medicinal plants, plants used as natural medicines. This practice has existed since prehistoric times. There are three ways in which plants have been found useful in medicine. First, they may be used directly as teas or in other extracted forms for their natural chemical constituents. Second, they may be used as agents in the synthesis of drugs. Finally, the organic molecules found in plants may be used as models for synthetic drugs. Historically, the medicinal value of plants was tested by trial and error, as in the Doctrine of Signatures. Modern approaches to determining the medicinal properties of plants involve collaborative efforts that can include ethnobotanists, anthropologists, pharmaceutical chemists, and physicians. Many modern medicines had their origin in medicinal plants. Examples include aspirin from willow bark (Salix spp.), digitalis from foxglove (Digitalis purpurea), and vinblastine from Madagascar periwinkle (Vinca rosea) for the treatment of childhood leukemia. See also herbal medicine.
"medicinal plants." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (May 26, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/medicinal-plants
"medicinal plants." The Columbia Encyclopedia, 6th ed.. . Retrieved May 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/medicinal-plants
Plants can not run away from their enemies nor get rid of troublesome pests as humans or other animals do, so what have they evolved to protect themselves? Whatever this protection is it must be successful, for the diversity and richness of green plants is extraordinary, and their dominance in most ecosystems of the world is unquestioned. Plant successes are closely intertwined with the evolution and production of highly diverse compounds known as secondary metabolites, compounds that are not essential for growth and reproduction, but rather, through interaction with their environment, enhance plant prospects of survival. These metabolites are therefore plant agents for chemical warfare, allowing plants to ward off microorganisms, insects, and other animals acting as predators and pathogens . Such compounds may also be valuable to humans for the same purposes, and therefore may be used as medicines.
What Characterizes Medicinal Plants
There are twenty thousand known secondary plant metabolites, all exhibiting a remarkable array of organic compounds that clearly provide a selective advantage to the producer, which outweighs their cost of production. Humans benefit from their production by using many of them for medicinal purposes to fight infections and diseases. An estimated two-fifths of all modern pharmaceutical products in the United States contain one or more naturally derived ingredients, the majority of which are secondary metabolites, such as alkaloids, glycosides, terpenes, steroids, and other classes grouped according to their physiological activity in humans or chemical structure. To illustrate the breadth of human reliance on medicinal plants, the accompanying table provides a list of the most significant plants, their uses in modern medicine, and the major secondary metabolites responsible for their activities. This list grows annually as new plants are found with desired activities and remedies to become pharmaceuticals for use in medicine.
|COMMON MEDICINAL PLANTS AND THEIR USES|
|Scientific Name||Common Name||Family||Compounds||Compound Class||Uses|
|Atropa belladonna, Duboisia myoporoides||Belladonna||Solanaceae||Atropine, scopolamine||Alkaloid||Anticholinergic, motion sickness, mydriatic|
|Cassia/Senna species||Senna||Fabaceae||Sennoside||Glycoside, anthraquinone||Laxative|
|Catharanthus roseus||Madagascar periwinkle||Apocynaceae||Vincristine, vinblastine||Alkaloid||Anticancer (antileukemia)|
|Chondrodendron tomentosa, Curarea toxicofera||Curare||Menispermaceae||(+)-Tubocurarine||Alkaloid||Reversible muscle relaxant|
|Cinchona calisaya, Cinchona officinalis||Jesuits' bark||Rubiaceae||Quinine, quinidine||Alkaloid||Antimalaria (quinine), antiarrhythmia (quinidine)|
|Colchicum autumnale||Autumn crocus||Liliaceae||Colchicine||Alkaloid||Gout|
|Digitalis lanata, Digitalis purpurea||Foxglove||Scrophulariaceae||Digoxin, digitoxin, lanatosides||Cardiac glycoside (steroidal)||Heart failure and irregularity|
|Dioscorea species||Yam||Dioscoreaceae||Diosgenin, precursor of human hormones and cortisone||Saponin glycoside (steroidal)||Female oral contraceptives, topical creams|
|Ephedra sinica||Ephedra, Ma huang||Ephedraceae||Ephedrine||Alkaloid||Bronchodilator, stimulant|
|Podophyllum peltatum||May-apple||Berberidaceae||Podophyllotoxin, etoposide||Resin||Anticancer|
|Rauwolfia serpentina||Apocynaceae||Reserpine||Alkaloid||Antihypertensive, tranquilizer|
