Chemistry Therapeutic Action of Different Classes of Drugs : II
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Topics Covered :

● Neurologically Active Drugs
● Tranquilizers
● Analgesics
● Antimicrobials
● Antibiotics
● Antiseptics and Disinfectants
● Antifertility Drugs

Neurologically Active Drugs :

`=>` Tranquilizers and analgesics are neurologically active drugs.

`=>` These affect the message transfer mechanism from nerve to receptor.

Tranquilizers :

`=>` Tranquilizers are a class of chemical compounds used for the treatment of stress, and mild or even severe mental diseases.

`=>` These relieve anxiety, stress, irritability or excitement by inducing a sense of well-being.

`=>` They form an essential component of sleeping pills.

`=>` There are various types of tranquilizers.

● They function by different mechanisms.

● `color{red}("Example ")` : Noradrenaline is one of the neurotransmitters that plays a role in mood changes.

`->` If the level of noradrenaline is low for some reason, then the signal-sending activity becomes low, and the person suffers from depression. In such situations, `text(antidepressant drugs)` are required.

`->` These drugs inhibit the enzymes which catalyse the degradation of noradrenaline.

`->` If the enzyme is inhibited, this important neurotransmitter is slowly metabolised and can activate its receptor for longer periods of time, thus counteracting the effect of depression.

`->` Iproniazid and phenelzine are two such drugs.

`=>` Some tranquilizers namely, chlordiazepoxide and meprobamate, are relatively mild tranquilizers suitable for relieving tension.

`=>` Equanil is used in controlling depression and hypertension.

`=>` Derivatives of barbituric acid viz., veronal, amytal, nembutal, luminal and seconal constitute an important class of tranquilizers.

● These derivatives are called `color{green}("barbiturates")`.

● Barbiturates are hypnotic, i.e., sleep producing agents.

`=>` Some other substances used as tranquilizers are valium and serotonin.

Analgesics :

`=>` Analgesics reduce or abolish pain without causing impairment of consciousness, mental confusion, incoordination or paralysis or some other disturbances of nervous system. These are classified as follows :

(i) Non-narcotic (non-addictive) analgesics
(ii) Narcotic drugs

(i) `color{green}("Non-narcotic (non-addictive) Analgesics ")` : Aspirin and paracetamol belong to the class of non-narcotic analgesics.

● Aspirin is the most familiar example.

● Aspirin inhibits the synthesis of chemicals known as prostaglandins which stimulate inflammation in the tissue and cause pain.

● These drugs are effective in relieving skeletal pain such as that due to arthritis.

● These drugs have many other effects such as reducing fever (antipyretic) and preventing platelet coagulation.

● Because of its anti blood clotting action, aspirin finds use in prevention of heart attacks.

(ii) `color{green}("Narcotic Analgesics ")` : Morphine and many of its homologues, when administered in medicinal doses, relieve pain and produce sleep.

● In poisonous doses, these produce stupor, coma, convulsions and ultimately death. Morphine narcotics are sometimes referred to as opiates, since they are obtained from the opium poppy.

`=>` These analgesics are chiefly used for the relief of postoperative pain, cardiac pain and pains of terminal cancer, and in child birth.

Antimicrobials :

`=>` Diseases in human beings and animals may be caused by a variety of microorganisms such as bacteria, virus, fungi and other pathogens.

`=>` An antimicrobial tends to destroy/prevent development or inhibit the pathogenic action of microbes such as bacteria (antibacterial drugs), fungi (antifungal agents), virus (antiviral agents), or other parasites (antiparasitic drugs) selectively.

`=>` Antibiotics, antiseptics and disinfectants are antimicrobial drugs.

Antibiotics :

`=>` Antibiotics are used as drugs to treat infections because of their low toxicity for humans and animals.

`=>` Initially antibiotics were classified as chemical substances produced by microorganisms (bacteria, fungi and molds) that inhibit the growth or even destroy microorganisms.

`=>` The development of synthetic methods has helped in synthesising some of the compounds that were originally discovered as products of microorganisms.

`=>` Also, some purely synthetic compounds have antibacterial activity, and therefore, definition of antibiotic has been modified.

`color{green}("Antibiotic ")` : An antibiotic now refers to a substance produced wholly or partly by chemical synthesis, which in low concentrations inhibits the growth or destroys microorganisms by intervening in their metabolic processes.

`=>` The search for chemicals that would adversely affect invading bacteria but not the host began in the nineteenth century.

`=>` Paul Ehrlich, a German bacteriologist, conceived this idea. He investigated arsenic based structures in order to produce less toxic substances for the treatment of syphilis.

● He developed the medicine, `text(arsphenamine)`, known as `color{green}("salvarsan")`.

● Paul Ehrlich got Nobel prize for Medicine in 1908 for this discovery.

● It was the first effective treatment discovered for syphilis.

● Although salvarsan is toxic to human beings, its effect on the bacteria, spirochete, which causes syphilis is much greater than on human beings.

