The hunt for active constituents in plants–the so called "silver bullets" that characterize modern, single chemical entity drugs—is just over 2 centuries old. This method of drug development, which emphasizes chemical simplicity, has resulted in important—but sometimes temporary—treatment options. Multi-component remedies have often been neglected by researchers in favor of silver bullet drugs, due to difficulties in understanding their activity, biases within the medical establishment, and other factors. Some research indicates, however, that chemically-complex mixtures can be safer and more effective than single isolates, and they may also be less likely to result in drug resistance. With multidrug resistance becoming a leading obstacle to curing malaria and protecting against infection,1 it is critical both to understand the history of analytical chemistry as it has impacted the modern view on antimalarial drugs and to reevaluate the potential of using multi-compound treatments such as herbal remedies.
Sweet Annie Artemisia annua. Photo ©2009 Martin Wall
The Rise of Silver Bullet Screening within Pharmacology
As chemistry was maturing in the 19th century, the developing field of analytical chemistry, with its ability to isolate and purify the active ingredients of plants, was foundational in early drug research and development.2 Alkaloids, a widely diverse group of constituents, were some of the first principals isolated from plants. The historical record suggests that Charles Derosne was the first to extract plant alkaloids; he extracted a mixture of 2 alkaloids from opium (from Papaver somniferum, Papaveraceae) in 1803.3 During the same time period, Friedrich Wilhelm Adam Sertürner was purifying constituents from opium and in 1817 succeeded in isolating morphine.3,4 Over the next 5 years, Pierre Joseph Pelletier and Joseph Bienaimé Caventou, 2 French pharmacist-chemists at the Ecole de Pharmacie of Paris, isolated a number of noticeably active compounds from plants.3 One of the alkaloids, quinine, from the South American cinchona tree (Cinchona spp., Rubiaceae), would later become an antimalarial drug that would change the political and economic landscape of Africa and other tropical areas.3,5,6
Also during the 1800s, François Magendie, known as the father of experimental pharmacology and a teacher of the renowned French physiologist Claude Bernard, began experimenting with Javanese arrow-poisons and eventually found that the active constituent was strychinine. He and Pelletier later demonstrated that emetine was the primary active substance of ipecac, although they were unable to isolate a pure substance. (It was later shown that their emetine was a mixture of at least 3 alkaloids.7) Magendie took pharmacology further into a reductionist direction by promoting the use of isolated principles from plants. In 1821 he published a pocket formulary for practicing physicians entitled (translated from French) “Formulary for the preparation and use of several new drugs, such as nux vomica, morphine, prussic acid, strychinine, veratrine, the cinchona alkaloids, emetine, iodine.”7 This work was essentially a guide to using isolated alkaloids in clinical medicine. The silver bullets of modern pharmacology had arrived.
Half a century later, the physician Thomas MacLagan successfully used salicylic acid, a metabolite of salicin (from the bark of willow [Salix spp., Salicaceae]) in a clinical trial on patients with rheumatism.8 By the late 19th century, clinical trials such as MacLagan’s and the groundbreaking physiology experiments of Claude Bernard and Magendie had fertilized the medical sciences to the point that pharmacology, which had formerly been seen as having limited relevance to the medical sciences, was elevated to a respectable ranking among the medical disciplines. Further, Oswald Schmiedeberg and his students at the University of Strasbourg laid many of the intellectual and experimental foundations of pharmacology,2,9 while Friedrich Bayer8 and Charles Frederic Gerhardt,10 through the production of acetylsalicylic acid, laid the foundation for the synthetic processing practices of what would become the pharmaceutical industry.
One of the foundations of pharmacology thus came to be the isolation and purification of constituents from plant medicines, which were already being used in various non-purified forms.2,9 In fact, about half of the United States Pharmacopoeia (USP) at the beginning of the 20th century were still “impure” multi-constituent plant medicines.11 Many of the 19th century and early 20th century medical journals documented case studies substantiating the effectiveness of plant medicines in their crude form.
