PHARMACOLOGY

 IMPORTANCE OF STRUCTURE ACTIVITY RELATIONSHIP 

With a change in the chemical constitution of a compound one can bring about a radical change in the pharmacological actions of the compound for example strychnine as such is a very strong convulsant but if CH3 (methyl) radical is introduced in its molecule the action of the resultant compound brucine is diametrically opposite i.e.relaxant in nature 

        If benzoic acid is converted into salicylic acid by further introducing OH i.e. hydroxyl group the new compound salicylic acid partly retain its antiseptic character but acquires two new pharmacological actions viz antipyretic and more important antirheumatic action if now to salicylic acid a new NH2 group is added in the para position an antitubercular drug called p-aminosalicylic acid is obtained 

        The disinfectant phenol can be converted to cresol by replacing one H by CH3 radical The new disinfectant so formed is more potent and less toxic a disinfectant to distinct advantage 

    The knowledge of structure activity relationship may also help in decreasing or increasing the duration of action of drugs as required for example the duration of action of procaine can be increased by addition of amide group i.e. procainamide This is essential for the drug to be useful as an antiarrhythmic agent Similarly synthesis of homatropine from atropine is necessary to decrease its duration of action This is essential if homatropine is required for mydriasis i.e .dilatation of the pupil of eye 

    With the understanding of structure activity relationship it has been possible to synthesize a number of competitive antagonists for example p-aminobenzoic acid is growth factor for micro -organisms and p-aminosalicylic acid is an antagonist Similarly isoprenaline and propranolol are agonist and antagonist respectively structure activity relationship also helps in understanding of the basic chemical group responsible for drug action many potent drugs have been synthesized based on chemical structure e.g. dexamethasone and prednisolone Lastly while synthesizing a compound for one purpose new compounds have been discovered that had quite different action than the original for example pethidine was synthesized with the assumption of anti spasmodic action but it turned out to be a potent analgesic