Summary of discussions on tissue schizontocidal drugs against malaria*

Bulletin of the World Health Organization, 59 (3): 481-487 (1981) Summary of discussions on tissue schizontocidal drugs against malaria-principal lacunae in knowledge and recommendations for future research* The third meeting of the Scientific Working Group on the Chemotherapy ofMalaria was devoted to the subject of tissue schizontocidal drugs. Thepaperspresented at the meet- ing covered topics such as clinical problems associated with the use of 8-aminoquinolines, their pharmacology, pharmacokinetics, metabolism, toxicity, and safety assessment. This papersummarizes the Group's discussionsandconclusions on theprincipal lacunae in know- ledge in this field, and outlines the research programme to be implemented by the Steering Committee and the Secretariat of the Scientific Working Group. The papers presented at the third meeting of the Scientific Working Group on the Chemotherapy of Malaria reviewed the present state of knowledge on all aspects of the use of 8-aminoquinolines as tissue schizontocidal drugs. On the basis of these reports, the Scientific Working Group then prepared a plan of research aimed at producing improved drugs to help cope with one of the outstanding problems of malaria chemotherapy and malaria control. This summary identifies the main lacunae in know- ledge of tissue schizontocides, and outlines the research programme to be implemented by the Steer- ing Committee and Secretariat of the Scientific Work- ing Group. PRINCIPAL LACUNAE IN KNOWLEDGE OF TISSUE SCHIZONTOCIDES Pharmacology and kinetics In general, information on the preclinical and clinical pharmacology and toxicology of 8-amino- quinolines in man is extremely limited. Bioavailability of primaquine. Although prima- quine is apparently absorbed rapidly, it is not known to what extent, or whether it is subject to a " first-pass effect" a in the gut wall or liver. Both intravenous and oral studies need to be carried out in the same individual to determine the bioavailability of the drug. This is of particular relevance to alternative formu- lations and routes of administration of primaquine. * Based on the report of the third meeting of the Scientific Work- ing Group on the Chemotherapy of Malaria of the UNDP/World Bank/WHO Special Programme for Research and Training in Trop- ical Diseases. A list of participants is given on pages 486-487. a Modification by chemical or enzymatic action. Distribution ofprimaquine and its metabolites in tissue. Although data have been produced in animals on the distribution of radioactivity in tissues following intravenous administration of primaquine (with noticeable accumulation of activity in the lung), it is not known whether this represents unchanged drug or metabolites. No mass balance studies have been carried out to identify routes of excretion of unchanged drug or of specific metabolite products. Studies on the tissue distribution of unchanged drug and metabolites are needed in animals before the radioactive drug can be given to man, since data are required for ethical clearance of these studies. Metabolism and toxicity While it has been found that quinacrine enhances the toxicity of 8-aminoquinolines by increasing their plasma concentration, this has not been studied in man using recently developed specific methods of drug measurement. Since primaquine is usually given together with other antimalarial drugs, their inter- action is a matter of great importance for future kinetic and toxicological studies. The effects of primaquine on the cardiovascular system (ECG, blood pressure, blood clotting) and central nervous system (EEG) should be studied more extensively. These data are needed in order to develop new formulations and alternative routes of adminis- tration. Furthermore, dosage schedules can only be determined through consideration of the kinetics and dynamics of the drug and its effect on tissue schizonts. Identity of the active compounds. Although some work has been done on the identification of prima- quine metabolites, it is not yet possible to demonstrate the substances responsible for its tissue schizonto- cidal, gametocytocidal, and sporontocidal activity or toxicity. 4077 --481- 482 SUMMARY OF DISCUSSIONS Not enough is known about the role and importance of functional units of the parent compound and its metabolites. Site of drug action. Knowledge is lacking concern- ing the site of action of the 8-aminoquinolines. It is not known how these drugs enter the hepatocyte and affect the hypnozoite, and liver or cell lines capable of supporting tissue stages of the parasite are inadequate for these studies. Toxicity data. There is a considerable volume of data on the toxicity of primaquine but it has not been collated. New modes of application and new compounds Tissue stages of malaria parasites. Knowledge con- cerning tissue stages of malaria parasites is scanty compared with that on the more accessible blood forms. There is almost no information on