Different antimalarial drugs and years/places of deployment and emergence of resistance [references in bracket].
Malaria remains a leading cause of mortality and morbidity in many low- and middle-income countries. Artemisinin combination therapies (ACTs) have contributed to the substantial decline in the worldwide malaria burden, renewing the optimism that malaria elimination is achievable in some regions of the world. However, this prospect is threatened by the emergence of artemisinin resistance in Plasmodium falciparum leading to clinical failure of ACTs in Southeast Asia. Historically, drug resistance in P. falciparum has emerged in SEA and spread to Africa. Today, resistance to ACTs could reverse all the achievements of control and elimination efforts globally. With no new drug available, P. falciparum malaria must be eliminated from the Greater Mekong before it becomes untreatable.
- falciparum malaria
- malaria elimination
- Southeast Asia
The emergence of artemisinin-resistant falciparum malaria along the Thai-Cambodian border follows a familiar pattern. History shows that chloroquine resistance had arisen from this region in the 1950s (Table 1) and leads to the failure of the Global
|Antimalarial drug||Year of first deployment||Place of first deployment||Year of resistance emerged||Place of emergence of resistance|
|Quinine||1630 ||South America ||1910*||Brazil [28, 29]|
|Chloroquine||1945||Global Malaria Eradication Campaign ||1957||Colombia, Cambodia-Thailand border [41, 128, 129, 130]|
|1961||Colombia [56, 57]|
|Atovaquone||1996||Thailand ||1996||Thailand [72, 73, 75]|
|Proguanil||1948||Various African countries ||1949||Aden Protectorate, Yemen |
|Sulfa + antifols°||1967||Thailand ||1967||Thailand |
|Mefloquine||1967||Vietnam ||1982||Thailand [7, 8, 43]|
|Piperaquine||1978||China ||1985||China |
|Artemisinin||1979||China ||2008||Cambodia |
|Mefloquine-artesunate||1994||Thailand ||2002α||Cambodia |
|Artemether-lumefantrine||1994||China ||2006α||Cambodia [143, 144]|
|Dihydroartemisinin-piperaquine||2001||Cambodia ||2013α||Cambodia [86, 146, 147]|
In 2007, the Bill and Melinda Gates Foundation announced that it was investing millions of dollars to revitalise the efforts of malaria elimination . Ten years later, this seems to be an achievable goal since the global malaria burden has diminished (Figure 1), an encouraging result attributed to the widespread deployment of long-lasting impregnated nets (LLINs), the ACTs and increased availability of malaria diagnostic tests . However, the failure of the ACTs, the extension of vector resistance to the insecticides and the recent increase in the number of malaria cases are clear reminders that malaria is a formidable foe. Without new strategies, the same causes will lead to the same consequences  .
3. Mechanisms and emergence of antimalarial drug resistance
Causal stimuli of antimalarial resistance consist of spontaneous mutations in the parasite genome, antimalarial pharmacokinetics and the magnitude of parasite gene pool, which is proportionate to transmission intensity.
Primarily, as an innate survival strategy of microorganisms, mutation(s) occur
Spontaneous mutations, in the particular genes encoding the drug target, cause the reduction in drug accumulation or efflux (chloroquine, amodiaquine, quinine, mefloquine, halofantrine resistance) or reduced affinity of the drug target (pyrimethamine, cycloguanil, sulphonamide, atovaquone resistance), which finally enables the parasite to withstand the antimalarial treatment. Afterwards, the drug pressure facilitates the resistant parasites to propagate by eliminating the susceptible parasites, which are usually more fit and would outcompete the resistant ones in the absence of the drug. Eventually resistance becomes established and can persist or be reintroduced. In the absence of drug pressure, the resistant parasites have no longer any survival advantage and can be overtaken by wild-type (sensitive) parasites [18, 19]. But as soon as the abandoned drug is reintroduced, the resistant isolates regain their survival advantage and expand rendering the drug inefficient within a short time .
