AccessScience from McGraw-Hill Education Page 1 of 5 www.accessscience.com

Pharmacognosy

Contributed by: Richard A. Deno Publication year: 2014

The general biology, biochemistry, and economics of nonfood natural products of value in medicine, pharmacy, and other health professions. The products studied are of biologic origin, either plant or animal. They may consist of entire organs, mixtures obtained by exudation or extraction, or chemicals obtained by extraction and subsequent purification.

Pharmacognosy literally means knowledge of drugs, as do pharmacology and pharmacy. The center of interest in pharmacology, however, is on the mode of action of all drugs on the animal body, particularly on humans. In pharmacy major attention is directed toward provision of suitable dosage forms, their production and distribution. Pharmacognosy is restricted to natural products with attention centered on sources of drugs, plant and animal, and on the biosynthesis and identity of their pharmacodynamic constituents.

Sources of materials

Organs, or occasionally entire plants or animals, are dried or frozen for preservation and are termed crude drugs. They may be used medicinally in essentially this form, as in the case of the cardiac drug, digitalis, or the endocrine drug, thyroid, or as sources of mixtures or of chemicals obtained by processes of extraction.

Mixtures obtained by exudation from living plants include such drugs as opium, turpentine, and acacia. Processes of extraction are required to obtain such mixtures as peppermint oil (steam distillation), podophyllum resin (percolation), and parathyroid extract (solution). For a discussion of classes of natural products with medically significant members of this type See also: ANIMAL AND VEGETABLE WAX ; ESSENTIAL OILS; FAT AND OIL (FOOD); GUM; TERPENE.

Pure chemicals may be extracted from a crude drug (for example, the glycoside digitoxin from digitalis or the hormone insulin from pancreas), from a mixture obtained by exudation (for example, the alkaloid morphine from opium), or from an extracted mixture (for example, the terpene menthol from peppermint oil). For a discussion of natural products of this type See also: ALKALOID; GLYCOSIDE; HORMONE.

Vitamins as a class of natural products are within the scope of pharmacognosy, although many are obtained commercially by laboratory synthesis. Included also are antibiotics and biologicals (serums, vaccines, and diagnostic biological products). See also: ANTIBIOTIC; VITAMIN. AccessScience from McGraw-Hill Education Page 2 of 5 www.accessscience.com

The general biology of pharmacognosy is largely descriptive. It includes the taxonomic position of the natural source of the product, the part of the plant or animal yielding the drug, the scientific and common names of the biologic source, the gross and histologic anatomic characterization of the part used, and the principal uses of the product in the health professions.

The biochemistry is both descriptive and experimental. It includes the chemical nature and percentage of the medically significant constituent, the mechanisms of biosynthesis of the constituent, and the role of the constituent in the economy of the plant. Attention is also given to mechanisms of biosynthesis by the use of radioactive precursors of medically active constituents to follow biosynthesis step by step. The isolation and chemical identification of new, potentially useful plant and animal constituents are an important aspect of biochemical research in pharmacognosy.

The economic aspects include the discovery and study of natural sources of crude drugs and their derivatives, development of cultivated sources where feasible, improvement of the yield of useful constituents, and protection of medically useful crop plants from their natural enemies. Methods of harvesting, drying, curing or other processing treatment, storing, packaging, and shipping enter into the commerce of drugs.

For a single drug, for example, menthol from peppermint oil, these several aspects are frequently inseparable. The commercial grower must know the species of Mentha yielding commercially profitable quantities of the essential oil and of menthol, percentage yields from various species, conditions suitable for growth, natural enemies of the growing plant such as specific viruses, methods of extraction of the oil and of isolation of the menthol, and proper conditions for packaging, storing, and shipping the purified drug.

Uses of materials

Medical uses are chiefly as therapeutic, prophylactic, or diagnostic agents. Prior to the twentieth century the materia medica of all countries was preeminently of natural products; it still is in some underdeveloped countries. Research in the twentieth century has contributed many synthetic and semisynthetic drugs to the modern materia medica, but a significant number of the crude drugs are still the drugs of choice in therapy or serve as the source of widely used purified mixtures or chemicals. Digitalis and its glycosides, the alkaloids of opium and belladonna, penicillin, thyroid, insulin, and poliomyelitis vaccine are examples.

