Artemisia annua: A Vital Partner in the Global Fight against Malaria

UPDATE (June 27, 2014): I receive more correspondence on this story than any other! This post is from 2012. If you aren't satisfied by that, tweet at me (@ericmsawyer) to persuade me to make a new, up-to-date post.

Last year I wrote a brief post on artemisinin, often touted as the synthetic biology success story. Here is a much more thoroughly researched take on the topic, including recent news that this "miracle" drug is becoming susceptible to malaria parasite resistance.

Artemisia annua, known commonly as sweet wormwood, sweet Annie, and qinghao, is a shrub native to China long used both ornamentally and for medicinal purposes. A. annua is now cultivated globally as the only source of a potent anti-malarial drug, artemisinin. The drug is part of a cocktail of phytochemicals stored in glands on the leaves epidermis ("glandular trichomes,") which are used to deter browsers. Artemisinin has proved effective in the onslaught against the highly adaptable malaria parasite, which has already become resistant to many other drugs.

Cultivation

A. annua is native to parts of Asia within the temperate climate regions, particularly the northern parts of China's Chahar and Suiyuan provinces. However, wild populations have been established across the world, including in the United States, Argentina, and much of Europe. A. annua is cultivated on all six permanently inhabited continents, and the plant appears highly adaptive to non-temperate climates. Further, selective breeding for shorter photoperiods has made cultivation on near-equatorial latitudes possible. Despite these advances, China and Vietnam remain by far the largest cultivators of A. annua (Ferreira et al., 2005).

Farmers typically start A. annua crops from seeds, which are viable for up to three years if kept cool and dry. Cuttings have a high rooting rate (nearly 100%), but the higher cost of this method makes it less feasible than starting directly from seed. Young seedlings are highly vulnerable to being out-competed by weeds, so early application of chemical herbicides in large scale operations is typically recommended. However, established plants usually shade out weeds before they can become a threat to the crop. Pesticides and fungicides are unnecessary, as A. annua plants are largely free of pests and disease. At present the greatest biological threat is a fungus which affects <1% of plants in certain cultivation areas (Ferreira et al., 2005).

Medicinal Properties

In the 1970s, efforts funded by the Chinese government to identify native plant natural products of medicinal value paid off. In the 1960s Mao Zedong had ordered Chinese scientists to examine China's plant genetic resources for anti-malarial natural products-to aid the North Vietnamese, amusingly (McNeil, 2012). Extracts of A. annua, long used as an ornamental and medicinal shrub in traditional Chinese culture, were found to kill the malaria parasite, Plasmodium falciparum. Artemisinin, known in China as qinghaosu (Fig. 2), was identified as the responsible compound (Enserink, 2005).

Malaria is a devastating global health crisis, with an estimated 300 million people succumbing to infection by the malaria parasite every year. The vast majority of cases occur in sub-Saharan Africa. Between 500,000 and 2.7 million people die each year from malaria, and children represent a disproportionately large fraction of the victims (Ferreira, 2005).

The initial symptoms following infection with the parasite are chills and fever that last for several hours and recur every few days. Progression of the disease is marked by the onset of headaches, muscle cramping, diarrhea, and vomiting. If left untreated, enlargement of the liver and spleen occurs, followed by anemia and jaundice. In late stages of the disease, clogging of the cerebral vessels leads to coma and death. Because of the disease's extent and the high costs of treatments, malaria also represents a significant economic cost to affected areas. It is estimated that malaria costs the combined GDP of the African continent approximately $12 billion every year (Ferreira, 2005).

A. annua is cultivated exclusively for the extraction of artemisinin, and the compound has been isolated from leaves, stems, buds, flowers, and seeds. However, leaves are the principal source in commercial operations since they contain by far the most artemisinin by mass. Still, there is a high variance in artemisinin yield by plant, even among clonal populations (Ferreira, 2005).

Leaves are separated from the lower-yield stems in the extraction process. Mechanical grinding of the plant mass is unnecessary, since the artemisinin is found exclusively in glandular trichomes. The typical extraction procedure is submersion in petroleum ether or hexane followed by purification with column chromatography. The application of green chemistry principles, specifically the use of supercritical CO₂ in place of organic solvents, has been shown to reduce processing cost and improve yield. However, initial installation costs for supercritical CO₂ systems are in many cases prohibitively high (Ferreira, 2005).

Meeting the Global Demand for Artemisinin

Despite the high efficacy of artemisinin-based malaria treatments, a low and highly volatile global supply has historically limited access to the resultant drugs. In 2005, the journal Science reported on the dire state of global artemisinin supply. The Bill and Melinda Gates Foundation announced a $40 million initiative to make bioreactor-derived artemisinin available through the work of Jay Keasling's lab. Another concurrent and exciting development was the discovery of OZ277, a compound derived from artemisinin with improved characteristics (Enserink, 2005), but which remains in clinical trials and might be superseded by an improved variant (Charman, 2011).

In 2006, Keasling's lab reported that they had successfully adapted previous work on terpenoid biosynthesis for the production of artemisinic acid, a precursor of artemisinin. Using the tools of synthetic biology, they programmed yeast cells to secrete artemisinic acid. They were able to achieve yields approaching 0.1 g/L in this preliminary study (Ro et al., 2006).

However, as Keasling's team and their commercial partner, Sanofi-aventis, continued to work on refining their yeast strain and production setup, the amount of A. annua crops cultivated globally exploded. Production of refined artemisinin grew 10-fold in only three years, from about 20 t (2×10⁴ kg) in 2004 to 200 t in 2007. That cut the price from over $1100 per kg of artemisinin to $200 per kg (Noorden, 2010).