the bio- chemistry, metabolism, and host-parasite relation- ships of exoerythrocytic forms, or on the mechanisms by which drugs exert their tissue schizontocidal effects. Furthermore, elucidation of these important basic principles is greatly handicapped by the lack of suitable methods of obtaining and isolating these organisms for study. Relapsing malaria models for drug screening. For the screening and evaluation of causal prophylactic compounds, a number of rodent and simian models have been developed and used effectively in identify- ing a number of potentially useful classes of drugs. With respect to radical cure, only the rhesus monkey/ Plasmodium cynomolgi model has been used effec- tively. It is not, however, suitable for primary screen- ing purposes because of the limited availability of rhesus monkeys and the cost of operating such a system. Insufficient attention has been given to the development of alternative laboratory models suitable for radical curative screening. Culture systemsfor exoerythrocyticparasite stages. The inability to grow exoerythrocytic forms in vitro is a major constraint on the ability to study the exoeryth- rocytic parasite and to examine the influence of chemotherapeutic agents on these stages. There is an urgent need for the development of techniques to maintain persistent and to grow non-persistent tissue stages in vitro. b Exoerythrocytic stages of avian malarias can be grown in vitro, but the similarities and differences between avian and mammalian forms have never been critically evaluated at the biochemical level. b Since this meeting was held, Hollingdale has reported, for the first time, the successful production of infective exoerythrocytic merozoites of P. berghei following the invasion of WI 38 lung cells by sporozoites, and the normal development of the complete asexual cycle in vitro (USAID Meeting on Malaria Immunology and Vaccine Development, Bethesda, USA, January 1981). Metabolic characterization of exoerythrocytic parasites. Studies of plasmodial metabolism and elucidation of host-cell-parasite relationships are beginning to shed light on unique features of the parasite, which may provide targets for chemothera- peutic intervention. To date, such studies have been limited largely to blood forms. It is important that such investigations be extended to exoerythrocytic parasites in order to determine how tissue stages and blood stages, and persistent and non-persistent stages differ. The nature of the stimulus that triggers schizogony in persistent stages is completely unknown. Mechanism ofaction of tissue schizontocides. With the exception of the tetrahydrofolate dehydrogenase (dihydrofolate reductase) inhibitors and 4-amino- benzoic acid antagonists, the mechanism of action of tissue schizontocides is unknown. Hypotheses regard- ing possible mechanisms of action of primaquine and several other classes have been proposed, but none has been subjected to critical experimental tests. The mechanisms underlying the development of resistance to causal prophylactic drugs are not understood, and methods to overcome or retard the development of resistance have not been explored. Production of alternative drugs. The development of a more suitable analogue of primaquine, with an improved therapeutic index, is being explored, and recent progress in this area suggests that productive avenues of investigation have not yet been exhausted. A considerable diversity of nuclear and side-chain analogues of primaquine has been synthesized and tested, and 5-, 6-, and 7-aminoquinolines have been screened for efficacy. These data have not been sub- jected to quantitative structure-activity analyses using modern methods, and it is suggested that such analy- ses might identify approaches that have been over- looked. The earlier work on side-chain variations has not been analysed using new computer graphic tech- niques, which might verify or disprove the notion that side-chain variations influence drug transport only. Characterization ofprimaquine metabolites. Work is now in progress to identify primaquine metabolites, but the biological properties of these metabolites have not been characterized. Promising new techniques for identification and quantification of primaquine and its metabolites have not yet been fully evaluated and alternative approaches have not been compared. Further work needs to be done with the quinone and quinonimine derivatives of primaquine to determine whether they are the active and/or toxic metabolites. Efficacy of drug combinations. Little work has been done to determine whether advantage can be gained in causal prophylaxis or radical cure by combinations of drugs. The possibility that a causal RESEARCH REQUIRED ON TISSUE SCHIZONTOCIDAL DRUGS prophylactic might enhance the activity of a radical curative drug against persistent tissue stages has not been investigated. Moreover, no consideration has been given to the potential value of drug combinations in limiting the development of drug