Novel induction of CD40 expression by tumor cells with RAS/RAF/PI3K pathway inhibition augments response to checkpoint blockade

Background: While immune checkpoint blockade (ICB) is the current first-line treatment for metastatic melanoma, it is effective for similar to 52% of patients and has dangerous side effects. The objective here was to identify the feasibility and mechanism of RAS/RAF/PI3K pathway inhibition in melanoma to sensitize tumors to ICB therapy. Methods: Rigosertib (RGS) is a non-ATP-competitive small molecule RAS mimetic. RGS monotherapy or in combination therapy with ICB were investigated using immunocompetent mouse models of BRAF(wt) and BRAF(mut) melanoma and analyzed in reference to patient data. Results: RGS treatment (300 mg/kg) was well tolerated in mice and resulted in similar to 50% inhibition of tumor growth as monotherapy and similar to 70% inhibition in combination with alpha PD1 + alpha CTLA4. RGS-induced tumor growth inhibition depends on CD40 upregulation in melanoma cells followed by immunogenic cell death, leading to enriched dendritic cells and activated T cells in the tumor microenvironment. The RGS-initiated tumor suppression was partially reversed by either knockdown of CD40 expression in melanoma cells or depletion of CD8(+) cytotoxic T cells. Treatment with either dabrafenib and trametinib or with RGS, increased CD40(+)SOX10(+) melanoma cells in the tumors of melanoma patients and patient-derived xenografts. High CD40 expression level correlates with beneficial T-cell responses and better survival in a TCGA dataset from melanoma patients. Expression of CD40 by melanoma cells is associated with therapeutic response to RAF/MEK inhibition and ICB. Conclusions: Our data support the therapeutic use of RGS + alpha PD1 + alpha CTLA4 in RAS/RAF/PI3K pathway-activated melanomas and point to the need for clinical trials of RGS + ICB for melanoma patients who do not respond to ICB alone.