PHARMACOLOGY

 AFFINITY AND INTRINSIC ACTIVITY ::

A drug combines with the receptor and many initiate a sequence of effects The ability of drug to combine with the receptor to form the drug -receptor complex is defined as affinity The ability of drug to initiate the effect is termed as intrinsic activity Agonist is the drug which possess both affinity as well as intrinsic activity 

        The affinity of an agonist can be determined by finding out the pD2 value which is defined as -ve log of molar concentration of an agonist which produces half (50%) of the maximum response (EC50) 

                                        pD2= -ve log EC50

    The intrinsic activity indicates the capacity of an agonist to produce the response The intrinsic activity of two agonists can be compared by finding out the ratio between the maximum responses (E max ) of each of them 

                        Intrinsic activity = E max (test)

                                                       ---------------------

                                                       E max (control) 

It is possible that a particular drug may have high intrinsic activity but low affinity or high affinity and low intrinsic activity for example impromidine an H2 receptor agonist has high affinity but low intrinsic activity as compared to histamine for its relaxant effect on rat uterus Such agents are described as partial agonists They are the agonists whose maximum effect is less than the maximum response of which the tissue is capable to produce