Nature and Sources of Drugs
Several drug candidates have been derived from various naturally occurring medicinal sources. They can be broadly divided in to four major categories:
• Plant sources
• Animal sources
• Microbial sources
• Marine sources.
✓ In addition to the above mentioned natural sources, two other sources need specific mention. A drug may be derived either as a semi-synthetic product from any of the naturally occurring resources or it may be of totally synthetic origin. Having knowledge about the receptor and Quantitative Structure-Activity Relationship (QSAR), one may predict possible structure of a drug. Subsequently, a drug with optimum biological action can be synthesised, tested and evaluated for possible clinical activity
• Plant sources.
✓ The use of plants as medicines has a long history. Opium has been the first known drug from plant sources. Plants continue to be a significant part of traditional medicine and herbal medicine. Several important drugs like Taxol, Camptothecin, Morphine and Quinine have been isolated from plant sources.
• Animal sources
✓ Animals have been sources for some drugs. Insulin and heparin are two common names of drugs of animal origin. Till last few decades insulin was obtained from pancreas of animals. Now insulin is available by genetic engineering from microbial sources. Heparin is obtained from lungs of animals. Epibatidine, obtained from the skin of an Ecuadorian poison frog, is ten times more potent than morphine. Teprotide, extracted from a Brazilian snake viper, has led to the development of Cilazapril and Comptopril, which are effective against hypertension.
• Microbial sources
✓ Microorganisms as a source of potential drug candidates were not explored till the discovery of penicillin in 1929. Subsequently several antibiotics have been obtained either from microbial sources or from their semi-synthetic derivatives. Marine organisms have also been sources for new drugs. The first active compounds to
be isolated from marine species were Spongouridine and Spongothymidine from the Carribean sponge Cryptotheca crypta in 1950s. These compounds are nucleotides and show a great potential as anticancer and antiviral agents. Discodermolide, isolated from the marine sponge, Discodermia gissoluta has a strong antitumour activity and has mode of action similar to that of Paclitaxol. Few antibiotics also originate from ocean.
• Marine sources.
✓ Several derivatives of antibiotics, β-blockers, anti-protozoal drugs, anti-malarial drugs are of totally synthetic origin. The examples of the categories of drugs are illustrative only. In recent years most of the new drugs are of synthetic origin based on the knowledge about the receptor.
