Quantifying primaquine effectiveness and improving adherence: a round table discussion of the APMEN Vivax Working Group

Liver-targeted polymeric prodrugs delivered subcutaneously improve tafenoquine therapeutic window for malaria radical cure

Approximately 3.3 billion people live with the threat of Plasmodium vivax malaria. Infection can result in liver-localized hypnozoites, which when reactivated cause relapsing malaria. This work demonstrates that an enzyme-cleavable polymeric prodrug of tafenoquine addresses key requirements for a mass administration, eradication campaign: excellent subcutaneous bioavailability, complete parasite control after a single dose, improved therapeutic window compared to the parent oral drug, and low cost of goods sold (COGS) at less than $1.50 per dose. Liver targeting and subcutaneous dosing resulted in improved liver:plasma exposure profiles, with increased efficacy and reduced glucose 6-phosphate dehydrogenase-dependent hemotoxicity in validated preclinical models. A COGS and manufacturability analysis demonstrated global scalability, affordability, and the ability to redesign this fully synthetic polymeric prodrug specifically to increase global equity and access. Together, this polymer prodrug platform is a candidate for evaluation in human patients and shows potential for P. vivax eradication campaigns.

Radical cure for Plasmodium vivax malaria after G6PD qualitative testing in four provinces in Cambodia, results from Phase I implementation

BACKGROUND

Cambodia aims to eliminate all forms of malaria by 2025. In 2020, 90% of all malaria cases were Plasmodium vivax. Thus, preventing P. vivax and relapse malaria is a top priority for elimination. 14-day primaquine, a World Health Organization-recommended radical cure treatment regimen, specifically targets dormant hypnozoites in the liver to prevent relapse. Cambodia introduced P. vivax radical cure with primaquine after glucose-6-phosphate dehydrogenase (G6PD) qualitative testing in 2019. This paper presents Cambodia's radical cure Phase I implementation results and assesses the safety, effectiveness, and feasibility of the programme prior to nationwide scale up.

METHODS

Phase I implementation was carried out in 88 select health facilities (HFs) across four provinces. Males over 20kgs with confirmed P. vivax or mixed (P. vivax and Plasmodium falciparum) infections were enrolled. A descriptive analysis evaluated the following: successful referral to health facilities, G6PD testing results, and self-reported 14-day treatment adherence. P. vivax incidence was compared before and after radical cure rollout and a controlled interrupted time series analysis compared the estimated relapse rate between implementation and non-implementation provinces before and after radical cure.

RESULTS

In the 4 provinces from November 2019 to December 2020, 3,239 P. vivax/mixed infections were reported, 1,282 patients underwent G6PD deficiency testing, and 959 patients received radical cure, achieving 29.6% radical cure coverage among all P. vivax/mixed cases and 98.8% coverage among G6PD normal patients. Among those who initiated radical cure, 747 patients (78%) completed treatment. Six patients reported side effects. In implementation provinces, an average 31.8 relapse cases per month were estimated signaling a 90% (286 cases) reduction in relapse compared to what would be expected if radical cure was not implemented.

CONCLUSIONS

Plasmodium vivax radical cure is a crucial tool for malaria elimination in Cambodia. The high coverage of radical cure initiation and adherence among G6PD normal patients demonstrated the high feasibility of providing radical cure at point of care in Cambodia. Incomplete referral from community to HFs and limited capacity of HF staff to conduct G6PD testing in high burden areas led to lower coverage of G6PD testing. Phase I implementation informed approaches to improve referral completion and patient adherence during the nationwide expansion of radical cure in 2021.

Effect of primaquine dose on the risk of recurrence in patients with uncomplicated Plasmodium vivax: a systematic review and individual patient data meta-analysis

BACKGROUND

Primaquine is used to eliminate Plasmodium vivax hypnozoites, but its optimal dosing regimen remains unclear. We undertook a systematic review and individual patient data meta-analysis to investigate the efficacy and tolerability of different primaquine dosing regimens to prevent P vivax recurrence.

METHODS

For this systematic review and individual patient data meta-analysis, we searched MEDLINE, Web of Science, Embase, and Cochrane Central for prospective clinical studies of uncomplicated P vivax from endemic countries published between Jan 1, 2000, and June 8, 2023. We included studies if they had active follow-up of at least 28 days, and if they included a treatment group with daily primaquine given over multiple days, where primaquine was commenced within 7 days of schizontocidal treatment and was given alone or coadministered with chloroquine or one of four artemisinin-based combination therapies (ie, artemether-lumefantrine, artesunate-mefloquine, artesunate-amodiaquine, or dihydroartemisinin-piperaquine). We excluded studies if they were on prevention, prophylaxis, or patients with severe malaria, or if data were extracted retrospectively from medical records outside of a planned trial. For the meta-analysis, we contacted the investigators of eligible trials to request individual patient data and we then pooled data that were made available by Aug 23, 2021. We assessed the effects of total dose and duration of primaquine regimens on the rate of first P vivax recurrence between day 7 and day 180 by Cox's proportional hazards regression (efficacy analysis). The effect of primaquine daily dose on gastrointestinal symptoms on days 5-7 was assessed by modified Poisson regression (tolerability analysis). The study was registered with PROSPERO, CRD42019154470.

FINDINGS

Of 226 identified studies, 23 studies with patient-level data from 6879 patients from 16 countries were included in the efficacy analysis. At day 180, the risk of recurrence was 51·0% (95% CI 48·2-53·9) in 1470 patients treated without primaquine, 19·3% (16·9-21·9) in 2569 patients treated with a low total dose of primaquine (approximately 3·5 mg/kg), and 8·1% (7·0-9·4) in 2811 patients treated with a high total dose of primaquine (approximately 7 mg/kg), regardless of primaquine treatment duration. Compared with treatment without primaquine, the rate of P vivax recurrence was lower after treatment with low-dose primaquine (adjusted hazard ratio 0·21, 95% CI 0·17-0·27; p<0·0001) and high-dose primaquine (0·10, 0·08-0·12; p<0·0001). High-dose primaquine had greater efficacy than low-dose primaquine in regions with high and low relapse periodicity (ie, the time from initial infection to vivax relapse). 16 studies with patient-level data from 5609 patients from ten countries were included in the tolerability analysis. Gastrointestinal symptoms on days 5-7 were reported by 4·0% (95% CI 0·0-8·7) of 893 patients treated without primaquine, 6·2% (0·5-12·0) of 737 patients treated with a low daily dose of primaquine (approximately 0·25 mg/kg per day), 5·9% (1·8-10·1) of 1123 patients treated with an intermediate daily dose (approximately 0·5 mg/kg per day) and 10·9% (5·7-16·1) of 1178 patients treated with a high daily dose (approximately 1 mg/kg per day). 20 of 23 studies included in the efficacy analysis and 15 of 16 in the tolerability analysis had a low or unclear risk of bias.

INTERPRETATION

Increasing the total dose of primaquine from 3·5 mg/kg to 7 mg/kg can reduce P vivax recurrences by more than 50% in most endemic regions, with a small associated increase in gastrointestinal symptoms.

FUNDING

Australian National Health and Medical Research Council, Bill & Melinda Gates Foundation, and Medicines for Malaria Venture.

Tafenoquine following G6PD screening versus primaquine for the treatment of vivax malaria in Brazil: A cost-effectiveness analysis using a transmission model

BACKGROUND

Malaria transmission modelling has demonstrated the potential impact of semiquantitative glucose-6-phosphate dehydrogenase (G6PD) testing and treatment with single-dose tafenoquine for Plasmodium vivax radical cure but has not investigated the associated costs. This study evaluated the cost-effectiveness of P. vivax treatment with tafenoquine after G6PD testing using a transmission model.