resistance. Immunosuppressive activity ofprimaquine. It has been suggested that, in view of the possible immuno- suppressive effects of primaquine, administration might be more appropriate only after the parasitaemia has been controlled by blood schizontocides. The implications of immunosuppression by primaquine have not been adequately explored. RESEARCH REQUIREMENTS Pharmacology and kinetics All the research recommendations in this section are regarded as having equally high priority. Production of new formulations and compounds. The synthesis of labelled primaquine (incorporating 14C, 13C, 3H, 2H, or 15N) in greater quantities than are currently available is required. The specific label and activity of these materials should be agreed by chemists and the Steering Committee. It is proposed to set up a central store for the most wanted labelled primaquines, and other products, which would be available to accredited investigators. The synthesis and provision of the most important metabolites of primaquine which may be present in optically active form, and of (+ )-primaquine is required. The metabolites to be synthesized will depend on further studies, as outlined below. The use of these metabolites would be twofold: (a) As authentic standards for drug metabolism studies in different laboratories. For this, relatively small quantities would be necessary. (b) For testing biological activity in various systems. For this, larger quantities would be needed, but the number of metabolites required might be less than for (a). A formulation of primaquine with proven slow release characteristics is required. It is anticipated that this may allow the total dose of primaquine adminis- tered to man to be reduced, resulting in a decrease in toxicity without compromising efficacy. As a prelim- inary to this, studies are needed on the efficacy of pro- longed administration of low doses of conventional primaquine, and on the metabolism of the drug. The provision of an injectable formulation of primaquine with slow release from the injection site is required, particularly in view of observed patient non- compliance with a 5, 10, or 14-day course of oral therapy. Application of theseproducts to specificproblems. A comprehensive study of the clinical pharmacology of primaquine using recognized methods in suitably equipped centres is mandatory. As new agents become available, similar rigorous clinical pharmacological studies, backed up by relevant animal investigations, should be carried out. Having reviewed the methods available for measur- ing primaquine levels, consideration was given to priorities in the development of new analytical tech- niques. Development of a radioimmunoassay (RIA) for primaquine was given a low priority, since the facilities available at peripheral centres are limited and this technique allows measurement of unchanged drug only. Of other alternatives, it was considered that thin-layer chromatography and high-pressure liquid chromatography (HPLC) are more flexible for measuring drug and metabolite levels in tissue and blood samples. It is important to investigate the bioavailability of primaquine in animal models, by giving the drug orally and intravenously to the same animal. Such a study will provide information on whether the drug has a first-pass effect. If the intravenous adminis- tration of the drug proves to be safe in animals, similar studies should be carried out in human subjects. Animal studies should be carried out to identify tissue distribution of primaquine and its metabolites after oral, intravenous, and intramuscular adminis- tration. In part, this is necessary to obtain data before tracer studies can be carried out in man. These data should be obtained in the animal species most relevant to human pharmacology. Mass balance studies should be carried out in experimental animal models after oral and intra- venous primaquine administration. Urine, faeces, and blood should be collected at appropriate intervals following administration of labelled drug, and the metabolites identified and quantified. Studies should therefore be carried out in uninfected volunteers and malaria patients. Studies should be performed to determine the bio- logical activity of the known metabolites. If thera- peutic advantage is predicted from animal studies, preclinical toxicology tests should be carried out so that the metabolite may be used in trials in man. These trials should involve assessment of causal prophylactic activity, radical curative activity, and gametocyto- cidal or sporontocidal activity. A study is needed of primaquine interactions with drugs, particularly other antimalarials, and environ- mental chemicals, such as hydrocarbon insecticides. Such studies should include consideration of prima- quine metabolites and alterations in toxicity. Since primaquine is often used in conjunction with other antimalarials, particularly chloroquine and pyrimethamine/sulfonamide associations, its meta- 483 SUMMARY OF DISCUSSIONS bolism should be examined in