3. Progress of study and results obtained
(a) In silico screening
The high-throughput three-dimensional structure-activity
relationship (SAR) technique in silico was studied for screening
based on the binding capacity to the receptor molecule of the
substance in question. The automatic docking model for estrogen
receptor Ώ (ERΏ), shown to be useful in preliminary studies, was
improved for virtual screening.
The screening sorted about 2,000 candidates of ER-binding
agents out of some 200,000 substances. The calculation technique
is under further refinement. Estimation of the relative binding
capacity with 176-estradiol is now being tested.
Kanno's group developed an in silico docking calculation
technique for ER6 and is now comparing the method involving ERΏ.
The background of the in silico technique presently used should
be mentioned here. Intensive preliminary investigation showed
that the docking technique, involving calculation of receptor-ligand
interaction, is preferable to various methods based on ligand
structure analysis and regression models, such as the commonly
used (by the U.S. EPA, for example) CoMFA technique.
The latter method is based on statistical analysis related to
ligand molecule geometry deduced from observation of ligand
activity. This permits examination of systems containing
receptors of unknown structure. On the other hand, the
predictive performance of the method depends on the "tutor"
compounds used to produce statistical data. In other words, it
is difficult to apply to molecules structurally dissimilar to
the tutor compounds.
In contrast, the docking technique needs no tutor compound
(although some tutors are used presently), and therefore has no
structural limits about ligands considered, although it has its
own limitations, such as the need for the knowledge of the
structure of the ligands, the complexity and incompleteness of
the theory used in calculating the interaction, and tedious
calculations. Comparison of computational predictions with
actual observations showed that this technique is a practical
one for the screening in which pseudo-positives are tolerated.