METHODS AND FINDINGS

We explored the cost-effectiveness of using tafenoquine after G6PD screening as compared to usual practice (7-day low-dose primaquine (0.5 mg/kg/day) without G6PD screening) in Brazil using a 10-year time horizon with 5% discounting considering 4 scenarios: (1) tafenoquine for adults only assuming 66.7% primaquine treatment adherence; (2) tafenoquine for adults and children aged >2 years assuming 66.7% primaquine adherence; (3) tafenoquine for adults only assuming 90% primaquine adherence; and (4) tafenoquine for adults only assuming 30% primaquine adherence. The incremental cost-effectiveness ratios (ICERs) were estimated by dividing the incremental costs by the disability-adjusted life years (DALYs) averted. These were compared to a willingness to pay (WTP) threshold of US$7,800 for Brazil, and one-way and probabilistic sensitivity analyses were performed. All 4 scenarios were cost-effective in the base case analysis using this WTP threshold with ICERs ranging from US$154 to US$1,836. One-way sensitivity analyses showed that the results were most sensitive to severity and mortality due to vivax malaria, the lifetime and number of semiquantitative G6PD analysers needed, cost per malaria episode and per G6PD test strips, and life expectancy. All scenarios had a 100% likelihood of being cost-effective at the WTP threshold. The main limitations of this study are due to parameter uncertainty around our cost estimates for low transmission settings, the costs of G6PD screening, and the severity of vivax malaria.

CONCLUSIONS

In our modelling study that incorporated impact on transmission, tafenoquine prescribed after a semiquantitative G6PD testing was highly likely to be cost-effective in Brazil. These results demonstrate the potential health and economic importance of ensuring safe and effective radical cure.

Effect of adherence to primaquine on the risk of Plasmodium vivax recurrence: a WorldWide Antimalarial Resistance Network systematic review and individual patient data meta-analysis

BACKGROUND

Imperfect adherence is a major barrier to effective primaquine radical cure of Plasmodium vivax. This study investigated the effect of reduced adherence on the risk of P. vivax recurrence.

METHODS

Efficacy studies of patients with uncomplicated P. vivax malaria, including a treatment arm with daily primaquine, published between January 1999 and March 2020 were identified. Individual patient data from eligible studies were pooled using standardized methodology. Adherence to primaquine was inferred from i) the percentage of supervised doses and ii) the total mg/kg dose received compared to the target total mg/kg dose per protocol. The effect of adherence to primaquine on the incidence of P. vivax recurrence between days 7 and 90 was investigated by Cox regression analysis.

RESULTS

Of 82 eligible studies, 32 were available including 6917 patients from 18 countries. For adherence assessed by percentage of supervised primaquine, 2790 patients (40.3%) had poor adherence (≤ 50%) and 4127 (59.7%) had complete adherence. The risk of recurrence by day 90 was 14.0% [95% confidence interval: 12.1-16.1] in patients with poor adherence compared to 5.8% [5.0-6.7] following full adherence; p = 0.014. After controlling for age, sex, baseline parasitaemia, and total primaquine dose per protocol, the rate of the first recurrence was higher following poor adherence compared to patients with full adherence (adjusted hazard ratio (AHR) = 2.3 [1.8-2.9]). When adherence was quantified by total mg/kg dose received among 3706 patients, 347 (9.4%) had poor adherence, 88 (2.4%) had moderate adherence, and 3271 (88.2%) had complete adherence to treatment. The risks of recurrence by day 90 were 8.2% [4.3-15.2] in patients with poor adherence and 4.9% [4.1-5.8] in patients with full adherence; p < 0.001.

CONCLUSION

Reduced adherence, including less supervision, increases the risk of vivax recurrence.

Supervised administration of primaquine may enhance adherence to radical cure for P. vivax malaria in India

The Plasmodium vivax lifecycle encompasses a dormant liver-stage known as 'hypnozoite' which serves as silent reservoirs of malaria, reactivation of which results in recurring episodes of relapse with varying periodicity. This contributes to continuous transmission of malaria unamenable to control methods. The prevention of relapse requires a "radical cure" by a hypnozoitcidal drug. Primaquine (PQ) has been the recommended radical cure for this malaria. However, adherence to 14 days PQ treatment remains poor. India accounts for majority of P. vivax burden globally. However, PQ administration is not supervised in the current national programme. Supervised administration of drugs ensures compliance and improves drug regime success rate. Trials across different countries have established the effectiveness of directly observed therapy (DOT) for prevention of relapses. As India aims to eliminate malaria by 2030, it is prudent to consider DOT to ensure complete treatment of the malaria affected populations. Therefore, we recommend that the Indian malaria control programme may consider DOT of primaquine for treatment of vivax malaria. The supervised administration would entail additional direct and indirect costs but will ensure complete treatment and hence minimize the probability of relapses. This will help the country in achieving the goal of malaria elimination.

A Review of the Current Status of G6PD Deficiency Testing to Guide Radical Cure Treatment for Vivax Malaria

Plasmodium vivax malaria continues to cause a significant burden of disease in the Asia-Pacific, the Horn of Africa, and the Americas. In addition to schizontocidal treatment, the 8-aminoquinoline drugs are crucial for the complete removal of the parasite from the human host (radical cure). While well tolerated in most recipients, 8-aminoquinolines can cause severe haemolysis in glucose-6-phosphate dehydrogenase (G6PD) deficient patients. G6PD deficiency is one of the most common enzymopathies worldwide; therefore, the WHO recommends routine testing to guide 8-aminoquinoline based treatment for vivax malaria whenever possible. In practice, this is not yet implemented in most malaria endemic countries. This review provides an update of the characteristics of the most used G6PD diagnostics. We describe the current state of policy and implementation of routine point-of-care G6PD testing in malaria endemic countries and highlight key knowledge gaps that hinder broader implementation. Identified challenges include optimal training of health facility staff on point-of-care diagnostics, quality control of novel G6PD diagnostics, and culturally appropriate information and communication with affected communities around G6PD deficiency and implications for treatment.

Individual variation in Plasmodium vivax malaria risk: Are repeatedly infected people just unlucky?

Extensive research has examined why some people have frequent Plasmodium falciparum malaria episodes in sub-Saharan Africa while others remain free of disease most of the time. In contrast, malaria risk heterogeneity remains little studied in regions where P. vivax is the dominant species. Are repeatedly infected people in vivax malaria settings such as the Amazon just unlucky? Here, we briefly review evidence that human genetic polymorphism and acquired immunity after repeated exposure to parasites can modulate the risk of P. vivax infection and disease in predictable ways. One-fifth of the hosts account for 80% or more of the community-wide vivax malaria burden and contribute disproportionally to onward transmission, representing a priority target of more intensive interventions to achieve malaria elimination. Importantly, high-risk individuals eventually develop clinical immunity, even in areas with very low or residual malaria transmission, and may constitute a large but silent parasite reservoir.

Plasmodium vivax in Children: Hidden Burden and Conspicuous Challenges, a Narrative Review

There has been progress towards decreasing malaria prevalence globally; however, Plasmodium vivax has been less responsive to elimination efforts compared with Plasmodium falciparum. P. vivax malaria remains a serious public health concern in regions where it is the dominant species (South and South-East Asia, the Eastern Mediterranean region, and South America) and is increasingly recognized for its contribution to overall morbidity and mortality worldwide. The incidence of P. vivax decreases with increasing age owing to rapidly acquired clinical immunity and there is a disproportionate burden of P. vivax in infants and children, who remain highly vulnerable to severe disease, recurrence, and anemia with associated developmental impacts. Diagnosis is sometimes difficult owing to the sensitivity of diagnostic tests to detect low levels of parasitemia. Additionally, the propensity of P. vivax to relapse following reactivation of dormant hypnozoites in the liver contributes to disease recurrence in infants and children, and potentiates morbidity and transmission. The 8-aminoquinolines, primaquine and tafenoquine, provide radical cure (relapse prevention). However, the risk of hemolysis in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency necessitates testing prior to administration of 8-aminoquinolines, which has limited their uptake. Additional challenges include lack of availability of pediatric dose formulations and problems with adherence to primaquine owing to the length of treatment recommended. A paucity of data and studies specific to pediatric P. vivax malaria impacts the ability to deliver targeted interventions. It is imperative that P. vivax in infants and children be the focus of future research, control initiatives, and anti-malarial drug development.