relation to these drugs. In addition, evidence of the suggested potentiation of 8-aminoquinolines by quinine should be confirmed. Similar studies should be conducted with mefloquine. In any proposed kinetic studies, involving either single or multiple dosage, a sufficient number of data points should be obtained for a full analysis. Con- sideration must also be given to the type of kinetic model used in the subsequent data analysis. Metabolism and toxicity Since it is not yet possible to demonstrate the meta- bolites of primaquine that are responsible for tissue schizontocidal, gametocytocidal, and sporontocidal activity or toxicity, it is recommended that much greater emphasis be given to studies on the separation, isolation, and identification of these metabolites in models relevant to man. These should be character- ized and then produced either by direct chemical synthesis or by the use of microbial or other systems. The metabolites should then be made available for other workers. Newer and more sophisticated tech- niques, such as mass spectrometry, nuclear magnetic resonance, and ion resonance, as well as HPLC systems, which may be of more value in peripheral centres, should facilitate these studies. Toxicity studies. Because little is known about the mode of action of primaquine, further basic and clinical studies should be undertaken to examine the activity and toxicity of both parent drug and its putative metabolites. With respect to clinical toxi- cology, the use of non-invasive methods should be encouraged, e.g., monitoring of EEG for assessment of CNS activity, and ECG and systolic time intervals for CVS activity. It is recommended that the toxicity data associated with the clinical use of primaquine be collected and critically assessed. Further Phase 3 studies (1) should be undertaken to produce an approved schedule for the use of primaquine and to facilitate the study of its toxicity in more detail. Emphasis should be placed on exploration of gen- etic and nutritional factors in man in relation to both antimalarial and toxic factors. These studies should include use of in vitro systems to compare the effects of primaquine and its metabolites on human erythro- cytes with various genetic aberrations, such as enzy- matic defects of the oxidative metabolism of glucose, including G6PD deficiency and haemoglobin vari- ants. Nutritional factors such as riboflavin or ascorbic acid deficiency may also be expected to affect the response to primaquine and its metabolites. New compounds and modes of application Development ofanimal models and in vitro culture systems. The only useful model currently available for radical curative screening is P. cynomolgi in the rhesus monkey. Because of the limited availability and cost of all species of subhuman primates, the development of non-primate models for primary screening would be of considerable value. Primate species, other than rhesus monkeys, susceptible to P. cynomolgi or to P. vivax would be useful in secondary testing. It was also stressed that new drugs derived from a rational or empirical approach should be tested for tissue schizontocidal activity. This can be examined initially in the causal prophylactic screen (rodent), but studies are required to develop a new tissue schizont (hypnozoite) test system, either in rodents or another suitable model, which need not necessarily be associ- ated with a malarial infection. Research on the biology of the latent tissue stages (hypnozoites) of relapsing malarias, and their signifi- cance in relation to chemotherapy, should be intensi- fied. Development of methods for growing or maintain- ing exoerythrocytic forms of mammalian plasmodia in vitro are an essential prerequisite to studies of the biochemistry, metabolism, and host-parasite rela- tionships of exoerythrocytic stages. Culture systems are almost indispensable to studies of mechanisms of drug action and would be of considerable value as a drug screening tool. Intensive efforts should be made to develop methods of growing both persistent and non-persistent tissue forms in vitro. Parasite metabolism and host-parasite relation- ships. Studies of exoerythrocytic forms are essential to provide a rational basis for tissue schizontocidal drug development. Progress in these studies will depend on development of animal and in vitro model systems. Elucidation of metabolic pathways and determination of the requirements of tissue parasites for preformed substrates provided or transported by the host cell are necessary if biochemical targets for possible chemo- therapeutic attack are to be identified. The antiparasitic action ofprimaquine. Studies are needed to determine the mechanisms by which prima- quine exerts its tissue schizontocidal action. For example, studies of the parasite electron transport sys- tem would appear to be of particular interest, especi- ally with respect to the role of quinones in promoting or antagonizing electron transfer. Consideration ought to be given also to the possibility that