Antimalarial Drugs

Antimalarial drugs are pharmacological agents designed to inhibit the growth, reproduction, or survival of Plasmodium parasites, which cause malaria. The therapeutic goal is to clear the parasite from the host to prevent morbidity and mortality. The primary species causing severe disease in humans is Plasmodium falciparum, while Plasmodium vivax is widespread and notable for its ability to cause relapse due to a persistent liver stage. Understanding the pharmacology of these agents requires dist

The clinical management of malaria requires a rigorous assessment of toxicity, particularly when balancing the efficacy of established treatments against potential host harm. Traditional 4-aminoquinolines, such as chloroquine, are historically favoured for their limited host toxicity. However, other classes, specifically the 8-aminoquinolines like primaquine, present significant risks. Primaquine is a potent tissue schizontocide, but its clinical utility is constrained by serious side effects, as it is known to induce hemolysis especially in glucose-6- phosphate dehydrogenase (G6PD)-deficient indivi duals.

In the context of pregnancy, the safety profile of antimalarials is often obscured by inconsistent reporting. In a comprehensive review of chemoprevention in pregnancy, only five trials explicitly stated that no adverse effects attributable to the drugs were observed in the mothers. In the majority of studies, information regarding adverse events was either unclear or entirely absent. Illustrative example: If a pregnant patient is prescribed sulfadoxine-pyrimethamine (SP) for intermittent preventive treatment, the clinician should be aware that while four trials reported adverse events following SP, the lack of systematic documentation across the literature suggests that the true incidence of adverse effects is likely underestimated. ^[7]

Modern drug design is actively seeking to overcome these toxicity limitations. Rational drug design has led to the development of hybrid molecules, which are single entities containing dual functionality or multiple targets. These molecules aim to achieve selective toxicity, ensuring the drug is safely delivered to the parasite while sparing the host cell. For instance, the trioxaquine prototype DU 1302 has demonstrated superior gametocytocidal activity compared to artesunate while lacking toxicity in both HCT cell lines and mouse models. Although these hybrid drugs have not yet reached clinical application, they represent a significant advancement in the effort to minimize host toxicity compared to traditional agents.

The Plasmodium life cycle involves distinct stages: the exoerythrocytic (liver) stage and the erythrocytic (blood) stage. Drugs target specific biochemical pathways or structural components of the parasite at these stages. Primaquine is currently the only drug that cures the parasite liver stage, but it requires 14 days to be effective and can cause haemolysis in people with G6PD deficiency. In addition, there is some evidence of parasite resistance to the drug. Tafenoquine is a new alternative with a longer half-life that addresses some of these limitations. The table below summarizes key antimalarial agents, their primary targets, and associated clinical considerations based on available evidence. ^{[4] [6]}

For travellers, the choice of antimalarial agent depends on individual assessment of adverse effects, pill burden, and cost. Currently, the baseline efficacy of doxycycline, atovaquone-proguanil, and mefloquine when used as prophylaxis is thought to be similar. Most guidelines recommend selecting prophylaxis based on individual choice, pre-existing conditions, side effect profile, and drug resistance patterns in the destination country. Some travellers prefer mefloquine for its once-weekly regim ^[3][5]

Artemisinin-based combination therapies (ACTs) are defined by the pairing of two antimalarial drugs that have different modes of action. This design aims to provide rapid parasite clearance from the artemisinin component while the partner drug eliminates remaining parasites, thereby reducing the chance of resistance emergence. The available evidence notes that atovaquone‑proguanil, although a two‑drug formulation, is not equivalent to an ACT because proguanil’s primary role is to potentiate atovaquone rather than to exert direct antimalarial activity itself. In comparative research, atovaquone ^[3]

Malaria chemoprophylaxis is defined as the use of antimalarial medication to prevent the clinical symptoms of malaria. This is because no drugs are able to prevent the introduction of infection by destroying the sporozoites injected by the female Anopheles mosquito. Chemoprophylaxis is one of several tools used to prevent malaria; other recommended measures include sleeping under insecticide‑treated bed nets, wearing insecticide‑treated clothing, and applying chemical repellent sprays to the skin surface. None of these methods provide complete protection and a combination of approaches is advised. ^[5]