Spatial patterns and climate drivers of malaria in three border areas of Brazil, Venezuela and Guyana, 2016-2018

In 2020, 77% of malaria cases in the Americas were concentrated in Venezuela, Brazil, and Colombia. These countries are characterized by a heterogeneous malaria landscape and malaria hotspots. Furthermore, the political unrest in Venezuela has led to significant cross-border population movement. Hence, the aim of this study was to describe spatial patterns and identify significant climatic drivers of malaria transmission along the Venezuela-Brazil-Guyana border, focusing on Bolivar state, Venezuela and Roraima state, Brazil. Malaria case data, stratified by species from 2016 to 2018, were obtained from the Brazilian Malaria Epidemiology Surveillance Information System, the Guyana Vector Borne Diseases Program, the Venezuelan Ministry of Health, and civil society organizations. Spatial autocorrelation in malaria incidence was explored using Getis-Ord (Gi*) statistics. A Poisson regression model was developed with a conditional autoregressive prior structure and posterior parameters were estimated using the Bayesian Markov chain Monte Carlo simulation with Gibbs sampling. There were 685,498 malaria cases during the study period. Plasmodium vivax was the predominant species (71.7%, 490,861). Malaria hotspots were located in eight municipalities along the Venezuela and Guyana international borders with Brazil. Plasmodium falciparum increased by 2.6% (95% credible interval [CrI] 2.1%, 2.8%) for one meter increase in altitude, decreased by 1.6% (95% CrI 1.5%, 2.3%) and 0.9% (95% CrI 0.7%, 2.4%) per 1 cm increase in 6-month lagged precipitation and each 1 °C increase of minimum temperature without lag. Each 1 °C increase of 1-month lagged maximum temperature increased P. falciparum by 0.6% (95% CrI 0.4%, 1.9%). P. vivax cases increased by 1.5% (95% CrI 1.3%, 1.6%) for one meter increase in altitude and decreased by  1.1% (95% CrI 1.0%, 1.2%) and 7.3% (95% CrI 6.7%, 9.7%) for each 1 cm increase of precipitation lagged at 6-months and 1 °C increase in minimum temperature lagged at 6-months. Each 1°C increase of two-month lagged maximum temperature increased P. vivax by 1.5% (95% CrI 0.6%, 7.1%). There was no significant residual spatial clustering after accounting for climatic covariates. Malaria hotspots were located along the Venezuela and Guyana international border with Roraima state, Brazil. In addition to population movement, climatic variables were important drivers of malaria transmission in these areas.

Variation in Glucose-6-Phosphate Dehydrogenase activity following acute malaria

Primaquine and tafenoquine are the only licensed drugs with activity against Plasmodium vivax hypnozoites but cause haemolysis in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency. Malaria also causes haemolysis, leading to the replacement of older erythrocytes with low G6PD activity by reticulocytes and young erythrocytes with higher activity. Aim of this study was to assess the impact of acute malaria on G6PD activity. Selected patients with uncomplicated malaria were recruited in Bangladesh (n = 87), Indonesia (n = 75), and Ethiopia (n = 173); G6PD activity was measured at the initial presentation with malaria and a median of 176 days later (range 140 to 998) in the absence of malaria. Among selected participants (deficient participants preferentially enrolled in Bangladesh but not at other sites) G6PD activity fell between malaria and follow up by 79.1% (95%CI: 40.4 to 117.8) in 6 participants classified as deficient (<30% activity), 43.7% (95%CI: 34.2 to 53.1) in 39 individuals with intermediate activity (30% to <70%), and by 4.5% (95%CI: 1.4 to 7.6) in 290 G6PD normal (≥70%) participants. In Bangladesh and Indonesia G6PD activity was significantly higher during acute malaria than when the same individuals were retested during follow up (40.9% (95%CI: 33.4-48.1) and 7.4% (95%CI: 0.2 to 14.6) respectively), whereas in Ethiopia G6PD activity was 3.6% (95%CI: -1.0 to -6.1) lower during acute malaria. The change in G6PD activity was apparent in patients presenting with either P. vivax or P. falciparum infection. Overall, 66.7% (4/6) severely deficient participants and 87.2% (34/39) with intermediate deficiency had normal activities when presenting with malaria. These findings suggest that G6PD activity rises significantly and at clinically relevant levels during acute malaria. Prospective case-control studies are warranted to confirm the degree to which the predicted population attributable risks of drug induced haemolysis is lower than would be predicted from cross sectional surveys.

Developing sero-diagnostic tests to facilitate Plasmodium vivax Serological Test-and-Treat approaches: modeling the balance between public health impact and overtreatment

BACKGROUND

Eliminating Plasmodium vivax will require targeting the hidden liver-stage reservoir of hypnozoites. This necessitates new interventions balancing the benefit of reducing vivax transmission against the risk of over-treating some individuals with drugs which may induce haemolysis. By measuring antibodies to a panel of vivax antigens, a strategy of serological-testing-and-treatment (PvSeroTAT) can identify individuals with recent blood-stage infections who are likely to carry hypnozoites and target them for radical cure. This provides a potential solution to selectively treat the vivax reservoir with 8-aminoquinolines.

METHODS

PvSeroTAT can identify likely hypnozoite carriers with ~80% sensitivity and specificity. Diagnostic test sensitivities and specificities ranging 50-100% were incorporated into a mathematical model of vivax transmission to explore how they affect the risks and benefits of different PvSeroTAT strategies involving hypnozoiticidal regimens. Risk was measured as the rate of overtreatment and benefit as reduction of community-level vivax transmission.

RESULTS

Across a wide range of combinations of diagnostic sensitivity and specificity, PvSeroTAT was substantially more effective than bloodstage mass screen and treat strategies and only marginally less effective than mass drug administration. The key test characteristic determining of the benefit of PvSeroTAT strategies is diagnostic sensitivity, with higher values leading to more hypnozoite carriers effectively treated and greater reductions in vivax transmission. The key determinant of risk is diagnostic specificity: higher specificity ensures that a lower proportion of uninfected individuals are unnecessarily treated with primaquine. These relationships are maintained in both moderate and low transmission settings (qPCR prevalence 10% and 2%). Increased treatment efficacy and adherence can partially compensate for lower test performance. Multiple rounds of PvSeroTAT with a lower performing test may lead to similar or higher reductions in vivax transmission than fewer rounds with a higher performing test, albeit with higher rate of overtreatment.

CONCLUSIONS

At current performance, PvSeroTAT is predicted to be a safe and efficacious option for targeting the hypnozoite reservoir towards vivax elimination. P. vivax sero-diagnostic tests should aim for both high performance and ease of use in the field. The target product profiles informing such development should thus reflect the trade-offs between impact, overtreatment, and ease of programmatic implementation.

