scope and practice of pharmacognosy

Chapter 2 The scope and practice of pharmacognosy

Until relatively recently pharmacognosy was regarded, almost exclusively, as a subject in the pharmaceutical curriculum focused on those natural products employed in the allopathic system of medicine. Coincident with the increasing attractiveness of alternative (complementary) therapies and the tremendous range of herbal products now generally available to the public, regulatory requirements covering medicinal herbs have been put in place by many countries in order to control the quality of these products. Monographs are now available on a large number of such drugs giving descriptions, tests for identity and purity and assays of active constituents. These monographs are being compiled by a number of bodies (see below). In this respect recognition should be given to the pioneering production of the British Herbal Pharmacopoeia, first produced in 1974 with the latest volume in 1996. Pharmacognosy is also important in those countries having their own systems of medicine in which plants are important components.

Many crude drugs once generally categorized as herbal remedies are now, in accordance with Continental European practice, described in the British Pharmacopoeia (BP). Chromatographic, chemical and physical tests, together with assay procedures, are given for many drugs for which previously there was no quantitative evaluation of the chemical constituents available. The importance of quality control is paramount, as the demand and the possibility of substitution has increased. The upsurge in the marketing of Chinese and Asian traditional medicines worldwide, for which there is a need for adequate control, adds a further dimension to pharmacognosy; pharmacopoeial monographs now include Liquorice for use in Chinese medicine, Chinese angelica root and Astragalus root. It is understood that further monographs on Chinese and Indian drugs for use in traditional medicine are to be included in the BP 2009.

Although pharmacognosy is principally concerned with plant materials, there are a small number of animal products which are traditionally encompassed within the subject; these include such items as beeswax, gelatin, woolfat, vitamins, etc. Other natural products such as the antibiotics, hormones and others may or may not be involved, depending on the teaching practice of a particular institution. Marine organisms, both plant and animal, with potent pharmacological actions are receiving increasing attention in the search for new drugs. Materials having no pharmacological action which are of interest to pharmacognosists are natural fibres, flavouring and suspending agents, colourants, disintegrants, stabilizers and filtering and support media. Other areas that have natural associations with the subject are poisonous and hallucinogenic plants, allergens, herbicides, insecticides and molluscicides.

Vegetable drugs can be arranged for study under the following headings.

1 Alphabetical. Either Latin or vernacular names may be used. This arrangement is employed for dictionaries, pharmacopoeias, etc. Although suitable for quick reference it gives no indication of inter-relationships between drugs.

2 Taxonomic. On the basis of an accepted system of botanical classification (Chapter 3), the drugs are arranged according to the plants from which they are obtained, in classes, orders, families, genera and species. It allows for a precise and ordered arrangement and accommodates any drug without ambiguity. As the basic botanical knowledge of pharmacy students decreases over the years this system is becoming less popular for teaching purposes.

3 Morphological. The drugs are divided into groups such as the following: leaves, flowers, fruits, seeds, herbs and entire organisms, woods, barks, rhizomes and roots (known as organized drugs), and dried latices, extracts, gums, resins, oils, fats and waxes (unorganized drugs). These groupings have some advantages for the practical study of crude drugs; the identification of powdered drugs (see Chapter 43) is often based on micro-morphological characters.

4 Pharmacological or Therapeutic. This classification involves the grouping of drugs according to the pharmacological action of their most important constituent or their therapeutic use. R. Pratt and H. W. Youngken Jr. were, in 1956, the first to use this approach for an English language textbook and now, with so many plant materials being screened for specific pharmacological activity, this type of listing is found increasingly in the literature. Its use is illustrated in Chapters 27–32. However, it is important to appreciate that the constituents of any one drug may fall into different pharmacological groups.

5 Chemical or Biogenetic. The important constituents, e.g. alkaloids, glycosides, volatile oils, etc., or their biosynthetic pathways, form the basis of classification of the drugs. This is a popular approach when the teaching of pharmacognosy is phytochemically biased. Ambiguities arise when particular drugs possess a number of active principles belonging to different phytochemical groups, as illustrated by liquorice, ginseng, valerian, etc. The scheme is employed in Chapters 19–26 for arranging the established pharmacopoeial drugs.

The following list of works, arranged in the above five groups, will serve as examples and also provide a useful list of textbooks and works of reference; those no longer in print may be found in established pharmaceutical libraries.