Early efforts in the development of pharmacological agents were based on observations of outcomes within living systems exposed to substances. This strategy provided such drugs as aspirin in the West, as well as both the Ayurvedic and Chinese pharmacopeias in Asia. As the understanding of pathogenesis advanced, research strategies moved to in vivo animal models, followed by in vitro cellular models, which produced such drugs as the antibiotic penicillin from fungi and the anticancer drug cisplatin from an inorganic potassium salt.12 However, with increasing technology came sharper focus on single etiological agents; currently most drug discovery is confined to single protein targets. Medicinal chemists insist on single target-based screens because the alternative—studying multiple interactions—until recently, was not technologically possible and was considered too complex.13
Safety and Effectiveness of Medicinal Plants vs Drugs
The medical sciences have drastically changed their focus over the last 200 years. Not only have they moved from complex molecular mixtures to single molecules, but they have also shifted their focus to disease models of decreasing complexity, from the living to the inanimate.12 Williamson points out that when complex extracts were simplified to one molecule, scientists did not realize until much later that the specific mode of activity and the adverse side effects were altered, sometimes producing more serious adverse effects. And Vickers comments that an unspoken oversight of the medical sciences is that the rationale for the approach of isolation and purification of active constituents from “crude drugs” has never been made explicit.11
Although it has been suggested that the isolation and purification of active constituents from plants can provide the advantage of precise dosing and decreases the possible adverse events induced by other plant constituents, statistics suggest that pure compounds have their own risks. Analysis of data suggests single chemical drugs produce in the realm of 1000-10,000 times higher toxicity than medicinal plant preparations (although such data are not always directly comparable). A 2006 report by the Institute of Medicine put the number of medication errors causing injury to Americans at
1.5 million per year.14 Considering that the reports of adverse events are estimated to be underreported by a factor of 10, this is a remarkable figure.15 Detailed analysis of data from 55 countries published in the British Medical Journal noted that adverse events from herbal remedies are “a tiny fraction of adverse events associated with conventional drugs,” and the risk of using herbal remedies is “fewer than synthetic drugs.”16
Moreover, the belief that “therapeutic reproducibility” is superior because of the precise dosing of an isolated chemical strongly ignores the large variation in drug metabolism. For example, caffeine and other drug metabolism varies at least 60-fold in healthy subjects.17 Thus, if an individual is exposed to a concentrated chemical and has a slow metabolism, severe toxicity may occur. The likelihood of this happening with a medicinal plant, which is inherently a dilute mixture of chemicals, is, relatively speaking, significantly less likely.16
Critics of medicinal plants argue that the low concentration of any one phytochemical in a plant creates a mixture of compounds too dilute to have an effect. However, Rajapakse et al. have demonstrated that very low concentrations of any one chemical will contribute to a chemical mixture’s activity, even if that chemical does not show activity when isolated.18 This notion particularly challenges the research on herbal medicine that has suggested that some are void of activity because of failure to find a single active constituent. This also challenges research that equates activity of a plant with a single isolated chemical contained within a plant or plant part.
Further, attempts as early as 1928 demonstrated that the pharmacological activity of combinations of constituents often had different activity that could not be predicted by the activity of the isolated constituents.1 In other words, the efficacy of medicinal plants often cannot be reduced to a single constituent. Thus much of the research on medicinal plants that seeks a pharmaceutical gem from a jungle of phytochemistry is incomplete as it neglects the possibility of synergic, additive or antagonist activity of multi-constituent remedies.19
The chemical complexity inherent to multi-constituents, however, is not without disadvantages. Medicinal plants/multi-component remedies represent a particular challenge in understanding molecular modes of activity. That this is a particularly complex issue is demonstrated by the attempts to use information theory to cope with the complexity of the multi-component nature of herbal remedies.20,21 This issue still remains to be solved and will continue to delay significant pharmacological research on medicinal plants. But until modes of activity are clearly elucidated, outcome studies provide meaningful data.
Pharmacological research in itself will also need to break out of its current research methodologies to fully understand medicinal plant activity. The inability of contemporary science to describe systems composed of diverse elements that engage in nonlocal interactions has limited pharmacology, as well as many other areas of science.22 Constructs such as complexity theory and information theory offer model systems that provide a more complete approximation of natural processes. In addition, systems biology and network pharmacology offer approximations of physiology one step closer to real time cellular interactions and as a result, may substantially increase the understanding of multi-component remedies interfacing with cellular networks.