Repeatability and reproducibility of a handheld quantitative G6PD diagnostic

BACKGROUND

The introduction of novel short course treatment regimens for the radical cure of Plasmodium vivax requires reliable point-of-care diagnosis that can identify glucose-6-phosphate dehydrogenase (G6PD) deficient individuals. While deficient males can be identified using a qualitative diagnostic test, the genetic make-up of females requires a quantitative measurement. SD Biosensor (Republic of Korea) has developed a handheld quantitative G6PD diagnostic (STANDARD G6PD test), that has approximately 90% accuracy in field studies for identifying individuals with intermediate or severe deficiency. The device can only be considered for routine care if precision of the assay is high.

METHODS AND FINDINGS

Commercial lyophilised controls (ACS Analytics, USA) with high, intermediate, and low G6PD activities were assessed 20 times on 10 Biosensor devices and compared to spectrophotometry (Pointe Scientific, USA). Each device was then dispatched to one of 10 different laboratories with a standard set of the controls. Each control was tested 40 times at each laboratory by a single user and compared to spectrophotometry results. When tested at one site, the mean coefficient of variation (CV) was 0.111, 0.172 and 0.260 for high, intermediate, and low controls across all devices respectively; combined G6PD Biosensor readings correlated well with spectrophotometry (rs = 0.859, p<0.001). When tested in different laboratories, correlation was lower (rs = 0.604, p<0.001) and G6PD activity determined by Biosensor for the low and intermediate controls overlapped. The use of lyophilised human blood samples rather than fresh blood may have affected these findings. Biosensor G6PD readings between sites did not differ significantly (p = 0.436), whereas spectrophotometry readings differed markedly between sites (p<0.001).

CONCLUSIONS

Repeatability and inter-laboratory reproducibility of the Biosensor were good; though the device did not reliably discriminate between intermediate and low G6PD activities of the lyophilized specimens. Clinical studies are now required to assess the devices performance in practice.

Plasmodium vivax - How hidden reservoirs hinder global malaria elimination

Plasmodium vivax is the most geographically widespread human malaria parasite. Global malaria efforts have been less successful at reducing the burden of P. vivax compared to P. falciparum, owing to the unique biology and related treatment complexity of P. vivax. As a result, P. vivax is now the dominant malaria parasite throughout the Asia-Pacific and South America causing up to 14 million clinical cases every year and is considered a major obstacle to malaria elimination. Key features circumventing existing malaria control tools are the transmissibility of asymptomatic, low-density circulating infections and reservoirs of persistent dormant liver stages (hypnozoites) that are undetectable but reactivate to cause relapsing infections and sustain transmission. In this review we summarise the new knowledge shaping our understanding of the global epidemiology of P. vivax infections, highlighting the challenges for elimination and the tools that will be required achieve this.

Antiprotozoal agents: How have they changed over a decade?

Neglected tropical diseases are a diverse group of communicable diseases that are endemic in low- or low-to-middle-income countries located in tropical and subtropical zones. The number and availability of drugs for treating these diseases are low, the administration route is inconvenient in some cases, and most of them have safety, efficacy, or adverse/toxic reaction issues. The need for developing new drugs to deal with these issues is clear, but one of the most drastic consequences of this negligence is the lack of interest in the research and development of new therapeutic options among major pharmaceutical companies. Positive changes have been achieved over the last few years, although the overall situation remains alarming. After more than one decade since the original work reviewing antiprotozoal agents came to light, now it is time to question ourselves: How has the scenario for the treatment of protozoal diseases such as malaria, leishmaniasis, human African trypanosomiasis, and American trypanosomiasis changed? This review covers the last decade in terms of the drugs currently available for the treatment of these diseases as well as the clinical candidates being currently investigated.

Low-level Plasmodium vivax exposure, maternal antibodies, and anemia in early childhood: Population-based birth cohort study in Amazonian Brazil

Background

Malaria causes significant morbidity and mortality in children under 5 years of age in sub-Saharan Africa and the Asia-Pacific region. Neonates and young infants remain relatively protected from clinical disease and the transplacental transfer of maternal antibodies is hypothesized as one of the protective factors. The adverse health effects of Plasmodium vivax malaria in early childhood–traditionally viewed as a benign infection–remain largely neglected in relatively low-endemicity settings across the Amazon.

Methodology/Principal findings

Overall, 1,539 children participating in a birth cohort study in the main transmission hotspot of Amazonian Brazil had a questionnaire administered, and blood sampled at the two-year follow-up visit. Only 7.1% of them experienced malaria confirmed by microscopy during their first 2 years of life– 89.1% of the infections were caused by P. vivax. Young infants appear to be little exposed to, or largely protected from infection, but children >12 months of age become as vulnerable to vivax malaria as their mothers. Few (1.4%) children experienced ≥4 infections during the 2-year follow-up, accounting for 43.4% of the overall malaria burden among study participants. Antenatal malaria diagnosed by microscopy during pregnancy or by PCR at delivery emerged as a significant correlate of subsequent risk of P. vivax infection in the offspring (incidence rate ratio, 2.58; P = 0.002), after adjusting for local transmission intensity. Anti-P. vivax antibodies measured at delivery do not protect mothers from subsequent malaria; whether maternal antibodies transferred to the fetus reduce early malaria risk in children remains undetermined. Finally, recent and repeated vivax malaria episodes in early childhood are associated with increased risk of anemia at the age of 2 years in this relatively low-endemicity setting.

Conclusions/Significance

Antenatal infection increases the risk of vivax malaria in the offspring and repeated childhood P. vivax infections are associated with anemia at the age of 2 years.

Plasmodium vivax malaria causes frequent hospital admissions of infants and toddlers in areas of intense transmission in the Asia-Pacific region, often due to severe anemia, but its epidemiology and burden have been understudied in children from other endemic settings. Here we characterize the cumulative impact of P. vivax infections in infants and toddlers exposed to relatively low levels of malaria transmission in the Brazilian Amazon. We have previously shown that vivax malaria in pregnancy is associated with increased risk of maternal anemia and impaired fetal growth in this population. Now we show that the adverse effects of malaria extend to early childhood. Children born to mothers who had one or more infections during pregnancy are at an elevated risk of P. vivax malaria in their early life, although the transfer of maternal antibodies to the fetus may provide some short-term protection. Children who are repeatedly infected with P. vivax since birth are more likely to be anemic at the age of 2 years. These findings further challenge the traditional view of vivax malaria as a relatively benign infection in pregnancy and early childhood in the Amazon.

Global economic costs due to vivax malaria and the potential impact of its radical cure: A modelling study

BACKGROUND

In 2017, an estimated 14 million cases of Plasmodium vivax malaria were reported from Asia, Central and South America, and the Horn of Africa. The clinical burden of vivax malaria is largely driven by its ability to form dormant liver stages (hypnozoites) that can reactivate to cause recurrent episodes of malaria. Elimination of both the blood and liver stages of the parasites ("radical cure") is required to achieve a sustained clinical response and prevent ongoing transmission of the parasite. Novel treatment options and point-of-care diagnostics are now available to ensure that radical cure can be administered safely and effectively. We quantified the global economic cost of vivax malaria and estimated the potential cost benefit of a policy of radical cure after testing patients for glucose-6-phosphate dehydrogenase (G6PD) deficiency.

METHODS AND FINDINGS

Estimates of the healthcare provider and household costs due to vivax malaria were collated and combined with national case estimates for 44 endemic countries in 2017. These provider and household costs were compared with those that would be incurred under 2 scenarios for radical cure following G6PD screening: (1) complete adherence following daily supervised primaquine therapy and (2) unsupervised treatment with an assumed 40% effectiveness. A probabilistic sensitivity analysis generated credible intervals (CrIs) for the estimates. Globally, the annual cost of vivax malaria was US$359 million (95% CrI: US$222 to 563 million), attributable to 14.2 million cases of vivax malaria in 2017. From a societal perspective, adopting a policy of G6PD deficiency screening and supervision of primaquine to all eligible patients would prevent 6.1 million cases and reduce the global cost of vivax malaria to US$266 million (95% CrI: US$161 to 415 million), although healthcare provider costs would increase by US$39 million. If perfect adherence could be achieved with a single visit, then the global cost would fall further to US$225 million, equivalent to $135 million in cost savings from the baseline global costs. A policy of unsupervised primaquine reduced the cost to US$342 million (95% CrI: US$209 to 532 million) while preventing 2.1 million cases. Limitations of the study include partial availability of country-level cost data and parameter uncertainty for the proportion of patients prescribed primaquine, patient adherence to a full course of primaquine, and effectiveness of primaquine when unsupervised.

