WHO/CDS/CSR/DRS/2001.4 ORIGINAL: ENGLISH DISTRIBUTION: GENERAL

Drug resistance in malaria Peter B. Bloland

Copies can be obtained from the CDS Information Resource Centre World Health Organization World Health Organization, 1211 Geneva 27, Switzerland fax: +41 22 791 42 85 • email: [email protected] WHO/CDS/CSR/DRS/2001.4 ORIGINAL: ENGLISH DISTRIBUTION: GENERAL

Drug resistance in malaria Peter B. Bloland Malaria Epidemiology Branch Centers for Disease Control and Prevention Chamblee, GA, United States of America

World Health Organization

A BACKGROUND DOCUMENT FOR THE WHOFOR GLOBAL CONTAINMENT STRATEGY OF ANTIMICROBIALRESISTANCE Acknowledgement The World Health Organization wishes to acknowledge the support of the United States Agency for Inter- national Development (USAID) in the production of this document.

© World Health Organization 2001 This document is not a formal publication of the World Health Organization (WHO), and all rights are reserved by the Organiza- tion. The document may, however, be freely reviewed, abstracted, reproduced and translated, in part or in whole, but not for sale or for use in conjunction with commercial purposes. The views expressed in documents by named authors are solely the responsibility of those authors. The designations employed and the presentation of the material in this document, including tables and maps, do not imply the expression of any opinion whatsoever on the part of the secretariat of the World Health Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted lines on maps represent approximate border lines for which there may not yet be full agreement. The mention of specific companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by WHO in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters. Designed by minimum graphics Printed in Switzerland WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANC IN MALARIA

Contents

1. Introduction 1 2. Disease incidence and trends 2 2.1 Geographical distribution and populations at risk 2 2.2 Causative agents 3 2.3 Diagnosis 3 2.3.1 Microscopy 3 2.3.2 Clinical (presumptive) diagnosis 3 2.3.3 Antigen detection tests 5 2.3.4 Molecular tests 5 2.3.5 Serology 5 2.4 Drugs available for treatment of malaria 5 2.4.1 Quinine and related compounds 5 2.4.2 Antifolate combination drugs 9 2.4.3 Antibiotics 9 2.4.4 Artemisinin compounds 9 2.4.5 Miscellaneous compounds 9 2.4.6 Combination therapy with antimalarials 10 2.5 Current status of drug-resistant malaria 10 3. Causes of resistance 12 3.1 Definition of antimalarial drug resistance 12 3.2 Malaria treatment failure 12 3.3 Mechanisms of antimalarial resistance 12 3.3.1 Chloroquine resistance 12 3.3.2 Antifolate combination drugs 13 3.3.3 Atovaquone 13 3.4 Factors contributing to the spread of resistance 13 3.4.1 Biological influences on resistance 13 3.4.2 Programmatic influences on resistance 15 4. Detection of resistance 16 4.1 In vivo tests 16 4.2 In vitro tests 17 4.3 Animal model studies 17 4.4 Molecular techniques 17 4.5 Case reports and passive detection of treatment failure 18 5. Treatment 19 6. The future: prevention of drug resistance 20 6.1 Preventing drug resistance 20 6.1.1 Reducing overall drug pressure 21 6.1.2 Improving the way drugs are used 21 6.1.3 Combination therapy 21

iii DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

7. Conclusions and recommendations 23 7.1 Priorities 23 8. Bibliography 24

Figure and tables Figure 1. Approximate distribution of malaria 2 Table 1. Comparative descriptions of available malaria diagnostic methods 4 Table 2. Antimalarial drugs for uncomplicated malaria 6 Table 3. Distribution of drug-resistant Plasmodium falciparum malaria 10

iv WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

1. Introduction

Malaria remains an important public health of the greatest challenges facing malaria control concern in countries where transmission occurs today. Drug resistance has been implicated in the regularly, as well as in areas where transmission has spread of malaria to new areas and re-emergence of been largely controlled or eliminated. Malaria is a malaria in areas where the disease had been eradi- complex disease that varies widely in epidemiology cated. Drug resistance has also played a significant and clinical manifestation in different parts of the role in the occurrence and severity of epidemics in world. This variability is the result of factors such some parts of the world. Population movement has as the species of malaria parasites that occur in a introduced resistant parasites to areas previously free given area, their susceptibility to commonly used of drug resistance. The economics of developing or available antimalarial drugs, the distribution and new pharmaceuticals for tropical diseases, includ- efficiency of mosquito vectors, climate and other ing malaria, are such that there is a great disparity environmental conditions and the behaviour and between the public health importance of the level of acquired immunity of the exposed human disease and the amount of resources invested in populations. In particular, young children, developing new cures (1, 2). This disparity comes pregnant women, and non-immune visitors to at a time when malaria parasites have demonstrated malarious areas are at greatest risk of severe or fatal some level of resistance to almost every anti- illness. Many malaria control strategies exist, but malarial drug currently available, significantly none are appropriate and affordable in all contexts. increasing the cost and complexity of achieving Malaria control and prevention efforts need to be parasitological cure. designed for the specific environment in which they The purpose of this review is to describe the state will be used and need to take into account the of knowledge regarding drug- resistant malaria and local epidemiology of malaria and the level of avail- to outline the current thinking regarding strategies able resources and political will. to limit the advent, spread, and intensification of Antimalarial drug resistance has emerged as one drug-resistant malaria.

1 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

2. Disease incidence and trends

2.1 Geographical distribution and (5) and the Medicines for Malaria Venture (6) a populations at risk history of unpredictable support for malaria-related Malaria occurs in over 90 countries worldwide. research and control activities in endemic countries According to figures provided by the World Health have left many of these countries with little techni- Organization (3), 36% of the global population live cal capacity for malaria control activities. in areas where there is risk of malaria transmission, Each year an estimated 300 to 500 million clini- 7% reside in areas where malaria has never been cal cases of malaria occur, making it one of the most under meaningful control, and 29% live in areas common infectious diseases worldwide. Malaria can where malaria was once transmitted at low levels be, in certain epidemiological circumstances, a or not at all, but where significant transmission has devastating disease with high morbidity and mor- been re-established (3). The development and tality, demanding a rapid, comprehensive response. spread of drug-resistant strains of malaria parasites In other settings, it can be a more pernicious pub- has been identified as a key factor in this resur- lic health threat. In many malarious areas of the gence and is one of the greatest challenges to world, especially sub-Saharan Africa, malaria is malaria control today. Although there is currently ranked among the most frequent causes of mor- an increase in attention and resources aimed at bidity and mortality among children and is often malaria, including such initiatives as Roll Back the leading identifiable cause. WHO estimates that Malaria (4), the Multilateral Initiative on Malaria more than 90% of the 1.5 to 2.0 million deaths

FIGURE 1. APPROXIMATE DISTRIBUTION OF MALARIA

2 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

attributed to malaria each year occur in African 2.3.1 Microscopy children (3). Other estimates based on a more Simple light microscopic examination of Giemsa- rigorous attempt to calculate the burden of disease stained blood films is the most widely practised and in Africa support this level of mortality (7). In useful method for definitive malaria diagnosis. addition to its burden in terms of morbidity and Advantages include differentiation between species, mortality, the economic effects of malaria infection quantification of the parasite density, and ability can be tremendous. These include direct costs for to distinguish clinically important asexual parasite treatment and prevention, as well as indirect costs stages from gametocytes which may persist with- such as lost productivity from morbidity and mor- out causing symptoms. These advantages can be tality, time spent seeking treatment, and diversion critical for proper case-management and evaluat- of household resources. The annual economic bur- ing parasitological response to treatment. Specific den of malaria infection in 1995 was estimated at disadvantages are that slide collection, staining, and US$ .8 billion, for Africa alone (8). This heavy toll reading can be time-consuming and microscopists can hinder economic and community development need to be trained and supervised to ensure con- activities throughout the region. sistent reliability. While availability of microscopic Malaria transmission occurs primarily in tropi- diagnosis has been shown to reduce drug use in cal and subtropical regions in sub-Saharan Africa, some trial settings (10), in practice, results are Central and South America, the Caribbean island often disregarded by clinicians (11). Any pro- of Hispaniola, the Middle East, the Indian subcon- gramme aimed at improving the availability of tinent, South-East Asia, and Oceania (figure1). In reliable microscopy should also retrain clinicians areas where malaria occurs, however, there is con- in the use and interpretation of microscopic siderable variation in the intensity of transmission diagnosis. and risk of malaria infection. Highland (>1500 m) A second method is a modification of light and arid areas (<1000 mm rainfall/year) typically microscopy called the quantitative buffy coat have less malaria, although they are also prone to method (QBCTM, Becton-Dickinson). Originally epidemic malaria when parasitaemic individuals developed to screen large numbers of specimens for provide a source of infection and climate condi- complete blood cell counts, this method has been tions are favourable to mosquito development (3). adapted for malaria diagnosis (12). The technique Although urban areas have typically been at lower uses microhaematocrit tubes precoated with fluo- risk, explosive, unplanned population growth has rescent acridine orange stain to highlight malaria contributed to the growing problem of urban parasites. With centrifugation, parasites are con- malaria transmission (9). centrated at a predictable location. Advantages to QBC are that less training is required to operate 2.2 Causative agents the system than for reading Giemsa-stained blood films, and the test is typically quicker to perform In humans, malaria infection is caused by one or than normal light microscopy. Field trials have more of four species of intracellular protozoan para- shown that the QBC system may be marginally site. Plasmodium falciparum, P. v i v a x , P. o v a l e , and more sensitive than conventional microscopy P. malariae differ in geographical distribution, under ideal conditions (13, 14). Disadvantages are microscopic appearance, clinical features (periodic- that electricity is always required, special equipment ity of infection, potential for severe disease, and and supplies are needed, the per-test cost is higher ability to cause relapses), and potential for devel- than simple light microscopy, and species-specific opment of resistance to antimalarial drugs. To date, diagnosis is not reliable. drug resistance has only been documented in two of the four species, P. falciparum and P. vivax. 2.3.2 Clinical (presumptive) diagnosis 2.3 Diagnosis (Table 1) Although reliable diagnosis cannot be made on the basis of signs and symptoms alone because of the Direct microscopic examination of intracellular non-specific nature of clinical malaria, clinical parasites on stained blood films is the current stand- diagnosis of malaria is common in many malarious ard for definitive diagnosis in nearly all settings. areas. In much of the malaria-endemic world, However, several other approaches exist or are in resources and trained health personnel are so scarce development and will be described here. that presumptive clinical diagnosis is the only real-

3 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

TABLE 1. COMPARATIVE DESCRIPTIONS OF AVAILABLE MALARIA DIAGNOSTIC METHODS

Method Sensitivity/specificity Advantages Disadvantages Cost* References

Rapid diagnostic test sens: • Differentiates P. falciparum from non- • Cannot differentiate between non- 1.00 based on pLDH: spec: falciparum infections. falciparum species. (OptiMal - Flow Inc) • Speed and ease of use; minimal training • Will not quantify parsitaemia requirements to achieve reliable result. (+/- only). • Reportedly does not remain positive after clearance of parasites. • No electricity, no special equipment needed; could be used in community outreach programmes.

Rapid diagnostic stick sens: 84% –97% • Speed and ease of use; minimal training • Will not diagnose non-falciparum 0.80 to (23) test based on PfHRP-II: spec: 81%–100% requirements to achieve reliable results. malaria although subsequent 1.00 (ParaSight-F – • No electricity, no special equipment generation tests will be able to do this. Becton – Dickinson; lower values probably needed; could be used at health post/ • Will not quantify parasitaemia (22) Malaria PfTest – due to low parasite community outreach. (+/- only). ICT Diagnostics) densities • Card format easier to use for individual • Can remain positive after clearance of tests; dipstick test easier to use for parasites. batched testing.

