03.2019 | THE SCIENTIST 27preserving the toxicity of this byproduct of
the parasite’s digestion of hemoglobin. Nor-
mally, the parasite polymerizes heme sub-
units into harmless clumps, but the binding
of chloroquine prevents this aggregation,
preserving the heme’s toxicity. Mutated
PfCRT appears to limit chloroquine’s access
to the digestive vacuole, allowing resistant
parasites to clean up the dangerous scraps
of their hemoglobin dinner without inter-
ruption. (See illustration on page 26.)
Before long, chloroquine was rendered
too unreliable to use as a regular treatment
against P. falciparum. Researchers devel-
oped synthetic alternatives, but these too
encountered resistant parasites soon after
their release.^5 Some drugs also carried
risks of severe side effects such as bad skin
reactions and liver problems; or, they had
complex dosing instructions that reduced
compliance, thereby limiting their use as
frontline treatments. Mortality rose at an
alarming rate, especially in Africa. In Sen-
egal, for example, death rates climbed as
much as sixfold in children under 10.^6
Enter artemisinin, which would
come to be known as a “miracle drug.”
Semisynthetic antimalarial formula-
tions of the natural compound, originally
developed in China in the 1970s, proved
incredibly effective at swiftly killing P.
falciparum with minimal side effects.
When activated by iron released by the
hemoglobin-digesting parasite, the drug
is thought to pummel P. falciparum at
multiple targets, including ATPases and
enzymes important for the redox cycle,
the core of fundamental biological pro-
cesses such as metabolism and cellular
respiration. A short, three-day pulse of
artemisinin wipes out the vast majority
of infecting parasites. When paired with
slower-acting partner drugs that mop
up any stragglers, artemisinin deriva-
tives such as artemether, artesunate,
and dihydroartemisinin (DHA) became
unstoppable. By the early 2000s, such
A C Ts were the go-to malaria treatment.
Mortality rates slowed globally, then
dropped by the millions.
The problem was that some of the
partner drugs had already been around
for decades as chloroquine alternatives.
That meant various P. falciparum strains
had already developed resistance to some
of these partner drugs. Because of this,
researchers began to see malaria infections
resurface after ACT treatment.The rise of ACT resistance
The first widely used ACT paired the
existing drug mefloquine, which had been
around since the late 1970s, with a blast of
artesunate. In the early 1990s, artesunate-
mefloquine (ASMQ) was used to treat
malaria in provinces on the Cambodia-
Thailand border, where existing meflo-
quine therapies were failing. With an ini-
tial dose of artesunate to wipe out the bulk
of offending parasites, ASMQ succeeded
in curing almost 100 percent of infected
patients in the area.^7By the mid-2000s, however, it became
clear that artesunate could no longer com-
pensate for the ever-increasing ability of P.
falciparum to resist mefloquine. By upreg-
ulating the multidrug resistance gene that
encodes the mutated transport protein
PfMDR1, the parasite likely pumps the
drug away from its cytosolic target and
into the digestive vacuole to be destroyed.
Many parasites that lingered after the arte-
sunate treatment were thus able to survive
the mefloquine mop-up. By 2008, ASMQ
was failing in about 20 percent of patients
in that region of Southeast Asia^8
A new AC T, which combines DHA
with the existing drug piperaquine (PP),
hit the market in 2007. DHA-PP worked
well against ASMQ-resistant parasites,
but soon failed against strains from a lin-eage called PLA1 that somehow resisted
P P. Like chloroquine, PP inhibits heme
detoxification, but it’s also thought to
target plasmepsins 2 and 3, which are
proteases the parasite requires to digest
hemoglobin for its peptides.^9 Without
these proteases, P. falciparum starves. By
making multiple copy numbers of plas-
mepsin 2 and 3 genes, PLA1 parasites can
overcome the effects of the drug.^10 By 2013,
25 percent of patients in western Cambo-
dia weren’t responding to DHA-PP.^11
In addition to resistance to the part-
ner drugs in A C Ts , P. falciparum has also
shown signs of evading artemisinin deriva-
tives. Around the same time that DHA-PP
was released, researchers in western Cam-
bodia noticed that it was taking patients
longer to clear parasites following the ini-
tial pulse of artesunate, DHA, and other
artemisinin-derived drugs.^12 P. falciparum
were found in the blood three or four days
after treatment, whereas it normally took
just one or two days for the drugs to reduce
the infection to below-detectable levels of
parasite. Malaria scientists dubbed this
phenomenon “partial” or “emerging” arte-
misinin resistance, because although the
treatment was taking longer to work, it
was still effective, with most resilient par-
asites being cleared within a week.^13
Investigations into delayed-clearance
mechanisms pointed to various muta-
tions in the kelch13 gene, which codes for
a poorly understood kinase-binding pro-
tein. In vitro studies show that the para-
site’s resistance is effective at a specific life
stage—the early ring stage, which occurs
soon after P. falciparum enters a red blood
cell but before it starts replicating. Arte-
misinin derivatives get metabolized by the
body very quickly, so “resistant” parasites
appear to evade the drug by lingering lon-
ger in the ring stage, biding their time.^14
(See illustration on page 28.)
Initially, cases of delayed clearance in
Western Cambodia were limited to small,
discrete geographic regions where the
parasites carried various different kelch13
mutations, none of which appeared to
have an advantage over the others, says
Roberto Amato, a population geneticist at
the Wellcome Sanger Institute. It wasn’tResistance
against malaria
drugs has been
a battle since
day one.