by Jin Ming KONG and Yinghong LIU
nterest has revived recently in the investigation of medicinal plants to identify novel active phytochemicals that might lead to drug development. Nature has generated such substances for millennia ó before modern synthetic chemistry developed in the mid 19th century. Because these substances arise from a more or less hostile environment, the percentage of biologically active natural substances is relatively high in comparison with substances from artificial sources. Currently more than 50% of drugs in clinical use have a natural-product origin, and about half of the worldís 25 best-selling pharmaceutical agents are natural-product derived.
Many countries are developing botanical-drug research and testing because of the importance of phytochemicals to human life and their huge market potential.
According to a new report, China has an estimated 139 innovative drugs in its pipeline, 9 of which come from traditional Chinese medicine. In the United States, more than 10 countries have collaborated on a project funded by the National Cancer Institute ó Natural Inhibitor of Carcinogenesis ó which has been collecting over 5,000 plant samples and has obtained more than 250 active compounds for cancer chemoprevention. Other countries such as India, Pakistan, and Brazil also conduct similar research. Accordingly, many countries have modified their regulations to facilitate the development and medical application of plant-derived drugs. For example, in December 2003, the European Parliament adopted new legislation to make it easier for traditional medicine makers to demonstrate efficacy in European Union member nations. Canada also opened the doors of its new Natural Health Products Directorate programme in January 2004. In the United States, there are about 10 to 20 botanical drugs in serious clinical development.
Anti-Cancer Drugs from Plants
Cancer, one of the most dreaded diseases, continues to spread with increasing incidence. Statisticians estimate that over 10 million new cases of cancer appeared worldwide, with over 6 million deaths in the year 2000. Since 1990 cancer incidence and mortality have increased 22% based on information until 2003; the four most frequently occurring cancers are lung, breast, colorectal, and stomach, and the four most deadly comprise lung, stomach, liver, and colorectal cancers. Only cardiovascular disease surpasses cancer, the second leading cause of death in the United States.
Of all available anti-cancer drugs developed from 1940 to 2002, 40% were natural products or natural-product-derived with another 8% considered natural-product mimics. Plant anti-cancer agents currently in clinical use can be categorised into four main classes of compounds: vinca (or Catharanthus) alkaloids, epipodophyllotoxins, taxanes, and camptothecins. Taxanes and camptothecins alone accounted for approximately one-third of the global anti-cancer market in 2002, with a market value of over US$2.75 billion.
Other Examples of Drugs from Plants
Chemists extract opium from exudates derived from seed pods of the opium poppy, Papaver somniferum. Civilizations as far back as ancient Persia, Egypt, and Mesopotamia cultivated the poppy plant. Frederick Serturner, a young clerk in a small German pharmacy, extracted morphine from opium; Serturner's extraction was the first alkaloid ever isolated, and he named it Morpheus after the Greek god of dreams and published his findings in 1817. Since then, drug discovery from medicinal plants led to the early isolation of such drugs as cocaine, codeine, digitoxin, and quinine, all of which are still prescription drugs.
Aspirin or acetylsalicylic acid, a historically important plant drug, was extracted from the bark of the willow tree (Salix alba), and went on to become one of the most widely available and used compounds for mild pain management, in use since its market debut in the year of 1899.
Malaria is caused by protozoan parasites of the genus Plasmodium. Chloroquine has been by far the most widely used anti-malarial drug in conventional malaria therapy. However, owing to widespread drug resistance, the drug has become increasingly ineffective in many parts of the world. Fortunately, qinghaosu or artemisinin, discovered by Chinese scientists in the 1970s, has been found to be an exceptionally effective anti-malarial agent with negligible toxicity and high effectiveness against all forms of the parasite, including the drug-resistant ones. (See "Traditional Medicine for Malaria" in this issue.)
Several million patients have used artemisinin derivatives and have tolerated them well. In China these drugs have already replaced quinine as the first-line of defence against P. falciparum malaria. Since 2001, the World Health Organization has begun to recommend the use of artemisinin derivatives in combinational therapies (with drugs such as mefloquine) in Africa as first-line therapy.
Another important plant drug is apomorphine, a semisynthetic opium alkaloid, long known for its ability to affect erectile activity at a dose of 2-6mg. Apomorphine exerts its erectile effect in the central nervous system. Around 1998, Takeda Abbott Pharmaceuticals developed a fast-acting sublingual formulation of
apomorphine for treatment of male erectile dysfunction. Its plant-derived and traditional-formula base delivers better treatment effect, lowers toxicity, and gives fewer side effects than would be expected from artificial counterparts.
The Laboratory for Natural Products Research at Nanyang Technological University, Singapore, has for many years carried out screening of the anti-oxidant potential of flavonoids and diterpenoids from local plants. Southeast Asia, an area rich in tropical fruits, vegetables, and other plant resources, provides many opportunities to develop natural drugs. According to the lab's experimental results, the flavonoid 2'O-(3,4,5-trihydroxybenzoyl) quercitrin (galloyl quercitrin) from local laksa plant (Polygonum odoratum) leaves has powerful anti-oxidant capacity with great potential. Scientists have also successfully isolated and purified new diterpeonids sincoesta A and B from Isodon coesta Hara, Lophanthodin G from Isodon Lophanthodies var. gerdianus (Bentham) Hara, and Unirubescensin from Isodon rubescens (Hemsl.) Hara. Cytotoxicity study of these isolated diterpenoids shows them to have strong anti-cancer properties.
The potential of natural products has been recognised since antiquity. They remain vital to modern life in such forms as antibiotics, anti-cancer drugs, cardiac drugs, and insecticides. The major achievements of natural-products research of the past decades have clearly demonstrated that natural products represent an unparalleled source of molecular diversity for drug discovery and development, especially where plant resources are fast diminishing. In the future the synergy of natural-products research and biotechnology will drive new plant-drug development.
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