’t kill you makes you stronger. Whoever coined that adage had probably never heard of Plasmodium.
It’s a microscopic parasite, invisible to the naked eye but common in tropical and subtropical regions throughout the world. Each year, millions of people are infected by Plasmodium and exposed to an even more debilitating—and often deadly—disease: malaria.
Malaria is one of the deadliest diseases known to man. It can lead to extreme illness, marked by fever, chills, headaches and fatigue. More than half the world’s population is at risk of contracting the disease, and those who develop relapsing infections suffer a host of associated costs.
Limited educational opportunities and wage loss lead to an often unbreakable cycle of poverty. Vulnerable populations are most at risk.
“When I’m teaching in an endemic area like Africa, it isn’t unusual to find a student who needs to sleep during part of the workshop because they have malaria,” researcher Jessica Kissinger said.
It’s a challenge she and her collaborators in the University of Georgia’s Center for Tropical and Emerging Global Diseases (CTEGD) are trying to combat.
When the Center was established in 1998, there were only a couple of faculty members studying Plasmodium. Now, 25 years later, it has become a world-class powerhouse of multidisciplinary malaria research. Scientists examine various species of the dangerous parasite, studying its life cycle and the mosquito that transmits it.
While Plasmodium seems to have superpowers that allow it to evade detection and resist treatment, CTEGD researchers are working together to innovate and transfer science from the lab to interventions on the ground.
A 50,000-piece puzzle with no edges
Plasmodium is a complex organism, and studying it is like putting together a jigsaw puzzle. Some researchers contribute pieces related to the blood or liver stages of the parasite’s lifecycle, while others provide insights about hosts interactions. One way UGA’s research connects with the global effort to eradicate malaria is PlasmoDb—a resource derived in part from Kissinger’s research that is now part of a host of databases under the umbrella of The Eukaryotic Pathogen, Vector and Host information Resource (VEuPathDB).
“Our group has been able to help many others when their research question crosses into an –omic,” Kissinger said, referring to in-house shorthand for domains like genomics, proteomics and metabolomics.
Kissinger, Distinguished Research Professor of genetics in the Franklin College of Arts & Sciences, became interested in malaria and Plasmodium during her postdoctoral training at the National Institutes of Health (NIH). Working from an evolutionary biology perspective, she’s interested in how the parasite has changed over time.
PlasmoDb, a database of Plasmodium informatics resources, is a tool developed in part by the work of Distinguished Research Professor Jessica Kissinger, who became interested in malaria during her postdoctoral training at the National Institutes of Health.
“I see it as an arms race,” Kissinger said. “I want to understand what moves they have and can make.”
To understand the parasite, you must dive deep into its genetic code.
Kissinger paired her work in Plasmodium genomics with her interest in computing by helping create the database with information from the Plasmodium genome project completed in 2002. The Malaria Host-Pathogen Interaction Center, one of her projects at UGA, was a seven-year, multi-institutional effort funded, in part, by NIH to create data sets that could be used in systems biology of the host-pathogen interaction during the development of disease.
“Wouldn’t it be neat if, from the beginning of infection all the way to cure, you knew everything that was going on in the organism all the time?” Kissinger said, noting the project’s goal.
They generated terabytes of data that, along with data from the global research community, are publicly accessible and reusable through PlasmoDB and other resources.
Being part of a group that is studying so many different aspects of malaria helps put Kissinger’s research into perspective. Now, in addition to understanding the parasite, she also thinks about tools needed to facilitate research from peers.
High-tech solutions rely on basic research
David Peterson, professor of infectious diseases in the College of Veterinary Medicine, noted that low-tech solutions have mitigated malaria’s human costs. He acknowledged, however, that their long-term goals required more.
“We have to acknowledge that low-tech solutions, such as mosquito nets, have saved lives,” Peterson said. “But to develop the high-tech solutions that will one day end malaria, we need basic research.”
Pregnant women are particularly vulnerable to malaria because their existing immunity to malaria fails to protect them during pregnancy. Placental malaria often results in premature birth and low birth weight.
