Keep in mind that in most cases, one drug enantiomer is much more potent than the other, reflecting a better 'fit' to the receptor molecule
For example, the dextrorotatory isomers
(the (+) enantiomers
) in the morphine series of drugs, such as dextromethorphan, are cough suppressants that possess the anti-tussive properties of codeine (which has a levorotatory, (-)
, stereochemistry), without the analgesic, addictive, central depressant, and constipating features
exhibited by the (-) form
. The (+)
form simply does not bind to the receptors involved in analgesic, constipative, addictive, and other actions exerted by the (-)
form. As a result, dextromethorphan
has largely replaced many older antitussives
, including codeine
, in prescription and nonprescription cough preparations.
- Another example is the local anesthetic levobupivacaine, which is the (S) isomer of bupivacaine. Bupivacaine is available on the market; it is the racemic mixture of the (R) and (S) isomers. Both the (R) and (S) isomers have good local anesthetic activity, but the (R) isomer causes
- myocardial depression and ventricular arrhythmias. In contrast, the (S) isomer shows much less cardiotoxicity. Consequently, the pure (S) isomer of bupivacaine is now available on the market because of its superior safety profile.