Global eradication of malaria seemed close at hand in 1979, when the research team of Chinese pharmacologist Youyou Tu published its study demonstrating the power of artemisinin. The antimalarial drug, obtained from the herb Artemisia annua, was effective against the most lethal species of malaria-causing parasite – the protozoan Plasmodium falciparum. The compound’s identification and laboratory synthesis drastically reduced the number of deaths from malaria, saving millions of lives and winning Tu the 1979 Nobel Prize in Physiology or Medicine (see story). In the 2000s, however, artemisinin and its derivatives started losing some of their potency against the disease in five countries in Southeast Asia.
“In these regions, artemisinin, which once eliminated the parasite from the patient’s blood by the second day of treatment, is now only able to do so after the third day,” says physician Marcus Vinícius Lacerda, researcher at the Dr. Heitor Vieira Dourado Foundation for Tropical Medicine (FMT-HVD), owned by the Amazonas State government and based in the city of Manaus. “There is already a small population of P. falciparum that is resistant to artemisinin, a problem that will start growing and spreading as the drug continues to eliminate the parasites that are still sensitive to its effect,” says Lacerda, who is also a researcher at the Leônidas and Maria Deane Institute of the Oswaldo Cruz Foundation (Fiocruz), in Manaus.
Lacerda has been studying a similar effect in the Brazilian Amazon, but this one involving Plasmodium vivax, the species responsible for 85% of all malaria cases in Brazil. In a study slated for publication in Lancet Global Health, Lacerda and his colleagues at FMT-HVD conducted a phase-3 clinical trial to assess the safety and effectiveness of a drug against P. vivax produced by pharmaceutical company Sanofi. The new drug is a combination of two compounds: artesunate, obtained from artemisinin, and amodiaquine. In the study, the effectiveness of this combination was compared with that of chloroquine, a drug used worldwide to treat malaria caused by P. vivax.
The researchers monitored two groups of patients in the city of Manaus for 42 days, each group receiving a different treatment. During this period, they assessed the drugs’ ability to reduce – or even completely eliminate – the number of parasites in the red blood cells of the patients. The study showed that the combination of artesunate and amodiaquine works better than chloroquine. More importantly, it also revealed that chloroquine was ineffective in 10% of cases.
This finding, presented at the XIV Brazilian Meeting on Malaria Research, held in early October 2015 in the city of São Paulo, supports the results of previous studies also conducted by Lacerda. He had already observed that 5% to 10% of all malaria cases caused by P. vivax in Manaus did not respond well to treatment with chloroquine, a relatively inexpensive drug when compared to derivatives of artemisinin.
“This is an alarming finding,” says physician Marcelo Urbano Ferreira, from the Institute of Biomedical Sciences at the University of São Paulo (ICB-USP). Ferreira is investigating the risk of resistance against antimalarial drugs in the Amazon region of Brazil, particularly in the state of Acre. As yet, his team has found no evidence that Plasmodium vivax has developed a resistance to chloroquine in Alto Juruá, currently the Brazilian region with the highest incidence of malaria. “The World Health Organization suggests that a treatment should be replaced when it fails in more than 10% of cases,” he says.
Ferreira notes, however, that the high level of resistance shown by P. vivax is probably restricted to Manaus. Together with Lígia Gonçalves, a visiting researcher at ICB-USP, and Pedro Cravo from the Federal University of Goiás, Ferreira reviewed the existing scientific literature in search of cases of chloroquine-resistant P. vivax in Latin America. Since the 1990s, there have been reports of such resistance in Brazil and Peru, as well as in some infected tourists in Guyana. In 2015, reports also emerged of P. vivax showing resistance in the Bolivian Amazon, near the border with the Brazilian state of Rondônia. “This is still a rare phenomenon in Brazil, but vigilance is crucial,” says Ferreira, who published the review in 2014 in the journal Memórias do Instituto Oswaldo Cruz.
Developed by German researchers in the 1930s as a replacement for quinine – which is still effective, but causes serious side effects –, chloroquine was the most widely used antimalarial drug in global campaigns for eradication of the disease after World War II. In the 1950s, the first reports of chloroquine-resistant P. falciparum started appearing in South America and Southeast Asia. In little over 40 years, this resistance spread around the world. “Today, the only regions in the world that still have chloroquine-sensitive P. falciparum are Central America, Haiti and the Dominican Republic,” says Ferreira.
