Elsevier

Phytochemistry

Volume 56, Issue 3, February 2001, Pages 237-243
Phytochemistry

Phytochemistry and medicinal plants

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Abstract

A truncated history of the contribution of plants to medicine is given with reference to some of the less well known ancestors of the Harborne family. Six of the top 20 prescriptions dispensed in 1996 were natural products and the clinical use of drugs such as artemisinin, etoposide and taxol has once more focussed attention on plants as sources of novel drug entities. High through-put robotic screens have been developed by industry and it is possible to carry out 50,000 tests per day in the search for compounds which have specificity of action against a key enzyme or a subset of receptors. Bioassay-guided fractionation of plant extracts linked to chromatographic separation techniques leads to the isolation of biologically active molecules whose chemical structures can readily be determined by modern spectroscopic methods. The role of academics in the search for new drugs is discussed by reference to some of our research into natural products with activity on the central nervous system, on pain receptors, the malaria parasite Plasmodium falciparum, the wound healing properties of the sap of species of Croton (Dragon's blood), and a traditional Chinese medicine used to treat eczema. Expertise in phytochemistry has been essential for this research and the strong lead shown by Professor Jeffrey Harborne is gratefully acknowledged.

Introduction

The use of plants as medicines goes back to early man. Certainly the great civilisations of the ancient Chinese, Indians, and North Africans provided written evidence of man's ingenuity in utilising plants for the treatment of a wide variety of diseases. In ancient Greece, for example, scholars classified plants and gave descriptions of them thus aiding the identification process. Theophrastus has been described by some as the father of botany (Fig. 1) but little, if anything, has been recorded on his distant relative J.B. Theophrastus1 who extolled the virtues of medicinal plants and forecast the possibility of discovering flavonoids. As Europe entered the dark ages much of this information would have been lost had it not been for the monasteries that acted as centres for the production of medicinal plants which were used to heal the suffering of mankind. There is still much we can learn from investigating the old herbals, particularly those less well known such as the one attributed to the monk J.B. Harbonus1.

It was not until the 19th century that man began to isolate the active principles of medicinal plants and one particular landmark was the discovery of quinine from Cinchona bark by the French scientists Caventou and Pelletier (Fig. 2). Much less is known about the isolation of quinine by J.B. Caventou1 and J.B. Pelletier1. Such discoveries led to an interest in plants from the New World and expeditions scoured the almost impenetrable jungles and forests in the quest for new medicines (Fig. 3). One of the lesser known intrepid explorers was J.B. van Harbon1 who was never happier than when he was able to hatchet his way through the jungle stripping off the barks from every tree in sight. Such expeditions would last for years and it was not until the plants arrived at a well equipped phytochemical laboratory that the real discoveries could be made (Fig. 4). Laboratories such as those of Professor J.B. de Harbonney1 became centres for the isolation of the active principles of medicinal plants from around the globe. Years of toil would be rewarded by the isolation of numerous flavonoids which were welcomed by the cognoscenti as well as the rapidly expanding pharmaceutical companies.

Section snippets

New drugs from nature

Prior to World War 2, a series of natural products isolated from higher plants became clinical agents and a number are still in use today. Quinine from Cinchona bark, morphine and codeine from the latex of the opium poppy, digoxin from Digitalis leaves, atropine (derived from (−)-hyoscyamine) and hyoscine from species of the Solanaceae continue to be in clinical use. The antibiotic era dawned during and after World War 2 due to the antibacterial effects of a whole series of natural products

Will further new drugs be developed from natural product research?

The clinical applications of taxol, etoposide and artemisinin have helped to revive an interest in higher plants as sources of new drugs (Phillipson, 1999a). Despite the belief that the majority of clinical drugs are synthetic in origin, it is interesting to note that 6 out of the top 20 pharmaceutical prescription drugs dispensed in 1996 were natural products and that over 50% of the top 20 drugs could be linked to natural product research. In recent years the development of sensitive

Malaria

In 1996 it was reported that there were between 1.5 and 2.7 million deaths annually and that the majority of these were children. There are in the order of 500 million new incidences of malaria annually. It is without doubt one of the major threats to mankind and chemotherapy is hindered by the increase in drug resistant strains, particularly of Plasmodium falciparum. In the mid 1980's we posed the question “ Are new antimalarial drugs awaiting discovery from plants?” Quinine, the first

Do traditional medicines necessarily contain a single active ingredient?

The isolation and use of natural products such as digoxin, morphine and quinine has resulted in replacing the plant extracts used with single chemical entities. There is a basic supposition that any plant possessing clinical effectiveness must contain an active principle which can completely replace the plant extract. Three examples from our research have shown that this may not necessarily be true (Phillipson, 1995).

Artemisinin is without doubt the potent antimalarial active principle of

Conclusions

Plants continue to be used world-wide for the treatment of disease and novel drug entities continue to be developed through research into their constituents. In the developed countries, high-throughput screening tests are used for bioassay-guided fractionation leading to the isolation of active principles that may be developed into clinical agents either as the natural product or a synthetic modification or a synthesised analogue with enhanced clinical action or reduced adverse side effects.

Acknowledgments

I am grateful to Parke, Davis and Company for permission to reproduce the figures from the book Great Moments in Pharmacy by G.A. Bender, Detroit, Northwood Institute Press, 2nd Edition, 1967.

David Phillipson is Emeritus Professor of Pharmacognosy at the Centre for Pharmacognosy and Phytotherapy at The School of Pharmacy, The University of London. He was formerly Professor and Head of Department of Pharmacognosy at The School before retiring in 1994. In 1995, he was appointed for 6 months at The Chinese University of Hong Kong as Wilson T.S. Wang Distinguished International Visiting Professor. He is an Honorary Professor at the Chinese Academy of Medical Sciences, Institute of

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David Phillipson is Emeritus Professor of Pharmacognosy at the Centre for Pharmacognosy and Phytotherapy at The School of Pharmacy, The University of London. He was formerly Professor and Head of Department of Pharmacognosy at The School before retiring in 1994. In 1995, he was appointed for 6 months at The Chinese University of Hong Kong as Wilson T.S. Wang Distinguished International Visiting Professor. He is an Honorary Professor at the Chinese Academy of Medical Sciences, Institute of Medicinal Plant Development, Beijing. For many years, he has been an active member of the Phytochemical Society of Europe and between 1977 to 1988 he held offices of Secretary, Vice-Chairman and Chairman. His research interests include the chemistry and biological activities of plants used in traditional medicine. He has received awards from the Phytochemical Society of Europe including the Tate and Lyle Award (1992), Medal (1994) and Pergamon Prize for creativity in plant biochemistry (1996). In 1989, he and four other European scientists in collaboration with Professor Meinhart Zenk (then of the University of Munich) were awarded the Korber Foundation Prize for achievement in European Science. The Pharmaceutical Society of Great Britain presented him with their Harrison Memorial medal in 1999.

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