Multiple Constituents of Herbs and the Importance of Synergy
It is a rare medicinal plant that has only one bioactive constituent.3,7 Rather, medicinal plants commonly contain numerous active constituents. Messina et al. point out that the allelochemicals of a single plant can have complementary and overlapping activities on human physiology, including alteration of biotransformation enzyme activities, anti-inflammatory effects, stimulation of the immune system, hormone metabolism, and antimicrobial effects.23
The medicinal plant sweet wormwood, also known as Sweet Annie (Artemisia annua, Asteraceae), source of the antimalarial drug artemesinin, contains constituents that improve pharmacokinetic parameters, as well as at least 9 other compounds that contain antimalarial activity.24 Some of the flavonoids of A. annua appear to potentiate the mode of activity of artemesinin.25 Two polymethoxyflavones, casticin and artemitin, although inactive against the malaria-causing protozoa Plasmodium spp., have been found via in vitro models to selectively enhance the activity of artemisinin against P. falciparum.26 Two additional flavones that show very little direct growth inhibitory activity, chrysosplenol and chrysoplenetin-D, appear to target the P-glycoprotein pumps known as multi-drug resistance (MDR) efflux inhibitors.27 This provides further possible potentiation of artemisinin against malaria,28 since resistance of P. falciparum to mefloquine and structurally related drugs has been found to be due to the P-glycoprotein pump.29,30
The same phenomenon is seen in plants from which alkaloidal drugs are extracted. Rarely do the alkaloidal plants limit their production to only one alkaloid; usually they yield a complex mixture, possibly dominated by one or 2 alkaloids, but often accompanied by literally dozens. For example, the Madagascar periwinkle (Catharanthus roseus, Apocynaceae), from which the cancer drugs vincristine and vinblastine are derived, contains close to 100 distinct alkaloids.3 Yet resistance to vincristine and vinblastine is a well known occurrence.31 It may be worth considering whether some of the co-occurring constituents of C. roseus could mitigate the development of resistance.
In Cinchona spp. there are at least 7 alkaloids, as well as other groups of constituents, that contribute to the antimalarial activity.32 During World War II, the US military experimented with a mixture of cinchona alkaloids named totaquine.33 Totaquine was easy to produce, even with cinchona bark of low quinine content, and it could have been a relatively inexpensive drug. The military concluded that totaquine was as effective as quinine in terminating acute attacks of malaria but had a slightly higher rate of nausea and blurred vision. They also found that the 2 alkaloids cinchonine and cinchonidine were less toxic than quinine.
A more recent study done with a mixture of 3 cinchona alkaloids—quinine, quinidine, and cinchonine—demonstrated a synergic effect against a culture of P. falciparum.34 Additionally, the Plasmodium strains that were resistant to quinine were up to 10 times more susceptible to the alkaloid mixture than any of the single alkaloids. It is possible that Plasmodium resistance could be at least delayed, if not avoided, with prudent use of such therapeutic mixtures.
As has been seen in multiple examples over the last 40 years, whether it be insect resistance to DDT, bacterial resistance to antibiotics, or Plasmodium spp. resistance to the antimalarial drugs chloroquine and mefloquine, resistance to a single agent is predictable.35 Resistance to mefloquine was found within 6 years in areas where it had been widely used (Thailand, Cambodia, and Vietnam).36 On the other hand, it has been shown in various research models that the development of resistance of microbes is greatly attenuated by multi-component remedies.27,37-41 For example, Heliobacter pylori exposed to the antibiotic clarithromycin for 10 exposures, sequentially, develops resistance. But when exposed 10 sequential times to essential oil of lemongrass (Cymbopogon citratus, Poaceae), consisting of at least 23 different terpenoids—16 of which have known antimicrobial activity24—H. pylori was unable to develop resistance.37
Although often led by clinical trial and error, the strategy of using multiple compounds is already being used in clinical medicine. Drug cocktails have proven successful in the treatment of other complex diseases. Cancer, hypertension, and psychiatric treatment protocols have taken to achieving maximum efficacy by targeting several biochemical pathways simultaneously, exploiting synergy, and minimizing toxicity.42 In addition, multicomponent remedies, or “multitargeting,” is becoming a theme of infectious diseases. Physicians are now using drug cocktails to compensate for resistance in tuberculosis found in the inner cities. Cocktail therapies for AIDS and bacterial, fungal, and viral infections suggest that multitarget perturbations are useful therapeutic strategies.43 Polypharmacy is increasingly being accepted as a reducer of microbial resistance.
