Yesterday, on October 5, 2015, one half of the Nobel Prize in Physiology or Medicine was awarded to scientist and pharmaceutical chemist Tu Youyou (alternatively, Tu Yo Yo, 屠呦呦 in Chinese), for her discovery of the anti-malarial Artemisinin. (The other half went jointly to William C. Campbell and Satoshi Ōmura, for their discovery of a novel therapy for roundworm infection.)
Professor Tu, 84, has the rare distinction of being the first native Chinese Nobel Laureate in Medicine. She was born (in the city of Ningbo of the Zhejiang province in the Northeast) and educated in China; she graduated in 1955 from the Department of Pharmaceutical Sciences of the erstwhile Beijing Medical University (now known as Peking University Health Science Center after several name changes), and trained thereafter in Traditional Chinese Medicine (TCM) for almost three years. Professor Tu has no postgraduate degree or study/research experience abroad, neither of which was possible in the 1960s and 70s when she was working, the era of Cultural Revolution in China; all of her professional work has been performed within China. She worked at what is now known as the China Academy of Chinese Medical Sciences after her graduation, and is currently a Lifetime Fellow and Chief Researcher at the Qinghaosu (Chinese name for Artemisinin) Research Center, Institute of Chinese Materia Medica of the Academy [Source: Youyou Tu’s 2011 Commentary in Nature Medicine]. Interestingly, she has not been granted the membership of any of the Chinese National Academies, a situation which has been criticized in view of the international recognition of her scientific accomplishments.
|Tu Youyou (right) and her tutor Lou Zhicen in China Academy of Chinese Medical Sciences in 1950s (Public Domain photo, via Wikimedia Commons)|
Artemisinin, for which Professor Tu also received the 2011 Lasker Prize, is considered a significant therapeutic breakthrough for Malaria, the tropical parasitic disease affecting 200 million people globally, leading to nearly 600 thousand deaths (2013 data, from WHO World Malaria Report 2014). Artemisinin-based drug combinations, included in WHO’s list of Essential Medicines, has become a standard of anti-malarial care, saving millions of lives in the world. The history of its discovery is quite fascinating.
In the late 1950s, the emergence of chloroquine-resistant Plasmodium falciparum parasite (the etiological agent of malaria) had put a serious dampener on WHO’s malaria eradication mission in the tropics – about 100 countries in Sub-Saharan Africa, Asia, Latin America, the Middle East and parts of Europe. North Vietnam, which was at war, faced devastation of its military and civilian population, and asked for help from China, where malaria was already a significant public health problem; thus was born the clandestine Project 523, whose beginnings and early endeavors are still undisclosed. Tu, then a researcher-phytochemist with experience in TCM-related herbs and folk remedies, was placed in charge of the Project in 1969, to lead a team of TCM practitioners, chemists, pharmacologists, and biological scientists, and work began in earnest to find a cure for the scourge of chloroquine-resistant malaria.
This brings me to two fascinating branches of biomedical science, known as pharmacognosy (the study of medicinal substances derived from plants and other natural sources) and ethnobotany (the study of plants traditionally used for medicinal purposes across different cultures). Historically, these have formed the basis of discovery of many new drugs used today in patient care. The finding of medicinal properties of natural substances is not new or even surprising. Natural products – derived from herbs, plants, organic sources originating in the oceans, and so forth – form the basis of countless medicines already in use or being tested for future use. But how does a natural product journey from its source to becoming a drug used by patients?
In modern medicine, the process of drug discovery follows a pathway which is laborious, time-consuming, fraught with frustrations, but necessary and ultimately productive. The first step in the process is called “lead identification and medicinal chemistry“, which is where ethnobotany primarily helps – as it did for Tu’s team.
Traditional therapies for various ailments, practised in different cultures, can offer valuable clues about the medicinal properties of a given natural product, such as a plant or a herb. In the laboratory, medicinal chemists would try to extract the biologically or pharmacologically active ingredient(s) from the product, say, a leaf, root, or bark of a particular plant. These ingredients are often secondary metabolites of the given plant, belonging to the chemical classes of either carbohydrates (e.g. glycosides and gums), proteins, amines, alkaloids, or various lipids (e.g. terpenes, steroids, essential oils, et cetera). Depending upon their chemical nature – acidic, alkaline, or neutral; hydrophilic (miscible with water) or hydrophobic (miscible in organic solvents, such as chloroform or ethyl alcohol) – these substances are separated into various fractions (or ‘extracts’), each of which are to be tested subsequently for chemical structure and screened for biological activity against an established or putative target. Substances with plausible or established biological activities are considered ‘leads’.