Large-scale and/or long-term distribution of several tons of medicated salt took place in many countries and was an important factor implicated in the emergence of both chloroquine and sulfadoxine/pyrimethamine (SP) resistance and accelerating their spread [21, 22, 23]. In WHO supported programs, the doses of antimalarial received by each individual were highly variable, and constant exposure to sub-parasiticidal (or even parasiticidal) drug concentrations might have eliminated the highly and moderately sensitive parasites, providing a selective advantage for less sensitive counterparts. Thus, the speed of selection of mutant parasites depends principally on the pharmacokinetics of the drug (slowly eliminated drugs with a long tail of sub-parasiticidal concentrations generally select faster) and the magnitude of drug use within a population (the higher the drug pressure per parasite, the faster the selection).
With ACTs, the newly emerged drug-resistant parasite has to overcome the parasiticidal action of the partner drug as well as the host immunity. At this point, with compromised efficacy of partner drug, along with declining immunity of the population, resistance to ACT combination is inevitable . This is the reason why artemisinin resistance has led to the clinical failure of mefloquine-artesunate and DHA-piperaquine combinations [25, 26].
The reason why antimalarial resistance always emerged in the same region of the world (SEA and specifically in Western Cambodia) is currently unknown. Some contributing factors have been proposed such as the low level of acquired immunity, the weak and seasonal transmission, the availability of antimalarial drugs, usage of monotherapies, sub-standard or counterfeit drugs, porous borders. The answer will probably be given by studies of the parasite population genetics, and recent work has shown the existence of “founding populations” favourable to the emergence of resistant parasites .
The emergence of drug resistance to various antimalarial compounds is mentioned by chronology in Table 1 (antimalarial drugs and years/places of deployment and emergence of resistance).
3.1. Quinine resistance
Quinine, initially as cinchona bark, was first used as a fever medicine and officially introduced into the London Pharmacopoeia in 1677 . The earliest resistance to quinine was reported in 1910 [28, 29]. Like chloroquine, quinine has been shown to accumulate in the parasite’s digestive vacuole inhibiting the haem detoxification process. Quinine resistance also seems to be associated with reduced drug uptake by the parasite. There is a weak association between quinine resistance and
3.2. Chloroquine resistance
Chloroquine, considered as one of the most successful medications ever deployed, saving several millions of lives, was developed in 1934 [2, 36] and replaced quinine for shorter regimen with better adherence. Single nucleotide polymorphisms in
3.3. Antifolate resistance
After the emergence of chloroquine resistance, sulfadoxine-pyrimethamine (SP) combination was deployed by the Thai Malaria Control Program as the first-line regimen for falciparum malaria in 1973. Afterwards, SP was extensively used throughout the country and was also available as an over-the-counter fever remedy in local dispensaries. Attributed to a number of reasons, including unrestricted usage, distribution of pyrimethamine medicated salt , superfluous drug pressure (prophylactic as well as presumptive use for fever) and poor compliance especially in migrant mobile population, the resistance to SP combination had emerged around 1980 in the Thai-Cambodian border [5, 44]. Then, in the early 1980s, even with an increased dose (i.e. three tablets of SP, instead of two tablets flat dosing), a cure rate of only 30–40% was achieved .