Support for Multi-Compound Remedies from Evolution and Biological Networks
Natural products have been described as a population of privileged structures selected by evolutionary pressures to interact with a wide variety of proteins and biological targets.44 From an evolutionary standpoint, many plant compounds are selected to enable plants to survive their biological environment. Depending on the mode of activity, allelochemicals (plant compounds generated for protection) must survive the metabolic processes of herbivores to be effective in their role against herbivory.28,45-47 Thus, these molecules, once absorbed, commonly have functional activity on various biochemical pathways of the herbivores that consume them. Natural selection would eventually eliminate plants that generated costly allelochemicals but could not effectively protect themselves by delivering these compounds to the herbivore.28 It follows that the phytochemical matrix surrounding these allelochemicals should, by natural selection, enhance absorption to allow the allelochemicals to reach their biochemical niche.48
Experimental models demonstrate that co-occurring compounds of medicinal plants play a role in enhancing the bioavailability and distribution of various phytochemicals. For example, the absorption of hypericin, the antiviral compound in St. John’s wort (Hypericum perforatum, Clusiaceae), is significantly enhanced in the presence of its naturally occurring flavonoid components.49 Similarly, artemisinin is absorbed faster in humans from a tea preparation of Artemisia annua than from tablets of pure artemisinin. This appears to be due to the co-occurring plant constituents, which seem to generate a high extraction efficiency of the lipophilic artemisinin in boiling water.50
Further, although one could argue that plant-human interactions were selected to repel and potentially harm humans, there are hypotheses suggesting the contrary. Plants may have selected compounds to encourage humans to feed on them and in the process enhanced early human health as well as encouraged propagation of themselves.51 Ehrlich and Raven suggested over 4 decades ago that the study of species interacting with one another have been narrow in scope and ignore the reciprocal aspects of these interactions.52 While much understanding of the interactions among species has been gained since Ehrlich and Raven’s work, their comment does seem pertinent to the idea of human and plant interactions as an evolutionary force influencing human physiology. An evolutionary perspective, when followed logically, would suggest that exclusively using isolated compounds to induce shifts in mammalian physiology is unsupported by the evolutionary process. On the contrary, exposure to one chemical at a time, from an evolutionary time scale, is completely novel to biology. For over 200 million years of evolving mammalian physiology, ingestion of foods and medicines, by way of plants, have always been multi-component mixtures of nutrients and secondary metabolites.
Systems biology, meanwhile, has shifted the investigational emphasis from the molecular level to the system level, recognizing that cellular physiology is organized as genes, proteins, and small molecules in intermolecular networks.53 The interactions among these components generate potential multi-step pathways, signaling cascades and protein complexes composed of redundant, convergent, and divergent pathways.1,54 In this paradigm, the core ecological milieu of cellular activity in health and disease, which were originally postulated as relatively disconnected linear pathways, are now recognized as a complex interdependent web of regulatory, structural, and metabolic signaling pathways among cells: Pharmacology is evolving from understanding the function of individual proteins to understanding how networks of proteins interact.53
In disease processes, there are numerous risk factors and defective proteins out of balance with each other that provide various pharmacological targets.55,56 In addition, many of the physiological systems and their interactions dynamically shift as a disease improves or worsens.57 There is often one major or easily definable defective target for a given disease, but collateral proteins that can act in a network are likely to be involved. With this in mind, the focus on a single protein to treat disease processes may not necessarily provide therapeutic efficacy.58
Illustrations of failed drugs that target a single protein, and ignore a sophisticated network system of disease processes, are numerous. For example, in the treatment of inflammatory bowel disease, many agents developed by targeting a specific molecule are proving to be either insufficiently effective or totally ineffective.58 Another example is the drug Iressa (gefitinib), which targets the protein EGFR to treat lung cancer. While this drug, which has been designed to have high selectivity for EGFR, generates an extraordinary response in 10% of those taking it, 90% of lung cancer patients show little-to-no response.59
Conversely, many highly efficient drugs, such as the non-steroidal anti-inflammatory drugs (NSAIDs), saliclyate, metformin, and the blockbuster drug Gleevec (imatib mesylate), affect many targets simultaneously.60 Ágoston et al, in a comparison of various pharmacological strategies, found that multiple but partial perturbations of selected targets in a network are almost always more efficient than the knockout of a single, carefully selected target.43 This is likely due to the redundant pathways of cellular networks that are not inhibited by a single chemical.60 (Case in point—Gleevec was originally formulated to target a single protein but has been found to hit multiple targets, which has been postulated to be the reason for its success.)
