This is frequently used in drug design:[2] the bioisostere will still be recognized and accepted by the body, but its functions there will be altered as compared to the parent molecule.
Non-classical isosteres do not obey the above classifications, but they still produce similar biological effects in vivo.
Non-classical isosteres may be made up of similar atoms, but their structures do not follow an easily definable set of rules.
[4][5][6] Classical isosteres are defined as being atoms, ions and molecules that had identical outer shells of electrons, This definition has now been broadened to include groups that produce compounds that can sometimes have similar biological activities.
Some evidence for the validity of this notion was the observation that some pairs, such as benzene, thiophene, furan, and even pyridine, exhibited similarities in many physical and chemical properties.