Bioisostere
In medicinal chemistry, bioisosteres are chemical substituents or groups with similar physical or chemical properties which produce broadly similar biological properties to another chemical compound. In drug design, the purpose of exchanging one bioisostere for another is to enhance the desired biological or physical properties of a compound without making significant changes in chemical structure. The main use of this term and its techniques are related to pharmaceutical sciences. Bioisosterism is used to reduce toxicity, change bioavailability, or modify the activity of the lead compound, and may alter the metabolism of the lead. Examples. Classical bioisosteres. Classical bioisosterism was  originally formulated by James Moir and refined by Irving Langmuir as a response to the observation that different atoms with the same valence electron structure had similar biological properties. For example, the replacement of a hydrogen atom with a fluorine atom at a site of metabolic oxidation in a drug candidate may prevent such metabolism from taking place. Because the fluorine atom is similar in size to the hydrogen atom the overall topology of the molecule is not significantly affected, leaving the desired biological activity unaffected. However, with a blocked pathway for metabolism, the drug candidate may have a longer  half-life. Another example are chalcones bioisosteres. By modifying certain substituents, the pharmacological activity of the chalcone and its toxicity are also modified. Non-classical bioisosteres. Non-classical bioisosteres may differ in a multitude of ways from classical bioisosteres, but retain the focus on providing similar sterics and electronic profile to the original functional group. Whereas classical bioisosteres commonly conserve much of the same structural properties, nonclassical bioisosteres are much more dependent on the specific binding needs of the ligand in question and may substitute a linear functional group for a cyclic moiety, an alkyl group for a complex heteroatom  moiety, or other changes that go far beyond a simple atom-for-atom switch. For example, a chlorine -Cl group may often be replaced by a trifluoromethyl -CF group, or by a cyano -C≡N group, but depending on the particular molecule used the substitution may result in little change in activity, or either increase or decrease affinity or efficacy depending on what factors are important for ligand binding to the target protein. Another example is aromatic rings, a phenyl -CH ring can often be replaced by a different aromatic ring such as thiophene or naphthalene which may improve efficacy, change  specificity of binding, or reduce metabolically labile sites on the molecule, resulting in better pharmacokinetic properties. Other applications. Bioisosteres of some patented compounds can be discovered automatically and used to circumvent Markush structure patent claims. It has been proposed that key force field features, that is the pharmacophore, be patented instead.