A "racemic switch" is the redevelopment in single-enantiomer form of a drug that was first approved as a racemate. Sometimes, the pharmaceutical activity is in only one enantiomer and the other is inactive, or the "other" enantiomer has a different kind of activity from the first.
Omeprazole is a Antiulcer drug (AstraZeneca) .It was marketed in U.S. as a racemic drug in 1995. The patent ran out in 2002. Since pharmacological property lies in (S)-enantiomer, the company has patented now (S)-enantiomer.