|Artemisia annua plant (Photo by Ton Rulkens of Mozambique, via Wikimedia Commons, under CC-BY-SA2.0)|
Following the same reasoning, Tu’s team scoured through ancient texts of TCM, investigating more than 2000 Chinese herbal preparations and identifying 640 leads for possible anti-malarial activity, eventually evaluating 380 extracts from about 200 herbs in mouse models of malaria – with no promising result in sight. Until, that is, they came across an extract of qinghao or Artemisia annua (a.k.a. “sweet wormwood” or “sweet mugwort”), which drastically reduced parasite growth in mice.
Did I mention that drug discovery can be quite frustrating? Subsequent experiments with qinghao extracts could not reproduce this beneficial effect. But this also marks the strengths of a good scientist. Rather than being discouraged, Tu went back to intensive review of the ancient literature; setting aside documents that didn’t describe relevant symptoms, she zeroed in on the single, almost 1700-year-old text which mentioned the use of qinghao for treating malaria-like symptoms, quickly realizing that the then-conventional, high temperature extraction via boiling was destroying the active principle, and went on to redesign the extraction process using the organic solvent, diethyl ether, at a low temperature. It additionally allowed her to separate an acidic portion of the extract with no anti-malarial activity from the neutral, pharmacoactive part – designated number 191. The anti-malarial substance was found to be present in the leaves, rather than elsewhere in the plant, and maximally in the upper portions of a newly growing plant. (NOTE: It was later found to be concentrated inside spiny, fluid-containing projections called Glandular Trichomes on leaves and floral stocks of A. annua.) Towards the end of 1971, Tu had finally obtained a non-toxic, neutral qinghao extract (“Qinghaosu”; ‘su’ = basic element in Chinese) that completely cleared parasitemia from mice and Cynomolgus monkeys experimentally infected with the appropriate strain of Plasmodium.
Offering themselves up as volunteers, Tu and her colleagues performed the first human safety trials; once safety was ascertained, Tu and her team tested the extract on subjects suffering from malaria in the Hainan province in South China. Encouragingly, regardless of the exact etiological agent (Plasmodium falciparum or P. vivax, a related species associated with recurrent fevers), patients receiving qinghaosu achieved rapid cure, in comparison to those who received chloroquine.
In the next step of the drug discovery process, Medicinal Chemistry plays a central role. The substance is isolated in larger quantities from the source, chemically purified, and investigated using physicochemical tools to determine the chemical structure. As and when the chemical structures are known, medicinal chemists try to artificially reproduce the substance and test each reproduction for biological activity; for instance, salicin, an alcoholic β-glucoside with anti-inflammatory activity present in the bark of white willow and some poplars, has been successfully reproduced and chemically enhanced into acetyl-salicylic acid, commonly known as Aspirin. The advantages to doing this is manifold; the amount of the natural product obtainable and its biological potency may be subject to various extraneous conditions, such as (a) source (specific part of the plant), (b) species or variety of the plant, (c) season and/or environmental conditions in a year, and (d) growth conditions, including pH and mineral content of the soil, hydration, and so forth. Artificial synthesis takes care of these variations, delivering consistent quantity and quality reproducibly. In addition, after primary screening for biological activity, medicinal chemists often chemically modify the leads in order to focus them more onto specific targets, as well as improve the interaction of the drug molecule with biological systems (such as absorption, solubility, and ability to penetrate individual cells). The leads are then rescreened iteratively.
A part of this process was already initiated by Tu when she redesigned the extraction process. In late 1972, Tu and her team identified and purified qinghaosu, a colorless, crystalline substance with a molecular structure (determined in 1975) unusual for then-known anti-malarial agents: a sesquiterpene lactone, containing a peroxyl (R-O-O-) group that was later found to be essential for its lethal anti-malarial effects. The crystal was tested on over 500 clinical cases of malaria, with most encouraging results that corroborated the pharmacological properties of qinghaosu.
