Bioisosteres are an essential tool in the medicinal chemistry toolkit.

Bioisosteres are chemical substituents or groups with similar steric or electrostatic properties which produce broadly similar biological properties to another chemical compound.

Bioisosteres won't always give improved properties, and sometimes we find that a transformation that improves metabolic stability in one series might have the reverse effect in another. However they provide useful (literature precedented) starting points for structural transformations that allow exploration of structure activity relationships. Sometimes they yield similar activity but offer alternative structural vectors for exploration, sometimes they simply improve solubility.

I've now updated the bioisosteres section of the Drug Discovery Resources including new examples people have kindly sent to me.