Drug Exerting Action as Undissociated Molecules

It has been observed that most drugs exert their pharmacodynamic action either as undissociated molecules or as ionized molecules. These two different aspects shall be discussed briefly. 

In a large number of potent medicinal compounds the dissociation constants play a vital role for their respective biological characteristics. 

The unusual structural groupings in the tetracyclines results three distinct acidity constants in aqueous solutions of the acid salts. The particular functional groups responsible for each of the thermodynamic pA a values has been determined by Lessen and co-workers as described below  

The approximate / Ka values for each of these groups in four commonly used tetracyclines are as shown below. 

Besides, the activity of several local anaesthetics, /-tubucurarine and phenol has also been proved to be related to their degree of ionization. 

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