This sequential approach to drug discovery is considered reductive, with a possible downside: biological activity of the raw extract may be greater than that of one or more specific ingredient(s) in the purified form, especially if they act synergistically. However, modern medical chemistry is able to use combinatorial approaches that allow testing of single ingredients or mixtures in various combinations. In addition, studies of Pharmacokinetics (PK; the body’s interaction with the drug molecule, including absorption, distribution, metabolism and excretion) and Pharmacodynamics (PD; biochemical and physiological effect of the drug and evaluation of the relation of these effects to its chemical structure) guide the medicinal chemists in refining the drug molecule to work in the body with maximum efficiency. For instance, pharmacoactive substances that can be destroyed by the high acidity of stomach or may irritate the stomach lining (‘gastric mucosa’) are often placed inside a coat of a special material (‘enteric coating’) that allows passage further downstream into the intestines, where the coat dissolves due to different pH and/or bacterial enzymatic action, disseminating the drugs. The first formulation Tu’s team tested in patients in form of tablets didn’t work as well due to incomplete disintegration; subsequently, they had much-improved results with a capsule of pure Artemisinin.
The information about this new drug molecule was released to the world in 1979 (the first English-language report, presented anonymously per Chinese custom at the time, was in December 1979), after a National Invention Certificate (similar to a patent) was granted to Tu by the China National Committee of Science and Technology for the discovery of Artemisinin. More than 2000 malaria patients, including some with chloroquine-resistant P. falciparum malaria, as well as some with cerebral malaria (a particularly severe and deadly form), had been treated across China with qinghaosu by that time, with astonishingly positive results and no reported adverse reactions. Tu presented her data in a 1981 Malaria conference in Beijing organized by the United Nations Development Program, the World Bank and WHO, to a great deal of international interest, and published them the next year, all anonymously under the name of “China Cooperative Research Group on qinghaosu and its derivatives as antimalarials”, in the English-language Journal of Traditional Chinese Medicine.
In the true tradition of proficient medicinal chemists, Tu wasn’t satisfied with existing qinghaosu; as early as in 1973, she had already chemically modified Artemisinin to dihydroartemisinin. Overcoming the initial concerns about its chemical stability, she found that this substance was stable, possessed ten-fold higher anti-malarial potential than Artemisinin, reduced the risk of recurrence, and provided the opportunity to develop newer Artemisinin derivatives through chemical modifications.
Tu’s exhaustive work involving pharmacognosy, ethnobotany, physical chemistry, biochemistry and clinical medicine has provided the foundation of today’s anti-malarial therapies; in 2005, WHO announced a shift to Artemisinin combination therapy in its redoubled malaria eradication efforts, which has already saved millions upon millions of lives, including children in sub-Saharan Africa who are the hardest hit. Quite a far cry from “A handful of qinghao immersed with 2 liters of water, wring out the juice and drink it all” – as appeared in the fourth century CE Handbook of Prescriptions for Emergencies by Ge Hong (284-346 CE), where it all started – am I right?
Tu studied and assimilated the heritage provided by 5000 years of the history of Chinese traditional medicine, but she didn’t stop there. Instead, she chose to utilize the techniques fashioned by modern science, and painstakingly apply the methodological rigor and empirical insights of the modern scientific process. THIS is what has brought the benefits of some obscure herbal medicinal plant to those who need them. I want to emphasize this observation. THIS is why Tu Youyou is the most deserving recipient of the Lasker award and the coveted Nobel Prize. In her 2011 Nature Medicine commentary, Tu provided many more examples of such evidence-based pharmacognostic bridges between ancient Chinese medical practices and modern medicine.
Unfortunately, this is also a fact that escapes, and/or is most likely ignored by, most practitioners of herbal medicine, including TCM, Amichi (from Tibet), Ayurveda, Siddha and Unani systems (from India); having craftily co-opted the genuine observations and the language from ethnobotany and pharmacognosy (two legitimate endeavors of scientific exploration) and prodigiously mixing it with dollops of Eastern mysticism, herbalists are more content to project themselves as valid, efficacious alternative or ‘integrative’ (the current buzzword) medical systems – rather than taking the pains to have their claims regarding ancient therapies tested and bolstered by empirical evidence. I consider this intellectual laziness, decidedly a tremendous pity. That consideration will not stop the proponents of herbal medicine from extrapolating Tu’s Nobel Prize as a validation of herbalism, but in effect, it would be a deplorable affront to Tu’s vision, hard work, as well as the sophisticated techniques she used to establish her hypotheses.
- Youyou Tu’s 2011 Commentary in Nature Medicine, written after her Lasker Award, has more detail of her journey along with most interesting photos of the original documents she had used.
- Additional details of Tu’s discovery are available from the Lasker Foundation’s award page.