Light microscopy Optimal conditions: • Species-specific diagnosis. • Requires relatively high degree of 0.03 to (22) sens: >90% • Quantification of parasitaemia aids training and supervision for reliable 0.08** (11) spec: 100% treatment follow-up. results. • Sensitivity and specificity dependent Typical field conditions: on training and supervision. sens: 25%–100% • Special equipment and supplies needed. spec: 56%–100% • Electricity desirable. • Time-consuming.

Fluorescent microscopy: AO: 42%–93% sens/ • Results attainable more quickly than • Special equipment and supplies needed. 0.03 (AO) (24) • Acridine orange [AO] 52–93% spec normal microscopy. • Sensitivity of AO poor with low parasite to 1.70 (22) stained thick blood densities. (QBC) smears); • Electricity required. • Quantitative Buffy QBC: 89% sens/ >95% • Unreliable species diagnosis; non-specific Coat (QBCTM) – spec staining of debris and non-parasitic cells. (Becton-Dickinson) • QBC will not quantify parasitaemia. • Acridine orange is a hazardous material.

Clinical, especially Variable depending on • Speed and ease of use. • Can result in high degree of misdiagnosis Variable (111) based on formal level of clinical • No electricity, no special equipment and over-treatment for malaria. depending algorithm such as competency, training, needed beyond normal clinical • Requires close supervision and on (112) Integrated Manage- and malaria risk equipment (thermometer, stethoscope, retraining to maximize reliability. situation. ment of Childhood (endemicity): otoscope, timer). Illnesses (IMCI) or with IMCI: similar algorithm low risk: sens: 87% spec: 8% high risk: sens: 100% spec: 0%

Table modified from Stennies, 1999, CDC unpublished document. * Approximate or projected cost given in US dollars per test performed and reflects only cost of expendable materials unless otherwise noted. ** Cost includes salaries of microscopists and expendable supplies; does not include cost of training, supervision, or equipment.

istic option. Clinical diagnosis offers the advantages symptoms of malaria and other frequent diseases, of ease, speed, and low cost. In areas where malaria especially acute lower respiratory tract infection is prevalent, clinical diagnosis usually results in all (ALRI), and can greatly increase the frequency of patients with fever and no apparent other cause misdiagnosis and mistreatment (16). being treated for malaria. This approach can iden- Attempts to improve the specificity of clinical tify most patients who truly need antimalarial treat- diagnosis for malaria by including signs and symp- ment, but it is also likely to misclassify many who toms other than fever or history of fever have met do not (15). Over-diagnosis can be considerable with only minimal success (17). The Integrated and contributes to misuse of antimalarial drugs. Management of Childhood Illnesses (IMCI) pro- Considerable overlap exists between the signs and gramme defined an algorithm that has been devel-

4 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

oped in order to improve diagnosis and treatment 2.3.4 Molecular tests of the most common childhood illnesses in areas Detection of parasite genetic material through relying upon relatively unskilled health care work- polymerase-chain reaction (PCR) techniques is ers working without access to laboratories or becoming a more frequently used tool in the diag- special equipment. With this algorithm, every nosis of malaria, as well as the diagnosis and sur- febrile child living in a “high-risk” area for malaria veillance of drug resistance in malaria. Specific should be considered to have, and be treated for, primers have been developed for each of the four malaria. “High risk” has been defined in IMCI species of human malaria. One important use of Adaptation Guides as being any situation where as this new technology is in detecting mixed infec- little as 5% of febrile children between the ages of tions or differentiating between infecting species 2 and 59 months are parasitaemic (18), a defini- when microscopic examination is inconclusive (27). tion that will likely lead to significant over-diagno- In addition, improved PCR techniques could prove sis of malaria in areas with low to moderate malaria useful for conducting molecular epidemiological transmission. investigations of malaria clusters or epidemics (28). Primary disadvantages to these methods are overall 2.3.3 Antigen detection tests (also known as rapid high cost, high degree of training required, need or “dipstick” tests) for special equipment, absolute requirement for A third diagnostic approach involves the rapid electricity, and potential for cross-contamination detection of parasite antigens using rapid immuno- between samples. chromatographic techniques. Multiple experimen- tal tests have been developed targeting a variety of 2.3.5 Serology parasite antigens (19‚ 20‚ 21). A number of com- Techniques also exist for detecting anti-malaria mercially available kits (e.g. ParaSight-F®, Becton- antibodies in serum specimens. Specific serologi- Dickinson; Malaquick®, ICT, Sydney, New South cal markers have been identified for each of the four Wales, Australia) are based on the detection of the species of human malaria. A positive test generally histidine-rich protein 2 (HRP-II) of P. falciparum. indicates a past infection. Serology is not useful for Compared with light microscopy and QBC, this diagnosing acute infections because detectable test yielded rapid and highly sensitive diagnosis of levels of anti-malaria antibodies do not appear P. falciparum infection (22, 23). Advantages to this until weeks into infection and persist long after technology are that no special equipment is re- parasitaemia has resolved. Moreover, the test is rela- quired, minimal training is needed, the test and tively expensive, and not widely available. reagents are stable at ambient temperatures, and no electricity is needed. The principal disadvan- tages are a currently high per-test cost and an 2.4 Drugs available for treatment inability to quantify the density of infection. of malaria Furthermore, for tests based on HRP-II, detect- There are only a limited number of drugs which able antigen can persist for days after adequate treat- can be used to treat or prevent malaria (Table 2). ment and cure; therefore, the test cannot adequately The most widely used are quinine and its deriva- distinguish a resolving infection from treatment tives and antifolate combination drugs. failure due to drug resistance, especially early after treatment (23). Additionally, a test based on detec- 2.4.1 Quinine and related compounds tion of a specific parasite enzyme (lactate dehydro- genase or pLDH) has been developed (OptiMAL®, Quinine, along with its dextroisomer quinidine, has Flow Inc. Portland, OR, USA) and reportedly only been the drug of last resort for the treatment of detects viable parasites, which if true, eliminates malaria, especially severe disease. Chloroquine is a prolonged periods of false positivity post-treatment 4-aminoquinoline derivative of quinine first syn- (24, 25, 26). Newer generation antigen detection thesized in 1934 and has since been the most widely tests are able to distinguish between falciparum and used antimalarial drug. Historically, it has been the non-falciparum infections, greatly expanding their drug of choice for the treatment of non-severe or usefulness in areas where non-falciparum malaria uncomplicated malaria and for chemoprophylaxis, is transmitted frequently. although drug resistance has dramatically reduced its usefulness. Amodiaquine is a relatively widely

5 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

TABLE 2. ANTIMALARIAL DRUGS FOR UNCOMPLICATED MALARIA

Drug name Use Half- Dosing (all per os) Contra- Cost Comments life indications (US$)* (hours) Adult Paediatric COMBINATION THERAPY Mefloquine • Treatment of non- M: (14– 15 mg (base)/kg to 15 mg (base)/kg See under 3.90 • Safety of artemisinins & MQ + severe falciparum 18 days) maximum of 1000 mg mefloquine on 2nd mefloquine during first trimester of Artesunate infections thought mefloquine base on day of treatment monotherapy. pregnancy not established. to be chloroquine Art: 0.5– second day of treat- followed by 10 mg/kg • Vomiting after mefloquine and SP resistant. 1.4 ment, followed by mefloquine on 3rd can be reduced by 10 mg (base)/kg to day. administering mefloquine maximum of 500 mg on the second and third day mefloquine base on after an initial dose of 3rd day. artesunate. • Artemisinin (10 mg/kg po 4 mg/kg artesunate 4 mg/kg artesunate daily for 3 days) can be daily for 3 days. daily for 3 days. substituted for artesunate.

Sulfadoxine/ • Treatment of non- S: 100– 25 mg/kg sulfa/1.25 By weight: Known SP 1.12 • This combination has not Pyrimethamine severe falicparum 200 mg/kg pyrimethamine 25 mg/kg sulfa/1.25 allergy. been evaluated as + infections thought per kg as single dose. mg/kg pyrimethamine extensively as MQ + Artesunate to be chloroquine P: 80– per kg as single dose. artesunate. resistant. 100 Average adult dose: • Safety of artemisinin during 3 tablets as a single By age: first trimester of pregnancy Art: 0.5– dose (equivalent to • < 1 year: 2 tablet not established. 3 1.4 1500 mg sulfa /75 mg • 1–3 years: /4 tablet • Artemisinin (10 mg/kg po pyrimethamine). • 4–8 years: 1 tablet daily for 3 days) can be • 9–14 years: 2 tablets substituted for artesunate. • >14 years: 3 tablets

4 mg/kg artesunate 4 mg/kg artesunate daily for 3 days. daily for 3 days.

Lumefantrine • Treatment of non- L: 3–6 Semi-immune patients: Tablets per dose by 7.30 • Fixed-dose combination + severe falciparum days 4 tablets per dose at 0, body weight: (Cameroon with each tablet containing Artemether infections thought 8, 24, and 48 hours street 20 mg artemether and to be chloroquine Art: 4–11 (total 16 tablet). 10–14 kg: 1 tablet price); 120 mg lumefantrine. Trade name: and SP resistant. 15–24 kg: 2 tablets • Safety during pregnancy Co-artem; Riamet Non-immune patients: 25–34 kg: 3 tablets 57 not established. 4 tablets per dose at 0 >34 kg: 4 tablets (Europe) and 8 hrs, then twice daily for 2 more days given in same schedule (total 24 tablets) as for adults, depend- ing on immune status.

SINGLE-AGENT THERAPY Chloroquine (CQ) • Treatment of non- (41±14 • Treatment: 25 mg • Treatment: 25 mg 0.08 • Widespread resistance in Trade names: falciparum infections. days) base/kg divided over base/kg divided P. falciparum in most Nivaquine, • Treatment of P. falci- 3 days. over 3 days. regions. Malaraquine, parum infections in • Resistance in P. v i va x occurs. Aralen, areas where chloro- Average adult: 1 g • Prophylaxis: 5 mg • Can cause pruritus in dark- many others quine remains chloroquine (salt), base/kg once per skinned patients, reducing effective. followed by 500 mg week. compliance. • Chemoprophylaxis (salt) in 6-8 hours and in areas where 500 mg (salt) daily for chloroquine remains 2 more days. effective. • Prophylaxis: 500 mg salt once per week.

6 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

TABLE 2 (continued)

Drug name Use Half- Dosing (all per os) Contra- Cost Comments life indications (US$)* (hours) Adult Paediatric SINGLE-AGENT THERAPY Amodiaquine (AQ) • Treatment of non- • Treatment: 25 mg • Treatment: 25 mg 0.14 • Cross-resistance with Trade names: severe falciparum base/kg divided base/kg divided over chloroquine limits useful- Camoquine, infections thought to over 3 days. 3 days. ness in areas with high rates others be chloroquine of chloroquine resistance. resistant. • Has been associated with toxic hepatitis and agranulocytosis when used as prophylaxis—risk when used for treatment unknown but likely to be low.