In clinical trials, specific formulations include chloroquine phosphate (300-mg tablets and 150-mg tablets), tafenoquine (150-mg tablets), and primaquine phosphate (15-mg encapsulated tablets). All drugs and placebo were administered orally with food and were provided without cost to all participants. Dosages are given for the free-base form. ^[6]

For special populations, such as children, pregnancy, older adults, or those in low-resource settings, the choice of ACT or prophylaxis must account for local resistance patterns. Since very large trials are necessary to exclude clinically important differences among antimalarial agents, local epidemiological data remains the primary guide. ^[5]

Quinolines constitute a major class of antimalarial agents, acting primarily by interfering with the parasite's ability to detoxify heme. While chloroquine remains a cornerstone for sensitive Plasmodium falciparum and P. vivax infections, resistance has driven the adoption of alternative quinolines and combination therapies. This highlights that monotherapies

Primaquine and tafenoquine are 8-aminoquinolines used specifically for the radical cure of P. vivax by targeting liver-stage hypnozoites. Tafenoquine offers a single-dose regimen, whereas primaquine requires a 14-day course. In the DETECTIVE trial, participants with normal G6PD activity were randomized to receive tafenoquine, primaquine, or placebo, alongside open-label chloroquine. The chloroquine regimen consisted of 600 mg on days 1 and 2, and 300 mg on day 3. Tafenoquine was administered as a single 300 mg dose, while primaquine was given at 15 mg daily for 14 days. Efficacy was measured by the percentage of patients free from recurrence at 6 months. The tafenoquine group achieved a 62.4% recurrence-free rate, compared to 69.6% for the primaquine group and 27.7% for placebo. The hazard ratio for recurrence with tafenoquine compared to placebo was 0.30, and for primaquine compared to placebo, it was 0.26. Tafenoquine was associated with asymptomatic declines in hemoglobin levels that resolved without intervention. Safety monitoring revealed that coadministration with chloroquine did not exacerbate QTcF prolongation. Ophthalmic examinations showed minor findings, such as unilateral retinal changes in 2 of 65 tafenoquine patients, but no clinically problematic ocular effects were identified. ^[6]

Antimalarial therapy relies on distinct pharmacological mechanisms to target the Plasmodium parasite at various stages of its life cycle. Quinine acts by interfering with the parasite’s ability to detoxify haem, leading to toxic accumulation that kills the organism. Atovaquone-proguanil operates via a dual mechanism: atovaquone inhibits mitochondrial electron transport, while proguanil blocks dihydrofolate reductase, an antifolate enzyme essential for nucleic acid synthesis. This combination provides a synergistic effect against blood-stage parasites. Antifolates, such as pyrimethamine or proguanil, specifically inhibit folate metabolism, disrupting DNA replication. Understanding these mechanisms is crucial for selecting agents based on resistance patterns and patient history. ^[3]

Pharmacokinetic properties influence dosing and efficacy. Atovaquone-proguanil is highly lipophilic, requiring food for optimal absorption, which ensures adequate plasma concentrations for prophylaxis and treatment. Quinine has a narrow therapeutic index, necessitating careful monitoring of serum levels to avoid toxicity. The half-life of these drugs varies significantly; for instance, mefloquine has a long half-life, allowing for weekly dosing, whereas doxycycline requires daily administration. These differences impact patient compliance and the window for effective prophylaxis after exposure.

