PerspectivePARASITOLOGY

Drugs to Combat Tropical Protozoan Parasites

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Science  19 Jul 2002:
Vol. 297, Issue 5580, pp. 343-344
DOI: 10.1126/science.1073126

The 130 scientists from 20 countries who gathered at a Keystone Symposium (1) to discuss development of drugs to combat tropical protozoan parasites [HN1] were inspired by alternate waves of desperation and hope. Desperation stemmed from the fact that tropical protozoan diseases such as malaria [HN2], leishmaniasis, and Chagas' disease affect 3 billion people (see the table), most of whom survive on less than $2 a day. Every minute, two people, usually children, die from malaria, and every year, more than 300 million persons suffer at least one malaria attack. In Latin America, millions of people are infected with the protozoan Trypanosoma cruzi, which causes Chagas' disease [HN3] and kills 10 to 20% of the people it infects. Meanwhile, Kala-azar, the most deadly form of leishmaniasis [HN4], is epidemic in the Bihar and Uttar Pradesh states of India. For most tropical diseases caused by protozoan parasites, there are either no safe efficacious drugs, or, as in the case of malaria, once effective and affordable drugs like chloroquine [HN5] are less widely used because the Plasmodium parasites that cause malaria have become resistant to them (2, 3).

Yet a wave of hope arrives with efforts to sequence the genomes of these protozoan parasites. As David Roos (Univ. of Pennsylvania) and Peter Myler (Seattle Biomedical Research Institute) [HN6] discussed, the genome of Plasmodium falciparum [HN7], the most deadly of the malaria parasites, is essentially complete, and those of Leishmania major, Trypanosoma brucei, T. cruzi, and Plasmodium vivax [HN8] are progressing rapidly with completion slated for 1 to 2 years' time. Of course, functional annotation of the parasite genomes will take considerably longer, but is already under way. Clearly, the complete genome sequences of protozoan parasites will accelerate efforts to develop cheap, effective drugs for treating the tragic diseases that they cause.

There was much discussion about different approaches to developing antiprotozoan drugs. Gary Posner (Johns Hopkins Univ.) discussed analogs of artemisinin [HN9], a natural antimalarial derived from Chinese traditional medicine. The analogs have improved solubility compared to the parent compound and a simpler structure; they decreased parasitemias in primates infected with malaria. Donald Krogstad (Tulane Univ.) demonstrated the efficacy of aminoquinolines even against chloroquine-resistant malaria, in vitro and in rodent models of malaria. Protease inhibitors received considerable attention, with inhibitors of the enzyme cruzipain (Jim McKerrow, UCSF) eliciting much enthusiasm, thanks to their fortunate property of being taken up selectively by T. cruzi. Compounds synthesized by Phil Rosenthal (UCSF) target cysteine proteases in the malaria parasite's food vacuole. Richard Tidwell (Univ. of North Carolina) disclosed the promise of pentamidine-type compounds as antiprotozoan agents. Henri Vial [HN10] (CNRS, Montpellier) described inhibitors of choline metabolism (required for phospholipid synthesis) that appear remarkably effective against plasmodial parasites (4).

View this table:

