Abstract B020: Sensitization of acute myeloid leukemia cells to the antileukemic activity of all- trans retinoic acid (ATRA) by decitabine is p53-independent

Abstract Background Our randomized phase II trial “DECIDER” showed that the addition of ATRA to decitabine (DAC) significantly improves overall survival in newly diagnosed elderly AML pts (Lübbert et al., J Clin Oncol. 2020). Subgroup analyses revealed that half of the TP53 mutated (MUT) pts responding to DAC+ATRA (DA) lived for >3 years (Bresser et al., Eur J Haematol. 2024, Thomas, Rehman et al., Clin Epigen. 2024). This notable in vivo cooperativity requires elucidation. Methods We employed a MOLM13 isogenic model of TP53+/+, R175H/-, M237I/-, R248Q/-, R273H/-, R282W/-, and TP53-/- clones treated with 100nM DAC by 3 daily pulses, 250nM ATRA on day 4, and studied for proliferation, viability, cell cycle (EdU, Ki67), differentiation (CD11b) at 24h, 48h and 72h post ATRA. RNA-sequencing was performed on TP53+/+ and TP53-/- with results used for gene set enrichment analyses (GSEA) on ‘Hallmark pathways’. Results DA significantly reduced proliferation, induced differentiation and apoptosis in TP53 isogenic model compared to the single agents. The global transcriptomic profile of DA- treated TP53+/+ and TP53-/- cells was distinct from single-agent treatments, as the majority of DEGs were unique to the combination, hence indicating that DAC sensitizes cells to ATRA’s effect. GSEA shows that DA activates the TP53 pathway, as most proliferative pathways were depleted in a p53-dependent manner. Interestingly, ‘Hallmark MYC target genes’ along with MYC transcript, were depleted in both clones, hence pointing towards MYC as an important downstream target for DA, independent of p53. DA treatment significantly reduced MYC+ population in TP53+/+ ( by 48%), R175H/- (55%), M237I/- (25%), R248Q/- (65%), R273H/- (32%), R282W/- (65%), and TP53-/- (25%) clones respectively, as early as 24 hours post ATRA, before any significant cell cycle inhibition (Ki67) was noted. DA increased pMYCT58 and global K48 ubiquitination compared to single agent, pointing towards increased proteasomal degradation. Loss of MYC in cells was followed by loss of Ki67. Heterogeneous and clone-dependent trends towards differentiation or apoptosis were observed in MYC- Ki67- populations. R248Q/- and R282W/- clones were more susceptible to apoptosis, while clones M237I/- and R175H/-, resistant to apoptosis, were susceptible to differentiation. R273H/- clone was vulnerable to apoptosis post differentiation. Conclusions The addition of ATRA to DAC enhances proteasomal degradation of MYC, resulting in reduced proliferation, enhanced apoptosis and increased differentiation in the TP53 isogenic model, with notable differences between the mutated clones studied. Functional association of DA with MYC reduction and validation of primary DECIDER samples with single cell omics is ongoing. Our results support the repurposing of ATRA for treatment of TP53 MUT AML pts. This is pursued in an ongoing phase III, placebo-controlled trial (DECIDER-2, accruing newly diagnosed AML pts with both TP53 WT and TP53 MUT genotypes) with a DAC+VEN backbone and an ATRA vs. placebo randomization. Citation Format: Usama-Ur Rehman, Steffen Boettcher, Michael Lübbert. Sensitization of acute myeloid leukemia cells to the antileukemic activity of all- trans retinoic acid (ATRA) by decitabine is p53-independent [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Methylation, Clonal Hematopoiesis, and Cancer; 2025 Feb 1-4; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2025;85(3 Suppl):Abstract nr B020.