Abstract IA011: Targeting the Palmitoylation Cycle in NRAS -Mutant Cancers

Abstract NRAS mutations are prevalent in acute myeloid leukemia (AML), juvenile myelomonocytic leukemia (JMML), melanoma, and thyroid cancer. Ras proteins are dependent on post-translational lipid modifications. While K-Ras4b farnesylation is sufficient for stable association with the plasma membrane (PM), farnesylated H-Ras, K-Ras4a, and N-Ras undergo palmitoylation before regulated translocation to the PM. N-Ras palmitoylation by palmitoyl acyl transferases and depalmitoylation by ABHD17 serine hydrolasess is a dynamic process. We hypothesized that co-targeting ABHD17 enzymes and the mitogen activated protein kinase (MAPK) pathway would exhibit chemical synthetic lethality in NRAS-mutant AML. ABD957 is an in vitro active ABHD17 inhibitor synthesized and characterized by Drs. Micah Niphakis (Lundbeck, Inc) and Benjamin Cravatt (Scripps Research Institute) that reduced the growth of NRAS mutant AML cells and was synergistic with the MEK inhibitor PD0325901 (PD901) (Remsberg et al, Nat Chem Biol 2021; 8:856-864). ABD778 is an analog of ABD957 with drug-like pharmacologic properties. ABD778 blocked N-Ras depalmitoylation in pulse-chase experiments (EC50 ∼53 nM), showed selective target engagement of ABHD17 enzymes, and induced sustained ABHD17 inhibition after in vivo dosing. ABD778 showed nanomolar potency - but incomplete growth inhibition - in NRAS-mutant AML cell lines (Emax ∼50%) but had no effect on KRAS-mutant AML cells. ABD778 and PD901 synergistically inhibited the growth of NRAS-mutant AML cell lines, which correlated with MAPK suppression. We confirmed these data in isogenic MOLM-14 AML cell line models that are dependent on oncogenic N-Ras or K-Ras proteins. Biochemical and BRET analyses support a working model whereby ABD778 reduces oncogenic N-Ras signal output and blunts the adaptive feedback response of RAS-mutant cancer cells to MEK inhibition. ABD778 also potently and selectively inhibited myeloid progenitor colony growth from primary NRAS-mutant JMMLs. ABD778 modestly extended the survival of mice transplanted with primary mouse AMLs harboring an Nras G12D driver mutation. Recipients of these Nras-mutant AMLs treated with ABD778 and PD901 had twice the median survival of mice dosed with PD901 alone (40 versus 20 days, p