CONCLUSIONS

Our modelling study highlights a substantial global economic burden of vivax malaria that could be reduced through investment in safe and effective radical cure achieved by routine screening for G6PD deficiency and supervision of treatment. Novel, low-cost interventions for improving adherence to primaquine to ensure effective radical cure and widespread access to screening for G6PD deficiency will be critical to achieving the timely global elimination of P. vivax.

Further evidence needed to change policy for the safe and effective radical cure of vivax malaria: Insights from the 2019 annual APMEN Vivax Working Group meeting

New diagnostics and treatment options for the radical cure of Plasmodium vivax malaria are now available. At the 2019 annual meeting of the Vivax Working Group of the Asia Pacific Malaria Elimination Network, participants took part in a roundtable discussion to identify further evidence required to introduce these new tools into policy and practice. Key gaps identified were accuracy and reliability of glucose-6-phosphate-dehydrogenase deficiency tests, health system capacity, and feasibility and cost effectiveness of novel treatment strategies in routine clinical practice. As expected, there were differences in the priorities between country partners and researcher partners. To achieve the 2030 target for the regional elimination of malaria, evidence to address these issues should be generated as a matter of priority. Review of global guidelines alongside locally generated data will help to ensure the timely revision and optimisation of national treatment guidelines that will be vital to meet regional elimination goals.

Estimated impact of tafenoquine for Plasmodium vivax control and elimination in Brazil: A modelling study

BACKGROUND

Despite recent intensification of control measures, Plasmodium vivax poses a major challenge for malaria elimination efforts. Liver-stage hypnozoite parasites that cause relapsing infections can be cleared with primaquine; however, poor treatment adherence undermines drug effectiveness. Tafenoquine, a new single-dose treatment, offers an alternative option for preventing relapses and reducing transmission. In 2018, over 237,000 cases of malaria were reported to the Brazilian health system, of which 91.5% were due to P. vivax.

METHODS AND FINDINGS

We evaluated the impact of introducing tafenoquine into case management practices on population-level transmission dynamics using a mathematical model of P. vivax transmission. The model was calibrated to reflect the transmission dynamics of P. vivax endemic settings in Brazil in 2018, informed by nationwide malaria case reporting data. Parameters for treatment pathways with chloroquine, primaquine, and tafenoquine with glucose-6-phosphate dehydrogenase deficiency (G6PDd) testing were informed by clinical trial data and the literature. We assumed 71.3% efficacy for primaquine and tafenoquine, a 66.7% adherence rate to the 7-day primaquine regimen, a mean 5.5% G6PDd prevalence, and 8.1% low metaboliser prevalence. The introduction of tafenoquine is predicted to improve effective hypnozoite clearance among P. vivax cases and reduce population-level transmission over time, with heterogeneous levels of impact across different transmission settings. According to the model, while achieving elimination in only few settings in Brazil, tafenoquine rollout in 2021 is estimated to improve the mean effective radical cure rate from 42% (95% uncertainty interval [UI] 41%-44%) to 62% (95% UI 54%-68%) among clinical cases, leading to a predicted 38% (95% UI 7%-99%) reduction in transmission and over 214,000 cumulative averted cases between 2021 and 2025. Higher impact is predicted in settings with low transmission, low pre-existing primaquine adherence, and a high proportion of cases in working-aged males. High-transmission settings with a high proportion of cases in children would benefit from a safe high-efficacy tafenoquine dose for children. Our methodological limitations include not accounting for the role of imported cases from outside the transmission setting, relying on reported clinical cases as a measurement of community-level transmission, and implementing treatment efficacy as a binary condition.

CONCLUSIONS

In our modelling study, we predicted that, provided there is concurrent rollout of G6PDd diagnostics, tafenoquine has the potential to reduce P. vivax transmission by improving effective radical cure through increased adherence and increased protection from new infections. While tafenoquine alone may not be sufficient for P. vivax elimination, its introduction will improve case management, prevent a substantial number of cases, and bring countries closer to achieving malaria elimination goals.

Rolling out the radical cure for vivax malaria in Asia: a qualitative study among policy makers and stakeholders

BACKGROUND

Wide-spread implementation of treatment regimens for the radical cure of vivax malaria is hindered by a range of factors. This has resulted in an increase in the relative proportion of vivax malaria and is an important obstacle in the achievement of global malaria elimination by 2030. The main objective of this study was to explore the current policies guiding the treatment plans on vivax malaria, and the factors affecting the implementation of radical cure in South/South East Asian and Asian Pacific countries.

METHODS

This was a qualitative study among respondents who represented national malaria control programmes (NMCPs) or had a role and influence in the national malaria policies. 33 respondents from 17 countries in South/South East Asia and Asia Pacific participated in interviews between October 15 and December 15, 2020. Semi-structured interviews were conducted virtually except for two face to face interviews and audio-recorded. Transcribed audio-records underwent thematic analysis using QSR NVivo.

RESULTS

Policies against vivax malaria were underprioritized, compared with the focus on falciparum malaria and, in particular, drug resistant Plasmodium falciparum strains. Despite the familiarity with primaquine (PQ) as the essential treatment to achieve the radical cure, the respondents contested the need for G6PD testing. Optional G6PD testing was reported to have poor adherence. The fear of adverse events led health workers to hesitate prescribing PQ. In countries where G6PD was mandatory, respondents experienced frequent stockouts of G6PD rapid diagnostic kits in peripheral health facilities, which was compounded by a short shelf life of these tests. These challenges were echoed across participating countries to various degrees. Most respondents agreed that a shorter treatment regimen, such as single dose tafenoquine could resolve these problems but mandatory G6PD testing will be needed. The recommendation of shorter regimens including tafenoquine or high dose PQ requires operational evidence demonstrating the robust performance of point of care G6PD tests (biosensors).

CONCLUSION

There was sparse implementation and low adherence to the radical cure in South/South East Asian and Asian pacific countries. Shorter treatment regimens with appropriate point of care quantitative G6PD tests may resolve the current challenges. Operational evidence on point of care quantitative G6PD tests that includes the feasibility of integrating such tests into the radical cure regimen are critical to ensure its implementation.

Tafenoquine: a toxicity overview

Introduction: A century-long history in 8-aminoquinolines, the only anti-malaria drug class preventing malaria relapse, has resulted in the approval of tafenoquine by the U.S. Food and Drug Administration (FDA) and the Australian Therapeutic Goods Administration (TGA) and to date registration in Brazil and Thailand. Tafenoquine is an alternative anti-relapse treatment for vivax malaria and malaria prophylaxis. It should not be given in pregnancy, during lactation of infants with glucose-6-phosphate dehydrogenase (G6PD) unknown or deficient status, and in those with G6PD deficiency or psychiatric illness.Areas covered: This systematic review assesses tafenoquine associated adverse events in English-language, human clinical trials. Meta-analysis of commonly reported adverse events was conducted and grouped by comparison arms.Expert opinion: Tafenoquine, either for radical cure or prophylaxis, is generally well tolerated in adults. There is no convincing evidence for neurologic, ophthalmic, and cardiac toxicities. Psychotic disorder which has been attributed to higher doses is a contraindication for the chemoprophylaxis indication and psychiatric illness is a warning for the radical cure indication. Pregnancy assessment and quantitative G6PD testing are required. The optimal radical curative regimen including the tafenoquine dose along with its safety for parts of Southeast Asia, South America, and Oceania needs further assessment.

