Reactivation of p53 in TP53-Y220C Mutant AML: Mechanism of Action and Mechanism-Based Drug Combinations

TP53 is the most frequently mutated gene in cancer, and until recently, no agents that selectively re-activate specific mutant p53 have been developed. Y220C is a recurrent hotspot TP53 mutation found in various tumors. In hematological malignancies, it is observed predominantly in AML and MDS and associated with poor outcomes due to limited therapeutic options. In a cohort of patients at MD Anderson, Y220C was the most frequent TP53 mutation in MDS and the third most frequent in AML (Gener-Ricos G et al., Leuk Lymphoma 2024). PC14586 (rezatapopt, PMV Pharmaceuticals) is a small molecule designed to fill the structural crevice in the p53-Y220C mutant protein and restore wild-type (WT) p53 conformation and transcriptional activity. We previously showed that PC14586 stabilized Y220C mutant p53 in the active form and selectively induced p53 downstream targets, including MDM2 and p21 in TP53-Y220C AML cells. In this experimental setting, PC14586 as a single agent decreased cell viability but had limited apoptogenic activities in TP53-Y220C AML cells (Carter B et al., ASH 2023). Here, we characterize transcription-dependent and -independent functions of PC14586-reactivated p53.

We hypothesized that MDM2, induced by PC14586-reactivated p53, limits p53 activity and that MDM2 inhibition will stabilize reactivated p53 and enhance its apoptogenic activity in TP53-Y220C cells. To test the hypothesis and better understand the underlying mechanisms, we treated TP53-WT and Y220C Molm13 cells with PC14586, the MDM2 inhibitor nutlin3a, or both. As expected, PC14586 had no activity in WT cells, while nutlin3a had no activity in Y220C cells, and PC14586 activated p53 transcriptional activity in Y220C cells but to a lesser degree than nutlin3a activated p53 in WT cells. PC14586 and nutlin3a, in combination, greatly increased p53 transcriptional activity in TP53-Y220C cells, but did not further induce p53 activity in WT cells. These results were further validated at the protein level. The combination of PC14586 and nutlin3a synergistically induced cell death in TP53-Y220C Molm13 cells, primary patient samples, and PDX cells that did not respond to PC14586 as a single agent, significantly prolonging the survival of NSG mice harboring the PDX cells (p=0.0041).

To understand why nutlin3a by itself induces apoptosis in TP53-Y220C Molm13 cells, but PC14586 alone lacks apoptogenic activity in these cells, we investigated if PC14586-reactivated p53 regains the transcription-independent activity to bind to Bcl-2 (Wei H et al., Nat Communication 2023). We discovered that nutlin3a-activated WT p53 co-precipitates with Bcl-2, Bcl-xL, and Mcl-1; while PC14586-reactivated p53 did not bind to Bcl-2, Bcl-xL, or Mcl-1. Consequently, PC14586 alone is unable to displace enough Bax from antiapoptotic proteins to induce apoptosis. PC14586, in combination with venetoclax (VEN), can activate Bax and synergistically induced cell death in TP53-Y220C Molm13, primary patient samples, and PDX cells that did not respond to PC14586 as a single agent, and prolonged survival in Molm13 TP53-Y220C xenografted mice (p=0.0026).

In summary, we demonstrate for the first time that MDM2 induced by PC14586-reactivated p53 restrains p53 transcriptional activity. In addition, PC14586-reactivated p53 does not bind antiapoptotic Bcl-2 proteins, resulting in impairment of transcription-independent apoptosis. Both contribute to the diminished apoptogenic activity of PC14586 in TP53-Y220C AML cells, which can be overcome by combinations of PC14586 with MDM2 or/and Bcl-2 inhibitors. Clinical trials of PC14586 with VEN or VEN/HMA combinations are planned in TP53-Y220C AML/MDS.