Exciting results were also obtained by “redirecting” compounds developed for other diseases toward tropical protozoa (also called “therapy switching” or “piggybacking”). Protein farnesyltransferase inhibitors, under vigorous development as anticancer agents, show promise (Fred Buckner, Wes Van Voorhis, Mike Gelb, Univ. of Washington, Seattle; Andrew Hamilton, Yale; Bill Windsor, Schering-Plough). As always, serendipity is a crucial player in drug discovery—Buckner reported his accidental discovery of an extremely potent inhibitor of the T. cruzi enzyme lanosterol 14-demethylase, which is essential for sterol biosynthesis in the parasite. The sterol biosynthetic pathway was also the topic of talks by Eric Oldfield (Univ. of Illinois) and Julio Urbina (IVIC, Venezuela), who demonstrated that inhibitors of isoprenoid and sterol biosynthesis were effective against T. cruzi in vitro and in rodent models of Chagas' disease (5). Vern Schramm (Albert Einstein College of Medicine, New York) discussed highly potent inhibitors that target the purine nucleoside phosphorylase of P. falciparum. Clearly, almost all of these promising compounds still need to undergo extensive testing for safety and efficacy before they will be useful in the field. However, Simon Croft (London School of Hygiene and Tropical Medicine) described the first orally active compound against cutaneous and visceral leishmaniasis (developed by Zentaris in Germany). The drug, a phospholipid analog called miltefosine, originally developed as an anticancer agent, has passed phase III clinical testing and was registered in March this year for treating visceral leishmaniasis patients in India [HN11] (6, 7).

Other groups talked about compounds that are not as far along the development pathway—for example, the xanthones that reduce the protective formation of hemozoin by Plasmodium (Michael Riscoe, Oregon Health Sciences Univ.), and adenosine analogs that block energy generation in trypanosomatids (Gelb, Hol, Van Voorhis, Buckner, Michels). Meanwhile, availability of the crystal structure of T. brucei ornithine decarboxylase has assisted in the generation of inhibitors that block this enzyme (Meg Phillips, Southwestern Medical Center). Also, crystal structures reveal that covalent inhibitors of trypanothione reductase are stacked one on top of the other in the active site, each covalently bound to the enzyme in different ways (Bill Hunter, Univ. of Dundee). Gerhard Klebe (Philipps Univ., Marburg) reported his collaborative work with Jomaa Pharmaceuticals (8) to synthesize inhibitors of deoxyxylulose phosphate (DOXP) reductoisomerase. This essential enzyme is part of the nonmevolanate isoprenoid pathway of the apicoplast, a unique organelle in the malaria parasite. Klebe's group has just solved the crystal structure of Escherichia coli DOXP reductoisomerase, along with that of glutamate dehydrogenase from P. falciparum. Glaucius Oliva (Univ. of Sao Paulo, Brazil) reported the crystal structures of T. cruzi phosphoenolpyruvate carboxykinase and glyceraldehyde-3-phosphate dehydrogenase in a complex with a natural product inhibitor derived from a plant in the Brazilian Atlantic forest. A recently created network of centers in Brazil is set to tap the rich biodiversity of this country's flora for the discovery of lead compounds.

Another encouraging development is NIH funding for the recently formed Structural Genomics of Pathogenic Protozoa [HN12] (SGPP) consortium (http://www.sgpp.org/, sgpp{at}u.washington.edu). This consortium aims to develop and apply high-throughput methods to express large numbers of genes and to elucidate 3D crystal structures of proteins from P. falciparum, T. brucei, T. cruzi, and Leishmania species. This initiative is likely to have a significant impact on drug design by making 3D crystal structures available to all researchers.

Functional genomics is crucial for identifying protein targets for structural genomics and drug development. David Roos—who oversees the P. falciparum genome database (PlasmoDB) (9)—has used sophisticated computer analysis to unravel all of the metabolic pathways in the plasmodial apicoplast (10). Terry Gaasterland's group (Rockefeller Univ., New York) is combining powerful bioinformatics techniques to annotate protozoan genes. State-of-the-art mass spectrometry is being used to analyze collections of proteins expressed in the various life cycle forms of malaria (Daniel Carruci, Naval Medical Research Center; John Yates, Scripps).

