It is sometimes observed that the enantiomers of a chiral drug may have opposite biological activity. One example of this is (-)-dobutamine, which is an agonist at a-adrenoceptors while (+)-dobutamine is an antagonist (Figure 5.9). However, (+)-dobutamine is 10-fold more potent than the (-)-isomer as a pi-adrenoceptor agonist and is used to treat cardiogenic shock. The individual enantiomers of the 1,4-dihydropyridine analogue BayK8644 (Figure 5.9) have opposing effects on L-type calcium channels with the (S)-enantiomer being an activator and the (R)-enantiomer being an antagonist.
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