Sulfadoxine/ • Treatment of non- SD: 100– 25 mg/kg sulfa/1.25 By weight: • Known sulfa 0.12 • Efficacy for vivax infections pyrimethamine severe falciparum 200 mg/kg pyrimethamine 25 mg/kg sulfa/1.25 allergy may be poor. (SP); infections thought per kg as single dose. mg/kg pyrimethamine • Widespread resistance in to be chloroquine SL: 65 per kg as single dose: P. falciparum in some Sulfalene/ resistant. Average adult: regions. pyrimethamine P: 80–100 1500 mg sulfa/75 mg By age: • Can cause severe skin 1 (Metakelfin) pyrimethamine as • < 1 year: /2 tablet disease when used prophy- 3 single dose. • 1–3 years: /4 tablet lactically; risk when used as • 4–8 years: 1 tablet treatment unknown but (Equivalent to 3 tablets • 9–14 years: 2 tablets likely to be very low. as a single dose.) • >14 years: 3 tablets

Mefloquine (MQ) • Treatment of non- (14–18 • Treatment: 750 mg Treatment: 15 mg • Known 1.92 • Vomiting can be a common Trade names: severe falciparum days) base to 1500 mg (base)/kg to 25 mg or suspected problem among young Lariam, infections thought to base depending on (base)/kg depending history of children. Mephaquine be chloroquine and local resistance pat- on local resistance neuro- • In some populations (e.g. SP resistant. terns. Larger doses patterns. Larger doses psychiatric very young African • Chemoprophylaxis (>15 mg/ kg) best (>15 mg/kg) best disorder. children), unpredictable in areas with chloro- given in split doses given in split doses • history of blood levels, even after quine resistance. over 2 days. over 2 days. seizures appropriate dosing, can • concomitant produce frequent • Prophylaxis: 250 mg • Prophylaxis: 5 mg use of treatment failure. once per week. base/kg once per halofantrine. • Use of lower dose may week. facilitate development of resistance.

Halofantrine • Treatment of 10–90 8 mg base/kg 8 mg base/kg every • Preexisting 5.31 • Cross-resistance with suspected multidrug- every 6 hours for 6 hours for 3 doses. cardiac mefloquine has been resistant falciparum. 3 doses. disease. reported. • Congenital • Reported to have highly Average adult: prolongation variable bioavailability.

1500 mg base of QTc interval. • Risk of fatal cardiotoxicity. divided into 3 doses • Treatment as above. with meflo- quine within prior 3 weeks. • Pregnancy.

Quinine • Treatment of severe 10–12 • Non-severe malaria: • Non-severe malaria: 1.51 • Side-effects can greatly malaria. 8 mg (base)/kg 3 8 mg (base)/kg 3 reduce compliance. • Treatment of multi- times daily for 7 days. times daily for 7 days. • Used in combination with drug-resistant • Average adult: • Severe: see section tetracycline, doxycycline, P. falciparum. 650 mg 3 times per on treatment of clindamycin, or SP (where • Treatment of malaria day for 7 days. severe malaria. effective) and in areas during 1st trimester • Severe: see section on where quinine resistance of pregnancy. treatment of severe not prevalent; duration of malaria. quinine dosage can be reduced to 3 days when used in combination.

7 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

TABLE 2 (continued)

Drug name Use Half- Dosing (all per os) Contra- Cost Comments life indications (US$)* (hours) Adult Paediatric SINGLE-AGENT THERAPY Tetracycline • In combination with Tetra: 10 Tetra: 250 mg/kg Tetra: 5 mg/kg • Age less than • Used only in combination (tetra)/ Doxy- quinine, can increase 4 times per day for 4 times per day for 8 years. with a rapidly acting cycline (doxy) efficacy of treatment Doxy: 16 7 days. 7 days. • Pregnancy. schizonticide such as in areas with quinine quinine. resistance and/or Doxy: 100 mg/kg Doxy: 2 mg/kg reduce likelihood of 2 times per day for twice per day quinine-associated 7 days. for 7 days. side-effects by reducing duration of Prophylaxis: 100 mg Prophylaxis: 2 mg/kg quinine treatment. doxy per day. doxy per day up to • Prophylaxis. 100 mg.

Clindamycin • For patients unable 3 300 mg 4 times per 20 to 40 mg/kg/day • Severe hepatic • Is not as effective as to take tetracycline. day for 5 days. divided in 3 daily or renal tetracycline, especially • In combination with doses for 5 days. impairment. among non-immune quinine, can increase • History of patients. efficacy of treatment gastrointestinal • Used only in combination in areas with quinine disease, with a rapidly acting resistance and/or especially schizonticide such as reduce likelihood of colitis. quinine. quinine-associated side effects by reducing duration of quinine treatment.

Atovaquone/ • Treatment of Atv: 59 1000 mg atovaquone No pediatric formula- 35.00 • Fixed dose combination. proguanil multidrug resistant + 400 mg proguanil tion currently available, • Reportedly safe in P. falciparum Prog: 24 daily for 3 days. but for patients pregnancy and young Trade name: infections. between 11 and 40 kg children. Malarone body weight: • Drug donation program exists 1 11–20 kg: /4 adult dose • Pediatric formulation in 1 21–30 kg: /2 adult dose development. 3 31–40 kg: /4 adult dose

Artesunate • Treatment of multi- 0.5–1.4 4 mg/kg on the first 4 mg/kg on the first 1.50–3.40 • Safety for use in pregnancy drug resistant day followed by day followed by not fully established, P. falciparum 2 mg/kg daily for 2 mg/kg daily for especially for use in first infections. total of 5 to 7 days. total of 5 to 7 days. trimester (available data suggest relative safety for Artemisinin 2–7 20 mg/kg on the first 20 mg/kg on the first 1.50–3.40 second or third trimester). day followed by 10 day followed by • Other artemisinin mg/kg daily for 10 mg/kg daily for derivatives include total of 5 to 7 days. total of 5 to 7 days. arteether, dihydro- artemisinin, artelinate. Artemisinin compounds Artemether 4–11 4 mg/kg on the first 4 mg/kg on the first 3.60–4.80 day followed by day followed by 2 mg/kg daily for 2 mg/kg daily for total of 5 to 7 days. total of 5 to 7 days.

Primaquine • Treatment of P. vivax 6 • 14 mg base per day • 0.3 mg (base)/kg • G6PD • Primaquine has also been infections (reduce for 14 days. daily for 14 days. deficiency. investigated for prophy- likelihood of relapse). • 45 mg once per • Pregnancy. laxis use. • Gametocytocidal week for 8 weeks. • Shorter courses have been agent. used for falciparum infections for gameto- cytocidal action.

* Cost is given for a full adult (60kg) treatment course. Prices have been derived from a variety of sources including Management Sciences for Health, World Health Organization, drug companies, published reports, and personal communication and are presented for relative comparison purposes only—actual local prices may differ greatly.

8 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

available compound closely related to chloroquine. 2.4.4 Artemisinin compounds Other quinine-related compounds in common use A number of sesquiterpine lactone compounds have include primaquine (specifically used for eliminat- been synthesized from the plant Artemisia annua ing the exoerythrocytic forms of P. vivax and P. ovale (artesunate, artemether, arteether). These com- that cause relapses), and mefloquine (a quinoline- pounds are used for treatment of severe malaria and methanol derivative of quinine). have shown very rapid parasite clearance times and faster fever resolution than occurs with quinine. In 2.4.2 Antifolate combination drugs some areas of South-East Asia, combinations of artemisinins and mefloquine offer the only reliable These drugs are various combinations of dihydro- treatment for even uncomplicated malaria, due to folate-reductase inhibitors (proguanil, chlorpro- the development and prevalence of multidrug- guanil, pyrimethamine, and trimethoprim) and resistant falciparum malaria (32). Combination sulfa drugs (dapsone, sulfalene, sulfamethoxazole, therapy (an artemisinin compound given in com- sulfadoxine, and others). Although these drugs have bination with another antimalarial, typically a antimalarial activity when used alone, parasitologi- long half-life drug like mefloquine) has reportedly cal resistance can develop rapidly. When used in been responsible for inhibiting intensification of combination, they produce a synergistic effect on drug resistance and for decreased malaria transmis- the parasite and can be effective even in the pres- sion levels in South-East Asia (32, 33) (see section ence of resistance to the individual components. 6.1.3). Typical combinations include sulfadoxine/ pyrimethamine (SP or Fansidar1), sulfalene- pyrimethamine (metakelfin), and sulfametho- 2.4.5 Miscellaneous compounds (not exhaustive) xazole-trimethoprim (co-trimoxazole). Halofantrine is a phenanthrene-methanol com- A new antifolate combination drug is currently pound with activity against the erythrocytic stages being tested in Africa. This drug, a combination of of the malaria parasite. Its use has been especially chlorproguanil and dapsone, also known as Lap- recommended in areas with multiple drug-resist- Dap, has a much more potent synergistic effect on ant falciparum. Recent studies have indicated, how- malaria than existing drugs such as SP. Benefits of ever, that the drug can produce potentially fatal this combination include 1) a greater cure rate, even cardiac conduction abnormalities (specifically, in areas currently experiencing some level of SP prolongation of the PR and QT interval), limiting resistance, 2) a lower likelihood of resistance devel- its usefulness (34). Atovaquone is a hydroxynaptho- oping because of a more advantageous pharmaco- quinone that is currently being used most widely kinetic and pharmacodynamic profile, and 3) for the treatment of opportunistic infections in probable low cost (currently estimated at less than immunosuppressed patients. It is effective against US$ 1 per adult treatment course) (29). chloroquine-resistant P. falciparum, but because, when used alone, resistance develops rapidly, 2.4.3 Antibiotics atovaquone is usually given in combination with proguanil (35, 36). A new fixed dose antimalarial Tetracycline and derivatives such as doxycycline are combination of 250 mg atovaquone and 100 mg very potent antimalarials and are used for both treat- proguanil (MalaroneTM) is being brought to ment and prophylaxis. In areas where response to market worldwide and is additionally being distrib- quinine has deteriorated, tetracyclines are often used uted through a donation programme (37, 38). Two in combination with quinine to improve cure rates. drugs originally synthesized in China are currently Clindamycin has been used but offers only limited undergoing field trials. Pyronaridine was report- advantage when compared to other available anti- edly 100% effective in one trial in Cameroon (39); malarial drugs. Parasitological response is slow to however, it was only between 63% and 88% effec- clindamycin and recrudescence rates are high (30, tive in Thailand (40). Lumefantrinel, a fluoro- 31). Its efficacy among non-immune individuals methanol compound, is being produced as a fixed has not been fully established. combination tablet with artemether (41, 42).

1 Note: Use of trade names is for identification only and does not imply endorsement by the Public Health Service or by the US Department of Health and Human Services.

9 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

2.4.6 Combination therapy with antimalarials translate into prolonged useful life span for either The use of two antimalarials simultaneously, drug. especially when the antimalarials have different Another combination therapy approach, com- mechanisms of action, has the potential for inhib- bining an artemisinin derivative with other, longer iting the development of resistance to either of the half-life antimalarials, is discussed in section components. The efficacy of a combination of a 6.1.3. 4-aminoquinoline drug (either chloroquine or amo- diaquine) with sulfadoxine/pyrimethamine has 2.5 Current status of drug-resistant been reviewed (43). The results of this review sug- malaria gested that the addition of either chloroquine or Resistance to antimalarial drugs has been described amodiaquine to SP marginally improved for two of the four species of malaria parasite that parasitological clearance (compared with SP alone). naturally infect humans, P. falciparum and P. vivax. The studies reviewed were mostly done in areas and P. falciparum has developed resistance to nearly all at times when both SP and chloroquine/amodi- antimalarials in current use, although the geo- aquine retained a fair amount of efficacy, and it is graphical distribution of resistance to any single not clear from these studies how well such a antimalarial drug varies greatly (Table 3). P. v i v a x combination would act in areas where one of the infection acquired in some areas has been shown components was significantly compromised. to be resistant to chloroquine and/or primaquine Additionally, to date, there are no data to suggest (44, 45). whether this slightly improved clearance would Chloroquine-resistant P. falciparum malaria has

TABLE 3. DISTRIBUTION OF DRUG-RESISTANT PLASMODIUM FALCIPARUM MALARIA

Region Resistance reported1 Comments

CQ SP MQ Others PLASMODIUM FALCIPARUM INFECTIONS Central America (Mexico, N N N North-west of Panama Canal only Belize, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, NW Panama)

Caribbean (Haiti and N N N Dominican Republic only)

South America (SE Panama, YYY QN Resistance to MQ and QN, although reported, is considered to occur infrequently Columbia, Venezuela, Ecuador, Peru, Brazil, Bolivia, )

Western Africa Y Y Y Incidence of resistance to CQ variable, but very common in most areas

Eastern Africa Y Y N Incidence o f resistance to SP highly variable, with some reports of focally high incidence, but generally uncommon

Southern Africa Y Y N Resistance to SP, although reported, is considered to be generally uncommon

Indian Subcontinent Y NN

South-East Asia and Oceania YYYHAL, Border areas of Thailand, Cambodia, and Myanmar highest risk for multiple-drug-resistant QN infections; in other areas, incidence of resistance to SP and MQ highly variable and absent in many areas

East Asia (China) YY? Resistance greatest problem in southern China

1 Reports of resistance to a given agent occurring in an area does not necessarily mean that occurrence is frequent enough to pose a significant public health risk. Bold “Y” indicates agents to which significant resistance occurs in a given area (although actual risk may be highly focal, e.g. South-East Asia, where MQ resistance, while very frequent in some limited areas, is infrequent or absent in most others). Regular “Y” indicates that, while resistance to agent has been reported, it is not believed to occur frequently enough to pose a significant public health risk.