The top 1%: quantifying the unequal distribution of malaria in Brazil

Background

As malaria endemic countries strive towards elimination, intensified spatial heterogeneities of local transmission could undermine the effectiveness of traditional intervention policy.

Methods

The dynamic nature of large-scale and long-term malaria heterogeneity across Brazilian Amazon basin were explored by (1) exploratory analysis of Brazil’s rich clinical malaria reporting database from 2004 to 2018, and (2) adapting Gini coefficient to study the distribution of malaria cases in the region.

Results

As transmission declined, heterogeneity increased with cases clustering into smaller subpopulations across the territory. In 2004, the 1% of health units with the greatest number of cases accounted for 46% of all reported Plasmodium vivax cases, whereas in 2018 52% of P. vivax cases occurred in the top 1% of health units. Plasmodium falciparum had lower levels of transmission than P. vivax, and also had greater levels of heterogeneity with 75% of cases occurring in the top 1% of health units. Age and gender stratification of cases revealed peri-domestic and occupational exposure settings that remained relatively stable.

Conclusion

The pathway to decreasing incidence is characterized by higher proportions of cases in males, in adults, due to importation, and caused by P. vivax. Characterization of spatio-temporal heterogeneity and risk groups can aid stratification for improved malaria control towards elimination with increased heterogeneity potentially allowing for more efficient and cost-effective targeting. Although distinct epidemiological phenomena were clearly observed as malaria transmission declines, the authors argue that there is no canonical path to malaria elimination and a more targeted and dynamic surveillance will be needed if Brazil decides to adopt the elimination target.

Liver-targeted polymeric prodrugs of 8-aminoquinolines for malaria radical cure

Primaquine and tafenoquine are the two 8-aminoquinoline (8-AQ) antimalarial drugs approved for malarial radical cure - the elimination of liver stage hypnozoites after infection with Plasmodium vivax. A single oral dose of tafenoquine leads to high efficacy against intra-hepatocyte hypnozoites after efficient first pass liver uptake and metabolism. Unfortunately, both drugs cause hemolytic anemia in G6PD-deficient humans. This toxicity prevents their mass administration without G6PD testing given the approximately 400 million G6PD deficient people across malarial endemic regions of the world. We hypothesized that liver-targeted delivery of 8-AQ prodrugs could maximize liver exposure and minimize erythrocyte exposure to increase their therapeutic window. Primaquine and tafenoquine were first synthesized as prodrug vinyl monomers with self-immolative hydrolytic linkers or cathepsin-cleavable valine-citrulline peptide linkers. RAFT polymerization was exploited to copolymerize these prodrug monomers with hepatocyte-targeting GalNAc monomers. Pharmacokinetic studies of released drugs after intravenous administration showed that the liver-to-plasma AUC ratios could be significantly improved, compared to parent drug administered orally. Single doses of the liver-targeted, enzyme-cleavable tafenoquine polymer were found to be as efficacious as an equivalent dose of the oral parent drug in the P. berghei causal prophylaxis model. They also elicited significantly milder hemotoxicity in the humanized NOD/SCID mouse model engrafted with red blood cells from G6PD deficient donors. The clinical application is envisioned as a single subcutaneous administration, and the lead tafenoquine polymer also showed excellent bioavailability and liver-to-blood ratios exceeding the IV administered polymer. The liver-targeted tafenoquine polymers warrant further development as a single-dose therapeutic via the subcutaneous route with the potential for broader patient administration without a requirement for G6PD diagnosis.

Monitoring Plasmodium vivax resistance to antimalarials: Persisting challenges and future directions

Emerging antimalarial drug resistance may undermine current efforts to control and eliminate Plasmodium vivax, the most geographically widespread yet neglected human malaria parasite. Endemic countries are expected to assess regularly the therapeutic efficacy of antimalarial drugs in use in order to adjust their malaria treatment policies, but proper funding and trained human resources are often lacking to execute relatively complex and expensive clinical studies, ideally complemented by ex vivo assays of drug resistance. Here we review the challenges for assessing in vivo P. vivax responses to commonly used antimalarials, especially chloroquine and primaquine, in the presence of confounding factors such as variable drug absorption, metabolism and interaction, and the risk of new infections following successful radical cure. We introduce a simple modeling approach to quantify the relative contribution of relapses and new infections to recurring parasitemias in clinical studies of hypnozoitocides. Finally, we examine recent methodological advances that may render ex vivo assays more practical and widely used to confirm P. vivax drug resistance phenotypes in endemic settings and review current approaches to the development of robust genetic markers for monitoring chloroquine resistance in P. vivax populations.

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Plasmodium vivax resistance to chloroquine may undermine malaria elimination efforts.

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Plasmodium vivax resistance to schizontocides has been mostly monitored in therapeutic efficacy studies.

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In vivo studies to determine the anti-relapse efficacy of primaquine are challenging to design and execute.

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Ex vivo assays to determine Plasmodium vivax resistance to schizontocides remain limited to research settings.

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Robust molecular markers to monitor Plasmodium vivax drug resistance are currently lacking.

Quantifying and preventing Plasmodium vivax recurrences in primaquine-untreated pregnant women: An observational and modeling study in Brazil

Each year, 4.3 million pregnant women are exposed to malaria risk in Latin America and the Caribbean. Plasmodium vivax causes 76% of the regional malaria burden and appears to be less affected than P. falciparum by current elimination efforts. This is in part due to the parasite's ability to stay dormant in the liver and originate relapses within months after a single mosquito inoculation. Primaquine (PQ) is routinely combined with chloroquine (CQ) or other schizontocidal drugs to supress P. vivax relapses and reduce the risk of late blood-stage recrudescences of parasites with low-grade CQ resistance. However, PQ is contraindicated for pregnant women, who remain at increased risk of repeated infections following CQ-only treatment. Here we apply a mathematical model to time-to-recurrence data from Juruá Valley, Brazil's main malaria transmission hotspot, to quantify the extra burden of parasite recurrences attributable to PQ ineligibility in pregnant women. The model accounts for competing risks, since relapses and late recrudescences (that may be at least partially prevented by PQ) and new infections (that are not affected by PQ use) all contribute to recurrences. We compare recurrence rates observed after primary P. vivax infections in 158 pregnant women treated with CQ only and 316 P. vivax infections in non-pregnant control women, matched for age, date of infection, and place of residence, who were administered a standard CQ-PQ combination. We estimate that, once infected with P. vivax, 23% of the pregnant women have one or more vivax malaria recurrences over the next 12 weeks; 86% of these early P. vivax recurrences are attributable to relapses or late recrudescences, rather than new infections that could be prevented by reducing malaria exposure during pregnancy. Model simulations indicate that weekly CQ chemoprophylaxis extending over 4 to 12 weeks, starting after the first vivax malaria episode diagnosed in pregnancy, might reduce the risk of P. vivax recurrences over the next 12 months by 20% to 65%. We conclude that post-treatment CQ prophylaxis could be further explored as a measure to prevent vivax malaria recurrences in pregnancy and avert their adverse effects on maternal and neonatal health.

