PHARMACOGNOSY - SOURCES AND COMPOSITION OF DRUGS

            A large number of drugs are used in the clinics for the cure of various ailments. These drugs are diverse in chemical structure and are obtained from a wide variety of sources. In earlier days, simple chemical substances and different parts of plants were employed as medicinal agents, but most drugs currently used in therapeutics are synthetic in nature. However, natural sources are still used for obtaining some drugs mainly because their synthesis is difficult and uneconomical. Drugs are mainly obtained from following sources.

I. NATURAL SOURCE

A. Organic drugs. Organic drugs are mainly obtained from

1. Vegetable source

2. Animal source

3. Microorganisms.

B. Inorganic drugs. These are mainly obtained from

1.  Metallic source

2.  Non-metallic source.

II. SYNTHETIC SOURCE

(I) NATURAL SOURCES OF DRUGS

Active principles of drugs are mainly present in crude form in minute amounts. These active principles are separated by various techniques or may be used as such in some cases.

(A) Organic drugs: Main natural sources of organic drugs are vegetable, animal and microorganisms.

1. Vegetable source: Substances obtained from obtaining several important these :

It is very old and important source of drugs, various parts of the plants are still used for drugs. The following table summarizes some of

 

Part of the plant           Name of the plant         Active Principle                        Use

Root                             Rauwolfia                      Reserpine                     Antihypertensive

                                    pecacuanha                  Ipecac                          Emetic

Bulbs                           Urginea             Squill                            Emetic, rodenticide

Bark                             Cinchona                      Quinine             Antimalarial

Rhizome                      Ginger                          Gingerol                        Carminative

Wood                           Sandal wood                 Sandal wood oil            Urinary antiseptic

                                    Quassia                        Quassin                        Stomachic

Leave                           Belladona                     Atropine                        Anti-muscarinic

                                    Digitalis                        Digitoxin                                   Cardiac stimulant

Flower                          Clove                Eugenol                                    Local anaesthetic,

                                                                                                            rubefacient

                                    Pyrethrium                    Pyrethrin                       Insecticide

Fruit                             Senna                           Senegrin                       Purgative

                                    Anise                            Anethole                       Carminative

Seed                            Nux vomica                   Strychnine                     Rodenticide

                                    Physostigma                 Physostigmine              Anti-glaucoma

Corn                             Colchicum                     Colchicine                     Anti-gout

The following is the short description of the active constituents of crude drugs.

(a) Alkaloid: These are complex, alkaline, nitrogenous compounds mostly obtained from plants and also animals. Their properties are as follows:  They are nitrogenous organic compounds. They are alkaline in reaction (so named alkaloids). They combine with acids to form crystalline salts without production of water. They are readily soluble in alcohol but sparingly soluble in water. But their salts are soluble in water. A few of them are liquid, which nearly contain C, H and N only. The solid alkaloids contain oxygen   in addition and are colourless crystalline in nature. Most of the alkaloids are closely related to pyridine and some may be prepared synthetically from pyridine bases. They are mostly bitter in taste. In higher concentration they are potent poisons. Their names mostly end with -ine.

Examples :       Solid alkaloid : Atropine, morphine, quinine, etc.

Liquid alkaloid : Arecholine, nicotine, lobeline

Semi-synthetic/synthetic : Apomorphine, homatropine.

Animal alkaloid : Adrenaline.

(b) Glycosides : They are mostly non-nitrogenous bodies mainly occurring in plants. They are non-nitrogenous compounds having sugar attached to non-sugar part by ether linkage. They are neutral in reaction.  They mainly contain C, H and O. Some may have in addition N and few S. They do not combine with acids to form salts.  They are mostly soluble in alcohol, less soluble in water and insoluble in ether. Some are highly active, while others are practically inert. These are hydrolysed by acids and liberate aglycone (non-sugar part). Their names usually end with -in.

Examples : Digitoxin, scillarin, digoxin, etc.

(c) Saponins : These are plant glycosides which have distinctive property of frothing.  They are a group of non-nitrogenous substances usually glycosides. They are soluble in water and form froth when shaken.  On hydrolysis they  split up into sugar and aglycone (sapogenin).  Toxic saponins are called sapotoxins.

Examples : Digitonin, senegin, glycyrrhizin, etc.

(d) Resins : These are rosin-like substances which are oxidative products of volatile oils. They are produced by some plants. They are invariably composed of a large number of substances which may be acid, alcohol or ester in chemical composition. They are secretions of plant tissues. They are bitter and amorphous solids. They are insoluble in water, but soluble in alcohol, ether, etc. They are soluble in alkalies forming non-detergent resin soaps.

Examples : Resin of Jalap, podophyllin.

Oleo-resins : They are natural plant exudates which are semisolid mixtures of resins and volatile oils, e.g. crude turpentine.

Gum-resins : They are mixture of resins and gums, e.g. asafoetida.

