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Artemisinin
Artemisinin (pronounced /ɑrtɨˈmɪsɨnɨn/) is a drug used to treat multi-drug resistant strains of
falciparum malaria. The compound (a sesquiterpene lactone) is isolated from the plant Artemisia
annua. Not all plants of this species contain artemisinin. Apparently it is only produced when the
plant is subjected to certain conditions, most likely biotic or abiotic stress. It can be synthesized
from artemisinic acid.[1] The drug is derived from a herb used in Chinese traditional medicine,
though it is usually chemically modified and combined with other medications.
Use of the drug by itself as a monotherapy is explicitly discouraged by the World Health
Organization as there have been signs that malarial parasites are developing resistance to the
drug. Combination therapies that include artemisinin are the preferred treatment for malaria and
are both effective and well tolerated in patients. The drug is also being studied as a treatment for
cancer.
History
Artemisia has been used by Chinese herbalists for more than a thousand years in the treatment of
many illnesses, such as skin diseases and malaria. The earliest record dates back to 200 BC, in
the "Fifty-two Prescriptions" unearthed from the Mawangdui Han Dynasty Tombs. Its
antimalarial application was first described in Zhouhou Beji Fang ("The Handbook of
Prescriptions for Emergencies"), edited in the middle of the fourth century by Ge Hong.
In the 1960s a research program was set up by the Chinese army to find an adequate treatment
for malaria. In 1972, in the course of this research, Tu Youyou (Chinese: 屠呦呦)[2] discovered
artemisinin in the leaves of Artemisia annua (annual wormwood). The drug is named Qinghaosu
(Chinese: 青蒿素) in Chinese. It was one of many candidates then tested by Chinese scientists
from a list of nearly 200 traditional Chinese medicines for treating malaria. It was the only one
that was effective, but it was found that it cleared malaria parasites from their bodies faster than
any other drug in history. Artemisia annua is a common herb and has been found in many parts
of the world, including along the Potomac River, in Washington, D.C.
Images of the original scientific papers are available online[3] and a book, Zhang Jianfang, "Late
Report – Record of Project 523 and the Research and Development of Qinghaosu", Yangcheng
Evening News Publisher 2007(張劍方. 遲到的報告五二三項目與青蒿素研發紀實. 羊城晚報
出版社, 2007),[4] was published in 2006, which records the history of the discovery.
It remained largely unknown to the rest of the world for about ten years, until results were
published in a Chinese medical journal. The report was met with skepticism at first, because the
Chinese had made unsubstantiated claims about having found treatments for malaria before. In
addition, the chemical structure of artemisinin, particularly the peroxide, appeared to be too
unstable to be a viable drug.
Artemisinin derivatives
Because artemisinin itself has physical properties such as poor bioavailability that limit its
effectiveness, semi-synthetic derivatives of artemisinin, including artemether and artesunate,
have been developed.
Chemically modified analogues
There are a number of derivatives and analogues within the artemisinin family:
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Artesunate (water-soluble: for oral, rectal, intramuscular, or intravenous use)
Artemether (lipid-soluble: for oral, rectal or intramuscular use)
Dihydroartemisinin
Artelinic acid
Artenimol
Artemotil
There are also simplified analogs in preclinical research.[5]
Purely synthetic analogues
To counter the present shortage in leaves of Artemisia annua, researchers have been searching
for a way to develop artemisinin artificially in the laboratory.
In 2003, Jay Keasling, a professor of biochemical engineering at the University of California at
Berkeley, published a paper in Nature Biotechnology that described how he and others had
discovered a pathway to create artemisinin in bacteria, by inserting genes from three organisms
into E. coli, one of the world’s most common bacteria. That research helped Keasling secure a
$42.6-million grant from the Bill and Melinda Gates Foundation. Keasling helped start a new
company, Amyris Biotechnologies, to refine the raw organism, then figure out how to produce it
more efficiently. Within a decade, Amyris had increased the amount of artemisinic acid that each
cell could produce by a factor of one million, bringing down the cost of the drug from as much as
ten dollars for a course of treatment to less than a dollar
A 2006 paper in Nature[6] presented a genetically engineered yeast that can synthesize a
precursor called artemisinic acid which can be chemically converted to artemisinin.
The compound called OZ-277 (also known as RBx11160), developed by Jonathan Vennerstrom
at the University of Nebraska, has proven to be even more effective than the natural product in
test-tube trials. A six month trial of the drug on human subjects in Thailand was started in
January 2005.
There are also plans to have the plant grow in other areas of the world outside Vietnam and
China (Kenya, Tanzania, Madagascar).
Indications
Malaria
Artemisinins can be used alone, but this leads to a high rate of recrudescence (return of parasites)
and other drugs are required to clear the body of all parasites and prevent recurrence. The World
Health Organization is pressuring manufacturers to stop making the uncompounded drug
available to the medical community at large, saying it would be a significant loss if the malaria
parasite developed resistance to Artemisinin.[7]
The World Health Organisation has recommended that a switch to artemisinin combination
therapies (ACT) be made in all countries where the malaria parasite has developed resistance to
chloroquine. Artemisinin and its derivatives are now standard components of malaria treatment
in China, Vietnam, and some other countries in Asia and Africa, where it has been proven to be a
safe and effective anti-malarial treatment. Fixed-dose combinations are preferred as this
guarantees that the partner drug is present to eradicate the last parasites while the artemisinin
component removes the majority at the start of the treatment[8].