|Taxus brevifolia||Pacific yew||Taxaceae||Taxol||Diterpene||Anticancer (ovarian, breast)|
How Plant Pharmaceuticals Are Discovered
The search for new pharmaceuticals from plants is possible using a number of distinct strategies. Random collecting of plants by field gathering is the simplest but least efficient way. The chances are much greater that new compounds of medicinal value will be discovered if there is some degree of selectivity employed by collecting those plants that a botanist knows are related to others already having useful or abundant classes of secondary metabolites. Even more relevant is to collect plants already targeted for specific medicinal purposes, possibly among indigenous or ethnic peoples who use traditional, plant-derived medicines often with great success to provide for their well-being. Such data are part of ethnobotany , when researchers often obtain detailed information on the plants people use to treat illnesses, such as the species, specific disease being treated, plant part preferred, and how that part is prepared and used for treatment. This strategy can provide rapid access to plants already identified by traditional practitioners as having value for curing diseases, and this shortcut often sets the researcher rapidly on the road to the discovery of new drugs.
Taking the ethnobotanical approach, a specific part of the targeted ethnomedicinal plant is extracted, usually in a solvent like ethanol, and then studied in biodirected assays or tests to determine its value using, for instance, tissue cultured cells impregnated with the organism known to cause the disease. For example, to assay for malaria the procedure could involve culturing red blood cells infected with the malarial-causing protozoan Plasmodium falciparum, placing a few drops of extract into the culture, and examining after a few days what effect, if any, the addition of the extract had on the protozoa. One final step in this process leading to the discovery of a new drug is to establish the mechanism of action of the compound, reactions in the body, and side effects or toxicity of taking it. The whole process from field discovery to a new pharmaceutical takes up to ten years and requires a multidisciplinary-interactive approach involving ethnobotanists , natural products chemists, pharmacognosists (those who study the biochemistry of natural products), and cell and molecular biologists.
Medically Important Compounds Derived From Plants
About ninety species of plants contribute the most important drugs currently used globally, and of these about 75 percent have the same or related uses as the plant from which each was discovered. Two examples provide additional details of their discovery and development as drugs.
Eastern North American Indians long used the roots and rhizomes (underground stems) of the native May-apple (Podophyllum peltatum, Berberidaceae) as a drastic laxative. By the nineteenth century, white "Indian Doctors" used extracts of these parts to treat cancerous tumors and skin ulcers, perhaps learned from Indians or by direct observation of its corrosive and irritating nature. The plant's main secondary metabolite is podophyllotoxin, a resin responsible for May-apple's antitumor effects. It is a mitotic poison that inhibits cell division and thus prevents unregulated growth leading to cancerous cells and tumors. However, in clinical trials podophyllotoxin proved too toxic for use as a cancer chemotherapeutic agent, although it remains the drug of choice as a caustic in removing venereal warts and other benign tumors.
Attempts to find safer compounds led chemists to manipulate the molecule, and by trial and error they discovered a semisynthetic derivative that proved at least as effective as the original compound without the same level of toxicity. (Semisynthetics are products of chemical manipulation using the naturally occurring plant compound as a base.) A compound called etopo-side was eventually found most valuable in treating a type (non-small cell) of lung cancer, testicular cancer, and lymphomas (cancer of lymphoid tissue), and particular (monocytic) leukemias (cancer of blood-forming organs) by preventing target cells from entering cell division. Etoposide was approved for use in the United States in 1983, twelve years after its discovery. Peak annual sales of the compound reached approximately $300 million in the late 1980s and early 1990s, and thousands of lives have been prolonged or saved during nearly two decades of its use as a leading anticancer drug derived from plants. It is possibly the most important pharmaceutical originating from a plant species native to eastern North America.