Point mutations at codons 51, 59, 108 and 164 in the
3.4. Amodiaquine resistance
Amodiaquine is structurally related to chloroquine but these amino-4-quinolines have different resistance patterns. Amodiaquine is effective against chloroquine-resistant isolates. However, parasites carrying the
3.5. Mefloquine resistance
Mefloquine was first produced in 1969 by the US Army Antimalarial Drug Development Program, primarily for the chemoprophylaxis in the military. The early therapeutic efficacy trial of mefloquine in Thailand showed 100% efficacy in 1976  and in combination with SP where 97% efficacy was proven in a large-scale trial during 1983–1985 [60, 61]. Then, in 1991, mefloquine monotherapy was used as the first line regimen for
Resistance to mefloquine was proven to be mediated by
3.6. Atovaquone resistance
Atovaquone was trialled as a monotherapy as well as in combination with proguanil between 1990 and 1996 in Thailand, and the therapeutic efficacy of atovaquone-proguanil was proven to be superior to mefloquine monotherapy, chloroquine, amodiaquine monotherapy and SP [72, 73]. A single point mutation (codon 268 in the
3.7. Pyronaridine resistance
Pyronaridine is a quinoline derivative compound with similar molecular structure as chloroquine and amodiaquine. There was a strong correlation between
3.8. Piperaquine resistance
Piperaquine (PPQ) has no cross resistance with chloroquine, and susceptibility is not associated with mutations on the
3.9. Artemisinin resistance
Artemisinins are thought to be inhibitor of
There are two main proposed pathways for artemisinin resistance with the involvement of Kelch (
In Kelch (propeller) mutant alleles, the mutations may alter the topology of the Kelch protein probably by modification of surface charges that disrupt interactions with other enzymes such as
Possible mechanisms proposed by transcriptomic study  is through upregulation of genes involved in the UPR pathway (especially two putative chaperonin complexes,
The role of Kelch non-propeller mutation (before the amino acid position 441) is still unclear. Some SNPs like E252Q emerged earlier along the Thai-Myanmar border and associated with reduced efficacy of ACT  but are being taken over by the propeller SNPs particularly C580Y . All these findings indicate that artemisinin resistance is likely to be multi-locus and that other genetic changes, such as P623T polymorphism in
4. Resistance facilitates the transmission potential
For the newly selected resistant parasites to be propagated, the recrudescent infection is essential . The threshold for successful transmission of malaria is around six viable gametocytes in one blood meal . Post-treatment gametocytaemia is a composite of ongoing gametocytogenesis despite treatment (especially with ineffective drug) and the release of sequestered gametocytes, which is enhanced by drug-induced stress . If the malaria infection is treated with partially effective drugs, post-treatment gametocytaemia is more likely. This was clearly shown for drugs such as CHQ and SP  as evidenced in patients with slower parasite clearance after artesunate treatment . Moreover, mutant isolates were also related to pre- and post-treatment gametocytaemia [107, 108, 109, 110] and hence possess transmission advantage (Figure 3).
5. Prospects of elimination
With the declining transmission of malaria, the geographic clustering of both clinical and asymptomatic infections has become more apparent. Asymptomatic carriers represent a “reservoir” of parasites that are difficult to detect because the density of parasites is often below the sensitivity threshold of conventional diagnostic tools (Rapid Diagnostic Tests and microscopy). The size of these reservoirs of sub-microscopic infections (also called “hot-spots”) can vary from a few households to large geographical areas. Clustering of these hotspots becomes more pronounced as transmission declines . While considering malaria elimination, radical depletion of parasite reservoir (asymptomatic carriers with sub-microscopic parasitaemia) and gametocytes is a necessity. This can be achieved by two functional components: (1) early diagnosis with treatment (EDT) of the symptomatic patients (preferably within 48 hour of symptoms before the development of gametocytaemia) and (2) early detection and treatment targeting the reservoirs of sub-microscopic infections through Mass Drug Administration (MDA) [112, 113].
The intervention for the first element is to set up or reinforce and sustain malaria control program hence reducing the number of clinical episodes as much as possible through increased access to EDT where the use of efficacious antimalarial regimen is critical . As the drug resistance worsen, the rising number of clinical cases due to increasing gametocyte carriage in the community will be inevitable. MDA or mass screening and treatment (MSAT) is only accelerating the malaria elimination alongside EDT, by eliminating the sub-microscopic reservoir . The effectiveness of MDA or MSAT significantly relies on the therapeutic efficacy of the drug in use, the coverage and the total number of rounds of MDA. In turn, this means that a careful and well-conducted community engagement is primordial for enhanced coverage [115, 116].