CQ = chloroquine QN = quinine SP = sulfadoxine-pyrimethamine HAL = halofantrine MQ = mefloquine

10 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

been described everywhere that P. falciparum is increasingly being relied upon as a replacement malaria is transmitted except for malarious areas of for chloroquine. Mefloquine resistance is frequent Central America (north-west of the Panama in some areas of South-East Asia and has been Canal), the island of Hispaniola, and limited areas reported in the Amazon region of South America of the Middle East and Central Asia. Sulfadoxine- and sporadically in Africa (46). Cross-resistance pyrimethamine (SP) resistance occurs frequently in between halofantrine and mefloquine is suggested South-East Asia and South America. SP resistance by reduced response to halofantrine when used to is becoming more prevalent in Africa as that drug treat mefloquine failures (47).

11 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

3. Causes of resistance

3.1 Definition of antimalarial drug incorrect dosing, non-compliance with duration of resistance dosing regimen, poor drug quality, drug interac- Antimalarial drug resistance has been defined as the tions, poor or erratic absorption, and misdiagnosis. “ability of a parasite strain to survive and/or multi- Probably all of these factors, while causing treat- ply despite the administration and absorption of a ment failure (or apparent treatment failure) in the drug given in doses equal to or higher than those individual, may also contribute to the development usually recommended but within tolerance of the and intensification of true drug resistance through subject”. This definition was later modified to increasing the likelihood of exposure of parasites specify that the drug in question must “gain access to suboptimal drug levels. to the parasite or the infected red blood cell for the duration of the time necessary for its normal action” 3.3 Mechanisms of antimalarial resistance (48). Most researchers interpret this as referring only In general, resistance appears to occur through to persistence of parasites after treatment doses of spontaneous mutations that confer reduced sensi- an antimalarial rather than prophylaxis failure, al- tivity to a given drug or class of drugs. For some though the latter is a useful tool for early warning drugs, only a single point mutation is required to of the presence of drug resistance (49). confer resistance, while for other drugs, multiple This definition of resistance requires demonstra- mutations appear to be required. Provided the tion of malaria parasitaemia in a patient who has mutations are not deleterious to the survival or received an observed treatment dose of an antima- reproduction of the parasite, drug pressure will larial drug and simultaneous demonstration of remove susceptible parasites while resistant para- adequate blood drug and metabolite concentrations sites survive. Single malaria isolates have been found using established laboratory methods (such as high to be made up of heterogeneous populations of performance liquid chromatography) or in vitro parasites that can have widely varying drug response tests (see section 4.2). In practice, this is rarely done characteristics, from highly resistant to completely with in vivo studies. In vivo studies of drugs for sensitive (51). Similarly, within a geographical area, which true resistance is well known (such as chlo- malaria infections demonstrate a range of drug sus- roquine) infrequently include confirmation of drug ceptibility. Over time, resistance becomes estab- absorption and metabolism; demonstration of per- lished in the population and can be very stable, sistence of parasites in a patient receiving directly persisting long after specific drug pressure is re- observed therapy is usually considered sufficient. moved. Some drugs, such as mefloquine, are known to pro- The biochemical mechanism of resistance has duce widely varying blood levels after appropriate been well described for chloroquine, the antifolate dosing and apparent resistance can often be ex- combination drugs, and atovaquone. plained by inadequate blood levels (50). 3.3.1 Chloroquine resistance 3.2 Malaria treatment failure As the malaria parasite digests haemoglobin, large A distinction must be made between a failure to amounts of a toxic by-product are formed. The clear malarial parasitaemia or resolve clinical dis- parasite polymerizes this by-product in its food ease following a treatment with an antimalarial drug vacuole, producing non-toxic haemozoin (malaria and true antimalarial drug resistance. While drug pigment). It is believed that resistance of resistance can cause treatment failure, not all treat- P. falciparum to chloroquine is related to an in- ment failure is due to drug resistance. Many fac- creased capacity for the parasite to expel chloro- tors can contribute to treatment failure including quine at a rate that does not allow chloroquine to

12 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

reach levels required for inhibition of haem polym- 3.4.1 Biological influences on resistance erization (52). This chloroquine efflux occurs at a Based on data on the response of sensitive parasites rate of 40 to 50 times faster among resistant para- to antimalarial drugs in vitro and the pharma- sites than sensitive ones (53). Further evidence sup- cokinetic profiles of common antimalarial drugs, porting this mechanism is provided by the fact that there is thought to always be a residuum of para- chloroquine resistance can be reversed by drugs sites that are able to survive treatment (57). Under which interfere with this efflux system (54). It is normal circumstances, these parasites are removed unclear whether parasite resistance to other quino- by the immune system (non-specifically in the case line antimalarials (amodiaquine, mefloquine, of non-immune individuals). Factors that decrease halofantrine, and quinine) occurs via similar mecha- the effectiveness of the immune system in clearing nisms (52). parasite residuum after treatment also appear to increase survivorship of parasites and facilitate de- 3.3.2 Antifolate combination drugs velopment and intensification of resistance. This mechanism has been suggested as a significant con- Antifolate combination drugs, such as sulfadoxine tributor to resistance in South-East Asia, where + pyrimethamine, act through sequential and parasites are repeatedly cycled through populations synergistic blockade of 2 key enzymes involved with of non-immune individuals (58, 59); the non- folate synthesis. Pyrimethamine and related com- specific immune response of non-immune individu- pounds inhibit the step mediated by dihydrofolate als is less effective at clearing parasite residuum than reductase (DHFR) while sulfones and sulfonamides the specific immune response of semi-immune inhibit the step mediated by dihydropteroate individuals (60). The same mechanism may also synthase (DHPS) (48). Specific gene mutations explain poorer treatment response among young encoding for resistance to both DHPS and DHFR children and pregnant women (60). have been identified. Specific combinations of these The contribution to development and intensi- mutations have been associated with varying de- fication of resistance of other prevalent immuno- grees of resistance to antifolate combination drugs suppressive states has not been evaluated. Among (55). refugee children in the former Zaire, those who were malnourished (low weight for height) had signifi- 3.3.3 Atovaquone cantly poorer parasitological response to both Atovaquone acts through inhibition of electron chloroquine and SP treatment (61). Similarly, evi- transport at the cytochrome bc1 complex (56). dence from prevention of malaria during pregnancy Although resistance to atovaquone develops very suggests that parasitological response to treatment rapidly when used alone, when combined with a among individuals infected with the human second drug, such as proguanil (the combination immunodeficiency virus (HIV) may also be poor. used in MalaroneTM) or tetracycline, resistance de- HIV-seropositive women require more frequent velops more slowly (35). Resistance is conferred by treatment with SP during pregnancy in order to single-point mutations in the cytochrome-b gene. have the same risk of placental malaria as is seen among HIV-seronegative women (62). Para- sitological response to treatment of acute malaria 3.4 Factors contributing to the spread among HIV-seropositive individuals has not been of resistance evaluated. The current prevalence of malnutrition Numerous factors contributing to the advent, among African children under 5 years has been es- spread, and intensification of drug resistance exist, timated to be 30% and an estimated 4 to 5 million although their relative contribution to resistance is children are expected to be infected with HIV at unknown. Factors that have been associated with the beginning of this new century (63). If it is antimalarial drug resistance include such disparate proven that malnutrition or HIV infection plays a issues as human behaviour (dealt with in detail else- significant role in facilitating the development or where), vector and parasite biology, pharmacoki- intensification of antimalarial drug resistance, the netics, and economics. As mentioned previously, prevalence of these illnesses could pose a tremen- conditions leading to malaria treatment failure may dous threat to existing and future antimalarial also contribute to the development of resistance. drugs. Some characteristics of recrudescent or drug- resistant infections appear to provide a survival

13 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

advantage or to facilitate the spread of resistance drug is not chemically related to drugs previously conferring genes in a population (32). In one study, experienced (72). The underlying mechanism of patients experiencing chloroquine treatment fail- this plasticity is currently unknown, but this ure had recrudescent infections that tended to be capacity may help explain the rapidity with which less severe or even asymptomatic (64). Schizont South-East Asian strains of falciparum develop maturation may also be more efficient among re- resistance to new antimalarial drugs. sistant parasites (65, 66). The choice of using a long half-life drug (SP, There is some evidence that certain combina- MQ) in preference to one with a short half-life (CQ, tions of drug-resistant parasites and vector species LapDap, QN) has the benefit of simpler, single- enhance transmission of drug resistance, while other dose regimens which can greatly improve compli- combinations inhibit transmission of resistant para- ance or make directly observed therapy feasible. sites. In South-East Asia, two important vectors, Unfortunately, that same property may increase the Anopheles stephensi and A. dirus, appear to be more likelihood of resistance developing due to prolonged susceptible to drug-resistant malaria than to drug- elimination periods. The relative contribution of sensitive malaria (67, 68). In Sri Lanka, research- low compliance versus use of long half-life drugs ers found that patients with chloroquine-resistant to development of resistance is not known. malaria infections were more likely to have Parasites from new infections or recrudescent gametocytaemia than those with sensitive infections parasites from infections that did not fully clear will and that the gametocytes from resistant infections be exposed to drug blood levels that are high enough were more infective to mosquitos (64). The reverse to exert selective pressure but are insufficient to is also true—some malaria vectors may be some- provide prophylactic or suppressive protection (57). what refractory to drug-resistant malaria, which When blood levels drop below the minimum in- may partially explain the pockets of chloroquine hibitory concentration (the level of drug that fully sensitivity that remain in the world in spite of very inhibits parasite growth), but remain above the EC5 similar human populations and drug pressure (e.g. (the concentration of drug that produces 5% inhi- Haiti). bition of parasite growth), selection of resistant Many antimalarial drugs in current usage are parasites occurs. This selection was illustrated in closely related chemically and development of one study in Kenya that monitored drug sensitiv- resistance to one can facilitate development of re- ity of parasites reappearing after SP treatment. Para- sistance to others. Chloroquine and amodiaquine sites reappearing during a period when blood levels are both 4-aminoquinolines and cross-resistance were below the point required to clear pyrimeth- between these two drugs is well known (69, 70). amine-resistant parasites, but still above that level Development of resistance to mefloquine may also required to clear pyrimethamine-sensitive parasites, lead to resistance to halofantrine and quinine. were more likely to be pyrimethmaine-resistant than Antifolate combination drugs have similar action those reappearing after levels had dropped below and widespread use of sulfadoxine/ pyrimethamine the level required to clear pyrimethamine-sensitive for the treatment of malaria may lead to increased parasites (73). This period of selective pressure lasts parasitological resistance to other antifolate com- for approximately one month for mefloquine, bination drugs (29). Development of high levels of whereas it is only 48 hours for quinine (57). SP resistance through continued accumulation of In areas of high malaria transmission, the prob- DHFR mutations may compromise the useful life ability of exposure of parasites to drug during this span of newer antifolate combination drugs such period of selective pressure is high. In Africa, for as chlorproguanil/dapsone (LapDap) even before instance, people can be exposed to as many as 300 they are brought into use. This increased risk of infective bites per year (in rare cases, even as much resistance due to SP use may even affect non- as 1000 infective bites per year), and during peak malarial pathogens; use of SP for treatment of transmission, as many as five infective bites per malaria increased resistance to trimethoprim/sufa- night (74, 75). methoxazole among respiratory pathogens (71). Mismatched pharmacokinetics can also play a There is an interesting theory that development role in facilitating the development of resistance. of resistance to a number of antimalarial drugs The elimination half-life of pyrimethamine is be- among some falciparum parasites produces a level tween 80 and 100 hours, and is between 100 and of genetic plasticity that allows the parasite to 200 hours for sulfadoxine, leaving an extended rapidly adapt to a new drug, even when the new period when sulfadoxine is “unprotected” by

