Brief History and Outlook

Early QSAR smdies were mainly focused on analyzing the effect of aromatic substituents on activity in congeneric compound series using substiment constants to describe the steric, electronic and lipophilic characteristics of the substituents. Hansch and Fujita proposed a method to describe quantitatively relationships between biological activity and chemicnl descriptors. This cnn be expressed as follows  

The Hansch-Fujita approach is also called the linear free-energy relationship (LFER) or extrathermodynamic approach, since most of the descriptors are derived from rate or equilibrium constants. 

There are numerous examples of traditional Hansch QSAR studies in the literature. Some include large sets of descriptors, while others explore just a few. If physicochemical descriptor values are not readily available, indicator variables, denoting presence or absence of a certain structural feature, may be of help. This is the basis of the Free-Wilson (FW) method. 

The FW model was proposed in 1964 at the same time as the Hansch mcxiel, but is far less widely used. It uses 

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