The active isomer of a drug with the desired biological effect is called the eutomer, whilst the less active isomer is called the distomer.
Eudismic ratio
is defined as the ratio of the biological activity of the eutomer to that of the distomer.
Individual stereoisomers of a chiral drug should be tested as well as the racemate before a drug is marketed. This is because the presence of the distomer in a racemic drug can have important consequences on the biological activity.
For example - the eutomer may be active whilst the distomer may be inactive
- the distomer may possess harmful side effects e.g. thalidomide.
- the distomer may be converted into the eutomer by the body e.g. ibuprofen.
Three-point attachment theory
: only one enantiomer of a drug can form three intermolecular bonds with the
receptor, this explains drug selectivity.
The drug must adopt a particular orientation to interact with the receptor.
