in reactions of this type the biotransformation results in a loss of chirality, the reaction taking place at the stereogenic center. Examples of interest are provided by the 1,4-dihydropyridine calcium-channel blocking agents. e.g., nitrendipine and nilvadipine. These agents undergo cytochrome P450-mediated oxidation to yield the corresponding achiral pyridine analogs. In the case of nilvadipine this reaction is srereoselective for the + enantiomer in the rat, but for the - enantiomer in dog and humans. Reduction of the chiral sulfoxide moiety in NSAID prodrug sulindac results in the formation of the cyclooxygenase inhibiting achiral sulfide metabolite. Similarly, sulfoxidation of sulindac results in the loss of chirality yielding the sulfone metabolite, which has recently been shown to have antiproliferative activity and may prove to be useful in the treatment of some forms of colon cancer. Thus, both achiral metabolites of sulindac appear to possess useful biological activity.