The global fight against malaria has, over the decades, repeatedly been compromised
by multidrug-resistant Plasmodium falciparum strains that first emerged in southeast
Asia.
1
Successively, these parasites have acquired resistance to chloroquine, sulphadoxine-pyrimethamine,
mefloquine, and more recently the artemisinins through point mutations or amplification
in genes (crt, dhps, dhfr, mdr1, and kelch13).
2–6
Following increased resistance to artesunate plus mefloquine,
7,8
an early artemisinin-based combined therapy, regional authorities turned increasingly
to dihydroartemisinin plus piperaquine. This drug combination was officially adopted
in the western provinces of Cambodia in 2008 and the rest of the country in 2010,
in Thailand in 2015, and in Vietnam in 2016 (although this artemisinin-based combined
therapy was available previously). Warning signs came in 2009 with reports of emerging
resistance to artemisinins,
9
manifesting clinically as delayed rates of parasite clearance
3
and placing increased selective pressure on the partner drug piperaquine.
10
In The Lancet Infectious Diseases, two studies illustrate the accelerated pace at
which resistance of P falciparum to dihydroartemisinin plus piperaquine has evolved
and spread across southeast Asia, decimating the efficacy of this drug combination.
The first report from Rob van der Pluijm and colleagues
11
presents interim clinical data from a multicentre, open-label, randomised controlled
trial (TRAC2; ) that was done to assess the efficacy, safety, and tolerability of
experimental triple artemisinin-based combined therapies compared with two-agent artemisinin-based
combined therapies in areas with multidrug-resistant P falciparum malaria. Clinical,
pharmacological, and genetic data were reported for a cohort of 140 patients with
acute P falciparum malaria who were treated in 2015–18 with dihydroartemisinin plus
piperaquine in sites in Cambodia, Thailand, and Vietnam. PCR-corrected clinical efficacy
rates at day 42 were 12.7% in northeastern Thailand, 38.2% in western Cambodia, 73.4%
in northeastern Cambodia, and 47.1% in southwestern Vietnam, averaging to a 50.0%
treatment failure rate across the region. Treatment failures were far more common
than reported a few years ago.
10,12
Significant increases were also observed across the region in the prevalence of P
falciparum markers of artemisinin resistance (with the kelch13 Cys580Tyr [C580Y] variant
now at 88%) and piperaquine resistance (plasmepsin 2 and plasmepsin 3 amplifications
and crt mutations, both at 74%), compared with prevalence data from 2011–13 (from
the earlier TRAC project). The most striking result was the rapid increase in crt
mutations, present at a combined prevalence of only 5% in the 2011–13 samples. The
risk of treatment failure was strongly associated with the individual crt mutations
Thr93Ser (T93S), His97Tyr (H97Y), Phe145Ile (F145I), or Ile218Phe (I218F), as well
as with plasmepsin 2 and plasmepsin 3 amplification. These data, supported by recent
clinical, genetic epidemiology, and gene-editing results,
13,14
provide compelling evidence that these new crt mutations can mediate high-grade piperaquine
resistance and are driving the increased rates of treatment failure with dihydroartemisinin
plus piperaquine.
The second, complementary study by William Hamilton and colleagues
15
provides detailed insight into the molecular epidemiology of P falciparum and the
evolution of the artemisinin-resistant and piperaquine-resistant KEL1/PLA1 co-lineage,
first identified in samples from 2008 in western Cambodia.
16
Genome data were analysed from 1673 P falciparum clinical samples collected between
2007 and 2018 from patients with malaria in Cambodia, Laos, northeastern Thailand,
and Vietnam, combining the TRAC2 and the Genetic Reconnaissance in the Greater Mekong
Subregion (GenRe-Mekong) projects. Results showed that KEL1/PLA1 parasites had spread
across all the surveyed countries, in several areas exceeding 80% of the local parasite
population. Genetic similarity between KEL1/PLA1-type parasites across borders was
greater than overall within-country parasite diversity, implying strong selective
pressures favouring KEL1/PLA1. Their aggressive expansion across the region was accompanied
by the diversification of the KEL1/PLA1 co-lineage into six different subgroups. The
three most abundant subgroups carried the mutually exclusive crt mutations T93S, H97Y,
F145I, or I218F, which had emerged on the crt Dd2 haplotype.
15
This founder haplotype had earlier swept across the region as the primary determinant
of chloroquine resistance, and harbours eight point mutations compared with the chloroquine-sensitive
wild-type crt isoform.
17
These new variant isoforms were found to co-exist simultaneously in Cambodia, Laos,
and Vietnam, suggesting they have a strong selective advantage over the other subgroups
(harbouring mostly the piperaquine-sensitive Dd2 isoform). An earlier study reported
that the F145I mutation conferred high-level piperaquine resistance in cultured parasites
yet had a substantially reduced growth rate in vitro.
14
Further studies will provide insight into how resistance and fitness contribute to
this evolving landscape of soft sweeps that often coalesce around a few isoforms or
only one, and will show how quickly this will change as countries move to alternative
first-line treatments and whether these new resistance traits expose vulnerabilities
in terms of other antimalarial drugs becoming more potent.
In the Greater Mekong Subregion, P falciparum parasite populations are highly structured
in fragmented forest areas (considered as hotspots of malaria transmission), yet remain
interconnected because of intensive human migration and parasite population flow.
This epidemiological context, wherein major changes can rapidly occur—for example,
following the introduction of new antimalarial drugs, and subsequent extinction or
adaptation and recolonisation by the relatively fittest parasite populations—could
explain the recent emergence and expansion of new KEL1/PLA1 subgroups flowing across
borders. In this regard, findings from these two studies highlight the urgent need
to adopt new and effective treatments (such as the triple artemisinin-based combined
therapies or the artemisinin-based combined therapy artesunate plus pyronaridine
18
). These findings also demonstrate the advantages of implementing a regional strategy
rather than country-specific programmes to address population movements and to integrate
regional clinical and genetic surveillance systems into a coordinated campaign, with
the goal of achieving malaria elimination in southeast Asia.