The bioisosterism concept Friedmans and Thornbers definitions

Recognizing the usefulness of the isosterism concept in the design of biologically active molecules, Friedman proposed to call bioisosteres compounds 

This definition received rapid acceptance and is now commonly used. Moreover, Friedman considers that isosteres that exhibit opposite properties (antagonists) have also to be considered as bioisosteres, since nsnally they interact with the same recognition site. This is the case for para-ami-nobenzoic acid and para-aminobenzene-snlfonamide ° and also for glutamic acid and its phosphonic analogs.  

The use of the word isosterism has been largely taken beyond its original meaning when employed in medicinal chemistry and Thomber proposes a loose and flexible definition of the term bioisostere  

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