Adverse effects profiles differ markedly among antimalarials, influencing clinical choice. Serious adverse effects have been reported for mefloquine and doxycycline, but not for atovaquone-proguanil. However, serious adverse effects are uncommon across all agents. While mefloquine is widely acknowledged to cause abnormal dreams and psychological adverse effects, atovaquone-proguanil is generally better tolerated regarding neuropsychiatric symptoms. Doxycycline is associated with increased risk of photosensitivity, dyspepsia, and vaginal thrush. Participants who received mefloquine were more likely to discontinue treatment due to adverse effects than participants who received atovaquone-proguanil. No difference was found in numbers of serious adverse effects with mefloquine and doxycycline. ^{[5] [3]}

The choice between antimalarial agents will therefore depend on how individual travellers rate the relative importance of specific adverse effects, pill burden and cost. For safety, nine trials used interviews with study personnel to elicit adverse events. Eleven trials actively sought adverse events, and all 13 collected data prospectively. In cohort studies reporting grouped adverse effects, there was no difference between groups for the overall number of adverse effects (RR 0.93, 95% CI 0.74 to 1.17; 12 cohort studies, 13,576 participants). Of the cohort studies, mefloquine users were more likely to experience 'cardiovascular' adverse effects (RR 7.32, 95% CI 1.06 to 50.42; 1 cohort study, 316 participants), 'constitutional' adverse effects (RR 13.53, 95% CI 1.89 to 96.60; 1 cohort study, 477 participants), 'gastrointestinal' adverse effects (RR 1.99, 95% CI 1.09 to 3.60; 2 cohort studies, 793 participants) and 'neuropsychologic' adverse effects (RR 8.48, 95% CI 3.18 to 22.62; 3 cohort studies, 1021 participants). Overall participants who took mefloquine were more likely to experience any adverse effect (RR 2.40, 95% CI 1.84 to 3.13; 10 cohort studies; 5404 participants). ^[5]

Randomized controlled trials (RCTs) in people with P. vivax malaria. Adverse effects of tafenoquine are assessed in populations where people with G6PD deficiency have been excluded, and in populations without screening for G6PD deficiency. Study authors often use the terms 'adverse event', 'adverse effect' or 'side effect' interchangeably and loosely. We found estimating the risk of serious psychological adverse effects from the studies was not straightforward. Study authors used the term 'serious' loosely, and often did not provide us with the detail required to determine whether these events met standardized definitions. Furthermore, the estimates of the absolute risk in both mefloquine and comparator arm varied considerably between trials, which may be related to data collection methods and the cut‐offs used rather than true differences among populations. Overall, we did not identify large differences in the risk of serious adverse effects among antimalarial agents; but what we did find was that the nature of these serious adverse effects corresponded with the known side effect profile of each drug. ^{[4] [5]}

Antimalarial drug resistance in Plasmodium falciparum continues to evolve, particularly in the Greater Mekong subregion, making treatment increasingly difficult and threatening elimination targets. Artemisinin resistance, first recognised in 2007 in western Cambodia, is closely associated with mutations in the pfkelch gene propeller region on chromosome 13. These mutations result in reduced ring stage susceptibility and slow parasite clearance. Initially, multiple independent emergences occurred, but hard selective sweeps by fitter parasites have gradually replaced the first soft selective sweeps. This reduced parasite killing increases selective pressure on artemisinin combination therapy (ACT) partner drugs, leading to subsequent resistance. For instance, amplification of pfplasmepsin2-3 and mutations in the pfcrt gene (positions 72–76 with Lys76Thr) are associated with piperaquine resistance. Similarly, pfmdr1 amplification is a well-established marker of mefloquine resistance, which also contributes to reduced lumefantrine susceptibility in southeast Asia. The spread of resistant lineages from eastern to western Greater Mekong subregion remains a concern, particularly regarding the role of intensive dihydroartemisinin–piperaquine deployment in mass drug administrations. However, large-scale investigations suggest that such mass treatments have not selected resistance further in the short term. ^[2]

For travellers, the choice of antimalarial agent depends on individual assessment of adverse effects, pill burden, and cost. Currently, the baseline efficacy of doxycycline, atovaquone-proguanil, and mefloquine when used as prophylaxis is thought to be similar. Most guidelines recommend selecting prophylaxis based on individual choice, pre-existing conditions, side effect profile, and drug resistance patterns in the destination country. Some travellers prefer mefloquine for its once-weekly regim