Since the mid-2000s, international health authorities have recommended that compounds derived from artemisinin always be administered in combination with another drug that has a different mechanism of action. This is intended to avoid dissemination of P. falciparum strains that are resistant to artemisinin. In Brazil, for example, malaria caused by P. falciparum is treated with a combination of artemether (an artemisinin derivative) and lumefantrine. “Even if the parasite develops a resistance to one of the drugs, it will not survive if it does not also develop a resistance to the other,” Ferreira explains. Despite this strategy, reports of resistance to these combined therapies have already started appearing in Southeast Asia. “We have to remain vigilant and seek new formulations of drugs, with different mechanisms of biochemical action.”
Some researchers are going down alternative paths to achieve the same goal. Pedro Melillo Magalhães, from the University of Campinas (Unicamp), coordinated a study that tested the use of an antimalarial tea made from leaves of an enriched variety of Artemisia annua, with a 100 times higher concentration of artemisinin than the wild form of the herb. The tea was given to 17 patients with mild cases of malaria caused by P. falciparum in the state of Pará. The alternative treatment eliminated the parasite in all patients by the third day of use. However, the protozoan reappeared in their bloodstreams within less than a month, even with continuing treatment. To preserve the health of participants in the study, conducted in partnership with researchers from the Evandro Chagas Institute in Pará and Oxford University in England, the patients received the conventional treatment against malaria (artemether and lumefantrine). “Used alone, the tea brought the same results as a treatment based exclusively on artemisinin,” says Magalhães, who works in the agrotechnology division of the Multidisciplinary Center for Chemical, Biological and Agricultural Research (CPQBA) at Unicamp. “But the dose was equivalent to one third of the recommended dose for artemisinin.”
To prevent the parasites from reemerging in patients, Magalhães advocates using the tea in combination with an antimalarial drug, similar to the current standard procedure for artemisinin. The next step in the Unicamp researcher’s studies will be to test this combination of tea and drug on patients in Pará with malaria caused by P. vivax.
Biochemist Rafael Guido, from the Laboratory of Medicinal and Computational Chemistry at the USP campus in São Carlos, emphasizes the need to find new points in the Plasmodium metabolism to be targeted by antimalarial drugs. “Almost all existing drugs converge on the same targets,” he explains. His group has been studying enolase, a protein used by the parasite to produce energy.
Recent studies have shown that the gene for enolase is active not only in the interior of the cell, where the “power plant” is located, but also in other places, like the cell membrane – where it plays a role in cell signaling. Having recently discovered a region that could reveal a new role for enolase, the group led by Guido has started testing the effect of different substances against this protein.
The test compounds were provided by the Medicines for Malaria Venture, an NGO that has curated pharmaceutical industry databases and selected drugs that look promising. “Their action against malaria is known, but the mechanism is not,” says Guido. In the tests, some of the compounds successfully blocked enolase. “Five compounds showed 100% inhibition, 10 inhibited 80% of enolase expression, and 38 achieved 50% inhibition,” he says. The researchers are now working on making these substances more potent without affecting human enolase.
One problem in finding new antimalarial drugs is that half of the approximately 5,000 genes of the two species of Plasmodium play unknown roles. “It is very difficult to study a biochemical pathway when you don’t know what genes code the protein involved in it,” says chemist Célia Garcia, from the Biosciences Institute at USP. Since the late 1990s, Garcia’s laboratory has been figuring out the workings of some of these biochemical pathways that are essential to the survival of malaria-causing parasites. They are groups of chemical reactions that enable the protozoan to perceive the environment around it, especially when it invades red blood cells and multiples within them.
Garcia and her team have shown, for example, that the parasite is able to control the concentration of calcium around it. This is essential for the protozoan to reproduce and to synchronize its reproductive phase by exploiting available melatonin, the compound that regulates the sleep-wake cycle in the human body. In a partnership with the research group led by biochemist Andrew Thomas at Rutgers University, in the United States, Garcia has been testing a series of compounds that may block the parasite’s ability to perceive melatonin. Garcia and other Brazilian chemists are also trying to identify compounds that act on other biochemical pathways of the parasite.