Several talks revealed the intricate biochemical pathways [HN13] of these devastating yet ingenious protozoa. Dan Goldberg (Washington Univ., St. Louis) described how the earlier discovery of one plasmodial aspartyl protease, plasmepsin—a key player in hemoglobin degradation in the plasmodial food vacuole—was followed by the identification in the P. falciparum genome sequence of nine other plasmepsins. One of these plasmepsins is a most unusual histidyl-aspartyl protease that may be amenable to selective inhibition; meanwhile, a unique plasmodial enzyme, the maturase, which processes the proplasmepsins into their active forms, may be an exciting new drug target. The complexities of the folate pathway in trypanosomatids discovered by Beverley's group (Washington Univ, St. Louis) explains why dihydrofolate reductase inhibitors have disappointingly little effect on Leishmania species. Buddy Ullman (Oregon Health Sciences Univ.) revealed the sophistication of the trypanosomatid purine salvage pathway. Meanwhile, studies on purine transporters are unveiling crucial new proteins. Sanjeev Krishna (St. George's Hospital Medical School, London) described several new hexose transporters in P. falciparum. The complexities of the malaria parasite's metabolism became clear in Akhil Vaidya's talk (MCP Hahnemann, Philadelphia) [HN14]. He pointed out the remarkable synergy required between the two components (atovaquone and proguanil) of the new antimalarial drug malarone [HN15], which collaborate to cause the collapse of the mitochondrial membrane potential of the parasite. Vaidya postulates that plasma membrane proton pumps and pyrophosphate are crucial for maintaining the energy metabolism of the parasite. Aloysius Tielens (Univ. of Utrecht) elaborated on the nonclassical biochemical pathways of trypanosomatid mitochondria and listed several specific drug targets. Paul Michels (Catholic Univ. Louvain, Belgium) discussed the properties of a unique trypanosomatid organelle, the glycosome, and indicated potential new pathways in this organelle that regulate glycolysis. Ching-Chung Wang (UCSF) described the fascinating and essential ubiquitin-proteasome pathway of T. brucei.

Talks from pharmaceutical company scientists made clear that large chemical libraries and experienced medicinal chemists (rarities outside the world of big pharma) are absolutely essential for drug development (11). This led participants to propose establishing a network of high-throughput synthesis centers that would synthesize chemical libraries and lead compounds against tropical protozoa. Each center would prepare chemical libraries in response to requests from scientists working on promising drug targets or lead compounds. Proposals would be solicited and ranked by review panels, and then the power of the synthetic teams would be made available for the high-priority projects. Such centers would be of immense benefit for translating the results of functional and structural genomics into drug candidates.

The conference also addressed the fascinating yet tragic phenomenon of drug resistance [HN16]. Point mutations provide resistance to a number of antimalarial folate inhibitors (3). Dyann Wirth (Harvard) described multidrug resistance in P. falciparum due to efflux protein pumps residing in the complex multivesicular tubule system of this malaria parasite. There seems to be no end to the array of tricks that these parasites use to combat drugs. Pradip Rathod (Univ. of Washington, Seattle) provided evidence for a specialized molecular machinery in P. falciparum that increases the mutation rate in highly specific areas of the malaria genome, thereby allowing rapid escape of the parasite from drug pressure without endangering its survival.

No wonder that participants ardently discussed ways to maintain the power of precious current (and future) drugs that have passed tests for safety and efficacy. The only way to safeguard the value of new drugs will be to bring them into the field in a well-controlled manner—possibly in paired combinations. A key requirement will be to maintain a large number of candidate drugs in the pipeline. Fortunately, several new funding sources have recently been created to achieve this end. Solomon Nwaka discussed the Medicines for Malaria Venture [HN17] (http://www.mmv.org/), and Victoria Hale described the fledgling nonprofit Institute for One World Health [HN18] (http://www.iowh.org/), which aims to fill gaps in the development of new drugs for neglected diseases.

We calculate that 20 to 30 new drugs will be needed for long-term control of the protozoan diseases rampant in the tropics. As it may take $200 million to bring each successful compound to patients, we will need $4 billion to $6 billion spread out over 10 to 20 years to achieve a goal of 20 to 30 effective new antiprotozoan drugs (a mere 10 cents per world citizen per year for a few decades). It is primarily a matter of organization, vision, and bringing together the right people and organizations to ensure that today's wealth of genomic knowledge will be smoothly translated into the new therapies of tomorrow.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Also in this issue is an Enhanced Perspective by C. A. Long and S. L. Hoffman titled “Malaria — from infants to genomics to vaccines.”

