Polymers, Composites and Nanocomposite Principles

Artemisinin derivatives are fast-acting, well-tolerated drugs that are often paired with longer-acting partner drugs as ACTs, which are now the first-line treatment for P. falciparum throughout the malaria-endemic world. Artemisinin resistance has emerged in Southeast Asia, manifesting itself as delayed clearance of parasitemia following treatment with artemisinin derivatives (Dondorp et al., 2009; Kyaw et al., 2013; Noedl et al., 2008; Phyo et al., 2012) Recently, this resistance has been shown to be associated with mutations within a Kelch protein located on P. falciparum chromosome 13 (k13 propeller), (Ariey et al., 2014;Straimer et al.,2015) and secondary loci may also be involved (Takala-Harrison et al., 2015). K13 propeller mutations have been associated with delayed parasite clearance in several South-East Asian countries, including Cambodia, Vietnam, and Myanmar, and have been shown to have spread

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between countries as well as to have emerged independently in different countries (Ashley et al., 2014; Takala-Harrison et al., 2015).

The emerging ACT resistance, led by P. falciparum resistance to the ART derivatives has threatened malaria control and elimination programme because it raises concerns about possible spread of resistance parasites into the African continent where malaria exerts its heaviest toll.

Reduced ART efficacy in turn places increased selective pressure on the ACT partner drugs, placing them at greater risk of failing. Indeed, piperaquine resistance has now emerged in k13-mutant parasites, resulting in treatment failure with dihydro-artemisinin+piperaquine—the first-line drug in Cambodia and several neighboring countries (Amaratunga et al., 2016; Leang et al., 2015; Saunders et al., 2014). Despite an active antimalarial drug discovery and development pipeline, there is at present no drug ready to take the place of ACTs should they begin to fail globally (Wells et al., 2015).

2.5.6.1 Thailand-Cambodia border: epicenter of drug resistance

Historically, the initial emergence of drug-resistant P. falciparum has been restricted to limited geographic regions, the so-called ``epicenters of resistance.'' The Greater Mekong sub-region is the most threatening focus of malaria in terms of antimalarial drug-based control (Cui et al., 2015). This area comprises six countries: Cambodia, Thailand, China's Yunnan province, Lao PDR, Myanmar, and Vietnam. P. falciparum isolates resistant to the conventional antimalarial drugs chloroquine, pyrimethamine, and sulphadoxine initially emerged from the Greater Mekong sub-region before subsequently migrating to the African continent.

The Thailand-Cambodia border has been regarded as the most important focal point for the emergence of drug-resistant parasites (Dondorp et al., 2009). In this region, population movements were frequent and extensive due to abundant mining of precious stones/gems

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(Dondorp et al, 2009). These mobile populations could carry artemisinin resistant parasites to other countries, where artemisinin resistance may be introduced and spread.

2.5.6.2 Why Mekong region is epicenter of drug resistance

Antimalarial drugs resistance emerged so often from the greater Mekong region of South East Asia reputed as an epicenter of antimalarial drug resistance possibly for the following reasons.

First, the region has a long history of antimalarial drug use. For example, chloroquine and pyrimethamine/sulphadoxine were deployed earlier in this area than in other endemic regions.

Thailand was the first country in which pyrimethamine/sulphadoxine was introduced as a first-line treatment (in the late 1960s), and resistant parasites were detected in the Thailand-Cambodia border region soon thereafter (Phillips-Howard and Bjorkman, 1990)

Second, the mass drug administration of chloroquine and pyrimethamine in form of the medicated salt project where pyrimethamine-containing salt for use in cooking were supplied along the Thailand-Cambodia border, might have played a role in the rapid development of parasite resistance (Verdrager, 1986). Resistance to pyrimethamine was observed soon thereafter. The antimalarial drug pressure imposed by the project might have created unique circumstances under which parasites were exposed to continuous and weak (sub-curative) doses of antimalarial drugs, thus providing optimal conditions for selection of drug resistance (Verdrager, 1986). Third, malaria transmission in the region is low. Individuals in this area do not develop sufficient level of protective immunity because malaria infections are occasional and inadequate to achieve protective immunity. This implies that most infections are symptomatic and must be treated leading to intermittent antimalarial drug use and the possibility of selecting drug resistant parasites (Hastings and Watkins, 2005).

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Fourth, the ecological and population genetic features of malaria parasites unique to the Greater Mekong sub-region may also be involved in rapid emergence of resistant parasites. In this region, the transmission intensity of malaria is much lower than that in highly endemic regions in Africa. The parasite population is small and well structured, compared to that of African endemic region (Iwagami et al., 2009).Drug resistant parasites are more readily selected under such ecological and population genetic conditions (Ariey and Robert, 2003). Finally, P. falciparum in the region could possess a genetic property predisposing the local strains to rapid generation of drug resistance (Rathod et al., 1997), such as a defect in DNA mismatch repair (Castellini et al., 2011).