A large number of fixed-dose ACTs are now available containing an artemisinin component and
a partner drug which has a long half-life, such as mefloquine (ASMQ), lumefantrine (Coartem),
amodiaquine (ASAQ), piperaquine (Duo-Cotecxin) and antifolates (Ariplus). Most are made to
GMP standard. A separate issue concerns the quality of some artemisinin-containing products
being sold in Africa and South-East Asia[9][10].
Artemisinins are not used for malaria prophylaxis (prevention) because of the extremely short
activity of the drug. To be effective, it would have to be administered multiple times each day.
Cancer treatment
Artemisinin is undergoing early research and testing for the treatment of cancer, primarily by
researchers at the University of Washington.[11][12] Chinese scientists have shown artemisinin has
significant anticancer effects against human hepatoma cells.[13] that Artemisinin has a peroxide
lactone group in its structure. It is thought that when the peroxide comes into contact with high
iron concentrations (common in cancerous cells), the molecule becomes unstable and releases
reactive oxygen species. It has been shown to reduce angiogenesis and the expression of vascular
endothelial growth factor in some tissue cultures.
Helminth parasites
Serendipitous discovery was made in China while searching for novel anthelmintics for
schistosomiasis. Artemisinin was effective against schistosomes, the human blood flukes, which
are the second most prevalent parasitic infections, after malaria. Artemisinin and its derivatives
are all potent anthelmintics.[14] They are later found to possess a broad spectrum of activity
against a wide range of trematodes including Schistosoma japonicum, S. mansoni, S.
haematobium, Clonorchis sinensis, Fasciola hepatica and Opisthorchis viverrini. Clinical trials
are also successfully conducted in Africa among patients with schistosomiasis.[15] A randomized,
double-blind placebo-controlled trial also revealed the efficacy against schistosome infection in
Cote d'Ivoire[16] and China.[17]
Resistance
A study published in 2008 by Noedl and colleagues in the New England Journal of Medicine
suggests a consensus among researchers that artemisinin is losing its potency in Cambodia and
increased efforts are required to prevent drug-resistant malaria from spreading across the
globe.[18]. These findings were subsequently supported by a detailed study from Western
Cambodia[19].
Adverse effects
Artemisinins are generally well tolerated at the doses used to treat malaria.[20] The side effects
from the artemisinin class of medications are similar to the symptoms of malaria: nausea,
vomiting, anorexia, and dizziness. Mild blood abnormalities have also been noted. One serious
adverse effect is an allergic reaction.[21] One case of liver inflammation has been reported.[22] The
drugs that are used in combination therapies can contribute to the adverse effects that are
experienced by those undergoing treatment. Adverse effects in patients with acute falciparum
malaria treated with artemisinin derivatives tend to be higher.[23]
Mechanism of action
There is no consensus regarding the mechanism through which artemisinin derivatives kill the
parasites. Their site of action within the parasite also remains controversial.
At the chemical level, one theory states that when the parasite that causes malaria infects a red
blood cell, it consumes hemoglobin within its digestive vacuole, liberating free heme, an ironporphyrin complex. The iron reduces the peroxide bond in artemisinin generating high-valent
iron-oxo species, resulting in a cascade of reactions that produce reactive oxygen radicals which
damage the parasite leading to its death[24].
Numerous studies have investigated the type of damage that oxygen radicals may induce. For
example, Pandey et al. have observed inhibition of digestive vacuole cysteine protease activity of
malarial parasite by artemisinin.[25] These observations were supported by ex vivo experiments
showing accumulation of hemoglobin in the parasites treated with artemisinin and inhibition of
hemozoin formation by malaria parasites. Electron microscopic evidence linking artemisinin
action to the parasite's digestive vacuole has been obtained showing that the digestive vacuole
membrane suffers damage soon after parasites are exposed to artemisinin.[26]
A study from French Guiana in field isolates of P. falciparum identified a mutation in the
parasite's SERCA pump that was associated with resistance to artemether[27]. However the
mutation has not achieved a meaningful prevalence in any location and the phenotypic
association has not been reported elsewhere (there is no evidence to suggest a role for PfSERCA
in the artemisinin resistance that appears to be emerging in Cambodia[18][19]). Critically, causality
has not been confirmed via a transfection study.
