1. (a) Drugs: Drugs are the chemical substances of low molecular masses (~100–500 u) which interact with macromolecular targets and produce a biological response.
(b) Medicines: Drugs which produce a therapeutic and useful biological response.
(c) Chemotherapy: The use of chemicals for treatment of diseases.
2. Classification of Drugs
(a) On the basis of pharmacological effect: This classification is based on pharmacological effect of the drugs. It is useful for the doctors because it provides them the whole range of drugs available for the treatment of a particular type of problem. For example, analgesics have pain killing effect, antiseptics kill or arrest the growth of microorganisms.
(b) On the basis of drug action: It is based on the action of a drug on a particular biochemical process. For example, all antihistamines inhibit the action of histamines which causes inflammation in the body.
(c) On the basis of chemical structure: It is based on the chemical structure of the drug. Drugs classified in this way share common structural features and often have similar pharmacological activity. For example, sulphonamides have common structural features as seen in Fig. 16.1.
(d) On the basis of molecular targets: Drugs usually interact with biological macromolecules such as carbohydrates, proteins, lipids and nucleic acids called target molecules. This classification is based upon the type of the molecular target with which the drug interact. This is the most useful classification for medicinal chemists.
3. Enzymes: Proteins which perform the role of biological catalysts in the body are called enzymes.
(a) Catalytic action of enzymes: In their catalytic activity, enzymes perform two major functions:
(i) The first function of an enzyme is to hold the substrate for a chemical reaction. Active sites of enzymes hold the substrate molecule in a suitable position, so that it can be attacked by the reagent effectively. Substrates bind to the active site of the enzymes through a variety of interactions such as ionic bonding, hydrogen bonding, van der Waals interaction or dipole–dipole interaction.
(ii) The second function of an enzyme is to provide functional groups that will attack the substrate and carry out chemical reaction.
(b) Drug–enzyme interaction: Drugs can block the binding site of the enzyme and prevent the binding of substrate, or can inhibit the catalytic activity of the enzyme. Such drugs are called enzyme inhibitors.
Drugs inhibit the attachment of substrates on active site of enzymes in two different ways:
(i) Drugs compete with the natural substrate for their attachment on the active sites of enzymes. Such drugs are called competitive inhibitors.
(ii) Some drugs do not bind to the enzyme’s active site. These bind to a different site of enzyme which is called allosteric site. This binding of inhibitor at allosteric site changes the shape of the active site in such a way that substrate cannot recognise it.
If the bond formed between an enzyme and an inhibitor is a strong covalent bond and cannot be broken easily, then the enzyme is blocked permanently. The body then degrades the enzyme–inhibitor complex and synthesises the new enzyme.
l Receptors: Proteins that are crucial to body’s communication process are called receptors. Receptor proteins are embedded in the cell membrane in such a way that their small part possessing active site projects out of the surface of the membrane and opens on the outside region of the cell membrane.
In the body, message between two neurons and that between neurons to muscles is communicated through certain chemicals. These chemicals, known as chemical messengers are received at the binding sites of receptor proteins. To accommodate a messenger, shape of the receptor site changes. This brings about the transfer of message into the cell. Thus, chemical messenger gives message to the cell without entering the cell.
l Antagonists: Drugs that bind to the receptor site and inhibit its natural function are called antagonists. These are useful when blocking of message is required.
l Agonists: Drugs that mimic the natural messenger by switching on the receptor are called agonists. These are useful when there is a lack of natural chemical messenger.
4. Antihistamines: Antihistamines are the drugs which interfere with the natural action of histamine by competing with histamine for binding sites of receptor where histamine exerts its effects. Brompheniramine, terfenadine, pheniramine maleate (avil), cetrizine and chlorpheniramine are some examples of antihistamines.
5. Antacids: These are the chemical substances which remove the excess acid in the stomach and raise the pH to appropriate level, e.g., sodium hydrogencarbonate, a mixture of aluminium and magnesium hydroxide, ranitidine, etc.
6. Neurologically Active Drugs: Analgesics and tranquilizers are neurologically active drugs. These affect the message transfer mechanism from nerve to receptor.
(a) Analgesic: Drugs which reduce or abolish pain without causing impairment of consciousness, mental confusion, incoordination or paralysis or some other disturbances of nervous system are called analgesics. These are classified as follows:
(i) Non-narcotic analgesics: These drugs are non-addictive. Aspirin and paracetamol are important examples of non-narcotic analgesics. These drugs are effective in relieving skeletal pain such as that due to arthritis. These drugs have many other effects such as reducing fever and preventing platelet coagulation.
(ii) Narcotic analgesics: These are the drugs which when administered in small doses relieve pain and produce sleep. Alkaloids like morphine, codeine and heroin belong to the class of narcotic analgesics. These are chiefly used for the relief of postoperative pain, cardiac pain and pains of terminal cancer, and in child birth.
(b) Tranquilizers: Tranquilizers are a class of chemical compounds used for the treatment of stress, fatigue, and mild or even severe mental diseases. These relieve anxiety, stress, irritability or excitement by inducing a sense of well-being.
Examples:
l Tranquilizers like, chlordiazepoxide and meprobamate are used for relieving tension.
l Iproniazid and phenelzine are used as antidepressant.
l Barbiturates, viz., veronal, amytal, nembutal, luminal and seconal are hypnotic, i.e., sleep producing agents.
l Equanil is used in controlling depression and hypertension.
7. Antimicrobial: An antimicrobial tends to destroy or inhibit the pathogenic action of microbes such as bacteria (antibacterial drugs), fungi (antifungal drugs) or other parasites (antiparasitic drugs), selectively. Antiseptics, disinfectants and antibiotics are antimicrobial drugs.
