Plants and medicine

Plant species are used for medicines in two ways: (1) as a major commercial use, whether by prescription or over-the-counter sales, and (2) as traditional medicines which may or may not attract a market price. In the rich world, perhaps 25% of all medical drugs are 'based' directly on plants and plant derivatives; this means that they remain linked directly to those plant forms for their production. In the poor world the proportion of drugs based on plants is closer to 75% (Principe, 1991). Clearly, both uses have an economic value.

Of course, most of the contributions of plants to the production of pharmaceuticals do not rely on their direct utilisation in production processes; they contribute instead by means of providing 'leads' or 'targets' which then serve as the foundation for future synthesis of a drug. Whereas some useful chemical compounds discovered within medicinal plants have not been reproduced synthetically (digitoxin, for example), and others have been reproduced but are less efficient than the original material (for example synthetic vincristine from Catharanthus), in most cases synthetic substitutes do exist. Therefore, the estimation of the value of plant-based pharmaceuticals clearly represents an attempt to value only a small subset of the total value of plant diversity for its contribution to the pharmaceutical industry.

This estimate of the value of plant-based pharmaceuticals is, in one sense, a serious underestimate of the total value. In another sense, however, it may be a serious overestimate on account of the problematic nature of using past and present usage as an indicator of future uses. The important issue here is: Are future drugs more, or less likely to be manufactured from plant-based materials? The answer to that question has been addressed elsewhere in this volume (see Aylward and Albers-Schonberg). Here, the economic literature on this question is surveyed.

Principe (1989) reports on a UN International Trade Centre study which suggests that, during the 1970s and early 1980s, pharmaceutical companies showed a decreasing interest in the development of new botanical products in favour of molecular biology and biotechnology applications to microorganisms. Processing plant genetic material is time-consuming and expensive, and simple comparative rates of return are higher from other routes. On the other hand, others in the industry appear to believe that plant-based resources will re-emerge, and one company, Merck, has entered into a licence and royalty agreement with Costa Rica. Merck's example does not appear to have been followed by any other company, but there are signs of a revived interest in plant material for drug development.

Principe (1989) reports several reasons why research based on microorganisms has limitations. The most important are: (1) the steps of identifying the chemical structure required to achieve a given effect and creating a proper genetic code structure are the most difficult stages of drug development, and these are not helped by microorganisms rather than plant-based genetic material, and (2) genetically engineered microorganisms can, so far, substitute for only some of the plant-based chemicals. Indeed, Principe reports that the vast majority of plant-based chemicals have not been successfully synthesised.

The future of drug development may also be more, rather than less, dependent on plant genetic material in light of the fact that plant-based research has gone in cycles. Findeisen (1991) reports that many thought that plant-based drug resources were exhausted in the early part of this century. The role of plants was, however, revived in the 1940s and 1950s with the discovery of the Vinca alkaloids (Catharanthus rosea) and reserpine (Rauwolfia serpentina). When the screening programmes at the National Cancer Institute (NCI) and elsewhere in industry failed to come up with significant discoveries, the industry lost interest and screening programmes were effectively halted in the 1970s. The disinterest was compounded by the difficulties of plant-based drug patents that have to relate to the process of manufacture or to some unanticipated use value. Natural compounds per se cannot be patented (see Walden, this volume). Thus, the Mexican government took control of Diosgenin resources in order to capture the rent from the production of Dioscorea, the main source of steroids in the early days of that drug. Attempts at the monopoly pricing of the resource forced pharmaceutical companies to search for synthetic substitutes. The case illustrates the problems of patenting and the problems facing countries that do seek to capture rents from biodiversity.

Some revival of interest in plant-based approaches in the last 5 years is accounted for by new techniques of purifying, analysing and assaying plant samples, including the use of robots for continuous assay of material. It is reported that the NCI, Monsanto, Smith Kline, Merck and Glaxo have revived plant screening programmes. Affymax and Shaman are new companies in the USA developing drugs solely from natural products, and with a lot of emphasis on traditional medicines. The other main source of a revival in interest in medicinal plants is consumer demand for 'natural products'. While consumers are unlikely to express a concern about the source material for major life-saving drugs, they do express a significant concern about the sources of over-the-counter drugs and cosmetics, as the success of some natural products shops reveals.

Clearly then, medicinal plant values are relevant to use value arguments for conserving biological resources, especially in the developing world. How far they have relevance in justifying conservation of biodiversity as such is more of a problem. Some commercial sources doubt that genetic engineering of microorganisms will totally displace plant-based research. This would suggest an insurance argument for conserving at least minimum diversity based on arguments related to the option values of the resource (see Swanson, this volume). These arguments are all the more powerful because of the extremely limited knowledge that exists about the medicinal properties of plants.

Evenson (1990) addresses these questions to an extent. He distinguishes between two fundamental values of genetic resources as producer goods: one in the general strategic search for new resources which justifies the maintenance of most materials, and another in the specialised search for genetic material to meet specific needs, which justifies the collection and preservation of 'fringe' genetic resources. His calculations for rice suggests that if there is an economic case for maintaining an ex-situ collection, the case for maintaining a near complete collection is stronger.

Overall, then, the economic value to medicinal plants falls into two general categories, one readily estimated and the other not. The first relates to the use values of plant-based drugs. These are drugs that remain closely connected to the plant form from which they were derived, and there exists a significant market in the drug; this value is appropriable and readily estimated. The other contribution of medicinal plants to the pharmaceutical industry is much more general and amorphous; it is the value of providing 'leads' in the creation of ultimately synthesised pharmaceuticals. This value is 'informational' in nature, and is very difficult to appropriate and to estimate. It is not the subject of this chapter. Here we attempt to provide a concrete estimate of the clearly attributable value generated by medicinal plants in the pharmaceutical industry. It is a 'floor' to the valuation of biodiversity, on which other values of diversity may then be constructed.

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