Pharmaceutical uses are chiefly in the production of palatable and stable dosage forms: gums and mucilages in emulsions and suspensions, starch and lactose in tablets, sugar and essential oils in elixirs, oils and waxes in ointments. Many natural products of insignificant or questionable therapeutic value continue to be used in home remedies.

Uses in other health professions include antiseptics, protectives, and local anesthetics used by dentists; rodenticides, insecticides, and other pesticides used in the protection of the public health; and a variety of prophylactic and therapeutic agents used by veterinarians. AccessScience from McGraw-Hill Education Page 3 of 5 www.accessscience.com

A large number of natural products of value in the health professions are used also in cosmetology (essential oils, gums, fats, and waxes), in the culinary arts (spices, essential oils, and condiments), and in industry (naval stores, mucilages, fats, and waxes).

The role of a medically active chemical produced and used by the animal organism is usually well understood. Physiologic function within the organism and therapeutic use by humans are usually closely related, as in the case of pepsin, thyroid, or the sex hormones.

Corresponding knowledge of medically active plant constituents is almost nonexistent. The role of menthol in the economy of Mentha piperita, of digitoxin in Digitalis purpurea,ofmorphineinPapaver somniferum,orof reserpine in Rauwolfia serpentina is unknown. Through the centuries, however, it has been discovered that certain plants relieve the symptoms of or cure certain diseases. With the discovery of alkaloids in the early nineteenth century and of the other major classes of medically active plant constituents, the chemicals responsible for therapeutic actions have been identified one by one, but not the function of these chemicals in their respective plant sources.

Types of materials

Classes of therapeutic agents have frequently been discovered by study of biosynthesized medicinal chemicals. Most such classes, in fact, have been developed from chemicals orginally known from crude drugs or from their exudates or extractives. The first uses of opium as a narcotic and analgesic drug are lost in antiquity, but its position in the medical practice of the day has been primary for over 2000 years. Morphine was among the first alkaloids to be isolated and has been widely used for more than 150 years; from its study has developed a class of analgesic drugs of wide application in medical practice. See also: ANALGESIC; MORPHINE ALKALOIDS.

An analogous pattern has given the modern classes of hypotensive drugs and tranquilizers. The Indian drug rauwolfia after centuries of use in folk medicine eventually found its way into scientific medical practice in the Orient. Study of its chemical derivatives during the 1930s and 1940s revealed the presence of many alkaloids, one of which, reserpine, was shown to be an effective antihypertensive agent. Subsequent therapeutic use of reserpine and other rauwolfia products demonstrated the tranquilizing action. A large class of drugs having hypotensive action, tranquilizing effect, or both developed rapidly. Various species of the genus have been characterized morphologically, and there has been intensive study of practical methods of culture. See also: TRANQUILIZER.

Another major class of modern drugs of natural origin, the antibiotics, has been developed largely since the beginning of World War II. The prototype, penicillin, was discovered in part as a result of fortuitous accident, but the many other commercially available antibiotics have been developed as a result of carefully planned systematic search. AccessScience from McGraw-Hill Education Page 4 of 5 www.accessscience.com

Not infrequently the clue that has led to collection and scientific investigation of a crude drug as a possible source of medically significant constituents has been use of the drug by natives for a nonmedical, but to them desirable, purpose such as narcosis or as a poison against wild animals or humans. The use of opium as a narcotic by the laity undoubtedly preceded its medical use. Coca was chewed or sucked by the Indians of South America to increase endurance; condemned by the Spanish conquerors, it nevertheless was introduced into medical practice in Europe. Discovery of the local anesthetic action of the alkaloid cocaine led to the development of a new and important class of therapeutic agents, the local anesthetics. See also: COCA; NARCOTIC; OPIATES.