● At the same time, Ehrlich was working on azodyes also. He noted that there is similarity in structures of salvarsan and azodyes.

● The `color{red}(–As = As–)` linkage present in arsphenamine resembles the `color{red}(–N = N –)` linkage present in azodyes in the sense that arsenic atom is present in place of nitrogen.

● He also noted tissues getting coloured by dyes selectively.

● Therefore, Ehrlich began to search for the compounds which resemble in structure to azodyes and selectively bind to bacteria.

● In 1932, he succeeded in preparing the first effective antibacterial agent, prontosil, which resembles in structure to the compound, salvarsan.

● Soon it was discovered that in the body prontosil is converted to a compound called sulphanilamide, which is the real active compound.

● Thus the sulpha drugs were discovered. A large range of sulphonamide analogues was synthesised. One of the most effective is sulphapyridine.

`=>` Despite the success of sulfonamides, the real revolution in antibacterial therapy began with the discovery of Alexander Fleming in 1929, of the antibacterial properties of a Penicillium fungus.

`=>` Isolation and purification of active compound to accumulate sufficient material for clinical trials took thirteen years.

`=>` Antibiotics have either cidal (killing) effect or a static (inhibitory) effect on microbes.

`=>` A few examples of the two types of antibiotics are as follows:

`color{red}(tt ( ( text{Bactericidal} , text{Bacteriostatic}) , ( text{Penicillin} , text{Erythromycin}) , ( text{Aminoglycosides} , text{Tetracycline}) , (text{Ofloxacin} , text{Chloramphenicol})))`

`=>` The range of bacteria or other microorganisms that are affected by a certain antibiotic is expressed as its spectrum of action.

`color{green}("Broad Spectrum Antibiotics ")` : Antibiotics which kill or inhibit a wide range of Gram-positive and Gram-negative bacteria are said to be broad spectrum antibiotics.

`color{green}("Narrow Spectrum Antibiotics ")` : Those effective mainly against Gram-positive or Gram-negative bacteria are narrow spectrum antibiotics.

`color{green}("Limited Spectrum Antibiotics ")` : If effective against a single organism or disease, they are referred to as limited spectrum antibiotics.

● Penicillin `color{red}(G )`has a narrow spectrum.

● Ampicillin and Amoxycillin are synthetic modifications of penicillins. These have broad spectrum.

● It is absolutely essential to test the patients for sensitivity (allergy) to penicillin before it is administered.

● In India, penicillin is manufactured at the Hindustan Antibiotics in Pimpri and in private sector industry.

`=>` Chloramphenicol, isolated in 1947, is a broad spectrum antibiotic.

● It is rapidly absorbed from the gastrointestinal tract and hence can be given orally in case of typhoid, dysentery, acute fever, certain form of urinary infections, meningitis and pneumonia.

● Vancomycin and ofloxacin are the other important broad spectrum antibiotics.

● The antibiotic dysidazirine is supposed to be toxic towards certain strains of cancer cells.

Antiseptics and disinfectants :

`=>` Antiseptics and disinfectants are also the chemicals which either kill or prevent the growth of microorganisms.

`=>` `text(Antiseptics)` are applied to the living tissues such as wounds, cuts, ulcers and diseased skin surfaces.

● Examples are `color{green}("furacine, soframicine")`, etc.

● These are not ingested like antibiotics.

● Commonly used antiseptic, dettol is a mixture of chloroxylenol and terpineol.

● Bithionol (the compound is also called bithional) is added to soaps to impart antiseptic properties.

● Iodine is a powerful antiseptic. Its `2-3` per cent solution in alcohol-water mixture is known as tincture of iodine. It is applied on wounds.

● Iodoform is also used as an antiseptic for wounds.

● Boric acid in dilute aqueous solution is weak antiseptic for eyes.

`color{green}("Disinfectants")` are applied to inanimate objects such as floors, drainage system, instruments, etc.

● Same substances can act as an antiseptic as well as disinfectant by varying the concentration.

● For example, `0.2` per cent solution of phenol is an antiseptic while its one percent solution is disinfectant.

● Chlorine in the concentration of `0.2` to `0.4` ppm in aqueous solution and sulphur dioxide in very low concentrations, are disinfectants.

Antifertility Drugs :

`=>` Antibiotic revolution has provided long and healthy life to people.

● The life expectancy has almost doubled.

● The increased population has caused many social problems in terms of food resources, environmental issues, employment, etc.

● To control these problems, population is required to be controlled.

● This has lead to the concept of family planning.

`=>` `color{green}("Antifertility drugs")` are of use in this direction.

● Birth control pills essentially contain a mixture of synthetic estrogen and progesterone derivatives.

● Both of these compounds are hormones.

● It is known that progesterone suppresses ovulation.

● Synthetic progesterone derivatives are more potent than progesterone.

● Norethindrone is an example of synthetic progesterone derivative most widely used as antifertility drug.

● The estrogen derivative which is used in combination with progesterone derivative is ethynylestradiol (novestrol).