Immune checkpoint blockade (ICB) is the current first-line treatment for metastatic melanoma. However, ICB fails in many patients and has dangerous side effects. Rigosertib (RGS), a non-ATP-competitive small molecule RAS mimetic, has the potential to block oncogenic RAS-RAF-MEK-ERK and/or PI3K-AKT-mTOR signaling pathways. Using immunocompetent mouse models of B16F10 (BRAFwt) and YUMM3.3 (BRAFmut) melanoma, we identified that RGS treatment (300mg/kg) of melanoma tumor-bearing mice is well tolerated and results in ~50% inhibition of tumor growth as monotherapy and ~70% inhibition in synergy with αPD1+αCTLA4. RGS-induced tumor inhibition depends on the induction of CD40 expression on melanoma cells, followed by immunogenic cell death, leading to an inflamed tumor microenvironment with enrichment of dendritic cells and activated CD8+ Tc cells. The RGS-initiated suppression of tumor growth was partially reversed by either suppression of CD40 expression in the melanoma cells by shRNA, or depletion of CD8+ Tc cells. Analysis of multiple published patient melanoma studies confirms that a high CD40 expression level correlates with CD80, ICOS-L, beneficial type-I T-cell responses, and better survival in melanoma patients. The CD40 levels in melanoma cells are prognostic for therapeutic response to RGS, RAF inhibitor and ICB. Notably, multiplex IHC analysis showed that BRAF inhibitor treatment significantly induces CD40+SOX10+ melanoma cells in the tumors of melanoma patients and patient-derived xenografts. Our preclinical data support the therapeutic use of RGS plus αPD1+αCTLA4 in RAS/RAF/MEK and/or PI3K pathway-activated melanoma tumors and point to the need for clinical trials to determine the clinical benefit of RGS plus ICB for metastatic melanoma patients who do not respond to ICB alone.

The authors sincerely thank Onconova Therapeutics, Newtown, PA 18940 for kindly supplying Rigosertib for this work.

Citation Format: Chi Yan, Nabil Saleh, Caroline Nebhan, Anna E. Vilgelm, E. Premkumar Reddy, Joseph T. Roland, Douglas B. Johnson, Rebecca L. Shattuck-Brandt, Sheau-Chiann Chen, Gregory D. Ayers, Ann Richmond. Heating it up: Targeting RAS/RAF/PI3K pathway to make melanoma tumors ‘immunologically hot' and suitable for checkpoint blockade immunotherapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1578.