ENHANCING EXPRESSION OF PRO-APOPTOTIC FACTORS THROUGH INHIBITION OF HDACS DRIVES P53-INDEPENDENT SYNERGY WITH VENETOCLAX FOR THE TREATMENT OF AML

Targeting BCL-2 with the BH3-mimetic venetoclax (VEN) has emerged as a highly efficacious treatment for elderly patients with acute myeloid leukaemia (AML), in combination with either low-dose Ara-C (Wei, JCO 2019) or hypomethylating agents (Di Nardo, Lancet Oncol 2018). Long-term survival outcomes remain poor among patients with adverse risk karyotype, who often harbour mutations in the TP53 gene. Therefore, identification of effective treatment options, with TP53 independent activity, represents an area of high unmet therapeutic need. We generated TP53-null AML cell lines using CRISPR-Cas9 gene editing and screened these cell lines against 48 compounds. The histone deacetylase inhibitors (HDACi) had substantial efficacy. Additional studies performed in factor dependent myeloid (FDM) cells demonstrated cell death from HDACi to be Bax/Bak dependent, but Bim and p53 independent. RNA-sequencing, following HDACi treatment, revealed a rapid upregulation of pro-apoptotic factors, including PMAIP1 (NOXA) and BBC3 (PUMA). These results indicate that treatment with HDACi upregulates pro-apoptotic factors, independently of P53, and led to our hypothesis that HDACi could sensitise tumor cells to BH3-mimetics. We then tested the activity of HDACi in combination with BH3-mimetics targeting BCL-2 against 61 primary AML patient samples. Overall, LC50 at 48h was reduced by >2 logs in 60% of cases. Importantly, TP53 mutant cases were sensitive to this combination. Patient-derived xenograft models confirmed that this combination was tolerable and able to suppress bone marrow AML in vivo. Our data supports the clinical use of combined HDAC and BCL-2 targeting for AML and represents a promising treatment option for patients with adverse risk genomic characteristics; in particular, those who have failed VEN treatment in combination with low dose chemotherapy, and cases defective in P53.