14 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

synergy with pyrimethamine (73). This sort of mis- gramme recommended 2 tablets (adult dose) of SP matched pharmacokinetics is even more apparent for treating malaria based on studies suggesting that in the mefloquine-sulfadoxine-pyrimethamine this was effective. Within a few years, this was no (MSP) combination used in Thailand (mefloquine longer effective and the programme increased the has an elimination half-life of approximately 336 regimen to 3 tablets. Although unproven, this may to 432 hours (48, 76). have contributed to the rapid loss of SP efficacy It is not clear what the relationship between there. Similar confusion over the proper SP dosing transmission intensity and development of resist- regimen exists in Africa. To simplify treatment, ance is, although most researchers agree that there many programmes dose children based on age seems to be such an association. It is apparent that rather than weight and, depending on the regimen there are more genetically distinct clones per per- being recommended, this has been shown to pro- son in areas of more intense transmission than in duce systematic underdosing among children of areas of lower transmission (77). However, the in- certain weight and age groups (ter Kuile, unpub- terpretation of this and its implications for devel- lished data, 1997). opment of resistance has variously been described The use of presumptive treatment for malaria as resistance being more likely in low-transmission has the potential for facilitating resistance by greatly environments (78, 79), high-transmission environ- increasing the number of people who are treated ments (80, 77, 81), or either low- or high- but not unnecessarily but will still be exerting selective pres- intermediate-transmission environments (82, 83). sure on the circulating parasite population. In some This relationship between transmission intensity areas and at some times of the year, the number of and parasite genetic structure is obviously complex patients being treated unnecessarily for malaria can and subject to other confounding/contributing be very large (15). factors (84, 83). What is clear is that the rate at Concurrent treatment with other drugs can in- which resistance develops in a given area is sensi- crease the likelihood of treatment failure and may tive to a number of factors beyond mere intensity contribute to development of drug resistance. Folate of transmission (such as initial prevalence of muta- administration for treatment of anaemia and pos- tions, intensity of drug pressure, population sibly when used as a routine supplement during movement between areas, the nature of acquired pregnancy (CDC, unpublished data,1998), can immunity to the parasite or its strains, etc.), but increase treatment failure rates (89). Similarly, that reducing the intensity of transmission will concurrent illness may have an influence, as was likely facilitate prolonging the useful life span of mentioned previously with regard to malnourish- drugs (81, 85). ment. Drug quality has also been implicated in in- effective treatment and possibly drug resistance. 3.4.2 Programmatic influences on resistance Either through poor manufacturing practices, in- Programmatic influences on development of anti- tentional counterfeiting, or deterioration due to malarial drug resistance include overall drug pres- inadequate handling and storage, drugs may not sure, inadequate drug intake (poor compliance or contain sufficient quantities of the active ingredi- inappropriate dosing regimens), pharmacokinetic ents. In an analysis of chloroquine and antibiotics and pharmacodynamic properties of the drug or available in Nigeria and Thailand, between 37% drug combination, and drug interactions (86). and 40% of samples assayed had substandard Additionally, reliance on presumptive treatment can content of active ingredients, mostly from poor facilitate the development of antimalarial drug manufacturing practices (90). Another study in resistance. Africa found chloroquine stored under realistic Overall drug pressure, especially that exerted by tropical conditions lost at least 10% of its activity programmes utilizing mass drug administration, in a little over a year (91). probably has the greatest impact on development of resistance (86, 87). Studies have suggested that resistance rates are higher in urban and periurban areas than rural communities, where access to and use of drug is greater (88). Confusion over proper dosing regimen has been described. In Thailand, the malaria control pro-

15 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

4. Detection of resistance

In general, four basic methods have been routinely Of the available tests, in vivo tests most closely used to study or measure antimalarial drug resist- reflect actual clinical or epidemiological situations ance: in vivo, in vitro, animal model studies, and (i.e. the therapeutic response of currently circulat- molecular characterization. Additionally, less rig- ing parasites infecting the actual population in orous methods have been used, such as case reports, which the drug will be used). Because of the influ- case series, or passive surveillance. Much discus- ence of external factors (host immunity, variations sion has occurred regarding the relative merits of of drug absorption and metabolism, and potential one test over another, with the implication always misclassification of reinfections as recrudescences), being that one type of test should be used prefer- the results of in vivo tests do not necessarily reflect entially. Careful consideration of the types of the true level of pure antimalarial drug resistance. information each yields should indicate, however, However, this test offers the best information on that these are complementary, rather than compet- the efficacy of antimalarial treatment under close ing, sources of information about resistance. to actual operational conditions—what can be ex- Recognition of drug resistance (or, more appro- pected to occur among clinic patients if provider priately, treatment failure) in individual patients is and patient compliance is high. made difficult in many settings by operational The original methods for in vivo tests required issues, such as availability and quality of microscopy. prolonged periods of follow-up (minimum of 28 Especially in Africa, where presumptive diagnosis days) and seclusion of patients in screened rooms and treatment for malaria is the rule, detection of to prevent the possibility of reinfection. These treatment failures also tends to be presumptive (per- methods have since been modified extensively and sistence or reappearance of clinical symptoms in a the most widely used methods now involve shorter patient recently receiving malaria treatment). periods of follow-up (7 to 14 days) without seclu- Because of the non-specific nature of clinical signs sion, under the assumption that reappearance of and symptoms of malaria and the many other causes parasites within 14 days of treatment is more likely of febrile disease, this can lead to a false sense that due to recrudescence than reinfection (92). Addi- a particular drug is not working when it is, or, tional modifications reflect the increased emphasis potentially, that an ineffective drug is working when on clinical response in addition to parasitological it is not. In cases where microscopy is used, pres- response. Traditionally, response to treatment was ence of parasitaemia in a supposedly fully treated categorized purely on parasitological grounds as patient may indicate treatment failure, but is not Sensitive, RI, RII, and RIII (48). Later modifica- necessarily evidence of drug resistance, as explained tions have combined, to varying extent, para- in section 3.1. sitological and clinical indicators (93). Because anaemia can be a major component of 4.1 In vivo tests malaria illness, in vivo methodologies allow inves- An in vivo test consists of the treatment of a group tigation of haematological recovery after malaria of symptomatic and parasitaemic individuals with therapy (94). This is obviously not possible with in known doses of drug and the subsequent monitor- vitro or molecular techniques. Failure of complete ing of the parasitological and/or clinical response parasitological clearance, even in situations where over time. One of the key characteristics of in vivo recurrence of fever is rare, can be associated with tests is the interplay between host and parasite. lack of optimal haematological recovery among Diminished therapeutic efficacy of a drug can be anaemic patients. masked by immune clearance of parasites among Unfortunately, these methodologies, while patients with a high degree of acquired immunity termed “standardized” are, in practice, not stand- (60). ardized. Major differences in sample size, enrolment

16 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

criteria, exclusion criteria, length and intensity of ferential die-off of parasites can occur. If, for in- follow-up, loss-to-follow-up rates, and interpreta- stance, resistant strains are less likely to adapt, the tion and reporting of results are apparent in pub- results would be biased towards sensitive responses. lished papers on in vivo trials. These differences are Because venous blood is typically needed, resist- at times so dramatic, that it is difficult, if not im- ance in the more vulnerable younger age groups is possible, to compare results from one study to often not studied. Finally, these tests are techno- another with any level of confidence (CDC, un- logically more demanding and relatively expensive, published data, 1999). which makes them potentially more difficult to The methodology currently being used and adapt successfully to routine work in the field. promoted, especially in sub-Saharan Africa, is a system that emphasizes clinical response over 4.3 Animal model studies parasitological response (95). Close adherence to this protocol does provide comparable data; how- This type of test is, in essence, an in vivo test con- ever, these data are not readily comparable to data ducted in a non-human animal model and, there- collected using other in vivo methods. Although fore, is influenced by many of the same extrinsic not called for in the protocol, categorization of the factors as in vivo tests. The influence of host im- parasitological response using the standard WHO munity is minimized by using lab-reared animals definitions (95) would allow some ability to com- or animal-parasite combinations unlikely to occur in nature, although other host factors would still pare to historical levels and provide useful para- be present. These tests allow for the testing of para- sitological results that would aid in interpreting the sites which cannot be adapted to in vitro environ- clinical results. ments (provided a suitable animal host is available) and the testing of experimental drugs not yet 4.2 In vitro tests approved for use in humans. A significant disad- From the point of view of a researcher interested in vantage is that only parasites that can grow in, or pure drug resistance, in vitro tests avoid many of are adaptable to, non-human primates can be the confounding factors which influence in vivo investigated. tests by removing parasites from the host and placing them into a controlled experimental envi- 4.4 Molecular techniques ronment. In the most frequently used procedure, These tests are in the process of being developed the micro-technique, parasites obtained from a and validated, but offer promising advantages to finger-prick blood sample are exposed in microtitre the methods described above. Molecular tests use plates to precisely known quantities of drug and polymerase chain reaction (PCR) to indicate the observed for inhibition of maturation into schizonts presence of mutations encoding biological resist- (96). ance to antimalarial drugs (98). Theoretically, the This test more accurately reflects “pure” anti- frequency of occurrence of specific gene mutations malarial drug resistance. Multiple tests can be per- within a sample of parasites obtained from patients formed on isolates, several drugs can be assessed from a given area could provide an indication of simultaneously, and experimental drugs can be the frequency of drug resistance in that area analo- tested. However, the test has certain significant dis- gous to information derived from in vitro meth- advantages. The correlation of in vitro response with ods. Advantages include the need for only small clinical response in patients is neither clear nor con- amounts of genetic material as opposed to live para- sistent, and the correlation appears to depend on sites, independence from host and environmental the level of acquired immunity within the popula- factors, and the ability to conduct large numbers tion being tested. Prodrugs, such as proguanil, of tests in a relatively short period of time. Disad- which require host conversion into active meta- vantages include the obvious need for sophisticated bolites cannot be tested. Neither can drugs that equipment and training, and the fact that gene require some level of synergism with the host’s im- mutations that confer antimalarial drug resistance mune system. Although adaptation of erythrocytic are currently known or debated for only a limited forms of P. v i v a x to continuous culture has been number of drugs (primarily for dihydrofolate achieved, non-falciparum erythrocytic parasites reductase inhibitors [pyrimethamine], dihydro- generally cannot be evaluated in vitro (97). In pteroate synthase inhibitors [sulfadoxine], and addition, because parasites must be cultured, dif- chloroquine) (98, 99). Confirmation of the asso-

