In this case the distomer is either inactive or displays no undesirable side effects. In the case of the antihypertensive agent (ß -blocker) propranolol (Figure 5.6) the (S)-enantiomer is 130-fold more potent than the (R) enantiomer as a ß-adrenoceptor antagonist (i.e., ER = 130). A number of other ß-blockers based on this structure show high ERs. These drugs are therefore marketed as racemates as the distomer displays no side effects. Despite this there would have been advantages in marketing the (S)-enantiomer if only to extend patent life.
FIGURE 5.6 The ER for (S)- and (R)-propranolol is 13C.
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