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Drugs and Medicines

Drugs are chemicals of low molecular mass (– 100 – 500 u) which interact with macro-molecular targets and produce a biological response. When the biological response is therapeutic and useful, these chemicals are called medicines and are used in treatment, diagnosis and prevention of diseases. Thus, chemical substances used for treatment, prevention and diagnosis of diseases and for reducing suffering from pain are called medicines.

Use of some chemical compounds as medicines has helped humanity in a number of ways. Medicines have been used to cure diseases, to reduce suffering from pain and to check population explosion. Medicines have increased the average life span of man and have helped him to lead a comfortable life, free from fear of pain.

The treatment of diseases by chemical compounds is known as chemotherapy. The chemicals so used for the cure are called chemotherapeutic agents. Each compound (or chemical) used in specific in its toxicity towards a particular micro-organism. The ideal requirement of the drugs is that on administration (to host) it should be localized at the site where it is required. Most of the medicines, if taken in doses higher than those recommended, are potential poisons.

Designing a drug

While designing a drug the two important considerations are:

1. Drug target: drugs usually interact with biomolecules such as carbohydrates, lipids, proteins and nucleic acids. These are called drug targets. The correct choice of the target for a drug is important in order to obtain its desired therapeutic effect.

2. Drug metabolism: a drug travels through the human system in order to reach the target. So the drug should be designed in such a way that it reaches the target without being metabolized in between. Moreover after its action, the side products of the drug should be excreted without causing harm to the body.

While designing a drug, the knowledge of physiological function of the drug target in the body is required. It helps in choosing a compound, which can interact with the target and is likely to be therapeutically active. Drugs are designed from the compounds. These compounds are called lead compounds and are chosen as starting points for drug designing. Lead compounds may be obtained from natural sources such as plants, trees, venoms and metabolites of micro-organisms or they may be synthesized.

Analogues of the lead compounds are synthesized to obtain compounds with improved activity and reduced side effects. The knowledge of mechanism of drug action and metabolic pathways in the biological systems helps in designing a drug with improved drug activity and minimum side effects.

Classification of drugs

Drugs can be classified in several ways. Some of these are as follows:

1. On the basis of pharmacological effect: This classification is based on pharmacological effect of the drugs. It is useful for doctors because it provides them the whole range of drugs available for the treatment of a particular type of health related problem. For example, analgesics have pain killing effect, antibiotics have antimicrobial properties, and tranquilizers reduce mental stress.

2. On the basis of action on a particular biochemical process: it is based on the action of a drug on a particular biochemical process. For example, all antihistamines inhibit the action of the compound, histamine which produces symptoms of allergic reaction such as inflammation in the body. There are various ways in which action of histamines can be blocked.

3. On the basis of chemical structure: It is based on the chemical structure of the drug. Drugs classified in this way have common structural features and often have similar pharmacological activity. For example, sulphonamides have common structural feature as given below and generally possess antibacterial properties.

4. On the basis of molecular targets: The classification based on molecular targets is the most useful classification for medicinal chemists. Various enzymes and receptors in the cell are some of the common drug targets.