Heart and vascular disease is the number one killer in the United States, a position held virtually every year in the twentieth century. Fluid accumulation or edema (dropsy) and subsequent congestive heart failure have been treated by European farmers and housewives as part of European folk medicine for a long time. Their remedy consisted of a concoction of numerous herbs that always contained leaves of foxglove (Digitalis species, Scrophulariaceae). In the 1700s William Withering, an English botanist and physician, observed in the countryside the successful use of this herbal mixture to treat dropsy and associated diseases. He eventually selected one plant from the mixture as the probable source of activity, and in 1785 Withering published his landmark book An Account of the Foxglove, and Some of Its Medicinal Uses in which he described how to determine the correct dosage (for foxglove was considered a potent poison that was ineffective medicinally unless used at near toxic levels) and how to prepare fox-glove, favoring the use of powdered leaves.
Withering's discovery revolutionized therapy associated with heart and vascular disease, and even today, powdered foxglove leaves are still prescribed and used much as they were more than two centuries ago. The active leaf metabolites are cardiotonic glycosides obtained mostly from two European species, Digitalis lanata and D. purpurea. They provide the most widely used compounds, digoxin (also available synthetically), digitoxin, and lanatosides. The magnitude of the need for cardiotonic therapy is suggested by the estimate that more than three million cardiac sufferers in the United States routinely use the preferred digoxin as one of several available drugs.
In congestive heart failure, the heart does not function adequately as a blood pump, giving rise to either congestion of blood in the lungs or backup pressure of blood in the veins leading to the heart. When the veins become engorged, fluid accumulates in the tissues, and the swelling is known as edema or dropsy. Cardiotonic glycosides increase the force of heart muscle contraction without a concomitant increase in oxygen consumption. The heart muscle thus becomes a more efficient pump and is better able to meet the demands of the circulatory system. If heart failure is brought on by high blood pressure or hardening (loss of elasticity) of the arteries, cardiotonic glycosides are also widely used to increase contractibility and improve the tone of the heart muscle, resulting in a slower but much stronger heart beat. If the heart begins to beat irregularity, again these cardioactive compounds will convert irregularities and rapid rates to normal rhythm and rate.
The search for new medicinal plants continues as remote regions of natural habitat are explored by botanists, plant systematists , and ethnobotanists. Further clinical studies of chemical components of these new discoveries may yield important novel drugs for the treatment of human diseases.
see also Alkaloids; Cannabis; Coca; Dioscorea; Economic Importance of Plants; Ethnobotany; Herbals and Herbalists; Opium Poppy; Pharmaceutical Scientist; Plant Prospecting; Psychoactive Plants; Systematics, Plant.
Walter H. Lewis
Balick, Michael J., and Paul Alan Cox. Plants, People, and Culture: The Science of Ethnobotany. New York: Scientific American Library, 1996.
Kreig, Margaret G. Green Medicine. New York: Rand McNally, 1964.
Lewis, Walter H., and Memory P. F. Elvin-Lewis. Medical Botany: Plants Affecting Man's Health. New York: John Wiley & Sons, 1977.
Nigg, Herbert N., and David Seigler, eds. Phytochemical Resources for Medicine and Agriculture. New York: Plenum Press, 1992.
Plotkin, Mark. Tales of a Shaman's Apprentice. New York: Viking, 1993.
Robbers, James E., and Varro E. Tyler. Tyler's Herbs of Choice: The Therapeutic Use of Phytochemicals. Binghamton, NY: Haworth Herbal Press, 1998.
"Medicinal Plants." Plant Sciences. . Encyclopedia.com. (May 26, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/medicinal-plants
"Medicinal Plants." Plant Sciences. . Retrieved May 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/medicinal-plants
me·dic·i·nal / məˈdisənl/ • adj. (of a substance or plant) having healing properties: medicinal herbs | humorous a large medicinal Scotch. ∎ relating to or involving medicines or drugs. • n. a medicinal substance. DERIVATIVES: me·dic·i·nal·ly adv.
"medicinal." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (May 26, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/medicinal-0
"medicinal." The Oxford Pocket Dictionary of Current English. . Retrieved May 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/medicinal-0
"medicinal." Oxford Dictionary of Rhymes. . Encyclopedia.com. (May 26, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/medicinal
"medicinal." Oxford Dictionary of Rhymes. . Retrieved May 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/medicinal