Confirmation by conformation: rescue of mutant p53 in AML Free

Somatic mutations in TP53 are associated with aggressive forms of acute myeloid leukemia (AML) that are resistant to treatment. In this issue of Blood, Carter et al¹ present findings that indicate that PC14586 (rezatapopt), a small molecule designed to target mutant p53, restores p53 wild-type conformation in TP53-Y220C–mutant AML.

Mutations in TP53 are the most common specific events in human cancer, and when they occur in AML, they are associated with an aggressive, treatment-resistant disease with poor outcomes.² PC14856 is the first clinically active drug reported to restore functional activity to a specific mutant p53, Y220C, by binding to and stabilizing its DNA binding active conformation.^3,4 The compound represents a genuine breakthrough in the field. Other drugs that act on the p53 pathway that have entered the clinic include compounds such as APR-246 that claim a more general ability to rescue a diverse set of mutant proteins⁵ and compounds that inhibit Mdm2, the negative regulator of p53, exemplified by idasanutlin (RG7388).⁶ Unfortunately, none of these compounds have been approved despite extensive trials.

The extreme specificity of PC14856 for the mutant protein^3,4 was again shown by Carter et al using panels of isogenic AML cell lines. Cells that contained wild-type p53 or other mutant alleles of p53 that are common in malignancies, such as R175H, were not affected by the drug, whereas all cells that contained the Y220C mutant form of the protein showed evidence of growth inhibition. Structural and biophysical studies of the drug-protein complex showed that this was because the compound bound to a specific pocket in the DNA binding region of the mutant p53 that is uniquely created by the mutation of tyrosine to cysteine. Although this specificity has the potential to mitigate adverse effects, it concomitantly restricts the number of patients eligible for treatment to a relatively small subset of patients with TP53-mutant AML.⁷ 

What constraints might limit the effectiveness of this new class of drug? In a surprising and important set of observations, the authors established that the rescued protein lacks the full functions of wild-type p53. Although AML cells that express wild-type p53 are induced to undergo growth arrest and apoptosis by the activation of p53 using the Mdm2 inhibitor nutlin, AML cells with PC14586-rescued, Y220C-mutant p53 undergo growth arrest but do not undergo apoptosis. The nature of this block against apoptosis induction is not fully resolved by the authors; however, it correlates with an inability of the rescued protein to directly interact with members of the BCL2 family of proteins. Although the wild-type protein binds BCL2, BCL-xL, and MCL-1, thereby promoting apoptosis, the rescued Y220C protein seems unable to do this. This new finding should be studied closely. The structure of the mutant proteins is restored, as shown by specific antibodies, and yet some function is missing. Could it merely be a quantitative effect? The authors used both Mdm2 inhibitors and nuclear export inhibitors to raise the level of the rescued p53, and although these drugs enhance its activity as a transcription factor and growth inhibitor, they do not restore its BCL-2 binding function.

The mentioned observations raise 2 very important questions. First, what is the biochemical basis of this apparent defect? Second, can the partial rescue of the wild- type function be sufficient for efficacy in AML? The authors showed that by combining PC14856 with venetoclax, a chemical Bcl2 inhibitor, they can see clear preclinical efficacy. That depends on the synergy between the compounds and such a combination trial is underway (ClinicalTrials.gov identifier: NCT06616636). However, the most important results may be in providing a system that fully explains the details of this long-described, somewhat mysterious, nontranscriptional function of p53.^8,9 A key question for the broader field is whether the defect in the induction of apoptosis by the rescued mutant protein will also affect its ability to induce other therapeutic mechanisms, such as immune sensitization, senescence, and ferroptosis, which may be important in clearing both myeloid and nonmyeloid malignancies. The future, nevertheless, is bright with many new chemical entities¹⁰ that emerge with almost antibody-like specificity and that may enable the modulation of the conformation of many mutant p53s and even wild-type p53 proteins to enhance their activity. Based on a solid history of clinical studies, these “super-rescued” p53s might finally make the tumor suppressor protein’s therapeutic potential a reality.

Conflict-of-interest disclosure: D.P.L. is the nonexecutive chairman of Chugai Pharmabody Research Pte Ltd, Singapore and the founder of Parabilis Medicines.