Bio-isosteric replacement is one of the most powerful and   versatile strategies in modern medicinal chemistry. It involves   substituting functional groups or structural fragments within a   molecule with alternative groups that possess similar physical or   chemical properties. The purpose of bio-isosterism is to enhance   potency, improve selectivity, increase metabolic stability,   modulate physicochemical properties, or reduce toxicity while   maintaining or improving the biological activity of a drug   candidate.