Adherence is critical, as nearly all cases of fatal malaria in travellers occur due to non-adherence with prophylactic measures. However, tolerability must be balanced against efficacy; the frequency of mild to moderate adverse drug reactions varies from 32% to 45%. Self-reported complete adherence was slightly higher in short-term travellers taking mefloquine compared to other agents (RR 1.10, 95% CI 1.03 to 1.18; 7 cohort studies, 7241 participants). However, the same effect was not seen in longer‐term travellers (RR 1.20, 95% CI 0.88 to 1.62; 4 cohort studies, 4890 participants, test for subgroup differences P = 0.61). ^[5]

The pharmacodynamic profile of antimalarial agents is defined by their ability to target specific stages of the parasite life cycle and their molecular interaction with parasite physiology. A primary mechanism involves the interference with hemoglobin digestion within the blood stages of the malaria parasite. This is the established mode of action for 4-aminoquinolines like chloroquine. Resistance to this class does not arise from an altered target, but rather through a compound efflux mechanism that prevents the drug from reaching its site of action. ^[8]

In contrast, the artemisinin derivatives, which include arteether, artemether, artelinic acid, and artesunate, function via an endoperoxide bridge. This 1,2,4-trioxane heterocycle is essential for their potent, rapid parasite clearance and broad stage specificity. Artemisinins are particularly effective because they target early asexual forms of the parasite, which contributes to their utility in combination therapy. While the detailed mechanism of action of sesquiterpene lactone artemisinin remains to be fully elucidated, it is widely accepted that its activity resides on the endoperoxide function. ^[8]

Modern drug design is increasingly focused on "covalent bitherapy," where two pharmacophores are linked into a single hybrid molecule. This approach aims to improve efficacy and reduce toxicity by controlling pharmacokinetic properties. For example, trioxaquines have demonstrated superior antimalarial activity compared to the individual components administered in double combination. Other agents, such as 5-fluoroorotate, target thymidylate synthase, although their clinical application is currently limited by significant toxicity concerns. ^[8]

Regarding stage-specific activity, certain compounds are classified as gametocytocides, meaning they target the sexual forms of the parasite to interrupt transmission. Notable examples with remarkable gametocytocidal activity include artesunate, synthetic trioxaquines like DU1302, and primaquine. In clinical practice, targeting these sexual stages is vital for public health, as it prevents the transmission of the parasite from the human host to the mosquito vector. ^[8]

The pharmacokinetic properties of antimalarial drugs are critical to their clinical success, particularly concerning the duration of action and the necessity for regular dosing. A major limitation of artemisinin semi-synthetics is their short half-life. This pharmacokinetic profile necessitates frequent dosing, which can lead to patient non-compliance and subsequent recrudescence of the infection. To mitigate this, clinicians rely on ACT, where the rapid-acting artemisinin is paired with a long-acting partner drug to ensure sustained therapeutic levels.

The development of hybrid molecules represents a strategic attempt to optimize these pharmacokinetic parameters. By covalently linking pharmacophores, researchers aim to create prodrugs that possess superior bioavailability compared to individual drugs used in combination. Furthermore, these hybrid structures can be engineered to bypass resistance mechanisms that rely on drug efflux, as seen in the ongoing efforts to improve the lipophilicity of 4-aminoquinolines by linking them with cationic amino acids like lysine and ornithine. This modification is necessary because, after bioactivation, standard analogs are often dealkylated, which significantly lowers their lipid solubility and reduces their ability to cross parasite membranes.

When evaluating new regimens, it is essential to systematically collect data on adverse effects to ensure clinical safety. While many compounds are evaluated for their potential, toxicity often prevents them from reaching clinical trials. Future chemotherapy aims to develop innovative compounds that utilize new chemical scaffolds and molecular targets to combat the global emergence of drug resistance. Malaria is a disease that affects nearly 40% of the global population, and chemotherapy remains the mainstay of its control strategy. ^[8]

Illustrative example: In a clinical setting, a patient with P. falciparum infection is treated with an ACT regimen. The artemisinin component provides rapid clearance of early asexual forms, while the long-acting partner drug provides sustained coverage to prevent recrudescence caused by the short half-life of the artemisinin component. If the parasite exhibits 4-aminoquinoline resistance, the clinician avoids monotherapy with that class, as the efflux mechanism renders it ineffective regardless of the dosage. In pregnant women, chemoprevention regimens are specifically evaluated for their protective efficacy against malaria and its consequences for both mother and baby, compared against placebo or case-management strategies.