prima- quine acts against tissue forms indirectly by inter- fering with the host cell's ability to support the parasite. Other aspects of primaquine action, such as its effects on nucleic acid metabolism, should also be explored. Development of new radical curative drugs. Since the only known radical curative drugs are primaquine, 484 RESEARCH REQUIRED ON TISSUE SCHIZONTOCIDAL DRUGS its close 8-aminoquinoline analogues, and some 6- aminoquinolines, no optimistic prediction can be made that novel alternative compounds will be found in the near future, through either rational or empirical approaches. It is recommended, therefore, that approaches be made to exploit fully the 8-amino- quinolines. Quantitative structure-activity analyses of historical radical curative and causal prophylactic data should be used to develop unifying physical- chemical concepts to guide further synthesis along productive lines. This will help in making best use of the data already available on the effect of substitution in different positions of the primaquine molecule. It is recommended that any such studies be undertaken in close collaboration with the Walter Reed Army Insti- tute of Research (WRAIR), with a view to gaining a greater insight into the role and importance of func- tional units of the parent compound and metabolites. This would promote a more rational approach to the preparation of more effective and safer new drugs. Recent studies on structure-activity relationships have shown that substitution in various positions can modify the toxicity and efficacy of primaquine, and that there is a distinct possibility of obtaining compounds with improved therapeutic indices. The finding that (+ )-primaquine might be less toxic than its (-)-enantiomer should be verified in man, and synthesis of chiral analogues and derivatives encour- aged. The possibility that useful radical curative activity may exist in 5-, 6-, and 7-aminoquinolines should also be explored more fully, using a rational approach based on results of current studies on drug metabolism and pharmacokinetics. The possibility that prodrugs of primaquine might be developed as a means of reducing toxicity and prolonging activity should be explored. Recent work has shown that this is a potentially useful approach, and a thorough exploration of the subject should be made. Recent work suggests that it may be feasible to develop delivery systems for primaquine which will improve its therapeutic index by delivering the drug more selectively to the target tissue. For example, by linking the primary amino group of primaquine to certain glycoprotein carrier groups, greater flexibility and specificity may be attained than has been possible with more complex and potentially less stable carrier systems, such as liposomes. Development of new causal prophylactic com- pounds. Recent work, particularly at WRAIR, has uncovered a large number of very active causal prophylactic compounds representing a number of distinct chemical classes. As there is a need for alternative causal prophylactic drugs, particularly for the prevention of drug-resistant falciparum malaria, these new leads should be fully explored with a view to possible clinical applications. Development of non-aminoquinoline radical curative drugs. The search for novel radical curative drugs outside the 8-aminoquinoline class should be extended. As far as possible, a rational approach should be adopted, based on knowledge of parasite metabolism. Establishment of drug-testingfacilities. Drug-test- ing requirements of the programme will necessitate the development and maintenance of at least two centres for drug screening with facilities for rodents, sub- human primates, and any new model systems that may be developed. RESUME RECAPITULATION DES DISCUSSIONS SUR LES MEDICAMENTS SCHIZONTOCIDES TISSULAIRES CONTRE LE PALUDISME-PRINCIPALES LACUNES DES CONNAISSANCES ET RECOMMANDATIONS EN VUE DES RECHERCHES FUTURES La troisieme reunion du Groupe de travail scientifique sur la chimiotherapie du paludisme (SWG-CHEMAL), sous les auspices du Programme special PNUD/Banque mondiale/ OMS de recherche et de formation concernant les maladies tropicales, a e entierement consacree aux medicaments schizontocides. La primaquine, une amino-8 quinoleine, est le schizontocide tissulaire le plus usite, mais sa toxicite, en particulier chez les sujets ayant un deficit en G6PD, et la necessite d'une administration prolongee dans le cas du traitement radical du paludisme a P. vivax et P. ovale restreignent son application pratique sur le terrain et donc son utilite dans la lutte contre le paludisme. Pourtant, aucun des autres schizontocides tissulaires disponibles n'est moins toxique ou plus efficace que la primaquine. De ce fait, les efforts pour ameliorer l'efficacite de cette derniee et abaisser sa toxicite, de meme que la recherche de nouveaux schizontocides tissulaires, sont consideres comme des domaines prioritaires du programme SWG-CHEMAL. La decouverte recente d'un stade tissulaire