Background
Malaria is a major public health problem, an estimated 215 million clinical cases and more than 400,000 malaria-related deaths occurred in 2015 alone [
1]. The World Health Organization (WHO) currently recommends artemisinin-based combination therapy (ACT) as the first-line treatment for all falciparum malaria [
2]. Worryingly, the efficacy of the artemisinins is declining due to the emergence of slow-clearing
Plasmodium falciparum parasites after artemisinin treatment in patients throughout Southeast Asia [
3,
4]. Widespread treatment failure of artemisinin derivatives is yet to be reported but previous first-line anti-malarial treatments, such as chloroquine and sulfadoxine-pyrimethamine have been phased out due to drug resistance and treatment failure [
5,
6].
Anti-malarial treatment outcome is determined, according to WHO criteria, as either adequate clinical and parasitological response (ACPR) or treatment failure, which can be further categorized as early treatment failure (ETF), late clinical failure (LCF), or late parasitological failure (LPF) [
7,
8]. The predominant cause of treatment failure is resistance to the active drug, or in the case of combination therapy, resistance to one or more of the active components. However, the efficacy of anti-malarials may be influenced by other factors independent of the parasites susceptibility to the drugs. For example, patients vary greatly in their drug concentration versus time profiles, the parasite burden and age distribution of the parasites at initial treatment, and the level of within-host immunity to malaria [
9].
Naturally acquired immunity to malaria develops in an age-dependant manner, after repeated exposure, in individuals living in malaria-endemic regions (reviewed in [
10,
11]). Antibodies targeting the blood stage of
Plasmodium spp. are acquired with age and are an important component of the anti-malarial immune response, acting by reducing parasite density and clinical symptoms [
12,
13]. Treatment efficacy improves with increasing age and intensified transmission, suggesting that acquired immunity may play a role in determining the efficacy of anti-malarial treatments [
14‐
17]. The direct role that naturally acquired immunity plays in influencing anti-malarial treatment outcome has been investigated in several studies with conflicting conclusions. The aim of this systematic review was to synthesize the evidence of studies investigating the relationship between
Plasmodium-specific blood-stage antibody responses and anti-malarial treatment failure. In addition, variations in the association according to the anti-malarial administered (which have different pharmacokinetic-pharmacodynamic profiles) and blood-stage antibody response (which can target different antigens and parasite life-cycle stages) was investigated.
Discussion
Identifying and quantifying host factors that determine anti-malarial treatment efficacy is essential for monitoring the occurrence of treatment failures and emerging resistance. An association between antibodies specific for P. falciparum blood-stage antigens and treatment failure was found for each of the anti-malarials investigated, with the largest magnitude of effect observed for artemisinin derivatives and chloroquine. Heterogeneity was observed in these associations according to the blood-stage antigen under investigation, with larger magnitudes of effect observed for variant surface antigens compared to merozoite antigens.
The anti-malarial treatments included in this review have different pharmacokinetic and pharmacodynamic profiles. The largest magnitude of effect with blood-stage immunity and anti-malarial treatment efficacy was observed with artemisinin derivatives and chloroquine. The 4-aminoquinolines such as chloroquine, amodiaquine and piperaquine, as well as the artemisinins, target early parasite forms [
20], particularly the ring stage in the case of artemisinins [
30]. The combined targeting of early intra-erythrocytic parasites by antibodies and drugs which preferentially target early forms is likely to provide swift clearance of IEs before cyto-adhesion and sequestration can occur (>18 h post merozoite invasion) [
31]. Conversely, treatment with dihydrofolate reductase inhibitors, such as pyrimethamine, interrupts late parasite stages (after the first 24 h of parasite life cycle) [
20], leaving parasites to mature, and
Pf-IEs to rosette and sequester regardless of treatment until the next parasite cycle. In addition to the variety of parasitic targets, the anti-malarials assessed in the included studies have different drug concentration–time profiles. The artemisinins, for instance, have very short elimination half-lives (between 0.7 and 1.4 h in the case of artesunate) (reviewed in [
32]), when compared with chloroquine which has a long terminal elimination phase, and may be detected in the patient months after administration [
33]. The difference observed in the drug elimination time profiles between treatments indicates that follow-up times recommended for determining treatments failures should vary in order to avoid underestimating treatment failure in those drugs with longer plasma half-lives [
34], however only two studies observed patients beyond 28 days despite a myriad of treatments used across included studies [
24,
28]. Immunity may therefore have a differential effect on the treatment failure of different anti-malarials, although this cannot definitively be concluded by this review as no individual study compared effects across different anti-malarials.
