Nutlin-3a is the active enantiomer of Nutlin-3, an MDM2 antagonist that inhibits MDM2-p53 interaction (Ki=90 nM) and activates p53. Nutlin-3a binds preferentially to the p53-binding pocket of MDM2, leading to stabilization of p53 and activation of the p53 pathway. Nutlin-3a has antitumor activity.