Thus, the hunt for high-affinity, high selectivity compounds, which has dominated pharmacological research, is not necessarily ideal for efficient perturbation of a cellular network.60 Low-affinity multi-target drugs such as plant extracts, on the other hand, may achieve significant alteration of a cellular network.43,60
Recent technological developments (e.g., Matrix Assisted Laser Desorption Ionization—Time of Flight, gene microarrays) to facilitate probes of interconnected pathways should add to the understanding of how cell networks interact by providing previously unattainable information about physiological processes. Information about protein abundance, phosphorylation state, and metabolite concentration are leading to more complex pharmacological models. Of great significance, “omics” experiments capture a glimpse of the activity of cellular networks by genomic, proteomic, and metabolomic profiles.54 Thus a snapshot of the cellular dynamics can be observed. Since natural products are often too complex to allow chemists to explore their structure-activity relationships,45 “omics” methodology may offer important insights into the mode of activity of complex traditional remedies.61,62
A union between systems biology, network pharmacology, and medicinal phytochemistry might reveal that nature’s strategy of activating multiple pathways simultaneously to elicit network regulation is likely safer and frequently more effective.
This strategy plays to the strength of plants, which have been selected, over millions of years, to modulate cellular networks and interconnected pathways. Medical sciences are increasingly able to appreciate, using network pharmacology models, the complex strategy of using multiple compounds. An evolutionary perspective, as well as a systems biology viewpoint, support the idea that chemical matrices are not only the evolutionarily established norm for shifting physiological processes, but may be superior for interfacing with the robust and complex cellular systems of life, including humans.60
The Treatment of Malaria with Medicinal Plants
Malaria, in addition to being the most pernicious parasitic disease of humans, is also the most prevalent. Current statistics suggest that malaria kills between 2.7 to 3 million people each year, with the majority being children under the age of 5 years.63 Plasmodium spp. has generated resistance to all classes of antimalarial drugs, and as a result there has been a doubling of malaria-attributable child mortality in eastern and southern Africa.64 Disturbingly, malaria is so common in certain tropical areas that “low transmission areas” are defined as a person acquiring Plasmodium spp. infection less than 3 times a year. Conversely, in some tropical areas new malaria infections are acquired more than once each day and can be asymptomatic.65 Current estimates suggest that approximately 300 million people on the planet are infected with Plasmodium spp.
Of the 4 species of malaria parasites that infect humans—P. falciparum, P. vivax, P. ovale, P. malariae—the most deadly is P. falciparum. If falciparum malaria is treated appropriately, the mortality is a mere 0.1%.65 However, P. falciparum parasites, especially from Southeast Asia, are particularly known for developing drug resistant strains and these strains can produce a mortality rate of 15-20%.66
Women sort Sweet Annie (Artemisia annua) leaves at Tooro Botanical Gardens in Fort Portal, Uganda. Photo ©2009 Keith Lindsey
Predictably, there are reports of in vitro resistance of Plasmodium spp. to artemisinin derivatives67,68 as well as reports of recrudescence in patients treated with artemisinin derivatives.69 This is of particular concern due to the increase in demand of artemisinin-derived drugs, from 22,000 treatment courses in 2001 to an estimated 200 million in 2008.
Treatment cost and income are important variables affecting the choice of malaria treatment and contributing to drug resistance. The majority of malaria-ridden countries spend less than US $10 per capita annually on health, creating a situation where even US 50 cents becomes a prohibitive cost of treatment.65 Perhaps partially due to such economics, the recommended treatment in many high-transmission areas are antimalarial drugs (i.e., chloroquine or sulfadoxine-pyrimethamin) that are partially or completely ineffective.70 As a result of cost and lack of access to healthcare facilities, medicinal plant preparations remain a popular choice for the rural poor.71 Studies report up to 75% of African patients with malaria use medicinal plants, while in French Guiana 33% report regular use of herbal remedies to prevent febrile illnesses and malaria.72 Mothers in rural Africa commonly start malaria treatment of their children with herbal therapies before they initiate pharmaceutical treatment.73
Treatment of malaria by the poor often involves buying whatever they can afford and not necessarily the correct dosage for effective treatment, which can contribute to drug resistance. Thus, it could be that new pricey pharmaceuticals (or even cheaper, older pharmaceutical antimalarials) combined with properly used medicinal plant preparations might stave off drug resistance. Considering that recent treatment strategies to reduce the emergence of de novo resistance relied on antimalarial drug combinations,70 it follows that if a plant contains compounds that are antimalarial (and antimalarial plants commonly have multiple antimalarial compounds), a combination of properly-dosed medicinal plant extracts with an inexpensive pharmaceutical antimalarial may greatly facilitate elimination of the malarial parasite.
