Therapeutic Efficacy of Artemether-Lumefantrine (AL) plus Single Low Dose Primaquine for the treatment of uncomplicated falciparum malaria in a high transmission setting, Western Ethiopia

Background

The development and spread of drug-resistant parasites continue to threaten the move toward malaria elimination. Therapeutic efficacy and resistant marker studies are needed to guide national control programs. In African settings, evidence of partial resistance to artemisinin combination therapies associated with Kelch13 is accumulating as World Health Organization (WHO) recommends a regular monitoring of first line antimalarial drugs to early detection of resistant parasites. In our study, we evaluated the efficacy of artemether-lumefantrine (AL) combined with a single low dose of primaquine (PQ) for treating uncomplicated Plasmodium falciparum malaria in a co-endemic area where P. falciparum is predominant.

Methods and findings

One hundred twenty-three cases with P. falciparum mono-infection were enrolled and treated with artemether-lumefantrine (AL) plus single low dose primaquine (PQ) as per the national guideline and followed up over 28 days. Pfmsp2 capillary electrophoresis (CE) genotyping was used to differentiate recrudescence from new infection and we found seven recrudescence as true treatment failure. More than half (56.1%) of the participants had high (>10,000 parasites/μL) parasitemia at enrollment. At day 3, 17% (20/118) remained parasitemic and of the 10 individuals with detectable gametocytes at enrollment, only 1 remained gametocytemic on day 3, and 100% parasite clearance was achieved on day 7 respectively, indicating no early treatment failure. Multiplicity of infection was 3.8 and 1.7 before treatment and the day of recurrence respectively. The adequate clinical and parasitological responses, the PCR uncorrected at 28-day was 73.7% (95% _CI 65.6 - 82.9), while the PCR adjusted efficacy was 91.3% (95% _CI 85.3 – 97.7). In our study assessment, no serious adverse event was recorded as AL plus single low-dose PQ is safe for the treatment of uncomplicated falciparum malaria in the study setting.

Conclusions

The low efficacy found in our study is concerning, which might pos a challenge for malaria control in the study setting and beyond. We suggest that regular monitoring and conducting further research using advanced molecular techniques such as next-generation sequencing (NGS) to enable early detection of mutant variants that reduce treatment efficacy.

Author Summary

Since 2022, Ethiopia’s progress toward malaria elimination has been challenged by a nationwide surge in malaria cases, prompting intensified efforts by the national malaria control program to address this upsurge (Fig 1). Of the factors attributed to the widespread outbreak include the emergence and spread of diagnostic and drug resistance mutations. Due to the spread of the HRP2/3 deletion, the region is moving away from a rapid diagnostic test that has high sensitivity and specificity. On top, of the emergence and spread of the Pfkelch13 mutation implicated in partial resistance to artemisinin-based combination, the first line regimen could be an impending challenge (Fig 2). The WHO recommends a regular therapeutic efficacy study to monitor antimalarial drugs with a target of 90% threshold for first line regimens.

Western Ethiopia has the highest malaria burden in the country. The Artemisinin Lumefantrine based combination is the first line for the treatment of uncomplicated falciparum malaria since the 2000s. In our study, we evaluated the efficacy of artemether-lumefantrine (AL) combined with a single low dose of primaquine (PQ) for treating uncomplicated P. falciparum malaria in a coendemic area where P. falciparum is predominant. A single arm partially directly observed 28days of follow up for one hundred twenty-three microscopically mono-infection were enrolled a therapeutic efficacy study, at Bambasi Health Center, Northwest Ethiopia. We found 91.3% PCR based adjusted efficacy (CE), a lower efficacy reported so far to the best of our knowledge in Ethiopia.

The PCR-corrected efficacy of 91.3%, just above the WHO’s 90% threshold, raises concerns about the declining effectiveness of artemether-lumefantrine (AL) in this high-transmission setting. We observed a high multiplicity of infection (MOI), with 98 Pfmsp2 allelic variants detected and an initial MOI of 3.8 at recruitment, which reduced to 1.7 on the day of recurrence, suggesting a complex parasite population that may contribute to treatment challenges. The WHO recommends amplicon-based sequencing in high MOI settings to distinguish recrudescence from new infections. Our findings, using Pfmsp2 CE genotyping, revealed a 28-day PCR-corrected efficacy of 91.3% for AL plus single low-dose PQ, which is below the 98.7% pooled efficacy reported in Ethiopian systematic reviews. Despite achieving 100% parasite clearance by day 7, the combination of AL and PQ showed enhanced gametocyte clearance compared to AL alone, though clearance was delayed, with one participant remaining gametocytemic on day 3. Pretreatment parasite density and hemoglobin levels likely influenced these outcomes. Notably, the high initial parasite load in 56.1% of participants (geometric mean of 13,513 parasites/μL) may have contributed to delayed clearance and elevated treatment tolerance risk. The efficacy of single low dose primaquine as WHO-recommended transmission blocking strategy requires further evaluation in a powered study, given the reduced efficacy observed in our study.