Cost-Effectiveness Analysis of Sex-Stratified Plasmodium vivax Treatment Strategies Using Available G6PD Diagnostics to Accelerate Access to Radical Cure

Tafenoquine has been licensed for the single-dose radical cure of Plasmodium vivax in adults; however, it is only recommended in patients with > 70% of normal glucose-6-phosphate dehydrogenase (G6PD) activity. Because this may hinder widespread use, we investigated sex-based treatment strategies in which all adult patients are tested with a qualitative G6PD rapid diagnostic test (RDT). Glucose-6-phosphate dehydrogenase normal males are prescribed tafenoquine in all three strategies, whereas G6PD normal females are prescribed either a low-dose 14-day primaquine regimen (PQ14, total dose 3.5 mg/kg) or a high-dose 7-day primaquine regimen (PQ7, total dose 7 mg/kg), or referred to a healthcare facility for quantitative G6PD testing before prescribing tafenoquine. Patients testing G6PD deficient are prescribed a weekly course of primaquine for 8 weeks. We compared the cost-effectiveness of these three strategies to usual care in four countries using a decision tree model. Usual care in Ethiopia does not include radical cure, whereas Afghanistan, Indonesia, and Vietnam prescribe PQ14 without G6PD screening. The cost per disability-adjusted life-year (DALY) averted was expressed through incremental cost-effectiveness ratios (ICERs). Compared with usual care, the ICERs for a sex-based treatment strategy with PQ7 for females from a healthcare provider perspective were $127 per DALY averted in Vietnam, $466 in Ethiopia, $1,089 in Afghanistan, and $4,443 in Indonesia. The PQ14 and referral options cost more while averting fewer DALYs than PQ7. This study provides an alternative cost-effective mode of rolling out tafenoquine in areas where initial testing with only a G6PD RDT is feasible.

Prevalence of Glucose 6-Phosphate Dehydrogenase Variants in Malaria-Endemic Areas of South Central Timor, Eastern Indonesia

Primaquine is an effective anti-hypnozoite drug for Plasmodium vivax and Plasmodium ovale. However, it can trigger erythrocyte hemolysis in people with glucose 6-phosphate dehydrogenase (G6PD) deficiency. In a previous report from South Central Timor (SCT), Indonesia, we described the prevalence of Vanua Lava, Chatham, and Viangchan variants; in this study, other G6PD variants (Kaiping, Coimbra, Gaohe, Canton, and Mahidol) were subsequently analyzed. For clarity, all of these results are described together. The 381 DNA samples from the previous study during 2013-2014 were analyzed for G6PD variants by using PCR-restriction fragment length polymorphism (RFLP). The prevalence of G6PD deficiency in SCT was 6.3% (24/381 cases), including 4.2% (16/381 cases), 0.5% (2/381 cases), and 1.6% (6/381 cases) for Coimbra, Kaiping, and Vanua Lava variants, respectively. No other variants were found in this population. A significant association was found between ethnicity and the distribution of G6PD Kaiping in female subjects. A positive association was shown between G6PD activity and heterozygous females carrying Coimbra genotype, hemizygous males carrying Vanua Lava, Plasmodium falciparum infection in female subjects, and P. vivax infection in male subjects. Further molecular analysis of heterozygous females, particularly in malaria-endemic areas, is needed for mapping distribution of G6PD deficiency status in Indonesia.

Plasmodium vivax in the Era of the Shrinking P. falciparum Map

Plasmodium vivax is an important cause of malaria, associated with a significant public health burden. Whilst enhanced malaria-control activities have successfully reduced the incidence of Plasmodium falciparum malaria in many areas, there has been a consistent increase in the proportion of malaria due to P. vivax in regions where both parasites coexist. This article reviews the epidemiology and biology of P. vivax, how the parasite differs from P. falciparum, and the key features that render it more difficult to control and eliminate. Since transmission of the parasite is driven largely by relapses from dormant liver stages, its timely elimination will require widespread access to safe and effective radical cure.

Novel Quinolinyl-pyrrolo[3,4-d]pyrimidine-2,5-dione Derivatives Against Chloroquine-resistant Plasmodium falciparum

INTRODUCTION

In this work DHPMs were combined with the quinoline nucleus to obtain new quinolinyl-pyrrolo[3,4-d]pyrimidine-2,5-dione compounds with improved antiplasmodial activity as well as decreased cytotoxicity. Nineteen quinolinyl-pyrrolo[3,4-d]pyrimidine-2,5-dione derivatives connected by a linker group to quinolone ring moieties with different substituents were synthesized and assayed against P. falciparum.

MATERIALS AND METHODS

Nineteen quinolinyl-pyrrolo[3,4-d]pyrimidine-2,5-dione derivatives connected by a linker group to quinoline ring moieties with different substituents were synthesized and assayed against chloroquine-resistant Plasmodium falciparum, along with the reference drug chloroquine. Among these compounds, the derivatives with two methylene carbon spacers showed the best activity accompanied by low cytotoxicity.

RESULTS

The derivative without substituents on the aromatic ring (2a) and the derivative with a chlorine group at position 4 (2d) provided the best results, with IC50 = 1.15 µM and 1.5 µM, respectively.

CONCLUSION

Compared to the parent drugs, these compounds presented marked decreases in cytotoxicity, with MDL50 values over 1,000 µM and selectivity indexes of >869.5 and >666.6, respectively. The quinolinyl-pyrrolo[3,4-d]pyrimidine-2,5-dione framework appears to be promising for further studies as an antimalarial for overcoming the burden of resistance in P. falciparum.

Mapping the global endemicity and clinical burden of Plasmodium vivax, 2000-17: a spatial and temporal modelling study

BACKGROUND

Plasmodium vivax exacts a significant toll on health worldwide, yet few efforts to date have quantified the extent and temporal trends of its global distribution. Given the challenges associated with the proper diagnosis and treatment of P vivax, national malaria programmes-particularly those pursuing malaria elimination strategies-require up to date assessments of P vivax endemicity and disease impact. This study presents the first global maps of P vivax clinical burden from 2000 to 2017.

METHODS

In this spatial and temporal modelling study, we adjusted routine malariometric surveillance data for known biases and used socioeconomic indicators to generate time series of the clinical burden of P vivax. These data informed Bayesian geospatial models, which produced fine-scale predictions of P vivax clinical incidence and infection prevalence over time. Within sub-Saharan Africa, where routine surveillance for P vivax is not standard practice, we combined predicted surfaces of Plasmodium falciparum with country-specific ratios of P vivax to P falciparum. These results were combined with surveillance-based outputs outside of Africa to generate global maps.

FINDINGS

We present the first high-resolution maps of P vivax burden. These results are combined with those for P falciparum (published separately) to form the malaria estimates for the Global Burden of Disease 2017 study. The burden of P vivax malaria decreased by 41·6%, from 24·5 million cases (95% uncertainty interval 22·5-27·0) in 2000 to 14·3 million cases (13·7-15·0) in 2017. The Americas had a reduction of 56·8% (47·6-67·0) in total cases since 2000, while South-East Asia recorded declines of 50·5% (50·3-50·6) and the Western Pacific regions recorded declines of 51·3% (48·0-55·4). Europe achieved zero P vivax cases during the study period. Nonetheless, rates of decline have stalled in the past five years for many countries, with particular increases noted in regions affected by political and economic instability.

INTERPRETATION

Our study highlights important spatial and temporal patterns in the clinical burden and prevalence of P vivax. Amid substantial progress worldwide, plateauing gains and areas of increased burden signal the potential for challenges that are greater than expected on the road to malaria elimination. These results support global monitoring systems and can inform the optimisation of diagnosis and treatment where P vivax has most impact.

FUNDING

Bill & Melinda Gates Foundation and the Wellcome Trust.

