Abstract B024: Combined inhibition of farnesyltransferase and MEK is effective in fusion-negative HRAS-mutant rhabdomyosarcoma

Abstract

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma of childhood, and RAS pathway mutations are the known driver mutations in the majority of fusion-negative (FN) RMS. Studies have demonstrated that HRAS mutations are enriched in infants with FN-RMS and can be associated with inferior outcomes. Using HRAS-mutant RMS models, we have demonstrated that tipifarnib (farnesyl transferase inhibitor, FTI) decreases ERK signaling and decreases in vitro and in vivo tumor growth. The effects of tipifarnib can be incomplete, and limitations may be due to adaptive or acquired resistance, or the presence of concurrent mutations that limit the response to single agents. These features suggest that HRAS-mutated FN-RMS may be sensitive to inhibition with combination therapy that prevents or delays the emergence of resistance. We examined the transcriptional effects of tipifarnib in FN-RMS cell lines using bulk RNA-sequencing to identify therapeutic vulnerabilities that may be exploited by RAS-directed therapies. Analysis of RNA sequencing data revealed downregulation of ERK transcriptional output genes upon treatment with tipifarnib, confirming the role of the MEK-ERK pathway in mediating the response to FTI. Trametinib (MEKi) inhibits tumor growth in xenograft models of FN-RMS but has only modest activity as a single agent. The efficacy of inhibition with FTI and MEKi has not been previously explored in RAS-driven FN-RMS. We therefore tested tipifarnib in combination with trametinib in HRAS-mutant FN-RMS cell lines and utilized in vitro assays to evaluate the effects of the combination on cell growth, differentiation and signaling via RAS effector pathways. In HRAS-mutant cells, we observed additive dose-dependent 2D and 3D growth inhibition in response to the combination. Further, tipifarnib and trametinib more potently reduced ERK phosphorylation than either drug individually, indicating effective RAS pathway inhibition. Additionally, we found that the combination of tipifarnib and trametinib induced myosin heavy chain expression, suggesting both inhibition of proliferation and promotion of myogenic differentiation. In an in vivo model, the combination of tipifarnib and trametinib led to tumor regression which was not seen with either drug alone. Our data suggest that the combination of tipifarnib and trametinib is active in models of HRAS-driven FN-RMS and may represent an effective therapeutic strategy for a genomically-defined subset of patients with FN-RMS. Citation Format: Patience Odeniyide, Alla Lisok, Elana J. Fertig, Alyza Skaist, Christine A. Pratilas. Combined inhibition of farnesyltransferase and MEK is effective in fusion-negative HRAS-mutant rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference: Targeting RAS; 2023 Mar 5-8; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Res 2023;21(5_Suppl):Abstract nr B024.