17 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

ciation between given mutations and actual drug help formulate recommendations for chemo- resistance is difficult, especially when resistance prophylaxis of non-immune travellers to endemic involves more than one gene locus and multiple areas (100). mutations. If these complexities can be resolved, Another method that has been used is passive molecular techniques may become an extremely detection of treatment failure. In this system, valuable surveillance tool for monitoring the patients are treated following usual treatment guide- occurrence, spread, or intensification of drug re- lines and told to come back to the clinic or hospi- sistance. tal if symptoms persist or return. Those cases which do return are considered to represent the popula- 4.5 Case reports and passive detection tion of treatment failures. Because this system does of treatment failure not ensure compliance with treatment regimens through directly observed therapy and does not Additional methods for identifying or monitoring attempt to locate and determine the outcome of antimalarial drug resistance include the use of case patients who do not return on their own, data are reports or case series of spontaneously reported seriously biased. In one study conducted in Ethio- treatment failure. In general, these methods require pia and Eritrea using this method, only 1706/39 far less investment in time, money, and personnel 824 (4.6%) patients returned to clinic (101). The and can be done on an ongoing basis by individual assumption was that those patients who did not health care centres. They suffer, however, from pre- return did not have resistant parasites, yielding a senting a potentially biased view of drug resistance very low prevalence of resistance (1.8% to 4.8%, primarily because denominators are typically un- depending on region). These results contrast known and rates of resistance cannot be calculated. dramatically with results from standard 7-day in Nonetheless, case reports can be useful and may vivo trials conducted at two sites in Eritrea in 1994 indicate a problem that should be confirmed using (CDC, unpublished data,1994) and one site in one of the other methods. In the United States, for Ethiopia in 1993–1994 which found between 58% instance, case reports, especially when occurring in and 86% RII/RIII level resistance (102). clusters, of prophylaxis failure have been used to

18 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

5. Treatment

In theory, recommended treatment regimens should Multidrug resistance (typically referring to re- be tailored specifically to a given region based on sistance to both chloroquine and SP, but may also resistance patterns found in that area. Other con- include resistance to other compounds as well) siderations include cost, cost-effectiveness, availabil- occurs frequently in Amazonia and South-East Asia. ity, ease of administration, capabilities of the health In these areas, a wide range of treatment options care infrastructure (i.e. do health care workers have are used. Quinine, either alone or in combination the equipment and training to safely use parenteral with tetracyclines, or mefloquine tend to be the routes of administration?), perceived efficacy, and drugs of choice for multidrug-resistant malaria, real and perceived safety of the drug (acceptability although declining quinine efficacy and high rates of the drug by the population). In practice, cur- of mefloquine resistance have been reported in some rently recommended treatment regimens often do areas of South-East Asia. In limited areas of Thai- not reflect the current state of antimalarial drug land, where falciparum is resistant to many of the resistance. available drugs, a combination of high-dose Chloroquine remains the drug of choice for mefloquine (25 mg/kg in a divided dose) and treatment of non-falciparum infections and non- artesunate (4 mg/kg daily for 3 days) or 7 days of severe falciparum infections acquired in areas of artesunate alone is required to achieve reliable clear- known chloroquine sensitivity. Because drug resist- ance of parasites. ance is not an all-or-nothing phenomenon, chlo- Table 2 summarizes various treatment options, roquine still retains adequate efficacy even in areas not all of which would be available or necessarily of known resistance for continued use to be justifi- appropriate in all contexts. One of the primary lim- able for the time being (for instance, some areas of iting factors to the use of a highly effective antima- West Africa continue to use chloroquine success- larial and a willingness to change policy to facilitate fully, although efficacy rates are declining). Much its use, is the cost of the drug itself. Although a of Africa, however, is currently investigating alter- number of evaluations have been able to show the natives to chloroquine. Malawi, Kenya, South cost-effectiveness of changing between certain Africa, and Botswana have moved away from chlo- drugs, in many cases, the total cost associated with roquine and are using sulfadoxine/ pyrimethamine use of a given drug may be prohibitively high (103). (SP) extensively or exclusively for treatment of non- Additional costs of interventions to improve use of severe falciparum infections. The United Republic drugs or patients’ adherence to treatment regimens of Tanzania recently indicated that it is moving (such a provider and user training, innovative pack- towards SP as first-line treatment of malaria, and aging) would further add to the total cost of using Ethiopia, Eritrea, Uganda, and others are in the some drugs or drug combinations. process of considering similar policy changes.

19 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

6. The future: prevention of drug resistance

The future of antimalarial drug resistance and household level (104). This approach is gaining efforts to combat it is defined by a number of support internationally. This approach is also in assumptions. First, antimalarial drugs will continue direct conflict with the primary methods for in- to be needed long into the future. No strategy in hibiting development of drug resistance, limited existence or in development, short of an unfore- access to and judicious use of chemotherapeutic seen scientific breakthrough or complete eradica- agents. Clearly, some middle ground will need to tion, is likely to be 100% effective in preventing be identified that will improve access to antimalar- malaria infection. Second, as long as drugs are used, ial drugs for those who need to be treated while at the chance of resistance developing to those drugs the same time reducing the inappropriate use of is present. P. falciparum has developed resistance to those same drugs. nearly all available antimalarial drugs and it is highly Prevention strategies can be divided into those likely that the parasite will eventually develop aimed specifically at preventing malaria infection resistance to any drug that is used widely. The and those aimed at reducing the likelihood of de- advent of P. v i v a x resistant to chloroquine and pri- velopment of drug resistance. Reduction of overall maquine may, in time, result in a resurgence of vivax malaria infection rates or transmission rates have malaria as has been seen with P. falciparum. Third, an indirect impact on development of drug resist- development of new drugs appears to be taking ance by reducing the number of infections need- longer than development of parasitological resist- ing to be treated (and therefore, overall drug ance. The development of resistance to antimalar- pressure) and by reducing the likelihood that re- ial drugs in South-East Asia has been far quicker sistant parasites are successfully transmitted to new than the estimated 12 to 17 years it takes to de- hosts. Full discussion of these strategies is beyond velop and market a new antimalarial compound the scope of this review but they include the use of (2). Fourth, affordability is an essential considera- insecticide-treated bednets, indoor residual insec- tion for any strategy to control drug-resistant ticide spraying, environmental control (mosquito malaria, especially in Africa. breeding site or “source” reduction), other personal The future, especially in Africa, will also be de- protection measures (e.g. use of repellent soap or fined by how well the central tenets of malaria con- screening windows) and chemoprophylaxis in trol can be reconciled with the central tenets of defined populations (use of mass prophylaxis is typi- control of drug resistance. One of the cornerstones cally not recommended). An effective and deliver- of the current approach to malaria control is the able vaccine would also be greatly beneficial. provision of prompt, effective malaria treatment. In much of Africa, easy access to public sector health 6.1 Preventing drug resistance care is limited and when it is accessible, health care staff are often inadequately trained, insufficiently Interventions aimed at preventing drug resistance, supplied and supported, ineffectively supervised per se, generally focus on reducing overall drug and/or poorly motivated. One response to this situ- pressure through more selective use of drugs; im- ation has been the intentional liberalization of proving the way drugs are used through improving access to drugs; instead of relying so heavily on the prescribing, follow-up practices, and patient com- formal public sector to distribute antimalarial drugs, pliance; or using drugs or drug combinations which some people are suggesting that the best way to are inherently less likely to foster resistance or have reduce the time between onset of illness and first properties that do not facilitate development or treatment with an antimalarial drug is by making spread of resistant parasites. these drugs widely available on the open market, from unofficial sources of health care, and at the

20 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

6.1.1 Reducing overall drug pressure. ture to locate patients in the community or a com- Because overall drug pressure is thought to be the munity willing to return on a given date, regardless single most important factor in development of re- of whether they feel ill or not. With this system, sistance, following more restrictive drug use and patients who fail initial treatment, for whatever prescribing practices would be helpful, if not reason, are identified quickly and re-treated until essential, for limiting the advent, spread, and in- parasitologically cured, decreasing the potential for tensification of drug resistance. This approach has spread of resistant parasites (107). gained support in North America and Europe for fighting antibacterial drug resistance (105, 106). 6.1.3 Combination therapy The greatest decrease in antimalarial drug use A strategy that has received much attention recently could be achieved through improving the diagno- is the combination of antimalarial drugs, such as sis of malaria. Although programs such as IMCI mefloquine, SP, or amodiaquine, with an aim to improve clinical diagnosis through well- artemisinin derivative (108). Artemisinin drugs are designed clinical algorithms, a large number of highly efficacious, rapidly active, and have action patients will continue to receive unnecessary anti- against a broader range of parasite developmental malarial therapy, especially in areas of relatively low stages. This action apparently yields two notable malaria risk (18). Basing treatment on the results results. First, artemisinin compounds, used in com- of a diagnostic test, such as microscopy or a rapid bination with a longer acting antimalarial, can rap- antigen test, however, would result in the greatest idly reduce parasite densities to very low levels at a reduction of unnecessary malaria treatments and time when drug levels of the longer acting antima- decrease the probability that parasites are exposed larial drug are still maximal. This greatly reduces to subtherapeutic blood levels of drug. both the likelihood of parasites surviving initial There are notable exceptions to this, however. treatment and the likelihood that parasites will be Presumptive therapy with SP during pregnancy has exposed to suboptimal levels of the longer acting been shown to be an operationally sustainable in- drug (32). Second, the use of artemisinins has been tervention that offers significant protection from shown to reduce gametocytogenesis by 8- to low birth weight associated with placental malaria 18-fold (33). This reduces the likelihood that (62). There may be a role for presumptive treat- gametocytes carrying resistance genes are passed ment or even mass drug administration in response onwards and potentially may reduce malaria trans- to an epidemic, although its cost-effectiveness has mission rates. Use of combination therapy has been not been proven. Prophylaxis programmes, how- linked to slowing of the development of mefloquine ever, should be used only among populations where resistance and reductions in overall malaria trans- compliance is likely to be high and where a highly mission rates in some parts of Thailand and has effective prophylactic drug can be used. been recommended for widespread use in sub- Saharan Africa (108). This interpretation and the 6.1.2 Improving the way drugs are used recommendation for rapid adoption of combina- Other disease control programmes, such as for TB, tion therapy in Africa, however, has been questioned STDs, and HIV, have begun to rely heavily on (109, 110). directly observed therapy (DOT) as a way to en- It should be noted that this argument contra- sure a high degree of compliance. While this has dicts a previously mentioned argument in that it not yet received serious consideration for malaria, promotes the use of a drug combination with the use of drugs with single-dose regimens (SP, grossly mismatched half-lives. Theoretically, in mefloquine) could potentially make DOT possi- areas where malaria transmission rates are quite low, ble. The benefits of using single-dose DOT need such as where this strategy has been most inten- to be weighed against the costs of using drugs with sively studied in Thailand, this is of minimal con- long half-lives. cern (i.e. the likelihood of being bitten by an Another approach that has not been widely infective mosquito during the period when drug adopted is the close follow-up and re-treatment, if levels are suboptimal is very low). In areas where necessary, of patients. The success of this approach transmission rates are very high (for example, is dependant on availability of reliable microscopy Africa where inoculation rates can be as high as five (to diagnose the illness initially as well as to con- infective bites per night), this likelihood is very high. firm treatment failure), and either an infrastruc- The relative contribution to development of resist-

21 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

ance of parasites surviving initial malaria treatment In the future, antimalarial therapy may be ex- compared with new parasites being exposed to sub- panded by combining chemotherapy with vaccines optimal drug levels is unknown. (or other drugs) specifically designed to inhibit As second concern about combination therapy transmission of malaria. These “transmission-block- is the extent to which the components might be ing” vaccines or drugs could reduce the potential used for monotherapy outside official health serv- for onward transmission of gametocytes carrying ices. Already, artemisinin compounds are available resistance genes, even if a relatively large number in the pharmacies and markets of Africa. As supply of parasites survive initial treatment. This could increases and the price drops, these drugs will be work through using drugs or vaccines with a high used increasingly for the treatment of fever and, degree of specific antigametocytocidal activity (such because of the rapidity of action, they may in fact as primaquine and related drugs), drugs that non- become the community’s drug of choice. It is un- specifically reduce the likelihood of gametocytes likely, in this scenario, that they would be used in developing (such as appears to be the case with the combination with another drug, whether SP or artemisinins), or drugs or vaccines that interfere mefloquine. Similarly, in Africa, SP is both widely with sexual reproduction and infection of the para- available and inexpensive and may continue to be site within the mosquitos when taken up with a used alone. Any benefits of combination therapy blood meal (although short acting, the combina- in preventing development or intensification of tion of atovaquone and proguanil has this type of resistance may be lost due to unofficial and incor- activity). rect use of the component drugs outside of official health services.