(a) Antiseptics: These are the chemical substances which prevent the growth of microorganisms or kill them but are not harmful to the living human tissues. Antiseptics are applied to wounds, cuts, ulcers and diseased skin surfaces. 0.2% solution of phenol, dettol (a mixture of chloroxylenol and terpineol), bithionol, tincture of iodine (2–3% solution of iodine in alcohol–water), hydrogen peroxide and boric acid solution are some of the common antiseptics.
(b) Disinfectants: These are the chemical substances which kill microorganisms or stop their growth but are harmful to living tissues. These are used to kill the microorganisms present in floors, drains, toilets, etc. 1% of phenol, chlorine in the concentration of 0.2 to 0.4 ppm in aqueous solution and SO2 in very low concentration act as disinfectants.
(c) Antibiotics: These are the chemical substances produced wholly or partly by chemical synthesis, which in low concentrations inhibit the growth or destroy microorganisms by intervening in their metabolic processes. Antibiotics are of two types:
(i) Bactericidal antibiotics, which have cidal (killing) effect on microbes. e.g., Penicillin, aminoglycosides, ofloxacin, etc.
(ii) Bacteriostatic antibiotics, which have static (inhibitory) effect on microbes. e.g., erythromycin, tetracycline, chloramphenicol, etc.
Spectrum: The full range of microorganism attacked by an antibiotic is called its spectrum. Antibiotics which kill or inhibit a wide range of Gram-positive and Gram-negative bacteria are called broad spectrum antibiotics. Those effective mainly against Gram-positive or Gram-negative bacteria are narrow spectrum antibiotics. Ampicillin, tetracycline, amoxycillin, chloramphenicol and ofloxacin are broad spectrum antibiotics. Penicillin G is a narrow spectrum antibiotic.
8. Antifertility Drugs: These are the chemical substances which are used to prevent unwanted pregnancies in women. For example, norethindrone, ethynylestradiol (novestrol) and mifepristone.
9. Artificial Sweetening Agents: These are the chemical compounds which are non-nutritive in nature and are used as substitutes for sugar in foods and beverages especially soft drinks. Some common artificial sweeteners are
n Saccharin (Ortho-sulphobenzimide): It is useful as a sugar substitute for diabetic persons and those who need to control their calorie intake.
n Aspartame: It is methyl ester of dipeptide formed from aspartic acid and phenylalanine. Aspartame is used only in cold foods and soft drinks as it is unstable at cooking temperature.
n Alitame: It is a high potency sweetener. The control of sweetness of food is difficult while using alitame.
Sucralose: It is trichloroderivative of sucrose. It is stable at cooking temperature.
10. Food Preservatives: These are the chemical substances which are added to the food materials to prevent their spoilage due to microbial growth. The most commonly used preservative include table salt, vegetable oil, sugar, potassium metabisulphite and sodium benzoate.
11. Antioxidants in Food: Antioxidants are important and necessary food additives that help in food preservation by retarding the action of oxygen on food. These are more reactive towards oxygen than the food material they are protecting. For example, butylated hydroxy toluene (BHT) and butylated hydroxy anisole (BHA). BHA is added to butter to increase its shelf-life.
12. Soaps: Soaps are sodium or potassium salts of long chain fatty acids. Soaps containing sodium salts are formed by heating fat (i.e., glyceryl ester of fatty acid) with aqueous sodium hydroxide solution. This reaction is known as saponification.
In this reaction, esters of fatty acids are hydrolysed and the soap is obtained in the colloidal form. It is precipitated from the solution by adding sodium chloride. The solution left after removing the soap contains glycerol, which can be recovered by fractional distillation. Soap is a good cleansing agent and is biodegradable. Soaps have two disadvantages:
n Soaps cannot be used in hard water as calcium and magnesium ions present in hard water form insoluble calcium and magnesium soaps, respectively.
The insoluble soaps separate as scum in the water and hence a part of soap is wasted. In fact, this scum creates hindrance to washing because the precipitates of these soaps adhere onto fibres of the cloth as gummy mass.
n Soap cannot be used in acidic solutions as acid present in the solution precipitate the insoluble free fatty acids which adhere onto the fabrics and thus decreases the ability of soaps to remove oil from fabrics.
13. Synthetic Detergents: These are the cleansing agents which have all properties of soaps, but actually do not contain any soap. These can be used both in soft and hard water as they give foam even in hard water. Synthetic detergents are of three types, namely anionic, cationic and non-ionic.
(i) Anionic detergents: These are so named because large part of their molecules are anions and it is the anionic part of the molecule which is involved in the cleansing action. These are sodium salt of sulphonated long chain alcohols or hydrocarbons. For example, sodium lauryl sulphate, sodium dodecylbenzene sulphonate, etc. Anionic detergents are used in household work and in toothpastes.
(ii) Cationic detergents: These are so called because large part of their molecules are cations and it is the cationic part of the molecule which is involved in the cleansing action. Cationic detergents are quaternary ammonium salts of amines with acetates, chlorides or bromides as anions. Cetyltrimethyl ammonium bromide is a cationic detergent and used in hair conditioners. Cationic detergents have germicidal properties and are expensive, therefore, these are of limited use.
(iii) Non-ionic detergents: Non-ionic detergents do not contain any ion. These are esters of high molecular mass alcohols. One such detergent is formed when stearic acid reacts with polyethyleneglycol. Liquid dishwashing detergents are non-ionic type.
n The problem in the use of detergents is that if their hydrocarbon chain is highly branched then bacteria cannot degrade them easily. Slow degradation of detergents leads to their accumulation and this causes water pollution. Unbranched chain can be biodegraded more easily and hence pollution is prevented.
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