6. Choice of drug for malaria elimination: is the pipeline empty?
The current malaria elimination program along the Thai-Myanmar border is using artemether-lumefantrine (AL) for treating the clinical cases at the village malaria posts or by malaria workers , whereas dihydroartemisinin-piperaquine (DP) is deployed in MDA activities . In this area, the third ACT, mefloquine-artesunate combination, is already failing , and the prospect of elimination program is highly dependent on the therapeutic efficacy of AL and DP. Recent emergence of piperaquine resistance following the artemisinin resistance has depleted the available ACTs to be deployed in malaria elimination programs. High failure rates of AL in Laos PDR and DP in Vietnam and Cambodia have cast doubts on the optimism of malaria elimination [10, 26, 87, 118].
There are very few new compounds in the development pipeline. The front runners are OZ439, a synthetic endoperoxide, structurally related to artemisinin, and KAF156 belonging to a new class of antimalarial (imidazolopiperazines) and the spiroindolone cipargamin (formerly KAE609). However, these short-acting drugs will have to be deployed in combination therapies and their full development will take many years.
As a stopgap measure, two triple ACTs (mefloquine plus DP and amodiaquine plus AL) are under multicentre trial, using the inverse correlation between susceptibility to amodiaquine and lumefantrine as well as between piperaquine and mefloquine. The trial has completed the patient recruitment and the results are promising with high cure rates. However, recent increasing prevalence of parasite isolates with potential resistance to both mefloquine and piperaquine has questioned the longevity of the triple ACT .
7. Drug resistance in
8. Regional artemisinin resistance initiative (RAI)
The six countries of the Greater Mekong Subregion (GMS), Thailand, Myanmar, Cambodia, Laos, Vietnam and China (Yunnan Province), are part of a larger community, the Association of Southeast Asian Nations (ASEAN). Despite political pledges to fight artemisinin resistance and eliminate malaria, coordination remains hampered by deep political, economic and geographical gaps. The WHO strategic plans to counter artemisinin resistance failed to prevent its spread to the entire sub-region. In 2013, the Global Fund launched the Regional Artemisinin-resistance Initiative to provide financial support to the five countries affected by this new treat. This initiative came in addition to the contributions of the Global Fund to the Malaria National Program and contributed to the decrease in malaria-related mortality and morbidity in the region. However, these efforts have been compromised by the fragmentation in the public health policies, the disparities in the infrastructures and human resources as well as corruption. In terms of treatment policies, all GMS countries had already adopted ACTs long before the emergence of resistance, but poor monitoring in some countries meant that monotherapies and sub-standard or counterfeit drugs continued to circulate until recently. The relative absence of entomological data in some parts of SEA explains that there is no coherent strategy for containment of local disease vectors. Large budgets continue to be spent on long-lasting impregnated nets (LLINs) despite the absence of evidence of their effectiveness.
Artemisinin resistance in
We acknowledge the insightful suggestions of two editors from InTech Open Access. The Shoklo Malaria Research Unit and Myanmar Oxford Clinical Research Unit are part of the Mahidol Oxford Tropical Medicine Research Unit-Tropical health network funded by the Wellcome Trust.
List of acronyms
|ACT||artemisinin combination therapy|
|ASEAN||Association of Southeast Asian Nations|
|CNV||(gene) copy number variation|
|EDT||early diagnosis and treatment|
|GMS||Greater Mekong Subregion|
|K-13||Kelch 13 gene of P. falciparum|
|LLINs||long-lasting impregnated nets|
|MDA||mass drug administration|
|MSAT||mass screening and treatment|
|Pfcrt||P. falciparum chloroquine resistance transporter|
|Pfmdr-1||P. falciparum multi-drug resistant gene-1|
|PfPI3K||P. falciparum phosphatidylinositol-3-kinase|
|Pgh1||P-glycoprotein homologue 1|
|RBC||red blood cell|
|SNP||single nucleotide polymorphism|
|UPR||unfolded protein response pathway (UPR)|
|WWARN||Worldwide Antimalarial Resistance Network|
|WHO||World Health Organisation|