Peterson is interested in a binding protein that allows the parasite to adhere to the placenta. While many P. falciparum parasites have only one gene copy that encodes the placental binding protein, Peterson is investigating Plasmodiumisolates with two or more slightly different copies.
But why isn’t one copy enough?
Professor David Peterson of the College of Veterinary Medicine acknowledges the importance of low-tech solutions like mosquito nets but said to mitigate its effects required better understanding at the genetic level.
That is the primary question Peterson is focused on. He wants to understand how Plasmodium uses extra copies to evade the immune system, distinguishing the role of each requires tools that Vasant Muralidharan, associate professor of cellular biology, has.
Muralidharan’s interest began when he contracted malaria himself. Through access to good health care, he made a full recovery, but the pain he endured remained. He wanted to understand this parasite. Even more, he wanted to make an impact with research.
His graduate training focused on biophysics, but soon his interest in Plasmodium resurfaced. He discovered there was a lack of tools to study the parasite on a genetic level.
“It’s like a house of cards, and each card is a gene,” Muralidharan said. “You can remove one and see what happens—does the house fall or remain standing?”
This is an illustration of the life cycle of the parasites of the genus, Plasmodium, that are causal agents of malaria.(Illustration by CDC/ Alexander J. da Silva, PhD; Melanie Moser)
In the days before CRISPR/Cas9, there wasn’t a precise way to remove genes. Muralidharan is among the pioneers of gene-editing techniques in Plasmodium.
Like Peterson, Muralidharan focuses on proteins secreted by the parasite. He studies the largely unknown process that allows the parasite to invade a red blood cell (RBC), replicate and escape. The lack of tools was a major hindrance, so Muralidharan created new ones.
These tools have been used by Muralidharan’s CTEGD and CDC colleagues to see how drugs might fail. Muralidharan’s laboratory can create mutant Plasmodium parasites that become resistant to a particular drug, and genome sequence databases allow researchers to check if that mutant is already circulating in malaria endemic regions.
Building a research bridge to endemic regions
Plasmodium vivax is the predominant malaria parasite in Southeast Asia. It causes “relapsing malaria” during which some parasites go “dormant” after entering the liver instead of reproducing. This phase is a major obstacle for current treatments.
CTEGD Director Dennis Kyle, GRA Eminent Scholar Chair in Antiparasitic Drug Discovery and head of the Department of Cellular Biology, became fascinated with the Plasmodium parasite early in his career, spending time living in Thailand and working in refugee camps where malaria is prevalent.
CTEGD Director Dennis Kyle was moved to follow through with his work as a researcher on a trip to a refugee camp in Thailand. Upon seeing the challenges residents faced, he thought perhaps he should have become a physician. Instead, a local leader impressed upon him the impact you could have in generating new treatments that could benefit everyone. (Photo by Andrew Davis Tucker/UGA)
“When I first got to the refugee camp and saw the situation people were living in, I questioned my decision to become a scientist in the lab instead of becoming a physician,” Kyle said, recalling a camp he worked in that housed about 1,300 kids between the ages of 2 and 15. “There was a guy who was a leader in the group who probably had no more than an early high school education. He said, ‘Look at what you can do—you might generate something that would benefit all of us. The physicians we have in the camp can only work on a few people at a time.’”
Kyle’s laboratory is looking to repurpose medications that have antimalarial properties, a safe way to reduce the development time from lab to clinical use. He’s optimistic we will see a drug treatment that eliminates vivax malaria.
“That’s where UGA is playing a major role,” he said. “The Gates Foundation funded us to develop tools to study the dormant parasite in the liver. And we’ve been successful.”
“We use mouse models to delve into the fundamental host-parasite interactions, which you cannot do practicallyin humans,” Kurup said. “Our understanding of these fundamental processes gives rise to newer and better vaccination approaches and drugs.”
