Plants have been used as the source of drugs for long ages, and as of today, approximately 70,000 species have been screened for their potential utility as medicines. In recent decades, several antitumor drugs have been derived from plants, including paclitaxel (from Taxus brevifolia) and camptothecin, using fractionation techniques based on bioactivity.
At present, about 8 out of 10 drugs used to treat infection, cardiovascular disease, or cancers, or as immunosuppressives, come from plants, directly or as derivatives. Between 1981 and 2006, approval was granted to 155 antitumor drugs, of which almost half were derived from natural products.
Only about one in 10,000 screened compounds are eventually proved to be safe and effective by regulatory authorities. Even at late stages of clinical trials, approximately one in two drug leads fail to make the mark. There have been many examples of drugs that are released commercially, only to be pulled within weeks, months or years due to unacceptable side effects.
This high degree of uncertainty has led to a focus on plant-based chemicals for drug discovery, especially since these are seen as safe and more effective than synthetic chemicals.
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Plant polysaccharides have immunomodulatory or immunostimulatory effects, such as Ganoderma lucidum, Cordyceps sinensis and Açaí fruit. Flavonoids, found in almost all plants, are compounds with a heterocyclic ring structure consisting of an aromatic ring and a benzopyran ring with a phenyl substituent and include flavones, isoflavone, flavonols, flavonones, and xanthones. They have a range of bioactive properties.
Proteins from some herbs are used as natural protein supplements for convalescent patients. Morphine and aspirin are pure natural and semi-synthetic products derived from the opium poppy (Papaver somniferum), and from willow bark, respectively. They were rapidly followed by codeine, digitoxin, quinine and pilocarpine.
The more recent plant-based drugs include Artemisinin from Artemisia annua, used to treat multidrug-resistant malaria, and Silymarin (used in liver disease) from the seeds of Silybum marianum. Alkaloids isolated earlier are now finding reapplication as nitric oxide activators, helping prevent obesity and atherosclerosis.
Again, galantamine is an alkaloid extracted from Galanthus nivalis, used to treat Alzheimer’s disease; apomorphine is a semisynthetic morphine derivative used in Parkinson’s disease; and Tiotropium, an atropine derivative from Atropa belladonna is useful in chronic obstructive pulmonary disease. The cannabis plant (Cannabis sativa), yields Dronabinol and Cannabidiol, while Capsicum annuum is a source of Capsaicin, all used as pain relievers.
Nitisinone is a derivative of the natural product Leptospermone (from Callistemon citrinus), used to treat antityrosinaemia. Such products are now being tapped again, with the aim of discovering new drugs, as well as addressing cost-effectiveness, quality control and standardization issues with older established medicines.
Modern technology is being harnessed to resolve these issues and bring plant-derived and chemical drugs under the same regulatory umbrella and thus give the former their rightful place in the sun.
This is the time when complementary and alternative medicine therapies (CAMTs) can also look forward to having their wealth of plant-based drugs recognized and utilized to help treat human disease more effectively.