7. Quantitative relationships between structure and activity

The above qualitative methods provide step-by-step access to compounds with biological activity. In some cases, the derivatives obtained are agonists with enhanced activity; in others, antagonists or partial agonists are obtained.

However, qualitative methods can lend themselves to quantification insofar as a chemical structural modification (change of a substituent or its position on a ring) results in a quantified biological property that can be represented by a mathematical equation. This "de novo" technique was developed by Free-Wilson.

Quantitative methods were also initiated by C. Hansch [23].

• Hansch method

  This method studies the relationship between the physical and chemical properties of a series of existing molecules and their biological properties, with...