Willcox has pointed out that there are 1,277 plant species from 160 families listed that have been used to treat malaria.74 (Of these, 5 were listed as “endangered,” 13 were listed as “vulnerable,” and 3 were listed as “near threatened.”) In northeast India, 65 medicinal plants from 38 different families have been reported to treat malaria,75 and in South Vietnam, 46 plants traditionally used for malaria have shown activity through in vitro testing.76 Approximately 64% of the traditional malaria remedies in Kenya have been found in an in vitro model to exhibit anti-plasmodial activity.77 Of the 1,277 plants Willcox listed,74 47 species are used on 2 continents and 11 species are used on all 3 tropical continents as antipyretics or antimalarials. The plants used on more than one continent for the treatment of malaria could provide an informed beginning for the search for effective antimalarials, whether they be low cost traditional remedies or high-tech combination cocktails made from isolates.
Notable mentions of medicinal plants include Terraplis interretis, which showed high rates of adequate clinical response* in testing to the point of clinical cure.74 Additionally, Cryptolepis sanguinolenta (Asclepiadaceae) has demonstrated activity roughly equal to that of chloroquine; Cryptolepis cleared fever 12 hours faster and cleared parasites within 24 hours.74 Bidens pilosa (Asteraceae) has shown activity against drug resistant P. falicparum parasites in vitro and in vivo in rodents. Strychnopsis thouarsii (Menispermaceae) appears to be useful for prevention of malaria due to activity against the hepatic stage of Plasmodium.78
Studies with plants traditionally used for malaria treatment from various parts of the world have intriguingly shown inhibitory activities against both chloroquine-sensitive and resistant strains of P. falciparum.79 Some of these medicinal plants, worthy of further research, include Coscinium fenestra (Menispermaceae), Psidium guajava (Mytraceae), Vangueria infausta (Rubiaceae), Struchium spargano-phorum (Asteraceae), Cinchona succirubra, Tithonia diversifolia (Asteraceae), Cedrela odorata (Meliaceae), and Pycnanthus angolensis (Myristicaceae).80 Traditional remedies of Kenya, which include Vernonia lasiopus (Asteraceae), Rhamnus prinoides (Rhamnaceae), and Ficus sur (Moraceae), also show notable antiplasmodium activity. Some, such as V. brachycalyx and V. lasiopus, showed a stronger effect on resistant Plasmodium strains than on nonresistant strains.77 V. lasiopus, which was found to potentiate chloroquine, also showed antiplasmodial activity comparable to Cinchona.77
Despite the prevalent use of traditional remedies for malaria, with or without pharmaceuticals, there seems to be few organizations dedicated to researching medicinal plant species as home remedies or sources of drugs to treat Plasmodium spp. infections. The Research Initiative on Traditional Antimalarial Methods (RITAM), Doctors for Life, Insect Centre of Insect Physiology and Ecology (ICIPE), Action for Natural Medicines (anamed), and the Plant Medicine Innovation Group, however, have dedicated their energies towards the political, economic, and research efforts of medicinal plants and other issues related to health and malaria. Many of these researchers believe that medicinal plants have the potential of solving the medical and societal issue of multi-drug resistance.41,81-86 Anamed’s work of training local people to cultivate A. annua and then treat malaria with the tea is reported to be successful and has likely led to a significant reduction in deaths. (K. Lindsey, personal communication, April 30, 2009). While some physicians are suggesting combinations of antimalarial drugs to prevent Plasmodium spp. resistance,38,87 the esteemed ethnobotanist James A. Duke—a veteran of malaria ridden areas—suggests that the use of teas or ethanolic extracts of A. annua, with its 9 different antimalarial compounds, might prove as efficacious as using multiple costly drugs.6,28 Duke’s suggestion, that extracts of A. annua are a natural “cocktail” therapy, could lead to self-reliance therapy that is readily available to impoverished areas where the death rates from malaria are high.