occulte, l'hypnozoIte, dans le cycle biologique d'un paludisme simien a rechutes que certaines constatations faites dans des cas d'infection humaine a P. vivax pouvaient laisser pre- sager, a montre qu'il etait n6cessaire et urgent de reorienter les systemes modeles, qui doivent refl6ter la veritable activite prophylactique causale et la veritable activite anti-rechutes. On a procede a une vaste recherche de composes doues 485 486 SUMMARY OF DISCUSSIONS d'une action schizontocide tissulaire parmi un grand nombre de groupements chimiques differents, mais sans parvenir jusqu'ici a mettre en evidence des substances admissibles convenables en dehors du groupe des amino-8 quinoleines. I1 est donc justifie de concentrer la recherche sur une amelioration de la primaquine et sur les tentatives en vue de decouvrir et de mettre au point d'autres amino-8 quinoleines. Tandis qu'il existe une masse considerable de renseigne- ments anciens sur la toxicite de la primaquine chez les animaux et chez l'homme, en particulier sur ses effets hema- totoxiques, on sait peu de chose sur la pharmacocinetique de ce compose et sur son metabolisme chez 1'h6te. En parti- culier, on ne sait pas encore avec certitude si c'est la primaquine elle-meme ou bien un ou plusieurs de ses meta- bolites qui sont responsables de la toxicite et de l'activite schizontocide tissulaire. Apres avoir identifie les principales lacunes dans les connaissances sur les medicaments schizontocides tissu- laires, le SWG-CHEMAL a prepare un programme de recherche qui accorde une priorite elevee aux etudes sur la pharmacologie, la pharmacocinetique, le metabolisme et la toxicite de la primaquine. Cela se justifie d'autant plus qu'il apparait maintenant que l'activite schizontocide tissulaire de la primaquine est liee a un ou plusieurs metabolites plutot qu'au medicament original. De meme, on procedera a l'etude de nouveaux modes d'administration de la prima- quine, en mettant principalement l'accent sur une reduction de la toxicite et sur la mise au point de formulations qui simplifieront l'utilisation de ce mrdicament sur le terrain. La mise au point de nouveaux medicaments schizontocides tissulaires sera poursuivie. A cet egard, il a e conclu qu'il convient de mettre au point sans delai des modeles experi- mentaux appropries pour etudier l'action prophylactique causale et l'action schizontocide tissulaire stricto sensu. REFERENCES 1. WHO Technical Report Series, No. 529, 1973, p. 119. LIST OF PARTICIPANTS N. Anand, Director, Central Drug Research Institute, Lucknow, India A. Breckenridge, Department of Clinical Pharmaco- logy, Liverpool School of Tropical Medicine, Liverpool L3 5QA, England A. Brossi, Chief, Medicinal Chemistry, NIAMDD, Department of Health and Human Services, Public Health Service, National Institutes of Health, Bethesda, MD 20014, USA C. J. Canfield, Director, Division of Experimental Therapeutics, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington, DC 20012, USA. P. E. Carson, Chairman, Department of Pharma- cology, Rush-Presbyterian St Luke's Medical Center, Rush University, Chicago, IL 60612, USA. D. Davidson, Division of Experimental Chemo- therapy, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington, DC 20012, USA M. Fernex, Hoffman-La Roche AG, Postfach 259, CH-4000 Basel 3, Switzerland K. A. Fletcher, Department of Tropical Medicine, Liverpool School of Tropical Medicine, Liverpool L3 5QA, England R. S. Grewal, CIBA-GEIGY Ltd, K-125.15.10, 4002 Basel, Switzerland C. C. Lee, Health Review Division, United States Environmental Protection Agency, Washington, DC 20460, USA J. D. McChesney, Department of Pharmacognosy, University of Mississippi, University, MS 38677, USA W. Peters, Department of Protozoology, London School of Hygiene and Tropical Medicine, London WClH 7HT, England W. H. G. Richards, The Wellcome Foundation Ltd, Berkhamsted, Hertfordshire HP4 2DY, England I. W. Sherman, Department of Biology, University of California, Riverside, CA 92521, USA A. Strother, Department of Pharmacology, School of Medicine, Loma Linda University, Loma Linda, CA 92354, USA A. Trouet, International Institute of Cellular and Molecular Pathology, B-1200 Brussels, Belgium WHO Secretariat D. F. Clyde, Regional Malaria Adviser, WHO Regional Office for South East Asia, New Delhi, India E. de Maar, Special Programme for Research and Training in Tropical Diseases, WHO, Geneva, Switzerland RESEARCH REQUIRED ON TISSUE SCHIZONTOCIDAL DRUGS 487 F. J. Lopez-Antuhano, Regional Research Malaria Adviser PAHO, WHO Regional Office for the Americas, Washington, USA P. I. Trigg, Special Programme for Research and Training in Tropical Diseases, Research and Technical Intelligence, Malaria Action Pro- gramme, WHO, Geneva, Switzerland (Secretary of Steering Committee) W. J. 0. van Dijk, Senior Malaria Adviser, WHO Regional Office for the Western Pacific, Manila, Philippines. W. H. Wernsdorfer, Chief, Research and Technical Intelligence, Malaria Action Programme, WHO, Geneva, Switzerland (Secretary)