For uncomplicated malaria, artemisinin-based combination therapies (ACTs) are the first-line treatment recommended by the WHO. A 2017 non-Cochrane systematic review estimated the efficacy of atovaquone-proguanil for treating uncomplicated malaria, including P. falciparum. This review combined randomised controlled trials (RCTs) and observational studies, using single-arm weighted means to estimate treatment success at day 28. The authors noted that meta-analysis suggests atovaquone-proguanil treatment success is equivalent to the use of ACT. However, the definition of 'treatment success' was unclear in this review. ^[3]

In contrast, some reviews have excluded certain studies due to protocol amendments or unclear reasons, highlighting the importance of rigorous study selection. The assessment of antimalarial drug efficacy often follows standard WHO approaches, which include clinical and parasitological response (ACPR). ACPR is defined as the absence of failure, ensuring that both clinical symptoms and parasitemia are resolved. This approach ensures consistency with previous Cochrane systematic reviews of malaria treatment. ^[3]

Safety profiles of antimalarials are crucial for long-term use, particularly in prophylaxis. Mefloquine has been associated with neuropsychiatric adverse events. Our findings are broadly consistent with previous versions of this Cochrane Review, which found higher rates of neuropsychiatric adverse events in mefloquine users compared with users of both atovaquone-proguanil and doxycycline^[1]. We expanded on this finding by providing estimated risks for specific neurological and psychiatric symptoms^[1]. Additionally, case reports of deaths associated with mefloquine have been addressed in separate reviews^[1]. ^[5]

Systematic reviews of antimalarials often face limitations in data pooling. One review included 4250 participants of varying ages and genders across Africa, South America, and South East Asia. However, the broad number of comparisons and outcomes, including antimalarials not currently recommended by the WHO, limited the extent to which data could be meaningfully pooled. Furthermore, standard WHO approaches for assessing efficacy may exclude a high number of randomized participants from the denominator for the final efficacy outcome. To restore integrity of randomization, sensitivity analyses are planned, though small trial numbers may limit their impact. ^[3]

When evaluating antimalarial therapies, laboratory investigations serve two distinct purposes: confirming the diagnosis and monitoring treatment efficacy or safety. For efficacy assessment, clinical trials often rely on polymerase chain reaction (PCR) genotyping to distinguish between recrudescent infections (the original parasite returning) and new infections acquired during the follow-up period. This differentiation is critical because unadjusted treatment failure rates can obscure the post-treatment prophylactic effect of partner drugs, whereas PCR adjustment provides a clearer picture of true drug failure. To capture most failures, guidelines advise a 28-day follow-up, extending to 42 days for drugs with longer elimination half-lives, such as mefloquine and piperaquine. ^{[3] [6]}

Safety monitoring requires specific laboratory tests, particularly for drugs that cause haemolysis. Glucose-6-phosphate dehydrogenase (G6PD) status is a key determinant for the safe use of certain antimalarials. In clinical studies, G6PD enzyme levels are often measured at screening and at specific intervals, such as days 60 and 120, with results averaged to ensure accuracy. For female participants, G6PD genotyping may be performed at a central laboratory to identify variants, while male participants are typically genotyped only if they exhibit a significant haemoglobin decline, defined as a drop of 3 g or more per deciliter. Although qualitative point-of-care tests for G6PD activity exist and may be used at screening, they are not always sufficient to determine eligibility for specific drug regimens. ^{[6] [4]}

Pharmacogenomic investigations also play a role in predicting drug metabolism and adverse events. For instance, whole-blood specimens collected in EDTA tubes can be used for genotype analysis of cytochrome P-450 2D6 (CYP2D6), which helps understand how patients metabolize certain antimalarial agents. These laboratory data are essential for assessing the risk of bias in study conduct, where the adequacy of the number and timing of tests is a standard criterion for evaluating data quality. ^[6]