2.5.6.3 Implications for the containment of artemisinin resistance

Since no efficient antimalarial drugs that can act on artemisinin-resistant parasites are currently available, there is an urgent need to prevent the spread of artemisinin-resistant P. falciparum, which was reported recently in a limited area of the Greater Mekong sub-region(Noedl et al., 2008). The WHO Global Malaria Programme has recently announced a ``Global Plan for Artemisinin Resistance Containment'' (GPARC) (WHO, 2011a) which aims to prevent the emergence and spread of artemisinin resistance. The program consists of five activities for successful management of artemisinin resistance: (i) stopping the spread of resistant parasites;

(ii) strengthening surveillance to evaluate the threat of artemisinin resistance; (iii) improvement of access to diagnostics and rational treatment with ACTs; (iv) investment in artemisinin resistance-related research; and (v) motivating action and mobilizing resources. Special emphasis is given to the cessation of unnecessary use of ACTs, oral artemisinin-based monotherapy, and counterfeit drugs from the market. Artemisinin have been used in western Cambodia and China, mostly as monotherapy, for more than 30 years.

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In addition, patients often have been treated by artemisinin without parasitological confirmation (Yeung and White, 2005). To limit such unnecessary ACT use, all malaria-suspected patients are strongly recommended to be parasitologically confirmed for malaria positivity and treated with affordable and quality-assured ACTs in areas where there is credible evidence of artemisinin resistance. The program also recommends intensive vector control using long-lasting insecticide nets and indoor insecticide residual spraying to minimize the transmission of resistant parasites by mosquitoes.

While there is persistence of chloroquine resistant malaria in South-East Asia and South America return of chloroquine sensitive parasites has been reported in Malawi many years after the removal of the drug pressure (Kublin et al., 2003). Persistence of chloroquine resistance in South-East Asia may be due to either continued drug pressure due to use of chloroquine to treat P. vivax or low malaria transmission leading to parasite population with unique characteristics.

Such small effective parasite population could have low diversity (single parasite genotype) leaving little opportunity for genetic recombination that could increase parasite diversity. It could also make it easier for specific alleles to become fixed leaving no room for drug sensitive parasites (Takala-Harrison and Laufer, 2015). In Africa, there is higher transmission with a high possibility of polyclonal infections and increase opportunity for recombination leading to competition between parasite strains. The degree of acquired immunity is high due to repeated exposure; this can produce a larger reservoir of sensitive parasites in untreated clinically immune individuals. (Takala-Harrison and Laufer, 2015)

In Nigeria, it is not known whether ACT use has removed chloroquine selective pressure on the parasites population. The reported return of chloroquine- sensitive parasites in Malawi may be due to resurgence and increase in frequency of diverse CQ sensitive parasites that survived drug

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pressure possibly in clinically immune hosts after removal of drug pressure. This suggests that there is a fitness cost to the parasites associated with Pfcrt mutations in the absence of chloroquine pressure.

2.5.6.4 Status of artemisinin resistance in Africa

Monitoring of parasite clearance rates following treatment with ACTs has occurred at various sites within Africa. Overall, P. falciparum parasites seem to clear rapidly in most African countries sampled to date (Ashley et al., 2014; Maiga et al., 2012) except in a study that reported increased rates of day-1 parasitemia and increased risk of parasite recrudescence following treatment with ACTs from 2005 to 2008 (Borrmann et al., 2011). The delayed clearance observed in the said study could suggest decreased susceptibility to ACTs, or a reflection of decreased clinical immunity within the study population (Borrmann et al, 2011).

Despite rapid clearance of parasites in all African studies conducted thus far, k13 propeller mutations have been observed at low levels in parasites from African study sites (Kamau et al., 2014; Taylor et al., 2015) Many of these mutations are within the propeller region of k13, but are different from the mutations observed in Southeast Asia, and occur at low frequencies, often within a single study site. The presence of k13 propeller mutations within Africa, where parasite clearance is generally rapid, suggests that not all k13 propeller mutations are associated with resistance, that secondary loci are involved in resistance and found in Asia but not in Africa thus far, or that high levels of antimalarial immunity mask the resistance phenotype. The spread or emergence of artemisinin resistance in Africa could have serious implications. The independent emergence of resistance in Myanmar (in contrast to spread from Cambodia) and the presence of existing k13 propeller mutations in African countries highlight the possibility of independent

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emergence of resistance in Africa. However, if secondary and/or background mutations required to augment resistance are not found in African parasites, then the level of resistance conferred by k13 propeller mutations may be less than that observed in parasites with an Asian genetic background (Takala-Harrison and Laufer, 2015). If k13 mutations on an Asian genetic background were to spread to Africa, any association with secondary loci on different chromosomes could be quickly unlinked as parasites reproduce with diverse African parasites.

Thus, it is possible that it may be more difficult for artemisinin resistance to become established in Africa compared to Asia (Takala-Harrison and Laufer, 2015) although this remains to be seen.

Further research is needed to understand the role of African k13 propeller mutations in artemisinin resistance. To reduce the probability of mutations in parasite populations, it is important to keep an eye on both the efficacy of both the artemisinin component and the partner drug to guarantee that only effective drugs are recommended for malarial control and treatment