**5. Molecular markers for resistance to atovaquone**

Atovaquone has been in use since the 1980s when it was first developed for the treatment of malaria. Despite its high efficacy in the past, it is faced with a high level of recrudescence of approximately 30% when used as a monotherapy [45, 46]. Currently, atovaquone is used in combination with proguanil as a prophylaxis or treatment of malaria [47].

Atovaquone acts by inhibiting the electron transport in the mitochondria by interacting with the cytochrome b1 complex [48]. This makes the cytochrome b gene a molecular marker for the monitoring of atovaquone resistance (**Table 1**) [49]. The cytochrome b Y268S/C/N mutations have been associated with resistance to atovaquone. These mutations have been validated to cause resistance to atovaquone, in a study that introduces the mutation Y302C in the bacterial cytochrome b (this mutation corresponds to the Y268C in the *P. falciparum*) rendered the bacterial cytochrome bc1 less sensitive to atovaquone [50].


**Table 1.**

*Current antimalarial drugs, and their molecular markers of resistance.*

#### **6. Molecular markers for drug resistance in** *P. vivax*

The study of antimalarial drug resistance in *P. vivax* is hindered by the lack of *in vitro* culture techniques for the culturing of the parasite. This has made knowledge about the genetic basis of resistance in *P. vivax* limited. Insights about the genetic basis of antimalarial drugs in *P. vivax* have been gained by comparing it with *P. falciparum*.

Orthologs of *pfcrt* and *pfmdr1* which are *pvcrt-o* and *pvmdr1* respectively in *P. vivax* have been reported. *P. vivax* isolates with the *pvmdr1* Y976F mutation are associated with higher CQ IC50 values. Studies show that the *pvmdr1* Y976F mutation has reached near fixation in parasite isolates from Papua New Guinea, Indonesia [62], and Brazil [63], but CQ is still highly efficacious in these countries. These provide weak evidence for using *pvmdr1* Y976F mutation as a CQ molecular marker of resistance in *P. vivax*, hence, CQ resistance in *P. vivax* may have a different genetic basis. Increased *pvndr1* copy numbers have been recorded in *P. vivax* in regions Thailand where mefloquine is used extensively, but not in regions where mefloquine is less used [62, 64].

Mutations in *dhfr* and *dhps* in *P. vivax* have been associated with decreased susceptibility to sulfadoxine-pyrimethamine [65]. Studies by [65] have identified more than 20 alleles in the *dhfr* and *dhps* genes in *P. vivax*. An example of such a mutation is the PvDHFR S58R/S117N which are homologous to PfDHFR C59R/S108N mutations. The PvDHFR S117N has been reported to prevent binding pyrimethamine [66] just like the PfDHFR S108N [67].

#### **7. Origins and spread of CQ resistance**

The notable mutations in *pfcrt* 72–76 are associated with certain geographical locations. Other mutations outside these positions have no clear geographical association

[68]. This makes it possible to identify or predict the evolution and geographical spread of chloroquine resistance-associated with mutations in *pfcrt* codons 72–76 by genotyping for these codons and the haplotype flanking this locus by microsatellite [68].
