I've been updating the Drug Discovery Resources. Over the last few days I've been expanding the section on bioisosteres.

A bioisostere is a molecule resulting from the exchange of an atom or of a group of atoms with an alternative, broadly similar, atom or group of atoms. The objective of a bioisosteric replacement is to create a new molecule with similar biological properties to the parent compound. The bioisosteric replacement may be physicochemically or topologically based. The replacement can attenuate toxicity, modify activity of lead, and/or alter pharmacokinetics or the toxicity of the lead.

Bioisosteres are an essential element in the Medicinal Chemists toolbox and the increasing variety reported is a testimony to the creativity of medicinal chemists.