In most cases anti-
Pf-IE antibodies were associated with the largest decrease in odds/risk of treatment failure compared to anti-merozoite antibodies, which suggests that immune mechanisms which contribute to
Pf-IE clearance (e.g., opsonic phagocytosis) rather than by reducing parasite multiplication rates [
35‐
38] may have a greater impact on measures of treatment failure. The varying magnitude of effect observed within merozoite antigens may also support a direct role of anti-merozoite responses in treatment failure. Antibodies specific for AMA-1, EBA-175 and MSPs antibodies, were found to reduce the odds of treatment failure [
24,
26,
27] and have been associated with protection from high density parasitaemia and symptomatic malaria in other studies [
12,
39], whereas anti-MSP1 Block 2 specific antibodies, were not associated with a reduced odds of treatment failure [
23] and in previous studies have not been shown to be protective against high density parasitaemia and symptomatic infections [
12]. Given that the antigen/parasite strain under investigation are potential sources of heterogeneity, both different antigens within study sites, and the same antigens across study sites (e.g., AMA1 and GLURP), further investigation into the relative utility of different antigens in assessing immunity in drug efficacy studies is warranted.
Any host mechanism capable of contributing to parasite clearance will have a profound effect in patients treated with drugs that are no longer or only partially efficacious by contributing to parasite clearance which may be wrongly interpreted as a direct effect of treatment. The frequency of drug-resistant parasites and malaria transmission. may also influence the association between antibodies and treatment failure. Pinder et al. [
26] and Enevold et al. [
27] examined the impact of immunity in a population where drug resistance was established but only one confirmed the presence of known molecular markers [
26]. Furthermore, the presence of resistant parasites may further influence results, as it has been recently demonstrated that the largest effect of immunity on parasite clearance after artemisinin treatment was observed in patients harbouring artemisinin resistant
kelch13 mutant rather than wild-type parasites [
40].. Differences in transmission intensity and acquisition of naturally acquired immunity between study sites may also be a source of heterogeneity. The majority of the included studies were conducted in moderate-high transmission settings [
22‐
28], with only one study assessing treatment efficacy in a low-transmission setting in Thailand [
29]. Despite being in an area of low transmission, and presumably of low naturally acquired immunity, this study by Mayxay et al. showed the highest magnitude of effect on the association between
Pf-IE antibodies and reduced odds of artemisinin treatment failure. Findings in this systematic review may be generalizable to populations of varying transmission but the generalizability of findings to areas of varying frequencies of genetic mutations are yet to be determined.
A strength of this review was that studies published in all languages were included and authors were contacted to provide estimates and data for inclusion in the review. A further strength is that the WHO classification of anti-malarial treatment failures was utilized to ensure the inclusion of rigorous studies and maximum comparability between studies. Importantly, the current WHO guidelines for the assessment of antimalarial treatment efficacy requires the use of molecular genotyping in regions of intense transmission to ensure recrudescent infections are accurately recorded and ensure reinfection if not mistaken for treatment failure and for inclusion in treatment failure analyses {WHO, 2009 #2599}. Two of the included studies (both of which utilised data acquired prior to the recommendation of PCR correction in 2003 [
7]) either did not complete or did not report molecular genotyping [
22,
29], the consequence being that treatment failures may have been overestimated in these studies. Furthermore, not all of the included studies categorized patients into the treatment failure sub-categories: ETF, LCF and LPF. This made the direct comparison of studies challenging, but also prevented analyses stratified for the different stages of treatment failure. Some studies did not include effect estimates stratified by the treatment given. For example Van Geertruyden et al. and Mayxay et al. provided estimates for combined patients treated by different drugs or the same drugs in mono- and combination therapy [
28,
29], making it difficult to determine the effect of antibody responses to treatment efficacy of specific anti-malarial regimens. Furthermore, analysis was stratified according to the potency of included treatments. However, the importance of partner drugs should not be underestimated in providing efficacious treatment, specifically in the ACTS where partner drugs provide essential and most importantly long-lasting anti-parasitic activity in combination with the more potent but short lived artemisinin derivatives.
Methodological heterogeneity meant that meta-analyses could not be performed and pooled estimates were unable to be calculated to quantify the overall effect of immunity on treatment efficacy, or assess publication bias. Furthermore, formal investigations of the presence of drug resistance markers and endemicity and other established cofactors influencing treatment success such as pharmacokinetic exposure, host genetics, and parasitaemia could not be assessed (and also rely on all included studies determining these parameters). Importantly, no study investigated the effect of acquired immunity on treatment outcomes for
P. vivax infection, which is the most widely distributed
Plasmodium species and is responsible for a significant proportion of the clinical burden of malaria in Southeast Asia [
41]. Future studies addressing the association between immunity and treatment of non-
falciparum cases are warranted.
This systematic review provides evidence that naturally acquired antibodies to blood-stage malaria are associated with reduced treatment failure to anti-malarials with different pharmacokinetic-pharmacodynamic properties. Immunity is therefore an important confounder in the assessment of treatment failures and emerging anti-malarial drug resistance in malaria endemic populations.
Authors’ contributions
JM performed the database searches, extracted the data and prepared the first manuscript with KO. KO performed data analysis, interpreted the data and prepared the final manuscript with FJIF and JAS. JM, JAS and FJIF conceived the systematic review protocol. All authors contributed to and approved the final version of the manuscript. All authors read and approved the final manuscript.