With growing food prices, many farmers opted not to plant artemisinin again in 2008, sending the global supply into a spiraling boom and bust cycle, with global production cut nearly in half. This cycle has been further exacerbated by the influence of year to year climate variation on crop yields (Noorden, 2010), as well as shifts in funding by major international health organizations from traditional therapies to artemisinin-based therapies (Kindermans et al., 2007).

Since artemisinin prices have remained low, synthetic yeast bioreactors are no longer the revolutionary cost-cutting solution they were initially marketed to be, and are unlikely to improve access to artemisinin therapies by virtue of cost alone (Noorden, 2010). Instead, Amyris, the company started by Keasling's group to develop synthetic biology based biofuels and medicines, has announced that the technology will be used to compensate for dips in supply. The current goal is to have the commercial artemisinin bioreactors up and running by 2013, which follows a series of production delays (Amyris, 2012).

Management and Policy

To safeguard the efficacy of the artemisinin drug family, the World Health Organization (WHO) developed a malaria management strategy structured around Artemisinin-based Combation Therapy (ACT). ACT combines an artemisinin drug with a second mainstay drug in an attempt to avoid the emergence of artemisinin resistance among parasite populations (Enserink, 2005).

However, ACTs have been slow to take to the market owing to their short shelf-life and a cost that exceeds that of traditional chloroquine or sulphadoxine-pyrimethamine therapies by a substantial margin. (Kindermans et al., 2007).

Sadly, the high cost and high demand of ACTs have paved the way for drug counterfeiting. In Southeast Asia, the total volume of artesunate (an artemisinin-derived drug) purchased was estimated to be between 33% and 53% counterfeits in 2008. The counterfeit tablets were found to contain a range of compounds, including unrelated drug precursors and even known carcinogens. A large proportion of the counterfeit supply was traced to southeastern China, which resulted in arrests (Newton et al., 2008).

Artemisinin is, undoubtedly, a case study in international disease management policy and ethics. How should large international biotech and pharmaceutical companies pursuing life saving treatments treat farmers who cultivate the basis of the treatments? Artemisinin drugs uniquely remain reliant on the original plants in which they were discovered for production due to the high cost of direct synthesis.

Yet, Sanofi-aventis, the company gearing up to produce biosynthetic artemisinin, has promised not to undercut farmers by lowering prices (Noorden, 2010). What side should companies pick in the tradeoff between protecting farmers' livelihoods and cutting drug prices to make life saving treatments available to the developing world?

ACT is by nature more expensive because two drugs are taken in concert. Since the average malaria patient might not benefit at all from this approach (if the parasites infecting them are not resistant to either drug), there is an enormous risk that noncompliance will emerge. Patients taking artemisinin monotherapy will, at present, receive much the same benefits of ACT but avoid its cost. That is a high risk scenario for the emergence of artemisinin resistant strains of the malaria parasite.

In only a few years, the global health community has gone from facing no observable artemisinin resistance among malaria parasites (Enserink, 2005) to a recent report in April 2012 documenting the slow emergence of artemisinin resistance in Southeast Asia (Cheeseman et al., 2012). In comparison, the first signs of quinine resistance did not appear until the 1960s, hundreds of years after its initial, albeit small scale, usage (Ferreira et al., 2005).

It is not clear why artemisinin resistance emerged so quickly. Perhaps ACT is simply ineffective in combating artemisinin resistance in a population of malaria parasites already highly resistant to the secondary drug. Alternatively, perhaps a sufficient number of patients are using cheaper artemisinin monotherapies against WHO recommendations to spur resistance. However, it is clear that resistance will be an ongoing and potentially derailing challenge for what is still an up-and-coming drug.

Image Credits (in order of appearance): Public domain (J. Ferreira, via Wikimedia Commons); Public domain (adapted); Fig. 3B from Ferreira & Janick (1995); Adapted from Artemisinin.pdb (via Jmol wiki: http://wiki.jmol.org); Nature 466, p. 672 (2010); Fig. 1C from Science 336, p. 80 (2012)

References:

Amyris. Artemisinin - Anti-malarial Therapeutic. Available from http://www.amyris.com/en/markets/artemisinin (2012).

Charman, S. A. et al. Synthetic Ozonide Drug Candidate OZ439 Offers New Hope for a Single-Dose Cure of Uncomplicated Malaria. PNAS 108, 4400-4405 (2011).

Cheeseman, I. H. et al. A Major Genome Region Underlying Artemisinin Resistance in Malaria. Science 336, 79-82 (2012).

Enserink, M. Source of New Hope Against Malaria is in Short Supply. Science 307, 33 (2005).

Ferreira, J. F. S. & Janick, J. Floral Morphology of Artemisia Annua with Special Reference to Trichomes. International Journal of Plant Sciences 156, 807-815 (1995).

Ferreira et al. Cultivation and genetics of Artemisia annua L. for increased production of the antimalarial artemisinin. Plant Genetic Resources 3, 206-229 (2005).

Kindermans, J.-M. et al. Ensuring Sustained ACT Production and Reliable Artemisinin Supply. Malaria Journal 6, 125 (2007).

LocalHarvest, Inc. Sweet Annie (Sweet Wormwood) Seed. Available from http://www.localharvest.org/sweet-annie-sweet-wormwood-seed-C9848 (2012).

McNeil, D. G. "For Intrigue, Malaria Drug Gets the Prize." The New York Times. January 16, 2012.

Newton, P. N. et al. A Collaborative Epidemiological Investigation into the Criminal Fake Artesunate Trade in South East Asia. PLOS Medicine 5, 0209-0219 (2008).

Noorden, R. V. Demand for Malaria Drug Soars. Nature 466, 672-673 (2010).

Ro, D.-K. et al. Production of the Antimalarial Drug Precursor Artemisinic Acid in Engineered Yeast. Nature 440, 940-943 (2006).