Like other Plasmodium researchers, Joyner became interested in parasites at an early age. During an undergraduate parasitology class, he discovered how little was known about P. vivax. He was already interested in how diseases develop, so for graduate school he focused on the liver stage of vivax malaria. However, it was a difficult task.
Samarchith Kurup is an assistant professor of cellular biology studying the human immune response to Plasmodium infection. (photo credit: Lauren Corcino)
Assistant Professor Chet Joyner discovered how little was known about Plasmodium vivax as an undergraduate student.
“At the time, the technologies weren’t there,” Joyner said. “Dennis was working on his system, but it wasn’t on the scene yet. I changed from studying the parasite to studying the animal model to understand pathogenesis and immunology in humans.”
Joyner joined UGA after completing his postdoctoral training at Emory University, where he developed a non-mouse animal model to study vivax malaria.
“We have to go to [Thailand] where people are infected and collect blood samples and then feed mosquitoes these samples to do the necessary studies,” Kyle said. “That’s been very impactful. We’ve gotten a lot of data out of it, and now with Chet’s model it all can be done under one roof.”
Joyner wants to understand the human immune response with a focus on vaccine development. Building on Muralidharan’s and other researchers’ findings of how the parasite interacts with the RBCs, Joyner’s vaccine program targets a specific protein in the parasite that inhibits the development of immunity.
“My colleagues have shown that if you knock this protein out in the parasite, the immune response in mice is actually great, and we are now working together to evaluate this in non-mouse models.” Joyner said.
Joyner also has collaborated with Belen Cassera, professor of biochemistry, to screen drug compounds. Cassera’s training focused on metabolism to find drug targets. She is particularly interested in how a drug functions.
“If we understand how the drug works, it will help us predict potential side effects in humans,” Cassera said. “We can’t predict everything, but knowing how it works gives you some confidence in whether it will work in humans.”
Cassera is focused on finding drugs that will treat the more lethal Plasmodium falciparum, the predominant species in Africa, which is rapidly becoming resistant to current treatments. Her work is complementary to Kyle’s.
“They run certain assays for the liver-stage infection, and our lab benefits because we want to know if the drug we are developing is specific for the blood stage or can tackle all stages,” Cassera said.
Professor Belen Cassera is identifying drugs that will treat the lethal Plasmodium falciparum, a predominant species of the parasite in Africa that has become resistant to many current treatments.
Don’t forget the mosquito
“Malaria is a vector-borne disease transmitted by a mosquito. You need to tackle not only the parasite in the human but also stop its transmission,” Cassera said. “CTEGD is unique because we can study the whole life cycle, including the mosquito.”
Michael Strand, H.M. Pulliam Chair of Entomology in the College of Agricultural and Environmental Sciences and a National Academy of Sciences Fellow, is an expert on parasite-host interactions. Instead of the human host, he is interested in mosquitoes. Recent work indicates blood feeding behavior of mosquitoes strongly affects malaria parasite development while the gut microbiota of mosquitos could lead to new ways to control populations. Having the SporoCore insectory on campus aids his research.
Michael Strand is an expert on parasite-host interactions. His research focuses on mosquitoes and their effects on malaria parasite development.
Established in 2020, SporoCore, under the management of Ash Pathak, assistant research scientist in the Department of Infectious Diseases, provides both uninfected and Plasmodium-infected Anopheles stephensi mosquitoes to researchers at UGA and other institutions. Like Joyner’s animal model, the insectory allows for research to be done in the U.S. that would otherwise require field work in an endemic country.
Old-school interventions like mosquito nets, combined with new drug therapies, have reduced the number of malaria deaths, which declined over the last 30 years before rising slightly during the COVID-19 pandemic. Great strides have been made to control and treat malaria—but not enough. New tools, like the ones being developed at CTEGD, are needed to keep pushing malaria’s morbidity and mortality rates in the right direction.
“The hard part—what can’t be done easily with the tools we already have—is being done,” Kyle said. “We just need new tools, which is one of the things that our center is really a leader in.”