Although Plasmoidal recurrence was an issue in one study using a tea of A. annua,88 as previously mentioned, the recrudescence issue could possibly be addressed by a different dosing strategy or extraction method. There are positive studies, at least in the short term, to support the use of an A. annua tea for the treatment of malaria.88,89 In addition, Willcox reports on Chinese studies performed with ethanolic extracts,90 which resulted in better outcomes than those studies using the teas. The recrudescence rate in the formal clinical trials using the tea of A. annua is likely due to the short half-life of artemisinin, which does not kill all stages of Plasmodium, and the short duration of treatment in these studies. This is of concern because recrudescence is a risk for resistance. On the other hand, de Ridder et al comment that A. annua’s traditional use in China for 2000 years for fevers is apparently without the emergence of resistance.91 Another option to avoid recrudescence might entail combining A. annua with Cinchona, or other medicinal plants, which have constituents with an extended half-life.
Considering the number of plant extracts that have shown activity against Plasmodium spp. and the research that has suggested promising results of some traditional remedies, it seems unlikely that there would not be more species that could be explored. Given that effective medicinal plant extracts could shift the benefit:cost ratio from dollars to pennies, and that many known antimalarial plants, including A. annua, grow prolifically in tropical equatorial climates, this could significantly change the societal and economic burden of disease in many parts of the world. In addition, properly planned cottage industries of producing plant-based remedies for the treatment of malaria and other disorders could generate income for rural communities. Nevertheless, until enough resources are marked for allowing research on the potential of medicinal plants as a low cost, easily accessible solution, this potential may never be known. It is this author’s opinion that if political and economic issues are removed from the labyrinth of malaria treatment, then medicinal plants, often readily available and affordable as opposed to pharmaceuticals, may provide at least a partial solution to one of the planet’s leading causes of mortality.
Opportunity for Pharmaceutical Companies
Unfortunately, most multinational pharmaceutical organizations have down-scaled, or terminated, their natural products operations. Basso points out that this is in spite of natural products having between a 25-50% share of the top-selling 35 ethical drugs from 2000-2003.80 Newman reports that between 1981-2002, 74% of drugs approved for cancer therapy were either natural products or based on natural products.92 Of 119 chemical compounds extracted and isolated from plants to make conventional drugs, an impressive 74% have the same or related use as the indigenous cultures that use them.93 Thus, it is obvious that, besides providing important drug leads, natural products exploration and the respectful observation of people that still rely on plants as medicine have much to offer, including economic incentive for pharmaceutical companies.
But modern pharmaceutics effectively severed the connection between plants, foods, and medicines during the 20th century journey in search for disease-curing silver bullets. The abandonment of searching natural products for drug leads has been accompanied by an inexorable rise in the cost of generating new drugs. Such methods as high-throughput screening have been reported as having not had a significant impact on the derivation of new drugs.94 Random searches through combinatorial libraries, which are typically not based on biologically relevant properties, according to one estimate, lead to hits at a rate of 1:10,000.95,96 Conversely, combinatorial libraries based on natural products—compounds that, by default, have been selected for biological activity through the high-throughput screening of the evolutionary process—increase the likelihood of finding active compounds.97 Similarly, ethnobotanical leads have yielded positive activity in the order of 2 to 5 times higher than random screening.98 Such statistics indicate that an obvious source of new drug discovery lies in natural products.99 It seems quite likely that the increasing cost of generating new-to-nature molecules will generate a gap in medical care that will reconnect plants and human health at a new level of technological sophistication.100
Current data leads away from the use of single compounds to treat infectious disease such as malaria and suggests that combinations of antimalarials that have different modes of activity will reduce the chance of plasmodial resistance.38 In spite of this research, there still exists a strong bias for reductionistic pharmacological models—structure- and function-based studies based on isolated compounds perturbing single targets. However, recently a number of research groups are screening compounds that stick to several targets and some are attempting to engineer “promiscuous drugs.”101 But this is a high tech, expensive solution that ignores the currently existing options of specific medicinal plant extractions, which are often overlooked because they contain promiscuous compounds. If science in the 21st century is to truly advance beyond drugs that temporarily cure, yet later induce microbial resistance, the public, as well as the scientific establishment, must actively acknowledge and support research—both theoretical and applied—utilizing network pharmacology models and the multi-constituent properties of indigenous plants, rather than single-constituent pharmaceuticals.
