Brief Review of QSAR Methods

The importance of QSAR in the prediction and design of biologically active compounds is now well recognized. There are quite a number of methods intended to determine QSAR. All of them are based on the concepts that there is a direct correlation between the molecular structure of the chemicals and their biological activity and that the QSAR obtained from a given set of compounds with known activity can be extrapolated to new compounds. 

Some methods aim at obtaining structural features that are common for all the active (or inactive) compounds of the analyzed series and formulating prognostic rules for biological activity. There are also efforts to find empirical functional relationships between physicochemical (and other) parameters of the systems and the level of biological activity by means of multiparameter regression analysis. An adequate combination of the qualitative and quantitative approaches seems to be most efficient. 

A number of approaches to QSAR are based on the ideas of pattern recognition theories (PRT). Using PRT, some classification rules are worked out, by means of which the molecules under consideration are divided into the classes of active and inactive compounds. Most important in these PRT approaches are the methods for describing molecules. Indeed, the better the description of the molecule in terms of parameters that represent its activity, the better the pattern recognition and separation of molecules into active and inactive classes. Therefore the QSAR methods differ mainly in the molecular descriptors used. 

At present, quite a number of works are using one of three methods vector description, description centers, or topological indexes. The vector description is the simplest one it is usually used when there is a series of N similar molecules, e.g., having the same skeleton with different substituents. In these cases, each (k) molecule of the series can be presented by a vector C,  

Some versions of the PRT used in the QSAR problems are based on the description of molecules by more physically grounded elements. These include methods of substructural analysis and logical-structural analysis. The former assumes that the activity under consideration is due to the presence of certain substructures in the molecule their relative contribution is estimated to be proportional to the number of active compounds containing the corresponding substructure. Some versions of this method are given in the literature, where statistical calculations of the substructure contributions and different classification procedures are given. 

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