Cripowellins from Crinum erubescens are known pesticidal and have potent antiplasmodial activity. To gain mechanistic insights to this class of natural products, studies to determine the timing of action of cripowellins within the asexual intraerythrocytic cycle of Plasmodium falciparum were performed and led to the observation that this class of natural products induced reversible cytostasis in the ring stage within the first 24 h of treatment. The transcriptional program necessary for P. falciparum to progress through the asexual intraerythrocytic life cycle is well characterized. Whole transcriptome abundance analysis showed that cripowellin B “pauses” the transcriptional program necessary to progress through the intraerythrocytic life cycle coinciding with the lack of morphological progression of drug treated parasites. In addition, cripowellin B-treated parasites re-enter transcriptional progression after treatment was removed. This study highlights the use of cripowellins as chemical probes to reveal new aspects of cell cycle progression of the asexual ring stage of P. falciparum which could be leveraged for the generation of future antimalarial therapeutics.
Joshua H Butler, Heather J Painter, Emily K Bremers, Priscilla Krai, Manuel Llinás, Maria B Cassera. Molecules. 2023 Mar 13;28(6):2600. doi: 10.3390/molecules28062600.
The recent development and mass administration of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) vaccines allowed for disease control, reducing hospitalizations and mortality. Most of these vaccines target the SARS-CoV-2 Spike (S) protein antigens, culminating with the production of neutralizing antibodies (NAbs) that disrupt the attachment of the virus to ACE2 receptors on the host cells. However, several studies demonstrated that the NAbs typically rise within a few weeks after vaccination but quickly reduce months later. Thus, multiple booster administration is recommended, leading to vaccination hesitancy in many populations. Detecting serum anti-SARS-CoV-2 NAbs can instruct patients and healthcare providers on correct booster strategies. Several in vitro diagnostics kits are available; however, their high cost impairs the mass NAbs diagnostic testing. Recently, we engineered an ACE2 mimetic that interacts with the Receptor Binding Domain (RBD) of the SARS-2 S protein. Here we present the use of this engineered mini-protein (p-deface2 mut) to develop a detection assay to measure NAbs in patient sera using a competitive ELISA assay. Serum samples from twenty-one patients were tested. Nine samples (42.8%) tested positive, and twelve (57.1%) tested negative for neutralizing sera. The data correlated with the result from the standard commercial assay that uses human ACE2 protein. This confirmed that p-deface2 mut could replace human ACE2 in ELISA assays. Using bacterially expressed p-deface2 mut protein is cost-effective and may allow mass SARS-CoV-2 NAbs detection, especially in low-income countries where economical diagnostic testing is crucial. Such information will help providers decide when a booster is required, reducing risks of reinfection and preventing the administration before it is medically necessary.
Bruna Andersen Pereira de Jesus, Anderson Albino Gomes, Alex E Clark, Tayse Andrade Rodrigues, Melissa Ledgerwood-Lee, Westley Van Zant, Howard Brickner, Meiqiao Wang, David L Blum, Maria B Cassera, Aaron F Carlin, Eliah S Aronoff-Spencer, Gustavo Felippe da Silva, Maria de Lourdes Borba Magalhães, Partha Ray. Viruses. 2022 Dec 18;14(12):2823. doi: 10.3390/v14122823.
Seventeen new cephalotane-type diterpenoids, fortalides A-Q (1-17), along with five known analogues, were isolated from the seeds of Cephalotaxus fortunei var. alpina. Their structures were determined by extensive spectroscopic methods, as well as electronic circular dichroism (ECD) and X-ray crystallographic data analyses. Some isolates exhibited unusual structural features that were first found in cephalotane-type diterpenoids, such as the occurrence of the 7-oxabicyclo[4.1.1]octane moiety in 14 and 15 and the cis-arrangement of 3-OH and Me-19 in 9. Besides, the antiplasmodial activity of these compounds was evaluated in this study.