22 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

7. Conclusions and recommendations

Because of the realities of health care infrastructure face of high levels of self-treatment and un- and the influence of the private sector, approaches official use of component drugs (or related to malaria therapy, especially in sub-Saharan compounds) as monotherapy and in various Africa, will probably favour increased access to epidemiological contexts (especially high- drugs (and, therefore, loss of control over how they transmission areas). are used) over restricted access (and, therefore, more 3. Investigate how a combination therapy strat- control over how they are used). If this proves to egy could be financed. This strategy, if proven be true, while only minor advances against anti- cost-effective, will nonetheless be more ex- malarial drug resistance can be expected, short-term pensive than current strategies. What mecha- reductions in malaria morbidity and mortality may nisms might be developed to assist countries be achieved. in adopting this strategy? Long-term success of this strategy, however, will depend on a continuous supply of new and afford- B. Invest significantly in identifying strategies to able drugs and on the development of effective and improve acceptance of and compliance with implementable control measures to reduce overall drug regimens, especially a combination therapy burden of disease. A more cautious approach would strategy, at all levels of official and unofficial be to avoid placing too much faith in future scien- health care systems, private sector, and com- tific advances and technology and to invest in meth- munity. Similarly, investigate to teach concepts ods to improve the way people and antimalarial of judicious use of antimicrobials (including drugs interact in an environment of essentially antimalarial drugs) to health care providers. uncontrolled use. The objective of this investment C. Investigate ways to improve effectiveness of drug would be to prolong the useful life span of drugs regulatory systems and ability to control intro- enough to increase the likelihood that new drugs duction of new antimalarials within endemic or other methods of malaria control will indeed be countries. This is required to avoid uncontrolled developed and implemented. use of new antimalarials resulting in develop- Significant advances against antimalarial drug ment of resistance before they are needed which resistance is probably unlikely without major could significantly compromise their efficacy change in health infrastructure leading to higher- when they are needed. quality services that are more readily available. D. Support new drug development. Investigate new approaches to drug delivery, such as time- 7.1 Priorities released formulations or novel delivery systems A. Investigate combination therapy: that would allow use of short half-life drugs while optimizing compliance. Investigate drugs 1. Fast-track a chlorproguanil/dapsone/ (or vaccines?) that have transmission-blocking artesunate fixed dose formulation. From a effect that could be used in combination with theoretical basis, this would offer the best drugs active against blood-stage parasites. combination of overall efficacy, synergy be- tween the antifolate-sulfa components, short E. Improve access to and use of definitive diagno- half-life, reasonably well-matched pharma- sis-based treatment. cokinetics, and probable cost. Because of F. Support more widespread use of insecticide- growing use of and resistance to SP, an treated materials or other appropriate vector urgency exists to field this promising agent. control strategies to reduce frequency of clini- 2. Investigate effectiveness of combination cal illness (and therefore, treatment) as well as therapy in terms of robustness of strategy in overall malaria transmission.

23 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

8. Bibliography

1. Foster SD. Pricing, distribution, and use of antima- sults in a significant risk of mistreatment of chil- larial drugs. Bulletin of the World Health Organiza- dren in urban Sahel. Transactions of the Royal Society tion 1991;69:349–363. of Tropical Medicine & Hygiene 1991;85:729–730. 2. Ridley RG. Plasmodium: Drug discovery and de- 16. Redd SC et al. Usefulness of clinical case-definitions velopment—an industrial perspective. Experimental in guiding therapy for African children with malaria Parasitology 1997;87:293–304. or pneumonia. Lancet 1992;340:1140–1143. 3. World Health Organization. World malaria situa- 17. Smith T et al. Attributable fraction estimates and tion in 1993, part I. Weekly Epidemiological Record case definitions for malaria in endemic areas. Statis- 1996;71:17–22. tics In Medicine 1994;13:2345–2358. 4. Nabarro DN, Talyer EM. The “Roll Back Malaria” 18. World Health Organization. Integrated Management campaign. Science 1998;280;2067–2068. of Childhood Illnesses Adaptation Guide. Part 2. C. Technical basis for adapting clinical guidelines, feed- 5. Nchinda TC. Malaria: A reemerging disease in ing recommendations, and local terms. Working Africa. Emerging Infectious Diseases 1998;4:398–403. Draft Version 3. Division of Child Health and 6. World Health Organization. MMV comes of age. Development, World Health Organization, 1997. TDR News 1999; No. 60:6. pp. 49–51. 7. Snow RW et al. Estimating mortality, morbidity and 19. Mackey LJ et al. Diagnosis of Plasmodium falciparum disability due to malaria among Africa’s non-preg- infection in man: detection of parasite antigens by nant population. Bulletin of the World Health ELISA. Bulletin of the World Health Organization Organization 1999;77:624–640. 1982;60:69–75. 8. Foster S, Phillips M. Economics and its contribu- 20. Fortier B et al. Enzyme immunoassay for detection tion to the fight against malaria. Annals of Tropical of antigen in acute Plasmodium falciparum malaria. Medicine & Parasitology 1998;92:391–398. European Journal of Clinical Microbiology 1987;6:596–598. 9. Knudsen AB, Slooff R. Vector-borne disease prob- lems in rapid urbanization: new approaches to vec- 21. Khusmith S et al. Two-site immunoradiometric tor control. Bulletin of the World Health Organization assay for detection of Plasmodium falciparum anti- 1992;70:1–6. gen in blood using monoclonal and polyclonal anti- bodies. Journal of Clinical Microbiology 1987; 10. Jonkman A et al. Cost-saving through microscopy- 25:1467–1471. based versus presumptive diagnosis of malaria in adult outpatients in Malawi. Bulletin of the World 22. Craig MH, Sharp BL. Comparative evaluation of Health Organization 1995;73:223–227. four techniques for the diagnosis of Plasmodium falciparum infections. Transactions of the Royal 11. Barat L et al. Does the availability of blood slide Society of Tropical Medicine and Hygiene 1997; microscopy for malaria at health centers improve the 91:279–282. management of persons with fever in Zambia? Ameri- can Journal of Tropical Medicine and Hygiene 23. World Health Organization. A rapid dipstick anti- 1999;60;1024–1030. gen capture assay for the diagnosis of falciparum malaria. WHO informal consultation on recent ad- 12. Levine RA, Wardlaw SC, Patton CL. Detection of vances in diagnostic techniques and vaccines for haematoparasites using Quantitative Buffy Coat malaria. Bulletin of the World Health Organization analysis tubes. Parasitology Today 1989;5:132–133. 1996;74:47–54. 13. Tharavanij S. New developments in malaria diag- 24. Makler MT, Palmer CJ, Ager AL. A review of practical nostic techniques. Southeast Asian Journal of Tropi- techniques for the diagnosis of malaria. Annals of Tropi- cal Medicine & Public Health 1990;21:3–16. cal Medicine & Parasitology 1998;92:419–433. 14. Rickman LS et al. Rapid diagnosis of malaria by acri- 25. Piper R et al. Immunocapture diagnostic assays for dine orange staining of centrifuged parasites. Lancet malaria using Plasmodium lactate dehydrogenase 1989;1:68–71. (pLDH). American Journal of Tropical Medicine and 15. Olivar M et al. Presumptive diagnosis of malaria re- Hygiene 1999;60:109–118.

24 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

26. Palmer CJ et al. Field evaluation of the OptiMAL7 42. van Vugt M et al. Efficacy of six doses of artemether- rapid malaria diagnostic test during anti-malarial lumefantrine (Benflumetol) in multidrug-resistant therapy in Guyana. Transactions of the Royal Society Plasmodium falciparum malaria. American Journal of of Tropical Medicine & Hygiene 1999;93:517–518. Tropical Medicine and Hygiene 1999; 60:936–942. 27. Beck HP. How does molecular epidemiology help 43. McIntosh HM, Greenwood BM. Chloroquine or to understand malaria? Tropical Medicine and Inter- amodiaquine combined with sulfadoxine-pyrimeth- national Health 1999;4:1–3. amine as a treatment for uncomplicated malaria—a systematic review. Annals of Tropical Medicine & 28. Freeman J et al. Effect of chemotherapy on malaria Parasitology 1998;93:265–270. transmission among Yanomami Amerindians: simu- lated consequences of placebo treatment. American 44. Murphy GS et al. Vivax malaria resistant to treat- Journal of Tropical Medicine and Hygiene ment and prophylaxis with chloroquine. Lancet 1999;60:774–780. 1993;341:96–100. 29. Watkins WM et al. The efficacy of antifolate anti- 45. Looareesuwan S et al. Primaquine-tolerant vivax malarial combinations in Africa: a predictive model malaria in Thailand. Annals of Tropical Medicine & based on pharmacodynamic and pharmacokinetic Parasitology 1997;91:939–943. analyses. Parasitology Today 1997;13:459–464. 46. Mockenhaupt FP. Mefloquine resistance in Plasmo- 30. Kremsner PG et al. Clindamycin treatment of dium falciparum. Parasitology Today 1995;11:248– falciparum malaria in Brazil. Journal of Antimicro- 253. bial Chemotherapy 1989;23:275–281. 47. ter Kuile FO et al. Halofantrine versus mefloquine 31. Kremsner PG et al. Clindamycin in combination in treatment of multidrug-resistant falciparum with chloroquine or quinine is an effective therapy malaria. Lancet 1993;341:1044–1049. for uncomplicated Plasmodium falciparum malaria 48. Bruce-Chwatt LJ et al. Chemotherapy of malaria, in children from Gabon. The Journal of Infectious Geneva, World Health Organization, 1986. Diseases 1994;169:467–470. 49. Lobel HO, Campbell CC. Malaria Prophylaxis and 32. White NJ et al. Averting a malaria disaster. Lancet distribution of drug resistance. Clinics in Tropical 1999;353:1965–1967. Medicine and Communicable Diseases 1986;1:225– 33. Price RN et al. Effects of artemisinin derivatives on 242. malaria transmissiblity. Lancet 1996;347:1654– 50. Slutsker LM et al. Mefloquine therapy for Plasmo- 1658. dium falciparum malaria in children under 5 years 34. Nosten F et al. Cardiac effects of antimalarial treat- of age in Malawi: in vivo/in vitro efficacy and corre- ment with halofantrine. Lancet 1993;341:1054– lation of drug concentration with parasitological 1056. outcome. Bulletin of the World Health Organization 1990;68:53–59. 35. Looareesuwan S et al. Clinical studies of atovaquone, alone or in combination with other antimalarial 51. Thaithong S. Clones of different sensitivities in drug- drugs for the treatment of acute uncomplicated resistant isolates of Plasmodium falciparum. Bulle- malaria in Thailand. American Journal of Tropical tin of the World Health Organization 1983;61: Medicine and Hygiene 1996;54:62–66. 709–712. 36. Radloff PD et al. Atovaquone plus proguanil is an 52. Foley M, Tilley L. Quinoline antimalarials: mecha- effective treatment for Plasmodium ovale and P. ma- nisms of action and resistance. International Journal lariae malaria. Transactions of the Royal Society of for Parasitology 1997;27:231–240. Tropical Medicine & Hygiene 1996;90:682 53. Krogstad DJ et al. Efflux of chloroquine from Plas- 37. Bloland PB et al. Malarone-donation programme in modium falciparum: mechanism of chloroquine Africa. Lancet 1997;350:1624–1625. resistance. Science 1987;238:1283–1285. 38. Foege WH. Malarone-donation programme. Lan- 54. Martin SK, Oduola AM, Milhous WK. Reversal of cet 1997;350:1628–1629. chloroquine resistance in Plasmodium falciparum by verapamil. Science 1987;235:899–901. 39. Ringwald P, Bickii J, Basco L. Randomised trial of pyronaridine versus chloroquine for acute uncom- 55. Plowe CV, Kublin JG, Duombo OK. P. falciparum plicated falciparum malaria in Africa. Lancet 1996; dihydrofolate reductase and dihydropteroate syn- 347:24–27. thase mutations: epidemiology and role in clinical resistance to antifolates. Drug Resistance Updates 40. Looareesuwan S et al. Pyronaridine. Lancet 1996; 1998;1:389–396. 347:1189–1190. 56. Ittarat I, Asawamahasakda W, Meshnick SR. The 41. Olliaro PL, Trigg PI. Status of antimalarial drugs effects of antimalarials on the Plasmodium falciparum under development. Bulletin of the World Health dihydroorotate dehydrogenase. Experimental Para- Organization 1995;73:565–571. sitology 1994;79:50–56.

