The relationship between the chemical structure of a molecule and its biological activity.

Structure-activity relationship (SAR) studies aim to identify the specific structural characteristics of a compound that are associated with its biological activity (e.g., enzyme inhibitory activity, antimicrobial activity, toxicity etc.).

At early stages of drug development, during hit-to-lead and lead optimization campaigns, multiple chemical modifications of a novel compound with antimicrobial properties (a hit) are made to understand which structural modifications improve or weaken the sought biological activity. As the SAR evolves for a class of compounds, it enables a medicinal chemist to design new molecules which enhance biological activity based on their molecular structure. Previous knowledge of SAR for other similar compounds in question can also suggest potential modifications to enhance the compound such as increasing its antimicrobial activity, improving its selectivity or reducing toxicity).