Zhan-Peng Ge, Bin Zhou, Flavia M Zimbres, Reagan S Haney, Qun-Fang Liu, Yan Wu, Maria B Cassera, Jin-Xin Zhao, Jian-Min Yue. Org Biomol Chem. 2022 Nov 4. doi: 10.1039/d2ob01748b
Malaria is a major global health problem which predominantly afflicts developing countries. Although many antimalarial therapies are currently available, the protozoan parasite causing this disease, Plasmodium spp., continues to evade eradication efforts. One biological phenomenon hampering eradication efforts is the parasite’s ability to arrest development, transform into a drug-insensitive form, and then resume growth post-therapy. Currently, the mechanisms by which the parasite enters arrested development, or dormancy, and later recrudesces or reactivates to continue development, are unknown and the malaria field lacks techniques to study these elusive mechanisms. Since Plasmodium spp. salvage purines for DNA synthesis, we hypothesized that alkyne-containing purine nucleosides could be used to develop a DNA synthesis marker which could be used to investigate mechanisms behind dormancy. Using copper-catalyzed click chemistry methods, we observe incorporation of alkyne modified adenosine, inosine, and hypoxanthine in actively replicating asexual blood stages of Plasmodium falciparum and incorporation of modified adenosine in actively replicating liver stage schizonts of Plasmodium vivax. Notably, these modified purines were not incorporated in dormant liver stage hypnozoites, suggesting this marker could be used as a tool to differentiate replicating and non-replicating liver forms and, more broadly, as a tool for advancing our understanding of Plasmodium dormancy mechanisms.
Alona Botnar, Grant Lawrence, Steven P Maher, Amélie Vantaux, Benoît Witkowski, Justine C Shiau, Emilio F Merino, David De Vore, Christian Yang, Cameron Murray, Maria B Cassera, James W Leahy, Dennis E Kyle. Int J Parasitol. 2022 Apr 18;S0020-7519(22)00066-2. doi: 10.1016/j.ijpara.2022.03.003.
The tetrahydro-β-carboline scaffold has proven fertile ground for the discovery of antimalarial agents (e.g., MMV008138 (1) and cipargamin (2)). Similarity searching of a publicly disclosed collection of antimalarial hits for molecules resembling 1 drew our attention to N2-acyl tetrahydro-β-carboline GNF-Pf-5009 ((±)-3b). Compound purchase, “analog by catalog”, and independent synthesis of hits indicated the benzofuran-2-yl amide portion was required for in vitro efficacy against P. falciparum. Preparation of pure enantiomers demonstrated the pharmacological superiority of (R)-3b. Synthesis and evaluation of D- and F-ring substitution variants and benzofuran isosteres indicated a clear structure-activity relationship. Ultimately (R)-3b was tested in Plasmodium berghei-infected mice; unfavorable physicochemical properties may be responsible for the lack of oral efficacy.
Hanan Almolhim, Sha Ding, Joshua H Butler, Emily K Bremers, Grant J Butschek, Carla Slebodnick, Emilio F Merino, Zaira Rizopoulos, Maxim Totrov, Maria B Cassera, Paul R Carlier. ACS Med. Chem. Lett. 2022, 13, 3, 371–376. https://doi.org/10.1021/acsmedchemlett.1c00697
Virtual ligand screening of a publicly available database of antimalarial hits using a pharmacophore derived from antimalarial MMV008138 identified TCMDC-140230, a tetrahydro-β-carboline amide, as worthy of exploration. All four stereoisomers of this structure were synthesized, but none potently inhibited growth of the malaria parasite Plasmodium falciparum. Interestingly, 7e, a minor byproduct of these syntheses, proved to be potent in vitro against P. falciparum and was orally efficacious (40 mg/kg) in an in vivo mouse model of malaria.
Jopaul Mathew, Sha Ding, Kevin A Kunz, Emily E Stacy, Joshua H Butler, Reagan S Haney, Emilio F Merino, Grant J Butschek, Zaira Rizopoulos, Maxim Totrov, Maria B Cassera, Paul R Carlier. ACS Med Chem Lett. 2022 Feb 23;13(3):365-370. doi: 10.1021/acsmedchemlett.1c00663.