Dictionaries and Glossaries

The On-line Medical Dictionary is provided by CancerWeb.

A medical dictionary is provided by MedicineNet.

D. Glick's Glossary of Biochemistry and Molecular Biology is provided on the Web by Portland Press.

Web Collections, References, and Resource Lists

The library of the Karolinska Institutet, Stockholm, Sweden, provides links to Internet resources on parasitic diseases, as well as links to resources related to microbiology and cell biology and molecular biology and genetics.

Amos' WWW links, available on the ExPASy Molecular Biology Server, provides pointers to information sources for life scientists with an interest in biological macromolecules. A collection of links to species specific databases is provided.

The World Wide Web Virtual Library: Parasitology is maintained by D. Gibson, Department of Zoology, Natural History Museum, London.

The Parasite-Genome Web site is made available by the European Bioinformatics Institute (EBI).

The Parasite Genomes Web Site, maintained by L. Simpson, Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, is a resource for information on advances in the various parasite genome projects and links to the parasite genome networks. The Molecular Parasitology Network Web Site is also provided.

The Division of Parasitic Diseases (DPD) of the Centers for Disease Control and Prevention (CDC) provides a collection of resource pages on parasitic diseases. The division's DPDx provides reviews and images of parasites and parasitic diseases.

The Atlas of Medical Parasitology is provided by the Carlo Denegri Foundation, Turin, Italy. A collection of Internet resources on parasitology and tropical medicine is provided.

C. Bennett's Ectoparasites and Endoparasites Web site provides a collection of medical endoparasite links.

Online Texts and Lecture Notes

J. Kimball maintains Kimball's Biology Pages, a Web textbook and glossary. A presentation titled “Games parasites play” is included.

Medical Microbiology is a Web textbook edited by S. Baron, Department of Microbiology and Immunology, University of Texas Medical Branch at Galveston.

Microbiology and Immunology On-line is a Web textbook provided by the Department of Pathology and Microbiology, University of South Carolina School of Medicine. A section on parasitology is included.

Parasites and Parasitological Resources, provided by the College of Biomedical Sciences, Ohio State University, provides information about parasites and their life cycles.

The Special Programme for Research and Training in Tropical Diseases (TDR) of the World Health Organization (WHO) provides resource pages on leishmaniasis, Chagas disease, malaria, and African trypanosomiasis.

The Parasitology Group at the Institute of Biological Sciences, Aberystwyth University, UK, provides teaching notes for a parasitology course. An overview of parasitic protozoa and a presentation on designing antiparasite drugs are included.

F. Opperdoes, Research Unit for Tropical Diseases, Christian de Duve Institute of Cellular Pathology, Brussels, provides lecture notes for a course on tropical parasitology.

M. Wiser, Department of Tropical Medicine, Tulane University School of Public Health, provides study guides and supplemental notes for a course on medical protozoology.

L. Simpson, D. Campbell, and P. Johnson, Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, provide lecture notes and other resources for a course on molecular parasitology.

General Reports and Articles

Excerpts from Fever Trail: The Hunt for the Cure for Malaria by M. Honigsbaum provide an introduction to the history of malaria and its treatment.

The 13 March 2002 issue of JAMA had an article by J. Stephenson titled “Sequencing of the malaria genome opens door to vaccines and new drugs.”

The 3 September 1999 issue of Science had a Perspective by R. Ridley titled “Planting the seeds of new antimalarial drugs.”

The Use of Antimalarial Drugs is a 2000 WHO report made available by the Roll Back Malaria Web site.

The January-March 1998 issue of Emerging Infectious Diseases had an article by P. Rosenthal titled “Proteases of malaria parasites: New targets for chemotherapy.”

