Enantiomers are the optical isomers which are non-superimposable mirror images to each other. They are also known as chiral molecules. It possess identical physical properties except rotation of plane polarized light in different directions and same chemical properties except when other chiral molecule is involved.
Example: (R)-1-Bromo-1-Chloroethane and (S)-1-Bromo-1-Chlorobutane
Enantiomers can have more than one chiral centres.
More than 500 currently useful drugs are racemic mixtures containing an active drug and its enantiomers in equal properties. Approximately 30% of marketed drugs are sold as a single isomeric form.
The enantiomer of the drug that we use for the specific disease has an effect on our receptors, and its other enantiomer will be totally inactive or may cause some other effect.
For e.g. one enantiomer of Thalidomide was used for morning sickness in pregnant women, its enantiomer caused abnormalities in new born babies. The S-isomer was used as sedative to calm nervousness and R-isomer is teratogen and caused birth defects in infants.
The anti-inflammatory drug naproxen is marketed as S-isomer but the R-isomer is liver toxin.
The (S,S)- form of the anti-tuberculosis drug ethambutol is active while (R,R)- form caused optical neuritis that can lead to blindness.