Records show drug use dating back to 2,700 B.C. in the Middle East and China. The drugs most commonly used were laxatives and anti-emetics To relieve pain, opium extract was used. Ephedrine was used for the treatment of respiratory tract disorders.
Until the beginning of twentieth century, the substances used for the treatment of diseases were obtained from natural sources. Natural sources include plants, animals, and minerals. Among the natural sources, plants were used most frequently. Sometimes minerals and occasionally animals were used for the same purpose. Today, most drugs are manufactured in the laboratory, i.e. synthetically. Microorganisms also serve as a source of a large number of drugs.
So, what are the sources of drugs?
To summarize the paragraph above, the sources of drugs can be grouped as follows:
1. Plant sources
2. Animal sources
3. Mineral sources
4. Laboratory sources
5. Microorganism sources
Now, let’s have a look at each of these drug sources individually.
Believe it or not, there used to be a time when the leaves that had the shape of the liver were used for the treatment of liver diseases. Subsequently, various parts of the plant such as root, bark, stem, leaf, seed, and flower were used.
Drugs don't all come from the same part of a specific plant. For example, atropine, caffeine, cocaine, digoxin, and pilocarpine are obtained from the leaves of specific plants. Castor oil, colchicine, morphine, strychnine, and theobromine are obtained from some plants' seeds. Cinnamon, quinidine, and quinine are found in the bark of certain plants, and reserpine and atropine are extracted from some plants' roots.
Today, with some exception (digitalis, belladonna), plants themselves are rarely considered for medical treatment. Rather their pharmacologically active constituents (e.g. atropine from the roots) are extracted and used.
The pharmacologically active constituents of different plants are grouped according to their physio-chemical properties. These include:
- Carbohydrates and related compounds
Some of these active constituents can be extracted by soaking the plant in alcohol.
The purpose of extracting the active constituents is:
- Identification of the active constituents
- Analysis of the pharmacodynamic and pharmacokinetic properties of the active constituents
- Ensuring a precise and constant dosage in the therapeutic use of chemically pure constituents
- The possibility of chemical synthesis
Now, let's discuss in detail each group of the active constituents obtained from plants.
The word alkaloid (alkali + oid) literally means alkali-like substance. An alkaloid is defined as a basic nitrogenous compound of plant origin that 1) produces salt when combined with acid, and 2) is physiologically active in plants and animals.
Some properties of alkaloids are:
- White crystalline substance
- Bitter taste
- Insoluble in water but its salt preparation is highly soluble in water
- Almost all aresoluble in alcohol, ether, chloroform, and oil
Names of alkaloids end in –ine. For example, atropine, cocaine, and morphine are all alkaloids.
The majority of alkaloids are extracted from the seeds and roots of flowering plants. Only a few alkaloids are obtained from flowerless plants or produced synthetically. Some examples of synthetic alkaloids are apomorphine and homatropine.
Classification of alkaloids
Alkaloids are broadly classified according to their plant source, i.e. the from which plant they are obtained. These are:
Fun fact: The word rauwolfa originated from the name of a German physician and botanist of the 16th century, Leonard Rauwolf.
1. Belladonna alkaloids
Belladonna alkaloids include:
- Scopolamine (hyoscine)
Atropine is an organic ester formed by the combination of tropine (an organic base) and tropic acid (an aromatic acid). On the other hand, scopolamine contains scopine and tropic acid.
Scopine is different from tropine because it has an oxygen bridge between the carbon atoms designated C6 and C7.
2. Cinchona alkaloids
The important cinchona alkaloids are:
Quinine is present abundantly in the bark of cinchona tree. It was used as an antimalarial, antipyretic and an analgesic.
3. Cocaine alkaloids
Cocaine alkaloids include:
Cocaine (the first discovered local anesthetic) is obtained in large amount (0.6 to 1.8%) in the leaves of Erythroxylon coca. It is an ester of benzoic acid and a nitrogen-containing base.
4. Ergot alkaloids
Ergot alkaloids include:
Ergot is the product of the fungus Claviceps purpurea that grows up on rye and other grains.
5. Opium alkaloids
Opium is obtained from the milky juice of the unripe seed capsule of the poppy plant (Papaver somniferum). There are at least 20 opium alkaloids, including the following list of clinically important ones:
6. Rauwolfia alkaloids
Some examples of rauwolfia alkaloids are:
7. Xanthine alkaloids
Xanthine alkaloids include:
That's all about alkaloids for now. Let's move on to glycosides, which, you may remember, are the second group in our list of the pharmacologically active constituents of plants.
Glycosides are non-nitrogenous, colorless, crystalline solids that split up into sugar (one to four molecules) and non-sugar parts. They do not form salts. Some are poisonous.
The non-sugar part of glycosides is termed aglycone or genin. Aglycone is made of a cyclopentanoperhydrophenanthrene nucleus (steroid nucleus) attached to an unsaturated lactone ring at the carbon atom designated C17. It is chemically related to bile acid, sterol, and steroid hormones.
The pharmacological activity of a glycoside resides in its aglycone, or non-sugar, part. The combination of sugar to the aglycone modifies the lipid/water partition coefficient, potency, and pharmacokinetic properties of glycosides. Aglycone can be separated from the sugar part of glycosides by adding an acid or an enzyme.
Classification of glycosides
Glycosides are classified as glucosides, glalactosides, or fructosides according to the type of sugar present in the compound.