Numbered Hypernotes

1. The Keystone Symposia Web site makes available a program listing for the symposium “Drugs against tropical protozoan parasites.”

2. Malaria. Malaria: An On-line Resource is provided by the Division of Laboratory Medicine at Royal Perth Hospital, Australia. The CDC's Division of Parasitic Diseases provides a resource page on malaria resources; DPDx provides an introduction to Plasmodium and malaria. A student project on malaria was prepared for a course on vaccine development at Brown University. M. Wiser provides a study guide on Plasmodium and malaria for a course on medical protozoology.

3. Trypanosoma cruzi and Chagas disease. The CDC's Division of Parasitic Diseases provides a fact sheet on Chagas disease. DPDx provides information about Trypanosoma cruzi and Chagas disease. Parasites and Parasitological Resources includes information about T. cruzi. D. Campbell provides lecture notes on T. cruzi and Chagas disease for a course on molecular parasitology. The National Institute of Allergy and Infectious Diseases (NIAID) offers a presentation on Chagas disease. A student research project on African and American trypanosomiasis was prepared for a course on vaccine development at Brown University. Microbiology and Immunology On-line makes available a presentation by R. Hunt on the molecular biology of Trypanosomes. G. Cross's Laboratory of Molecular Parasitology at Rockefeller University offers a presentation on molecular parasitology and Trypanosomes.

4. Leishmaniasis. The CDC's Division of Parasitic Diseases provides a fact sheet about Leishmania infection. DPDx provides information about leishmaniasis. Parasites and Parasitological Resources provide information about Leishmania. NIAID offers a presentation on leishmaniasis. L. Simpson provides lecture notes on Leishmania and leishmaniasis for a course on molecular parasitology. A student research project on leishmaniasis was prepared for a course on vaccine development at Brown University.

5. A section on chloroquine is included in The Use of Antimalarial Drugs. Radio Netherlands offers a feature about chloroquine and drug resistance. The 1 June 2001 issue of the Journal of Infectious Disease had an article by T. Nomura et al. titled “Evidence for different mechanisms of chloroquine resistance in 2 Plasmodium species that cause human malaria” (2).

6. D. Roos is in the Department of Biology, University of Pennsylvania; the Roos Lab has a Web page. P. Myler is at the Seattle Biomedical Research Institute.

7. Plasmodium falciparum genome. PlasmoDB is the database of the Plasmodium falciparum Genome Project. The 15 February 2002 issue of Science had a News of the Week article by M. Enserink and E. Pennisi titled “Researchers crack malaria genome.” The 12 November 1999 issue had a News of the Week article by E. Pennisi titled “Malarial genome comes into view” and a report by X. Su et al. titled “A genetic map and recombination parameters of the human malaria parasite Plasmodium falciparum.” NIH issued an 11 November 1999 press release about this research.

8. Trypanosoma brucei, T. cruzi, Leishmania major, and P. vivax genome projects. EBI's Parasite-Genome Web site provides links to the Trypanosoma brucei Genome Network, the Trypanosoma cruzi Genome Initiative, the Leishmania Genome Network, and NCBI's P. Vivax index, as well as links to related projects. Parasites and Parasitological Resources provides information about Trypanosoma and African trypanosomiasis. CDC's Division of Parasitic Diseases provides information about Trypanosoma brucei infections. DPDx provides information on T. brucei. D. Campbell provides lecture notes on T. brucei for a course on molecular parasitology.

9. Artemisinin. WHO's TDR provides a report on artemisinin. The Use of Antimalarial Drugs includes a section on artemisinin. An article titled “Artemisinin: An ancient remedy for modern malaria” is provided by the Howard Hughes Medical Institute Holiday Lectures on Science Program. The Artenam New Malaria Therapy Web site provides information on artemisinin for malaria. BBC News provides a 1 June 2001 article titled “Herb offers malaria treatment hope.”

