Abstract A08: Targeting HSP90 epichaperome in TP53 mutant AML

The Bcl-2 inhibitor venetoclax (VEN)/hypomethylation agent (HMA) combination achieves high response rates, has improved outcomes for many patients with AML and is now considered standard of care for patients who are older or unfit to receive intensive chemotherapy. However, the median overall survival is only 14.7 months on this regimen and only 2.5 months post relapse. Molecular analysis demonstrates that mutations in TP53 and oncogenic kinases are key determinants of lower response rates and early relapse. Preclinical studies also show that increased kinase signaling in AML stem/progenitor cells in TP53 mutant AML. The heat shock protein 90 (HSP90) chaperone, a key regulator of proteostasis, is responsible for the correct folding of kinases and transcription factors. HSP90-associated-epichaperomes, formed in malignant cells, are complexes consisting of HSP90, co-chaperones, and associated proteins that support the maturation, activity, and stability of many cancer-associated kinases and transcription factors including mutated TP53. Hence, HSP90 epichaperome inhibition has the potential of targeting TP53 mutant AML. In contrast to other HSP90 ATP inhibitors, PU-H71 (zelavespib) is a competitive inhibitor specific for the ATP binding site of HSP90 epichaperomes. We here investigate the therapeutic potentials of targeting HSP90 epitherachorme with PU-H71 in TP53 mutated AML. Western blot analysis found increased HSP90 and several signaling proteins in TP53 knockout and mutant Molm13 cells generated by CRISPR/cas-9 or by exposing to idasanutlin, compared to the isogeneic wild-type controls. Using a fluorochrome-labelled PU-H71 and flow cytometry, we demonstrate the presence of HSP90 epichaperomes in AML cells and AML stem/progenitor cells with TP53 mutations, but not in normal bone marrow and bone marrow stem/progenitor cells. PU-H71 effectively kills AML cells and AML stem/progenitor cells with various TP53 mutations, and prolongs survival in TP53-mutant AML xenograft mice with minimal effects on normal CD34+ bone marrow cells and hematopoiesis. PU-H71 increased Bim expression and enhanced VEN activity in AML cells and AML stem/progenitor cells with TP53 mutations. Importantly, in a mixture of TP53 wild-type/R248W Molm13 cells (1000:1), nutlin3a selectively killed TP53 wild-type but enriched TP53 mutant Molm13 cells; VEN treatment favored the outgrowth of TP53-mutant cells, while PU-H71 effectively killed TP53 wild-type and mutant cells. Furthermore, PU-H71 exhibited anti-leukemia activity against both TP53 WT and mutant AML cells, which was further enhanced by VEN in vivo in a xenograft model of mixed TP53 WT and mutated Molm13 cells (10:1). Our data support that the HSP90 epichaperome is essential for the growth and survival of AML and AML stem/progenitor cells harboring mutant TP53. Inhibition of HSP90 by PU-H71 targets AML cells/stem/progenitor cells enhances VEN activity and prevents outgrowth of VEN-resistant TP53 mutant AML cells. This concept warrants clinical evaluations. Citation Format: Bing Carter, Po Yee Mak, Wenjing Tao, Lauren B Ostermann, Yuki Nishida, Steffen Boettcher, Michael Andreeff. Targeting HSP90 epichaperome in TP53 mutant AML [abstract]. In: Proceedings of the AACR Special Conference: Acute Myeloid Leukemia and Myelodysplastic Syndrome; 2023 Jan 23-25; Austin, TX. Philadelphia (PA): AACR; Blood Cancer Discov 2023;4(3_Suppl):Abstract nr A08.