Abstract 1262: Depletion of dominant-negative mutant p53 improves the efficacy of mutant p53 reactivators.

Abstract TP53 (p53) forms a tetramer and functions as a transcription factor to upregulate mRNA expression of genes involved in cell cycle progression and apoptosis, thereby functioning as a tumor suppressor. P53 is the most commonly mutated gene in human cancers, and approximately 75% of p53 mutations are missense mutations. When p53 is mutated, the mutant p53 (mutp53) is no longer able to bind DNA, losing its transcriptional and tumor suppressor activities. Missense mutp53 is roughly classified as DNA contact mutants with a relatively intact p53 structure and conformation or structural mutants with a misfolded p53 structure. This cancer-specific missense mutp53 represents a critical target for the development of innovative precision therapies. Mutp53-targeting reagents include “depleters” that suppress mutp53-addicted tumors by reducing mutp53 protein levels and “reactivators” or “stabilizers” that restore the transcriptional activity of p53. Depleters include HSP90 inhibitors that induce degradation of both DNA contact and conformational mutp53, as well as statins or HSP40/DNAJA1 inhibitors that primarily deplete conformational mutp53. Reactivators include APR-246 (PRIMA1met) and arsenic trioxide (ATO). While APR-246 and ATO have been in several clinical trials, successful outcomes have not yet been reported, despite the evidence that these reactivators efficiently restore the intact conformation of p53 in culture. Thus, it remains unclear how to efficiently reactivate p53 in mutp53-expressing cancers and why the reactivated p53 cannot efficiently induce tumor suppression. We hypothesize that reactivated p53 (R-p53) is dominant-negatively bound by non-reactivated mutp53 (NR-mutp53), making the reactivator not fully effective; hence, the combination of reactivators with depleters to eliminate NR-mutp53 would improve the treatment efficacy of reactivators, leading to improved tumor suppression. To test this hypothesis, we used conformation-specific antibodies detecting NR-mutp53 and R-p53 to confirm their interactions in multiple cancer cell lines with conformational and DNA contact mutp53. Our study revealed that NR-mutp53 interacted with R-p53 in cells treated with the reactivator alone, while this interaction was eliminated when reactivators were combined with depleters. Moreover, our RT-PCR studies demonstrated that the combination of reactivator with depleters significantly increased mRNA expression of p53 downstream targets, as compared to that of the reactivator treatment alone. These results strongly suggest that the combination of reactivator drugs with depleter drugs could improve the treatment efficacy of reactivators to suppress mutp53-carrying tumors. Citation Format: Allison St. John, Mrinalini Bhosale, Alejandro Parrales, Atul Ranjan, Tomoo Iwakuma. Depletion of dominant-negative mutant p53 improves the efficacy of mutant p53 reactivators [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1262.