Resistance monitoring is another vital laboratory function. The development of drug resistance, particularly to atovaquone-proguanil, is a major barrier to antimalarial use. Strategies to delay this emergence include targeting treatment at areas with low levels of mutant genes, which requires molecular surveillance to track these genetic markers. While dried blood spots on filter paper are commonly used for PCR genotyping at screening and recurrence, the homology or heterology of the parasites is assessed using published markers to ensure accurate interpretation of treatment outcomes. ^{[6] [3]}

Malaria remains a significant global health challenge, with an estimated 216 million cases occurring globally in 2011. In 2010, approximately 655,000 deaths were attributed to the disease. While global eradication efforts have led to a 25% fall in mortality since 2000, the burden persists. Most cases are caused by Plasmodium falciparum and Plasmodium vivax. P. falciparum causes a more severe form of malaria with multi-organ involvement. In contrast, P. vivax is less virulent and seldom causes death, yet it causes substantive illness-related burden in endemic areas. The incidence of P. vivax infection has become particularly important in countries aiming for malaria elimination. ^[4]

For travellers, mefloquine is one of four antimalarial agents commonly recommended for preventing malaria in endemic areas. Despite its high efficacy, controversy exists regarding its psychological side effects. Mefloquine has been licensed for prevention in travellers since the late 1980s. The absolute risk of malaria during short-term travel appears very low with all three established antimalarial agents: mefloquine, doxycycline, and atovaquone-proguanil. The only commonly-used alternative drugs are doxycycline, which can cause skin problems and indigestion, and atovaquone-proguanil, which is often more expensive. ^[5]

In clinical trials, the percentage of people developing a malaria episode in the control arm varied from 1% to 82% (median 22%), while in the mefloquine group it ranged from 0% to 13% (median 1%). Most study populations had a degree of immunity to malaria. ^[5]

G6PD deficiency is another critical factor. Trials included populations where people with G6PD deficiency were excluded, as well as populations without screening for G6PD deficiency. This highlights the need for screening in certain contexts.

The management of malaria relies on a strategic combination of blood schizonticides and tissue schizonticides to achieve both immediate symptom resolution and long-term prevention of relapse. For Plasmodium vivax infections, the World Health Organization recommends a dual approach: first, a blood schizonticide such as chloroquine or an artemisinin-based combination therapy (ACT) is administered to clear the asexual parasites responsible for acute symptoms. This is followed by treatment with the 8-aminoquinoline primaquine for 14 days to kill hypnozoites in the liver, a process termed “radical cure”. Adherence to this 14-day regimen is critical; reported nonadherence rates range from 13.6 to 33.3%, which jeopardizes the effectiveness of the treatment and likely underestimates the real-world situation. To address adherence challenges, tafenoquine, a longer-acting 8-aminoquinoline with a half-life of approximately 15 days, has been registered as a single-dose treatment for the radical cure of P. vivax. ^[6]

In Plasmodium falciparum malaria, the focus shifts to artemisinin-based combination therapies (ACTs) to combat drug resistance. Mass treatment with dihydroartemisinin–piperaquine has been evaluated in Myanmar and Cambodia, where the proportion of isolates with pfplasmepsin2 amplification was low or zero before and after treatment. Crucially, there was no evidence for the selection of pfcrt mutations associated with piperaquine resistance; for instance, pfcrt Gly353Val was found in seven isolates before and seven after mass treatment in Cambodia. Similarly, no evidence of further selection of pfkelch mutations was observed after mass treatment. ^[2]

Clinical trial data highlights the variability in treatment failure rates. In studies comparing artemether-lumefantrine (AL) to artemether-piperaquine (AV+PG), there were fewer PCR-adjusted treatment failures at day 28 following treatment with AL, although the confidence intervals were very wide and crossed the line of no effect. When comparing AV+PG to artesunate-quinine (AS+AQ), there were more PCR-adjusted treatment failures at day 28 following treatment with AV+PG, but the confidence interva ^[3]