A 2005 study investigating the mode of action of artemisinin using a yeast model demonstrated
that the drug acts on the electron transport chain, generates local reactive oxygen species, and
causes the depolarization of the mitochondrial membrane.[28]
Dosing
The WHO approved adult dose of co-artemether (artemether-lumefantrine) for malaria is 4
tablets at 0, 8, 24, 36, 48 and 60 hours (six doses).[29][30] This has been proven to be superior to
regimens based on amodiaquine.[31] Artemesinin is not soluble in water and therefore Artemisia
annua tea was postulated not to contain pharmacologically significant amounts of artemesinin.[32]
However, this conclusion was rebuked by several experts who stated that hot water (85 oC), and
not boiling water, should be used to prepare the tea. Although Artemisia tea is not recommended
as a substitute for the ACT (artemisinin combination therapies) more clinical studies on artemisia
tea preparation have been suggested.[33]
Synthesis
In 2006 a team from UC Berkeley published an article claiming that they had engineered
Saccharomyces cerevisiae yeast that can produce the precursor artemisinic acid. The synthesized
artemisinic acid can then be transported out, purified and turned into a drug that they claim will
cost roughly 0.25 cents per dose. Details of the formation of artemisinic acid involves a
mevalonate pathway, expression of amorphadiene synthase, a novel cytochrome P450
monooxygenase (CYP71AV1) and its redox partner from A. annua. A three-step oxidation of
amorpha-4,11-diene gives the resulting artemisinic acid.[34] Amyris Biotechnologies is
collaborating with UC Berkeley and the Institute for One World Health to further develop this
technology.[35]
Using seed supplied by Action for Natural Medicine (ANAMED), the World Agroforestry
Centre (ICRAF) has developed a hybrid, dubbed A3, which can grow to a height of 3 m and
which produces 20 times more artemisinin than wild varieties. In northwestern Mozambique,
ICRAF is working together with a medical organisation, Médecins sans frontières (MSF),
ANAMED and the Ministry of Agriculture and Rural Development to train farmers on how to
grow the shrub from cuttings, and to harvest and dry the leaves to make artemisia tea. Cultivation
of this crop may well prove a valuable niche market for Africa, given the strong demand for the
plant from pharmaceutical laboratories.
The biosynthesis of artemisinin is expected to involve the mevalonate pathway (MVA) and the
cyclization of FDP (farnesyl diphosphate). Although it is not clear whether the DXP
(deoxyxylulose phosphate)pathway can also contribute 5-carbon precursors (IPP or/and
DMAPP), as occurs in other sesquiterpene biosynthetic system. The routes from artemisinic
alcohol to artemisinin remain controversial and they differ mainly in when the reduction step
takes place. Both routes suggested dihydroartemisinic acid as the final precursor to artemisinin.
Dihydroartemisinic acid then undergoes photoxidation to produce dihydroartemisinic acid
hydroperoxide. Ring expansion by the cleavage of hydroperoxide and a second oxygen-mediated
hydroperoxidation furnish the biosynthesis of artemisinin.
Figure 1. Biosynthesis of Artemisinin.
The total synthesis of artemisinin can also be performed using basic organic reagents. In 1982,
G. Schmid and W. Hofheinz published a paper showing the complete synthesis of artemisinin.
Their starting material was (-)-Isopulegol (2) which as converted to methoxymethyl ether (3).
The ether was hydroborated and then underwent oxidative workup to give (4). The primary
hydroxyl group was then benzylated and the methoxymethyl ether was cleaved resulting in (5)
which would be oxidized to (6). Next, the compound was protonated and treated with (E)-(3iodo-1-methyl-1-propenyl)-trimethylsilane to give (7). This resulting ketone was reacted with
lithium methoxy(trimethylsily)methylide to obtain two diastereomeric alcohols, (8a) and (8b). 8a
was then debenzylated using (Li, NH3) to give lactone (9). The vinylsilane was then oxidized to
ketone (10). The ketone was then reacted with fluoride ion that caused it to undergo desilylation,
enol ether formation and carboxylic acid formation to give (11). An introduction of a
hydroperoxide function at C(3) of 11 gives rise to (12). Finally, this underwent
photooxygenation and then treated with acid to produce artemisinin.[36]
The Artemisinin Project
The Artemisinin Project is an program by Sanofi-Aventis, Amyris Biotechnologies, the Institute
for OneWorld Health, and Jay Keasling, a researcher from the University of California, to
combat malaria by producing artemisinin at low cost.[37] Naturally derived artemisinin is
expensive and Keasling's proposal for its production by biotechnology is expected to reduce its
price. With the help of the Gates Foundation, Keasling, OneWorld Health and Amyris developed
a lab process for the production of artemisinin. Sanofi-Aventis was chosen to put the process into
mass-production.
[edit] Legal Status
For many years, access to the purified drug and the plant it was extracted from were restricted by
the Chinese government. It was not until the late 1970s and early 80s that news of the discovery
reached scientists outside China. The World Health Organisation (WHO) tried to contact
Chinese scientists and officials to find out more, but drew a blank. Dr Ying Lee, one of the
scientists involved in the research into artemisinin, said the Chinese distrusted the West. The
Chinese suspected the West just wanted to exploit the drug and sell it around the world slightly
altered and repatented. The fact that there were several Americans on the WHO's steering board
on malaria and that some were from the military did not help clear the distrust. It can be noted
Americans had just invested a lot into mefloquine, an analogue of chloroquine.