Nature and Sources of Drugs: Several drug candidates have been derived from various naturally occurring medicinal sources. They can be broadly divided into 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. Knowing the receptor and Quantitative Structure-Activity Relationship (QSAR), one may predict the possible structure of a drug. Subsequently, a drug with optimum biological action can be synthesized, tested, and evaluated for possible clinical activity.
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.
Animals have been sources for some drugs. Insulin and heparin are two common names of drugs of animal origin. Till the last few decades insulin was obtained from the pancreas of animals. Now insulin is available by genetic engineering from microbial sources. Heparin is obtained from the 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 Captopril, which are effective against hypertension.
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 Caribbean sponge Cryptotheca crypto in the 1950s. These compounds are nucleotides and show great potential as anticancer and antiviral agents. Discodermolide, isolated from the marine sponge, Discodermia dissolute has a strong antitumor activity and has a mode of action similar to that of Paclitaxel. Few antibiotics also originate from the ocean.
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.
Essential Drugs Concept
Although a large number of medicines are available in the market, all of them may not be necessary for the majority of the population. Based on this observation, World Health Organisation (WHO) proposed a concept of ‘essential medicines’ in 1977. According to this concept, essential medicines are those that satisfy the priority health care needs of the population. They are selected with due regard to public health relevance, evidence on safety and efficacy, and comparative cost-effectiveness. Essential medicines are intended to be available within the context of functioning health systems at all times in adequate times, in the appropriate dosage forms, with assured quality and adequate information, and at a price, the individual and the community can afford. The implementation of the concept of essential medicines is intended to be flexible and adaptable to many different situations; exactly which medicines are regarded as essential remains the responsibility of that country.
In the first, 1977 version of the list of essential medicines, 208 medicines were identified to address the global burden of disease at that time. The list is revised by a committee of independent experts after every two years to reflect new health challenges, pharmaceutical developments, and changing resistance patterns by microorganisms. In 2007, the 30th anniversary of WHO was celebrated. By then 156 of 193 WHO member states have official essential medicine lists, of which 127 countries have been updating it in the past 5-10 years. The list of essential medicines published in 2007 contains 340 medicines including drugs for malaria, HIV/AIDS, tuberculosis, reproductive health, chronic diseases like cancer, and diabetes. The list of essential medicines published in October 2007 contained for the first time “Essential Medicines for children”. The list of essential medicines for children is also updated every two years. In the year 2015, the 19th model list of essential medicines and the 5th revised list of essential medicines for children has been published by WHO. The 20th revision of essential medicines and the 6th revision of essential medicines for children are in the process.
The government of India has published a list of essential medicines. Few states have implemented it. In the year 2013, the eighth edition of the essential medicines list for the National Capital Territory of Delhi has been published. Few other states like Rajasthan, Kerala, Karnataka, and Madhya Pradesh have essential lists prepared by an expert committee and the concept is implemented in these states.
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