Chemical Instability and Promiscuity of Arylmethylidenepyrazolinone-Based MDMX Inhibitors

Targeting
the protein–protein interaction between p53 and
MDM2/MDMX (MDM4) represents an attractive anticancer strategy for
the treatment of p53-competent tumors. Several selective and potent
MDM2 inhibitors have been developed and entered the clinic; however,
the repertoire of MDMX antagonists is still limited. The arylmethylidenepyrazolinone
SJ-172550 has been reported as a selective MDMX antagonist; yet, uncertainties
about its mechanism of action have raised doubts about its use as
a chemical probe. Here, we show that, in addition to its unclear mode
of action, SJ-172550 is unstable in aqueous buffers, giving rise to
side products of unknown biological activity. Using an SJ-172550-derived
affinity probe, we observed promiscuous binding to cellular proteins
whereas cellular thermal shift assays did not reveal a stabilizing
effect on MDMX. Overall, our results raise further questions about
the interpretation of data using SJ-172550 and related compounds to
investigate cellular phenotypes.