For P. falciparum, the absence of resistance markers post-treatment is a positive indicator. In sites in Myanmar and Cambodia, the lack of pfplasmepsin2 amplification and stable pfcrt mutation frequencies suggest that mass treatment with dihydroartemisinin–piperaquine does not drive immediate selection of resistant phenotypes. However, clinicians must remain vigilant. In one study comparing AV+PG to AS+AV+PG, there were more PCR-adjusted treatment failures at day 42 following treatment with AV+PG, with a relative risk of 1.84. This underscores the need for rigorous post-treatment monitoring, typically at days 28 and 42, to detect recrudescence or reinfection. ^[3]

Mefloquine remains an option, particularly in pregnancy. Reviews have concluded there are no indications that mefloquine use during pregnancy carries an increased risk for the foetus. However, when used as intermittent presumptive treatment in pregnancy, the number of adverse events and serious adverse events was higher in participants who received mefloquine than in those who received sulphadoxine-pyrimethamine. Therefore, while safe for treatment, its use for prophylaxis in pregnancy requires careful risk-benefit assessment. ^[5]

The current paradigm of malaria therapy relies heavily on multicomponent drugs, where two or more agents are co-formulated into a single tablet to improve patient compliance. While this approach is effective, it inherently increases the risk of drug-drug adverse interactions compared to single-entity drugs. Future therapeutic strategies, such as the development of hybrid molecules, aim to mitigate these risks by combining different pharmacophores into a single chemical entity, thereby reducing the potential for adverse interactions while simultaneously addressing the rapid development of resistance. ^[8]

Contraindications are frequently dictated by the metabolic pathways of the parasite. Currently, all antifolate antagonists in clinical use target either dihydrofolate reductase or dihydropteroate synthase. Because no antimalarial antifolate currently targets thymidylate synthase (TS), there is a significant opportunity for the development of new, safer combinations that could be covalently linked to ensure selective delivery. Clinicians must also be cautious when extrapolating efficacy data from controlled settings to real-world practice. While evidence for the benefits of chemoprevention—such as reduced anaemia and antenatal parasitaemia—is considered high quality for women of low parity, these effects are often attenuated in observational studies. This attenuation is frequently attributed to the challenges of ensuring consistent delivery and patient adherence in non-trial settings. ^[7]

When managing patients in regions with high resistance, such as the Greater Mekong subregion, clinicians should be aware that the strategy of using "double drugs" or quinoline-chemosensitizers is designed to restore the efficacy of traditional agents like chloroquine. By exploiting the established benefits of 4-aminoquinolines—including low-cost synthesis and short-course therapy—these future therapies aim to provide a robust clinical profile that overcomes the limitations of current drug-resistant strains.

Understanding the pharmacological landscape of antimalarials requires distinguishing between prophylactic strategies and therapeutic interventions, as well as recognizing the specific life-cycle stages targeted by different agents. A critical distinction lies in the concept of 'causal' prophylaxis. Atovaquone-proguanil is commonly used to prevent malaria when travelling. It is a 'causal' prophylactic agent, meaning that it inhibits liver stage development of malaria. This mechanism offers a sign ^[3]

When considering treatment, the choice of agent is heavily influenced by the Plasmodium species involved. Malaria is a parasitic protozoal infection which is usually transmitted through the bite of female Anopheles mosquitoes. It is most common in tropical and subtropical regions. Clinical disease is caused by infection of red blood cells by one of four Plasmodium species: P. falciparum, P. vivax, P. ovale, and P. malariae. Humans can also become infected by forms of malaria that usually infect animals, such as P. knowlesi. While P. falciparum is the most dangerous, P. vivax and P. ovale present a unique challenge due to their ability to form dormant liver stages known as hypnozoites.