A second native drug from South America, curare, was first used by the Indians as an arrow poison to kill wild animals for food. The neuromuscular paralysis caused in game by the drug suggested therapeutic use as a muscular relaxant. Studies of the plants yielding crude curares revealed several species of two principal genera. Strychnos and Chondodendron, as the main sources. A number of crystalline alkaloids were isolated from crude curares; eventually the alkaloid tubocurarine was identified as having the therapeutic potentialities suggested by the paralyzing action of the native drug. The botanical source was established as C. tomentosum.

Comparable studies of the African arrow poisons inee and kombe added ouabain and strophanthin to the class of cardioactive glycosides, of which digitoxin is the most widely used. The arrow poisons are prepared from African species of Strophanthus and are used by natives of both the eastern and western coasts.

The alkaloid physostigmine, also from an African poison and useful in the treatment of glaucoma, was discovered as a result of use of its plant source as a human poison in the trial by ordeal of those accused of offenses. The alkaloid is the toxic constituent of the seeds of Physostigma venenosum, which were fed to the accused. Toxic symptoms were taken as evidence of guilt; those who vomited the material were considered guiltless.

Synthetic materials

Development of synthetic drugs related chemically to the active constituent of a natural product has frequently followed investigation of native use of the natural product as drug or poison. The objective of such development is usually to produce a drug having fewer undesirable side effects while retaining the useful therapeutic action. Substitutes for morphine, reserpine, cocaine, tubocurarine, and physostigmine are among a host of synthetic drugs which accomplish the objective to a greater or lesser degree and whose discovery depended on study of natural products.

Intermediates useful in the laboratory synthesis of drugs often exist as therapeutically inactive chemicals in natural products. Plants and animals biosynthesize many such compounds with chemical structures similar to, but not identical with, medicinally useful substances. A slight change in molecular configuration may yield a potent therapeutic agent. A simple example is pinene, a chemical abundant in turpentine oil and convertible by laboratory procedures into camphor. The resulting “synthetic” camphor is actually semisynthetic and possesses the therapeutic and most other properties of natural camphor. AccessScience from McGraw-Hill Education Page 5 of 5 www.accessscience.com

An important class of natural intermediates are the steroids, widely distributed in both plants and animals. Some chemical variations are active physiologically and as drugs, for example, sex and adrenal cortical hormones. Natural sources, glands of domesticated animals used as food by humans, are not available in quantities adequate to fulfill the drug needs for these products. Many plants contain steroids suitable as intermediates for sex hormones. Natural intermediates readily converted into adrenal cortical hormones are uncommon, and extensive search for such steroids has been made. Field studies involve collection and identification of plants judged to be potentially good sources of steroids, preliminary extraction and determination of the presence or absence of these intermediates, and further collection, drying, and preserving of larger quantities of promising species. See also: STEROID.

Systematic screening of plants of reputed therapeutic value and indigenous to a country or other restricted geographic area is a costly and time-consuming procedure—a major reason it has been done for relatively few regions. Notwithstanding, such surveys give promise of uncovering new sources for known classes of drugs, adding to knowledge of such little-known classes of hallucinogens and anticarcinogenic drugs, developing entirely new classes of therapeutic agents and providing profitable natural sources of intermediates useful in drug synthesis. See also: ANTIMICROBIAL AGENTS; BIOCHEMISTRY; BIOSYNTHESIS; PATHOLOGY; PHARMACOLOGY; PHARMACEUTICAL CHEMISTRY; PHARMACY; PLANT PHYSIOLOGY. Richard A. Deno

Bibliography

W. A. Creasy, Drug Disposition in Humans: The Basis of Clinical Pharmacology, 1979

G. E. Trease and W. C. Evans, Pharmacognosy, 12th ed., 1983

V. E. Tyler et al., Pharmacognosy, 9th ed., 1988

Additional Readings

S. K. Kim (ed.), Marine Pharmacognosy: Trends and Applications, CRC Press, Boca Raton, FL, 2013

B. Shah and A. Seth, Textbook of Pharmacognosy and Phytochemistry, Elsevier, New Delhi, India, 2010