Balsams : They are oleoresins containing benzoic acid or cinnamic acid, e.g. benzoin, balsam of Peru, etc.

(e) Tannins:  Tannins  are  non-nitrogenous  phenol  derivatives characterised by their astringent action on the mucous membrane. They mainly occur in leaves and barks of the plant. They have irritant or astringent action. They react with iron to form blue colouration. They precipitate metallic salts, alkaloids and proteins. Some are glycosides, i.e. occur in combination with sugar.

Example : Tannic acid obtained from nut galls.

(f) Gums : Gums are secretory products of plants which are used as emulsifying agents for oils and, suspending agents for insoluble substances. They are amorphous, colloidal, complex polysaccharides. They dissolve in water forming viscid adhesive fluid known as mucilage.

Examples : Acacia, tragacanth.

(g) Oils : Oils are obtained from vegetable, animal and mineral sources. Oils are of 3 types : Volatile, fixed and mineral.

Volatile oils : These are also called as essential, ethereal, aromatic or flavouring oils as they are responsible for the aroma and odour of plants and flowers. They are composed of diverse chemical compounds such as alcohols, aldehydes, ketones, esters, sulphur compounds, etc. They are mainly present in the flowering parts of plant, leaves and fruits and give characteristic smell to plants. They are mainly obtained by a process of distillation without being decomposed. They do not form soap with alkalies. They are less soluble in water but more soluble in organic solvents. Alcoholic solutions of these oils are known as essences and are used in perfumery. They do not leave a grease spot on paper. On exposure to air and light, they tend to oxidise and turn rancid.

Examples : Liquid volatile oil : Eucalyptus oil, clove oil. Solid volatile oil : Camphor, menthol, thymol.

Fixed oils : They are esters of higher fatty acids (oleic, palmitic, stearic acids) and glycerines. They are obtained from fruits, seeds and some other parts of the plants. They are non-volatile and as such cannot be distilled without decomposition, so obtained by process of expression. They are insoluble (immiscible) in water, sparingly soluble in alcohol and freely soluble in ether. They are liquid at ordinary temperature. They leave a permanent grease spot on paper. They turn rancid on heating. They form soap with alkalies.

Examples : Vegetable oils : Olive oil, castor oil, mustard oil. Animal oils : Cod liver oil, shark liver oil.

Fats : These are also oils containing more of palmitin and stearin making them solid at ordinary temperature, e.g. lard, lanolin, butter. Mineral oils : These are obtained by boring the earth and do not belong to organic class. Some are used in medicinal preparations and contain only C and H, e.g. liquid paraffin.

(h) Waxes : They are esters of higher fatty acids and higher monohydric alcohols. They are firmer in consistency and have higher melting , points, e.g. Yellow and white bees wax.

2. Animal sources : From animal source relatively few but important drugs are obtained. Following are some useful drugs:

Hormones        :   Insulin, thyroxin, gonadotropins, etc.

Vitamins         :   Cod liver oil (vit. A).

Vaccines/serra :   Anti-rabies vaccine (A.R.V), antitetanic

serum (A.T.S),,anti-diphtheric serum, etc. Replacement therapy : Liver extract.

3. Microorganisms : Microorganisms are also an important source of drugs. Certain bacteria, fungi and actinomycetes produce antibiotics which forms a major group of therapeutically useful drugs.

Fungi : Penicillin G from Penicillium notatum. Actinomycetes : Streptomycin from Streptomyces griseus Bacteria : Bacitracin from Bacillus subtilus.

(B) Inorganic Sources : Drugs obtained from inorganic sources have been simply classified into metals and non-metals. Metalloids which show intermediate properties are usually discussed along with metals. The elements either occur in native state or combined state.

1. Metals : Minerals are the main source of metals.

Agent                                       Use

Magnesium sulphate                  Purgative

Calcium carbonate                     Astringent

Copper sulphate                        Emetic

Ferrous sulphate                       Haematinic

Zinc sulphate                            Astringent

Bismuth subnitrate                     Antiseptic

Lead acetate                             Local sedative, antiseptic

2. Non-metals : These are also used for various purposes.

Agent                                       Use

Iodine (Pot. iodide)                    Expectorant

Bromine (Pot. bromide)             Sedative

Hydrogen peroxide                    Antiseptic

Sulphur                                     Insecticide, disinfectant

Carbon (charcoal)                      Adsorbant

(II) SYNTHETIC SOURCES OF DRUGS

Synthetic drugs are prepared in the laboratory with the help of inorganic and organic drugs. Today majority of drugs are obtained synthetically or semi-synthetically. Numerous drugs which were originally obtained from plants are now prepared synthetically. For example, ether and chloroform (Volatile anaesthetics), sulfonamides and quinolones (antimicrobial drugs), paracetamol (analgesic), pentobarbital and thiopental (hypnotic and parenteral anaesthetics), etc.