PHARMACOLOGY

 STRUCTURE - ACTIVITY RELATIONSHIP ::

Just as histological structure of an organ is generally correlated with its physiological functions the chemical formula of a drug is also corelated with its functions This correlation is known as Structure -Activity Relationship (SAR) This is one of the most important aspect of pharmacodynamics Had it been possible to establish laws relating to chemical constitution and drug action it would have been possible to make drugs which would produce the action as desired However very little success has been achieved There are many compounds having different structure formulae but similar action e.g.C N S stimulants nikethamide and picrotoxin Other common example is acetylcholine and pilocarpine -both cholinergic agents 

There are also some compounds with similar structural chemical formula but with different actions Pteroylglutamic acid and its antagonist aminopterin are the examples in point for they have similar structure but opposite action 

There are of course number of examples indicating that compounds having similar action produce identical pharmacological actions viz sulphathiazole and sulphadiazine adrenaline and noradrenaline etc 

Some other examples indicate that the stereospecificity of the compounds may affect the action following figure illustrates some examples 

Sometimes certain compounds may have different isomers i.e. the compounds have the same molecular formula but different molecular arrangement Catecholamine adrenaline has two isomers :(1) Levoadrenaline and (2) Dextroadrenaline The levoisomer is about 20-times stronger than the dextro form Another example is that of resorcin and pyrocatechin the former is sweet while the latter is bitter both having the formula C6H4(OH)2