One Enantiomer Is Converted into the Other in the Body

The interconversion of the enantiomers of thalidomide under physiological conditions has already been discussed. Another group of drugs, which are known to exhibit this phenomenon, are the a-arylpropionic acids, which are nonsteroidal anti-inflammatory drugs (NSAIDs). These drugs are used to treat rheumatoid arthritis and as analgesics. It is known in the case of naproxen and ibu-profen (Figure 5.11) that the desired activity resides in the (S)-enantiomer while the (^)-enantiomer undergoes metabolic inversion to the (S)-enantiomer. Ostensibly, these drugs are safe to give in the racemic form as the distomer is converted to the eutomer in the body, however, it is known that in the course of the metabolism of the (R)-enantiomer, ibuprofen accumulates in fatty tissue in the body. The (S)-enantiomer is not metabolized in the same fashion and therefore marketing the biologically active (S)-enantiomer could be advantageous.

(S)-ibuprofen (eutomer) (S)-Naproxen (eutomer)

FIGURE 5.11 Examples of drugs where one enantiomer is converted into the other in the body.

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