Cleves M, Nguyen XX, Nguyen TT, Tran DT, Le HX, Hens N, Rosanas-Urgell A, D’Alessandro U, Speybroeck N, Erhart A. Plasmodium vivax morbidity after radical cure: A cohort study in Central Vietnam

BACKGROUND

In Vietnam, the importance of vivax malaria relative to falciparum during the past decade has steadily increased to 50%. This, together with the spread of multidrug-resistant Plasmodium falciparum, is a major challenge for malaria elimination. A 2-year prospective cohort study to assess P. vivax morbidity after radical cure treatment and related risk factors was conducted in Central Vietnam.

METHODS AND FINDINGS

The study was implemented between April 2009 and December 2011 in four neighboring villages in a remote forested area of Quang Nam province. P. vivax-infected patients were treated radically with chloroquine (CQ; 25 mg/kg over 3 days) and primaquine (PQ; 0.5 mg/kg/day for 10 days) and visited monthly (malaria symptoms and blood sampling) for up to 2 years. Time to first vivax recurrence was estimated by Kaplan-Meier survival analysis, and risk factors for first and recurrent infections were identified by Cox regression models. Among the 260 P. vivax patients (61% males [159/260]; age range 3-60) recruited, 240 completed the 10-day treatment, 223 entered the second month of follow-up, and 219 were followed for at least 12 months. Most individuals (76.78%, 171/223) had recurrent vivax infections identified by molecular methods (polymerase chain reaction [PCR]); in about half of them (55.61%, 124/223), infection was detected by microscopy, and 84 individuals (37.67%) had symptomatic recurrences. Median time to first recurrence by PCR was 118 days (IQR 59-208). The estimated probability of remaining free of recurrence by month 24 was 20.40% (95% CI [14.42; 27.13]) by PCR, 42.52% (95% CI [35.41; 49.44]) by microscopy, and 60.69% (95% CI [53.51; 67.11]) for symptomatic recurrences. The main risk factor for recurrence (first or recurrent) was prior P. falciparum infection. The main limitations of this study are the age of the results and the absence of a comparator arm, which does not allow estimating the proportion of vivax relapses among recurrent infections.

CONCLUSION

A substantial number of P. vivax recurrences, mainly submicroscopic (SM) and asymptomatic, were observed after high-dose PQ treatment (5.0 mg/kg). Prior P. falciparum infection was an important risk factor for all types of vivax recurrences. Malaria elimination efforts need to address this largely undetected P. vivax transmission by simultaneously tackling the reservoir of P. falciparum and P. vivax infections.

Malaria morbidity and mortality following introduction of a universal policy of artemisinin-based treatment for malaria in Papua, Indonesia: A longitudinal surveillance study

BACKGROUND

Malaria control activities can have a disproportionately greater impact on Plasmodium falciparum than on P. vivax in areas where both species are coendemic. We investigated temporal trends in malaria-related morbidity and mortality in Papua, Indonesia, before and after introduction of a universal, artemisinin-based antimalarial treatment strategy for all Plasmodium species.

METHODS AND FINDINGS

A prospective, district-wide malariometric surveillance system was established in April 2004 to record all cases of malaria at community clinics and the regional hospital and maintained until December 2013. In March 2006, antimalarial treatment policy was changed to artemisinin combination therapy for uncomplicated malaria and intravenous artesunate for severe malaria due to any Plasmodium species. Over the study period, a total of 418,238 patients presented to the surveillance facilities with malaria. The proportion of patients with malaria requiring admission to hospital fell from 26.9% (7,745/28,789) in the pre-policy change period (April 2004 to March 2006) to 14.0% (4,786/34,117) in the late transition period (April 2008 to December 2009), a difference of -12.9% (95% confidence interval [CI] -13.5% to -12.2%). There was a significant fall in the mortality of patients presenting to the hospital with P. falciparum malaria (0.53% [100/18,965] versus 0.32% [57/17,691]; difference = -0.21% [95% CI -0.34 to -0.07]) but not in patients with P. vivax malaria (0.28% [21/7,545] versus 0.23% [28/12,397]; difference = -0.05% [95% CI -0.20 to 0.09]). Between the same periods, the overall proportion of malaria due to P. vivax rose from 44.1% (30,444/69,098) to 53.3% (29,934/56,125) in the community clinics and from 32.4% (9,325/28,789) to 44.1% (15,035/34,117) at the hospital. After controlling for population growth and changes in treatment-seeking behaviour, the incidence of P. falciparum malaria fell from 511 to 249 per 1,000 person-years (py) (incidence rate ratio [IRR] = 0.49 [95% CI 0.48-0.49]), whereas the incidence of P. vivax malaria fell from 331 to 239 per 1,000 py (IRR = 0.72 [95% CI 0.71-0.73]). The main limitations of our study were possible confounding from changes in healthcare provision, a growing population, and significant shifts in treatment-seeking behaviour following implementation of a new antimalarial policy.

CONCLUSIONS

In this area with high levels of antimalarial drug resistance, adoption of a universal policy of efficacious artemisinin-based therapy for malaria infections due to any Plasmodium species was associated with a significant reduction in total malaria-attributable morbidity and mortality. The burden of P. falciparum malaria was reduced to a greater extent than that of P. vivax malaria. In coendemic regions, the timely elimination of malaria will require that safe and effective radical cure of both the blood and liver stages of the parasite is widely available for all patients at risk of malaria.

Evaluation of Plasmodium vivax malaria recurrence in Brazil

BACKGROUND

Control of vivax malaria in endemic areas requires management of recurrence. The Brazilian National Malaria Surveillance System (SIVEP-Malária) records every case of malaria in Brazil, but is not designed to differentiate between primary and recurrent infections. The aim of this study was to explore whether the information provided by SIVEP-Malária could be used to identify Plasmodium vivax recurrences, its risk factors and evaluate the effectiveness of short course primaquine (7-9 days: total dose 3-4.2 mg/kg) in preventing relapses.

METHODS

In this observational retrospective cohort study, data matching of SIVEP-Malária records was undertaken using bloom filters to identify potential recurrences defined as microscopically-confirmed P. vivax episodes from the same individual occurring within a year. Generalized Estimation Equation (GEE) models were used to determine predictors of recurrence. Extended Cox-based conditional Prentice-Williams-Peterson models (PWP) models were used to evaluate time to recurrence.

RESULTS

Between June 1, 2014 and May 31, 2015, 26,295 episodes fulfilled the criteria of potential recurrence among 154,970 reported malaria episodes. Age ≤ 3 years, being male, literate, not-indigenous and having domestic working activities were identified as risk factors for recurrence. There was no difference in time to recurrence or recurrence frequency between patients treated with 14-day or 7-9 day primaquine regimens (HR = 1.02, 0.96-1.09) and RR = 0.97 (0.90-1.04), respectively. The use of chloroquine alone was associated with a 1.43 (1.29-1.58, p < 0.0001) increased risk of P. vivax recurrence compared to patients who used chloroquine combined with short-course primaquine, the Brazilian standard of care. This was RR = 2.06 (1.48-2.86, p < 0.0001), RR = 1.90 (1.60-2.25, p = 0.0001) and RR = 1.14 (1.00-1.29, p = 0.05) for recurrences occurring between 3-28, 29-60 and > 60 days, respectively. PWP models showed that the time to recurrence was longer in recipients of both primaquine and artemisinin-based combination therapy (ACT) compared to patients treated with chloroquine alone or with concomitant primaquine, HR = 2.2 (1.62-2.99, p < 0.0001), HR = 1.27 (0.97-1.66, p = 0.08), respectively.

CONCLUSION

Short course primaquine was as effective as 14-day regimens and associated with a halving of the risk and delay in time to recurrence of P. vivax infections in comparison to chloroquine alone. The study demonstrates the feasibility of using record linkage on routine surveillance data to identify potential P. vivax recurrences, associated risk factors and impact of treatment.