The eudysmic ratio (also spelled eudismic ratio) is a term commonly found in the fields of pharmacology, chemistry, and molecular biology which describes the difference in pharmacologic activity between the two enantiomers of a drug.
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In pharmacology it is frequently, though not always, the case that one enantiomer of a chiral drug has greater pharmacological activity than the other. The eudysmic ratio is a mathematical equation that quantitatively measures the difference in this activity. A eudysmic ratio significantly differing from 1 means that there is a difference in activity between the two enantiomers.
Terminology
The eutomer is the chiral enantiomer having the desired pharmacological activity, e.g., as an active ingredient in a drug.
The distomer, on the other hand, is the enantiomer of the eutomer which may have undesired bioactivity or may be bio-inert.
An equal mixture of both enantiomers is called racemate, which may be easier to manufacture than a single enatiomeric form.
It is often the case that only a single one of the enantiomers contains all of the wanted bioactivity, the distomer is often less active, has no desired activity or may even be toxic. In some cases the eudysmic ratio is so high that it is desired to separate out the two enantiomers instead of leaving it as a racemic product. It is also possible that the distomer is not simply completely inactive but actually antagonizes the effects of the eutomer. Alternatively, it is possible that the distomer converts in the body into the eutomer, at least partly.
Calculation
One way the eudysmic ratio is computed is by dividing the EC50 or the IC50 of the eutomer by the same measurement of the distomer. Whether one chooses to use the EC50 or IC50 depends on the drug in question.