25 DRUG RESISTANCE IN MALARIA WHO/CDS/CSR/DRS/2001.4

57. Wernsdorfer WH. The development and spread of 70. Hall AP et al. Amodiaquine resistant falciparum drug resistant malaria. Parasitology Today 1991; malaria in Thailand. American Journal of Tropical 7:297–303. Medicine & Hygiene 1975;24:575–580. 58. Verdrager J. Epidemiology of the emergence and 71. Feikin DR et al. Increased carriage of trimethoprim/ spread of drug-resistant falciparum malaria in South- sulfamethoxazole-resistant Streptococcus pneumoniae East Asia and Australasia. Journal of Tropical Medi- in Malawian children after treatment for malaria with cine & Hygiene 1986;89:277–289. sulfadoxine/pyrimethamine. Journal of Infectious Diseases 2000;181:1501–1505. 59. Verdrager J. Localized permanent epidemics: the genesis of chloroquine resistance in Plasmodium 72. Rathod PK, McErlean T, Lee PC. Variations in falciparum. Southeast Asian Journal of Tropical Medi- frequencies of drug resistance in Plasmodium cine and Public Health 1995;26:23–28. falciparum. Proceedings of the National Academy of Sciences of the United States of America 1997; 60. White NJ. Assessment of the pharmacodynamic 94:9389–9393. properties of antimalarial drugs in vivo. Antimicro- bial Agents and Chemotherapy 1997;41:1413–1422. 73. Watkins WM, Mosobo M. Treatment of Plasmodium falciparum malaria with pyrimethamine-sulfadoxine: 61. Wolday D et al. Sensitivity of Plasmodium falciparum selective pressure for resistance is a function of long in vivo to chloroquine and pyrimethamine- elimination half-life. Transactions of the Royal Soci- sulfadoxine in Rwandan patients in a refugee camp ety of Tropical Medicine & Hygiene 1993;87:75–78. in Zaire. Transactions of the Royal Society of Tropical Medicine & Hygiene 1995;89:654–656. 74. Trape JF, Rogier C. Combating malaria morbidity and mortality by reducing transmission. Parasitol- 62. Parise ME et al. Efficacy of sulfadoxine-pyrimeth- ogy Today 1996; 12:236–240. amine for prevention of placental malaria in an area of Kenya with a high prevalence of malaria and 75. Beier JC et al. Plasmodium falciparum incidence rela- Human Immunodeficiency Virus infection. Ameri- tive to entomologic inoculation rates at a site pro- can Journal of Tropical Medicine and Hygiene 1998; posed for testing malaria vaccines in western Kenya. 59:813–822. American Journal of Tropical Medicine and Hygiene 1994;50:529–536. 63. United Nations Children’s Fund. The State of the World’s Children 1998, Oxford and New York: Ox- 76. Palmer KJ, Holliday SM, Brogden RN. Mefloquine. ford University Press, 1998. A review of its antimalarial activity, pharmacokinetic properties and therapeutic efficacy. Drugs 1993; 64. Handunnetti SM et al. Features of recrudescent chlo- 45:430–475. roquine-resistant Plasmodium falciparum infections confer a survival advantage on parasites and have 77. Babiker HA, Walliker D. Current views on the popu- implications for disease control. Transactions of the lation structure of Plasmodium falciparum: implica- Royal Society of Tropical Medicine & Hygiene tions for control. Parasitology Today 1997; 13: 1996;90:563–567. 262–267. 65. Wernsdorfer WH et al. Chloroquine resistance of 78. Paul RE et al. Mating patterns of malaria parasite Plasmodium falciparum: a biological advantage? populations of Papua New Guinea. Science 1995; Transactions of the Royal Society of Tropical Medicine 269:1709–1711. & Hygiene 1995;89:90–91. 79. Paul RE, Day KP. Mating patterns of Plasmodium 66. Warsame M et al. Susceptibility of Plasmodium falciparum. Parasitology Today 1998;14;197–202. falciparum in vitro to chloroquine, mefloquine, 80. Mackinnon MJ. Survival probability of drug resist- quinine and sulfadoxine/pyrimethamine in Soma- ant mutants in malaria parasites. Proceedings of the lia: relationships between the responses to the dif- Royal Society of London, Series B, 1997;264:53–59. ferent drugs. Transactions of the Royal Society of Tropical Medicine & Hygiene 1991;85:565–569. 81. Mackinnon MJ, Hastings IM. The evolution of multiple drug resistance in malaria parasites. Trans- 67. Wilkinson RN, Noeypatimanondh S, Gould DJ. actions of the Royal Society of Tropical Medicine & Infectivity of falciparum malaria patients for anophe- Hygiene 1998;92:188–195. line mosquitoes before and after chloroquine treat- ment. Transactions of the Royal Society of Tropical 82. Hastings IM. A model for the origins and spread of Medicine & Hygiene 1976;70:306–307. drug-resistant malaria. Parasitology 1997;115:133– 141. 68. Sucharit S et al. Chloroquine resistant Plasmodium falciparum in Thailand: Susceptibility of Anopheles. 83. Hastings IM, Mckinnon MJ. 1998. The emerg2ence Journal of the Medical Association of Thailand of drug-resistant malaria. Parasitology;117:411–417. 1977;60:648–654. 84. Paul RE et al. Transmission intensity and Plasmo- 69. Basco LK. Inefficacy of amodiaquine against chlo- dium falciparum diversity on the northwestern roquine-resistant malaria. Lancet 1991;338:1460– border of Thailand. American Journal of Tropical 1460. Medicine & Hygiene 1998;58:195–203.

26 WHO/CDS/CSR/DRS/2001.4 DRUG RESISTANCE IN MALARIA

85. Molyneux DH et al. Transmission control and drug 99. Su X et al. Complex polymorphisms in an approxi- resistance in malaria: a crucial interaction. Parasitol- mately kDa protein are linked to chloroquine- ogy Today 1999;15:238–240. resistant P. falciparum in Southeast Asia and Africa. Cell 1997;91:593–603. 86. Wernsdorfer WH. Epidemiology of drug resistance in malaria. Acta Tropica 1994;56:143–156. 100. Lackritz EM et al. Imported Plasmodium falciparum malaria in American travelers to Africa. 87. Payne D. Did medicated salt hasten the spread of Implications for prevention strategies. Journal of chloroquine resistance in Plasmodium falciparum? the American Medical Association 1991;265:383– Parasitology Today 1988;4:112–115. 385. 88. Ettling, M, Bloland, PB, Ruebush, TK. Environmen- 101. Alene GD, Bennett S. Chloroquine resistance of tal Health Project Activity Report No. 15: Chloro- Plasmodium falciparum malaria in Ethiopia and quine efficacy study in Zambia, 9 May–9 July 1995. Eritrea. Tropical Medicine & International Health 89. van Hensbroek MB et al. Iron, but not folic acid, 1996;1:810–815. combined with effective antimalarial therapy pro- 102. Tulu AN et al. Failure of chloroquine treatment motes haematologic recovery in African children af- for malaria in the highlands of Ethiopia. Transac- ter acute falciparum malaria. Transactions of the Royal tions of the Royal Society of Tropical Medicine & Society of Tropical Medicine & Hygiene 1995;89:672– Hygiene 1996;90:556–557. 676. 103. Goodman CA, Coleman PG, Mills AJ. Cost- 90. Shakoor O, Taylor RB, Behrens RH. Assessment of effectiveness of malaria control in sub-Saharan the incidence of substandard drugs in developing Africa. Lancet 1999;354:378–384. countries. Tropical Medicine and International Health 1997;2:839–845. 104. Salako, LA. An African perspective. World Health 1998;3:24–25. 91. Ballereau F et al. Stability of essential drugs in the field: results of a study conducted over a two-year 105. Seppälä H et al. The effect of changes in the con- period in Burkina Faso. American Journal of Tropical sumption of macrolide antibiotics on erythromy- Medicine and Hygiene 1997;57;31–36. cin resistance in Group A streptococci in Finland. New England Journal of Medicine 1997;337:441– 92. Schapira A et al. The Plasmodium falciparum chlo- 446. roquine in vivo test: extended follow-up is more important than parasite counting. Transactions of the 106. Bauchner H, Pelton SI, Klein JO. Parents, physi- Royal Society of Tropical Medicine & Hygiene 1988; cians, and antibiotic use. Pediatrics 1999;103:395– 82:39–43. 401. 93. Brandling-Bennett AD et al. Chloroquine treatment 107. Wernsdorfer WH, Chongsuphajaisiddhi T, Salazar of falciparum malaria in an area of Kenya of inter- NP. A symposium on containment of mefloquine- mediate chloroquine resistance. Transactions of the resistant falciparum malaria in Southeast Asia with Royal Society of Tropical Medicine & Hygiene special reference to border malaria. Southeast Asian 1988;82:833–837. Journal of Tropical Medicine & Public Health 1994;25:11–18. 94. Bloland PB et al. Beyond chloroquine: implications of drug resistance for evaluating malaria therapy 108. White NJ et al. Averting a malaria disaster. Lancet efficacy and treatment policy in Africa. Journal of 1999;353:1965–1967. Infectious Diseases 1993;167:932–937. 109. Wongsrichanalai C, Thimasarn K, Sirichaisinthop 95. World Health Organization. Assessment of therapeu- J. Antimalarial drug combination policy: a caveat. tic efficacy of antimalarial drugs for uncomplicated Lancet 2000;355:2245–2247. falciparum malaria in areas with intense transmission. 110. Bloland PB, Ettling M, Meek S. Combination 1996, WHO/MAL/96.1077. therapy for African malaria: hype or hope? (in press) 96. Rieckmann KH et al. Drug sensitivity of Plasmo- 111. Weber MW et al. Evaluation of an algorithm for dium falciparum. An in-vitro microtechnique. Lan- the integrated management of childhood illness in cet 1978;1:22–23. an area with seasonal malaria in the Gambia. Bul- 97. Golenda CF, Li J, Rosenberg R. Continuous in vitro letin of the World Health Organization 1996;75:25– propagation of the malaria parasite Plasmodium 32. vivax. Proceedings of the National Academy of Sciences 112. Perkins BA et al. Evaluation of an alogrithm for of the United States of America 1997;94:6786–6791. integrated management of childhood illness in an 98. Plowe CV et al. Pyrimethamine and proguanil area of Kenya with high malaria transmission. Bul- resistance-conferring mutations in Plasmodium letin of the World Health Organization 1997;75:33– falciparum dihydrofolate reductase: polymerase chain 42. reaction methods for surveillance in Africa. Ameri- can Journal of Tropical Medicine & Hygiene 1995; 52:565–568.

27