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Willis KJ, editor. State of the World's Plants 2017. London (UK): Royal Botanic Gardens, Kew; 2017.
How many plant species are currently used as medicines? As traditional plant-based medicines become more widely accepted in mainstream health systems what are the mains issues and risks that need to be considered?
In many regions of the world, people still rely on traditional plant-based medicines for their primary healthcare[1,2]. This is especially true for many rural communities in Africa, parts of Asia, and Central and South America, where plants and knowledge of their traditional use are accessible and affordable. In other countries, many of these traditional plant-based medicines are being integrated through regulations into mainstream health systems[3,4]. For example, in December 2016 the Chinese government announced their aim to integrate Traditional Chinese Medicine (TCM) into their healthcare system by 2020[5], recognising improved scientific understanding of the plants and their value in treating chronic conditions. In Europe, there is also a trend towards using traditional plant-based (‘herbal’) medicines alongside pharmaceutical drugs; in Germany, for example, it is estimated that 90% of the population use herbal medicines[2].
Precise figures for the value of international trade in medicinal plants are difficult to obtain[6,7], but it is clear that the industry is growing fast[8,9]. In 2000, US$17 billion was spent in the US on traditional herbal medicines. In 2003, the World Health Organisation estimated the annual global market for herbal medicines to be worth US$60 billion[2] and by 2012 the global industry in TCM alone was reported to be worth US$83 billion[4].
In this chapter we examine the regulation of these herbal medicines and the importance of having an accurate plant species name linked to them. The need for more research evaluating the medicinal properties of these plants is also covered, as well as their potential as new drugs.
Although herbal medicines are becoming increasingly popular in the West[4], not all are safe, and health regulators seek to control their sale and ensure appropriate quality control. Many countries publish ‘pharmacopoeias’, which are official publications providing precise detailed descriptions and tests to identify and assess the quality of plants used in herbal drugs. The number of plants covered by pharmacopoeias, however, represents only a small percentage of the diversity used in traditional plant-based medicines (see Box 1). Kew’s Medicinal Plant Names Services (MPNS, see Box 2), collated information on the names of 28,187 species recorded as being used medicinally but found that only 4,478 are cited in regulatory publications[10].
HOW MANY MEDICINAL PLANTS ARE THERE?
MEDICINAL PLANT NAMES SERVICES (MPNS).
Correct labelling is also important. Product labelling is frequently misleading, with the trade name ‘ginseng’, for example, referring to 15 different species of plant, each with its own particular chemistry and therapeutic properties[10]. Substitution by a Belgian clinic of one Chinese medicinal herb (‘Fang Ji’) with another sharing the same name, led to over 100 patients requiring kidney dialysis for the remainder of their lives[15]. For another example, see Box 3.
THE SIGNIFICANCE OF AMBIGUOUS LABELLING FOR PUBLIC HEALTH.
Robust authentication of plant ingredients included in herbal products is also vital, along with their substitutes and adulterants[16]. A recent publication of 300 internationally traded Chinese medicinal plants[17] addressed this need by presenting illustrations and detailed descriptions of source plants and trade forms, enabling comparison with their common substitutes, adulterants and counterfeits. Going forward, adulteration could be reduced by sourcing plants from sustainable resources or cultivation[18] combined with reliable traceability procedures and effective authentication and quality control[17,19].
Historically, plants have often been selected for drug development programmes because they contain specific classes of compounds, such as alkaloids and terpenoids, that are known to be biologically active, or because of their traditional medicinal uses. To what extent does family membership influence the likelihood of a species having a medicinal use? For this year’s State of the World’s Plants, we carried out an analysis of the families of 28,187 species covered by MPNS to identify the families with the highest proportion of medicinal plants (See Table 1; Figure 1). From this it is apparent that some families contain significantly more medicinal plants than might be expected. Selecting families with a high number of medicinal plants along with classes of medicinally active compounds could possibly serve as a signpost for future drug discovery programmes (see Box 4).
PLANTS AS A SOURCE OF PHARMACEUTICAL DRUGS.
Plants with known medicinal uses have been a source of vital pharmaceutical drugs for the treatment of many diseases (see Boxes 5 & 6). For example, artemisinin (discovered in Artemisia annua) and quinine (from Cinchona officinalis), together with their synthetic analogues, remain among the most important weapons in our arsenal against malaria[20,21], of which 214 million cases and 400,000 deaths were recorded in 2015[22].
PLANTS AND DEMENTIA.
PLANTS AND DIABETES.
Will the next lead come from one or more of the 1,200 species used to treat malaria[23]? In Ghana, Nigeria, Mali and Zambia, about 60% of childhood malaria cases are first treated with herbal remedies[22]. Reviews from Cameroon and Guinea reported use of 217 and 113 species respectively[24], many proving to have antimalarial properties when tested. Of 24 species from one Latin American tree genus (Aspidosperma) tested in the laboratory, 19 showed activity against the Plasmodium malaria parasite[25]. As antimalarial drug resistance spreads[26], exploring and exploiting such under-utilised (and often under-researched) resources remains a high priority for science[27]. Success will depend both on equipping laboratories in endemic regions and establishing multidisciplinary research networks to bring together the diverse expertise necessary[28].
Increasing demand for herbal medicines (particularly for species covered by pharmacopoeias) threatens wild populations of many of these plants[29]. Of the 28,187 species recorded in MPNS, c.1,280 are under protection according to CITES (see chapter 12)[10,30]. The commercial value of particular herbal products leads to scarcity of populations of the species used in the product. This in turn increases the frequency of species substitution – when the species is, deliberately or otherwise, substituted with a different species – and thereby threatens patient safety[17,31–34].
For example, supply shortages of Eleutherococcus nodiflorus (‘Wu Jia Pi’), widely used in TCM to treat musculoskeletal pain and swelling[35], frequently result in its substitution with a similar-looking adulterant from the unrelated species Periploca sepium, which in inappropriate dosages, is toxic[17].
In summary, the focus of world trade on relatively few species of medicinal plants leads to sustainability and conservation issues, which ultimately lead to other plants being substituted, with potential risks to human health. More effective regulation can be achieved through more precise use of scientific plant names and greater awareness of the many alternative synonyms in use. However, clarity on which plants have or have not been studied in drug discovery programmes is also needed. This is now becoming possible through the collation of global data sources on medicinal plants and ‘big data’ analytics. Such approaches will be hugely important in improving our ability to realise current and future medicinal benefits from plants.
Monographs, or book chapters, which are outputs of Wellcome Trust funding have been made freely available as part of the Wellcome Trust's open access policy
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