Furthermore, large pharmaceutical companies have often focused research efforts exclusively on generating drugs that target conditions of wealthy, developed countries, while neglecting the needy in poverty-stricken countries, who are in need of life-saving drugs.80 More effort should be made by the pharmaceutical industry to address the healthcare problems of poverty-stricken areas. Fortunately, recent efforts by non-governmental organizations (NGOs), nonprofits, and other non-commercial entities have stepped into the area to fill the development gap. In addition, traditional healers and modern phytotherapists, who have provided a lifetime of work observing the effects of medicinal plant extracts on disease processes, perpetuate the knowledge of medicinal plant effects. These practitioners’ noteworthy efforts are often in the face of considerable resistance from much of the medical establishment as well as the social fabric of industrialized nations.
A Call to Action
Comprehensive evaluations of medicinal plants are urgently needed before more plant species are lost and knowledge of specific traditional medicines becomes irretrievable. While the study of a medicinal plant and its many components—some of them unidentified or having unknown properties—is theoretically, economically, and technically challenging, it should not be abandoned for sake of investigative expediency. Research into the multi-component nature of medicinal plant remedies offers a segue way into more complex therapeutics.34 Thus, the issue of using herbal remedies to alleviate human suffering is not one of merely assessing efficacy and safety,1 but a matter of the medical community’s struggle to understand a pharmacological paradigm that embraces the complexity of bio-molecular networks.
Changing research perspectives are leading to models that allow the observation of multiple perturbations of biological networks, in addition to multiple targets. This perceptual shift, coupled with the latest pharmacological models based on systems biology, build a paradigm in which multicomponent remedies, such as medicinal plants, are recognized as sophisticated pharmacological agents. Moreover, these multi-component remedies may offer improved efficacy and safety over isolated silver bullets.43,102
Implementation of network pharmacological models, which would lead to more complex therapeutic agents, could result in delayed antimicrobial resistance, decreased infectious morbidity, and less healthcare expenditures. But certain challenges have held drug therapeutics in the simplistic model that encourages the search for silver bullets. One obstacle, a limited collection of analytic tools, has been solved with the newest generation of high-tech analytical tools. Microarrays and related technologies are now economically feasible to the point that running hundreds of arrays are possible. Such an approach will demand more statistical, mathematically, and computational prowess. But if successful, this could generate improved therapeutics based on patient specific treatments and dietary guidelines, resulting in less human suffering and decreased economic burden. A second obstacle, a clashing of philosophies, is in the process of resolving. Ohno and collegues suggest that further progress will be made when all parties involved give up their subjective certainty and allow unbiased and more methodologically relevant investigations of medicinal plant species.37
After a hundred years of technological innovation, plants are still the primary source of leads for pharmacologically active compounds. The United Nations Convention on Biological Diversity takes the noteworthy stance that evolution has been selecting and perfecting diverse bioactive molecules for millions of years.103 The evolution of the science of pharmacology is likely to grow considerably beyond the current tenants of isolation, selectivity, and potency if it takes a cue from the 300 million years of plant evolution that have perfected a complex chemical means of defense against microbes and other predators. The study of phytochemical defense offers an opportunity to expand the foundational philosophy and techniques of the search for new drugs: They may best be utilized, not as expensively manufactured silver bullets hitting a single target, but as multi-component, broad-spectrum, pleiotropic molecular cocktails interfacing with cellular networks. This natural technology has been harnessed by traditional cultures for many centuries.
It is a scientific imperative for the progress of medicine that the time-tested methods of traditional medicine and the hi-tech modern pharmaceutical approaches coalesce. Both traditional and conventional healthcare systems seek to alleviate human suffering, both systems have merit, and both systems provide therapeutic options. All parties must learn to stretch pharmacological principles, beyond simplistic modeling and economic gain, to therapeutics based on improving the human condition. We must not let prejudice against therapeutics that are complex and not fully understood impede the use of life-saving remedies. Furthermore, where plant species intersect with medicine, we must keep an eye towards species preservation, sustainability, and the ethics of interfacing with traditional cultures.
Kevin Spelman, PhD, is currently a Marie Curie European Union Research Fellow at the Le Museum national d’Histoire naturelle in Paris, France. He is currently collaborating with several US and Parisian institutions investigating medicinal plants for anti-plasmodial activity.
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