10. H. Vial's research. The laboratory of H. Vial at the Département de Biologie Santé, University of Montpellier II, France, provides a research presentation titled “Membrane biogenesis in Plasmodium, the malaria causing parasite, and phospholipids: From molecular biology to antimalarial chemotherapy.” The 15 February 2002 issue of Science had a News of the Week article by G. Taubes titled “Candidate drug breaks down malaria's walls” about the report of Vial et al.'s research in that issue ("A class of potent antimalarials and their specific accumulation in infected erythrocytes” by K. Wengelnik et al.) (4). BBC News offers a 15 February 2002 article about this research titled “Malaria drug offers new hope.”

11. Miltefosine against leishmaniasis. WHO issued a 17 June 2002 press release titled “New therapy for ‘black fever’ is 95% effective.” Prous Science offers a November 2001 presentation on miltefosine as a molecule of the month. WHO's TDR offers a special June 2002 newsletter feature on miltefosine; included is an article by J. Engel of Zentaris titled “Miltefosine, the story of a successful partnership: Disease endemic country - TDR - pharmaceutical industry (Zentaris).” The INTEGRIS Health Web site makes available a 4 July 2002 Reuters Health feature by M. McKinney titled “New drug effective against ‘black fever’.”

12. The Structural Genomics of Pathogenic Protozoa Web site provides information about the project and the targeted diseases and presents a collection of solved protein crystal structures of pathogenic protozoa. The Gelb Laboratory Web page provides a research presentation about structure-based drug design.

13. Biochemical pathways of pathogenic protozoa. Malaria Parasite Metabolic Pathways is a resource on malaria parasite biochemistry presented by H. Ginsburg, Institute of Life Sciences, Hebrew University of Jerusalem, Israel. The Parasitology Group at the Institute of Biological Sciences provides teaching notes on parasite biochemistry for a parasitology course.

14. A. Vaidya is in the Department of Microbiology and Immunology, Drexel University College of Medicine.

15. Malarone. The Use of Antimalarial Drugs includes a section on atovaquone-proguanil (malarone). WHO's TDR includes a section on malarone in its 1999-2000 report. The CDC Traveler's Health Web site provides an information sheet on malarone for malaria treatment and prophylaxis. GlaxoSmithKline provides information about malarone; included are reviews of the pharmacology of atovaquone and proguanil in the treatment of malaria. RxList provides information on malarone.

16. Drug resistance. The CDC's National Center for Infectious Disease provides an Antimicrobial Resistance Web resource. NIAID provides a fact sheet on antimicrobial resistance and a 1999 presentation by A. Fauci titled “Antimicrobial resistance: The NIH response to a growing problem.” NIAID's Division of Microbiology and Infectious Diseases provides information about antimicrobial resistance. Antimicrobial Resistance: Issues and Options is a 1998 workshop report available from the National Academy Press. WHO's Communicable Disease Surveillance and Response division provides a resource page on drug resistance; a background document (in PDF format) by P. Boland titled “Drug resistance in malaria” is available. Roll Back Malaria makes available a December 2001 WHO report titled “Monitoring antimalarial drug resistance.” The Twentieth Report of the WHO Expert Committee on Malaria includes a section on drug resistance. The Department of Microbiology and Immunology, University of Leicester, makes available a presentation by T. Bradley titled “Malaria and drug resistance” prepared for course on microbiology.

17. The Medicines for Malaria Venture Web site offers a presentation on malaria and medicines as well as news and press releases; a glossary and a selection of Internet Links are also provided.

18. The Institute for One World Health Web site provides an overview of its programs. An interview with V. Hale is included.

19. M. H. Gelb is in the Department of Chemistry, University of Washington. M. Gelb's research group on medicinal enzymology has a Web page.

20. W. G. J. Hol (and research page) is in the Departments of Biochemistry and Biological Structure, University of Washington.

References and Notes

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