German biochemist Carsten Wrenger, from ICB-USP, is taking a different approach. When he was working in Hamburg, Wrenger and his colleagues identified two biochemical pathways in the known half of the genome of both species of Plasmodium that are essential to the parasite’s metabolism, but are absent from human cells. Like humans, Plasmodium needs vitamins B1 and B6 in order to survive. Without them, over 100 of its essential enzymes cease to work. However, while people can only obtain these vitamins through food, the protozoan makes its own.
In 2013, Wrenger and his colleagues synthesized a compound from which Plasmodium produces a defective version of vitamin B1. “This compound is inert to the human body and, when modified by the parasite, creates a non-functional version of the vitamin,” Wrenger explains. “Without the vitamin, the parasite’s metabolism stops.” Wrenger continues to seek and design compounds that will prevent the protozoan from producing vitamins B1 and B6. “Identifying this type of compound is complicated,” he says. “The upside is that it could affect over 100 targets simultaneously.”
At the Institute of Chemistry at Unicamp, chemist Luiz Carlos Dias and his team have been working with the MMV since 2013 to perfect a new class of promising compounds to fight malaria. Their research focuses on molecules that inhibit the activity of PI(4)K, an enzyme identified in 2013 by researchers at the Novartis pharmaceutical company. In tests run on laboratory animals, one of these compounds successfully eliminated the P. falciparum and P. vivax strains that are most resistant to currently available antimalarial drugs. According to Dias, the most interesting thing about this molecule is that it can kill the parasite at different stages of its life cycle in the mammal host. “No current drug can do that, only some compounds that are still in clinical trials,” he says.
Based on the structure of the PI(4)K inhibitor, Dias and his collaborators synthesized approximately 60 compounds and sent them to research institutions in different countries for testing. Although promising, an experiment conducted by the AbbVie biopharmaceutical company in the U.S. and at the University of Dundee in Scotland showed that they interact with one of the 140 human protein kinases tested. The researchers still do not know the consequences of this interaction, but, to avoid risks, they will need to alter the structure of this class of compounds. “We have to understand how they interact with the parasite kinase and with the human kinase in order to augment the first interaction and prevent the latter,” Dias explains. He plans to set up a consortium with teams from other universities in the state of São Paulo, to conduct some of the tests using cells and laboratory animals in Brazil.
1. Functional genomics in Plasmodium (nº 2011/51295-5); Grant Mechanism Thematic Project; Principal Investigator Célia Regina da Silva Garcia (IB-USP); Investment R$2,068,066.18.
2. Elucidation of vitamin B metabolism in the human malaria parasite Plasmodium falciparum and their validation as a target for chemotherapy (nº 2010/20647-0); Grant Mechanism Scholarships in Brazil – Young Investigators Program; Principal Investigator Carsten Wrenger (ICB-USP); Investment R$179,861.70.
3. Clinical research of plant extracts in the treatment of malaria from standardized raw material: Artemisia annua (VAR.CPQBA) (nº 2009/53639-3); Grant Mechanism Thematic Project – Pronex; Principal Investigator Pedro Melillo de Magalhães (CPQBA-Unicamp); Investment R$16,874.70.
4. Discovery and design of Plasmodium falciparum enolase inhibitors as new antimalarial candidates (nº 2014/26313-8); Grant Mechanism Scholarships in Brazil – Postdoctoral; Principal Investigator Rafael Victorio Carvalho Guido (IFSC-USP); Grant Recipient Lorena Ramos Freitas de Sousa; Investment R$ 169,558.00.
GONÇALVES, L. A. et al. Emerging Plasmodium vivax resistance to chloroquine in South America: an overview. Memórias do Instituto Oswaldo Cruz. V. 109 (5). Aug. 2014.
ALVES, E. et al. Encapsulation of metalloporphyrins improves their capacity to block the viability of the human malaria parasite Plasmodium falciparum. Nanomedicine. V. 11 (2). Feb. 2015.
CHAN, X. W. A. et al. Chemical and genetic validation of thiamine utilization as an antimalarial drug target. Nature Communications. May 28, 2013.