On the mechanism of action of SJ-172550 in inhibiting the interaction of MDM4 and p53

Abstract

SJ-172550 (1) was previously discovered in a biochemical high throughput screen for inhibitors of the interaction of MDMX and p53 and characterized as a reversible inhibitor (J. Biol. Chem. 2010; 285:10786). Further study of the biochemical mode of action of 1 has shown that it acts through a complicated mechanism in which the compound forms a covalent but reversible complex with MDMX and locks MDMX into a conformation that is unable to bind p53. The relative stability of this complex is influenced by many factors including the reducing potential of the media, the presence of aggregates, and other factors that influence the conformational stability of the protein. This complex mechanism of action hinders the further development of compound 1 as a selective MDMX inhibitor.

Figure 1. Structures of relevant compounds.

Panel A. Structure of SJ-172550 (1) and a non-reactive analog (2). Panel B. The potential mechanism of covalent adduct formation.

Figure 2. Formation of covalent adducts between compound 1 and MDMX.

Panel a. Mass spectrum arising from unmodified hMDMX (GST-tagged screening construct) showing unmodified mass of the protein. Panel b. Mass spectrum arising from treatment of 20 µM GST-hMDMX with 100 µM of compound 1 demonstrating multiple alkylation events. Note that 100 µM is well above the solubility limit of compound 1 and significant aggregation of compound exists. Panel c. Mass spectrum arising from treatment of 1 µM GST-hMDMX with 5 µM of compound 1 demonstrating no alkylation events. Panel d. Mass spectrum arising from unmodified hMDMX (untagged aa 23 to 111 construct) showing unmodified mass of the protein. Panel e. Mass spectrum arising from treatment of 20 µM hMDMX with 100 µM of compound 1 demonstrating partial alkylation. Panel f. Mass spectrum arising from treatment of 1 µM hMDMX with 5 µM of compound 1 demonstrating no alkylation.

Figure 3. Inhibition of MDMX-p53 peptide binding by compound 1 (IC₅₀ = 3 µM), compound 2 (IC₅₀>100 uM).

Figure 4. Variation in function of MDMX depending upon buffer conditions.

Panel a. SPR binding of hMDMX(23–111) to p53 peptide under non-reducing conditions. Note poor response indicating many of the protein molecules are not “active.” Panel b. The p53 binding curve generated in non-reducing conditions showing a K_d of 940 nM. Panel c. SPR binding of hMDMX(23–111) to p53 in the presence of 1 mM TCEP is much improved, indicating that more of the protein molecules are “active.” Panel d. The p53 binding curve generated under reducing conditions – the K_d is the same as that determined under non-reducing conditions.

Figure 5. Thermal stability equilibria of MDMX.

Panel a. Thermal shift data for MDMX (23–111) showing a 7 degree stabilization of the protein’s melting point by addition of compound 1. The panel shows individual data sampling points from 3 independent experiments from each condition. Panel b. Dose dependency and time dependency of the effect showing an apparent EC₅₀ of roughly 1 µM and minimal time dependency. Panel c. Dose dependent reversal of the effects of compound 1 by TCEP. Panel d. Dose dependent reversal of the effects of compound 1 by DTT.

Figure 6. Reversibility of the interaction of compound 1 with MDMX.

Panel a. SPR study of the binding of 1 (100 µM) to hMDMX (aa 23–111) under non-reducing conditions. While the off-rate is slow, the interaction is reversible. Panel b. SPR study of the binding of 1 (100 µM) to hMDMX (aa 23–111) under reducing conditions. No binding is observed.