Activation of p53 tumor suppressor by antagonizing its negative regulator murine double minute (MDM)2 has been considered an attractive strategy for cancer therapy and several classes of p53-MDM2 binding inhibitors have been developed. However, these compounds do not inhibit the p53-MDMX interaction, and their effectiveness can be compromised in tumors overexpressing MDMX. Here, we identify small molecules that potently block p53 binding with both MDM2 and MDMX by inhibitor-driven homo-and/or heterodimerization of MDM2 and MDMX proteins. Structural studies revealed that the inhibitors bind into and occlude the p53 pockets of MDM2 and MDMX by inducing the formation of dimeric protein complexes kept together by a dimeric small-molecule core. This mode of action effectively stabilized p53 and activated p53 signaling in cancer cells, leading to cell cycle arrest and apoptosis. Dual MDM2/MDMX antagonists restored p53 apoptotic activity in the presence of high levels of MDMX and may offer a more effective therapeutic modality for MDMXoverexpressing cancers.T he tumor suppressor p53 is a powerful growth-suppressive and proapoptotic protein tightly controlled by its negative regulators: murine double minute (MDM)2 and MDMX (1, 2). These proteins bind p53 with their structurally similar N-terminal domains and effectively inhibit p53 transcriptional activity (1, 3). They both possess a RING (really interesting new gene) domain in their C termini, but it is only functional in MDM2, which serves as a specific E3 ligase and main regulator of p53 stability (4, 5). Despite its RING domain, MDMX does not have an intrinsic ligase activity and does not affect directly p53 stability (6). However, MDMX can enhance ligase activity of MDM2 toward p53 by forming MDM2/MDMX heterodimers (7,8). It has been reported that the MDM2/MDMX complex is responsible for polyubiquitination of p53, whereas MDM2 alone primarily induces monoubiquitination (9). Targeted disruption of MDM2/MDMX heterocomplexes is embryonic-lethal in mice, suggesting that complex formation is essential for p53 regulation in vivo (10). On the other hand, MDM2 can also ubiquitinate MDMX and is, therefore, responsible for its stability as well (11,12). MDM2 is a transcriptional target of p53, and both proteins form an autoregulatory feedback loop by which they mutually control their cellular levels (13).The functional relationship between MDM2 and MDMX is still being refined at the molecular level, but it is well established that these two negative regulators play a critical role in controlling p53 tumor-suppressor function in normal cells (2,14). This is why they are frequently overproduced through gene amplification and/or overexpression in tumors that retain wildtype p53 (14). Therefore, antagonizing the binding of MDM2 and MDMX to p53 is expected to restore p53 function and may offer a strategy for cancer therapy (15). Recently identified small-molecule inhibitors of the p53-MDM2 interaction have validated this approach, and the first pharmacological MDM2 antagonists ar...