Primaquine (PQ), an 8-aminoquinoline, was first licensed for use in the 1950s by the Food and Drug Administration (FDA), United States, for treatment of vivax malaria. It is the only licensed drug capable of eliminating the vivax hypnozoites. Without administration of PQ in adequate doses, complete cure of patients with P. vivax infection is difficult, and patients often have relapses of clinical disease. However, PQ is a unique drug in combating vivax malaria but has side effects, which can sometimes be serious in people with G6PD deficiency. The long duration of treatment also leads to poor adherence. This adherence issue has driven research into alternatives, with tafenoquine (TQ) emerging as a promising candidate. TQ is an 8-aminoquinoline and is a synthetic analogue of PQ. It has potential to be useful in regimens for prophylaxis and radical cure of P. vivax malaria. TQ is a possible alternative that has shown promise in replacing PQ and it can be administered as a single dose or in much shorter treatment regimens. ^[4]

A frequent error among students and practitioners is underestimating the complexity of treatment failure assessment and the nuances of drug safety in specific populations. When evaluating the efficacy of antimalarials, particularly atovaquone-proguanil, one must look closely at treatment failure rates. Atovaquone-proguanil is efficacious against uncomplicated Plasmodium falciparum malaria, but treatment failure exceeded 5% in two studies, the level at which the WHO recommends avoiding adoption o ^[3]

Another significant pitfall involves the management of P. vivax malaria. Students often focus solely on clearing the blood-stage infection with chloroquine or artemisinins, forgetting that without radical cure, relapses are inevitable. It is important to include data for parasitaemia on day three as this is both part of the standardized definition for treatment response used for all levels of malaria transmission, and it is regarded as an indirect marker of artemisinin resistance. While this specific metric relates to artemisinin resistance in P. falciparum, the principle of monitoring early clearance applies broadly. For P. vivax, the failure to administer primaquine or tafenoquine after blood-stage clearance is the most common cause of apparent "treatment failure" or relapse.

Safety considerations also present traps, particularly regarding pregnancy and paediatric use. There are several situations in which mefloquine is potentially advantageous. All guidelines recommend that where avoidable pregnant women should not travel to areas where malaria is endemic. However, where travel is essential, mefloquine is often the preferred option. Mefloquine is widely considered to be safe within the second and third trimesters of pregnancy and guidelines increasingly recommend its use in the first trimester. Mefloquine is suitable for both children who weigh more than 5 kg and breastfeeding mothers. Confusing the safety profiles of different drugs can lead to suboptimal choices; for instance, doxycycline is generally avoided in young children and pregnant women due to effects on bone and teeth, whereas mefloquine offers a viable alternative in these groups when travel cannot be postponed. ^[5]

The application of antimalarial drugs is dictated by the purpose: prevention for travellers and treatment for patients. For prophylaxis, the choice depends on the destination's resistance patterns and the traveller's health profile. Atovaquone-proguanil is commonly used to prevent malaria when travelling. Its causal prophylactic action allows for a convenient one-week post-travel course. In contrast, mefloquine is often reserved for areas with chloroquine-resistant P. falciparum or for travellers who cannot take other agents. Can mefloquine prevent malaria during travel to areas where the disease is widespread? Yes, it remains a potent option, particularly given its safety in pregnancy and young children. ^[3]

In clinical practice, the diagnosis of malaria begins with recognizing that clinical presentation is nonspecific and varied; symptoms include fever, chills, headache, diarrhoea, muscle cramps, and abdominal pain. Severe disease is usually caused by infection with P. falciparum, but can also occur following infection with P. vivax and P. knowlesi. Host factors determining severity include genetics, host immune status, and age. Once diagnosed, the treatment pathway diverges. For uncomplicated P. falciparum, WHO-recommended ACTs are first-line. However, in regions where atovaquone-proguanil is used, clinicians must monitor for the aforementioned 5% failure threshold.

For P. vivax, the practice involves a two-step process: acute treatment followed by radical cure. Primaquine has been the standard for decades, but its long duration and G6PD-related risks are driving a shift towards tafenoquine. TQ is a possible alternative that has shown promise in replacing PQ and it can be administered as a single dose or in much shorter treatment regimens. Therefore, it is important to establish the effects of TQ from available data for preventing relapses of vivax malaria after an acute infection. This shift aims to improve adherence and reduce the burden of daily dosing.