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Background BRAF inhibitors (BRAFi) have significantly improved outcomes in BRAF V600E mutant melanoma, yet resistance often develops, largely due to an immunosuppressive tumor microenvironment (TME). Activation of the STING (Stimulator of Interferon Genes) pathway represents a promising strategy to restore innate immune responses and overcome this resistance. STING agonists can trigger type I interferon signaling and enhance natural killer (NK) cell activity. We hypothesized that combining the STING agonist ADU-S100 with the BRAFi vemurafenib would synergistically augment NK cell cytotoxicity and promote a more immunogenic TME.Methods Three human BRAF V600E mutant melanoma cell lines (SKMEL24, HTB66, SH4) were treated with vehicle, ADU-S100, vemurafenib, or the combination. NK cell-mediated cytotoxicity was assessed using co-culture assays with primary human NK cells (1:1 tumor:effector ratio), followed by Ethidium homodimer staining and quantification via ImageXpress at 48 hours. Cytokine profiling was conducted using a 60-plex Luminex assay to characterize immune shifts across treatment conditions.Results Combination therapy significantly enhanced NK cell-mediated cytotoxicity across all three melanoma cell lines, resulting in a marked increase in tumor cell death at 48 hours compared to monotherapy or control groups ( figure 1). Cytokine profiling revealed substantial remodeling of the TME. B2M, a molecule essential for antigen presentation and known to predict immune checkpoint inhibitor (ICI) response, was consistently upregulated in the combination group. IL-8, a pro-inflammatory cytokine associated with immunosuppression and metastasis, was significantly downregulated. VEGF, a key driver of angiogenesis and a contributor to ICI resistance, was also markedly decreased. Notably, CXCL13—a chemokine associated with tertiary lymphoid structure formation and favorable immunotherapy response—was upregulated, while CXCL5, implicated in melanoma invasiveness, was suppressed (figure 2). These findings together support a shift toward a more immunostimulatory and anti-tumor TME following combination treatment.Conclusions STING pathway activation via ADU-S100 synergizes with BRAF inhibition to both enhance NK cell cytotoxicity and reprogram the melanoma tumor microenvironment in a direction favorable for immune response. This study is the first to demonstrate enhanced NK cell killing following STING agonist treatment in BRAF V600E mutant melanoma. These findings provide a strong rationale for advancing STING agonist-BRAFi combinations as a novel immune-enhancing strategy to overcome therapeutic resistance and improve outcomes in targeted therapy-resistant melanoma.Abstract 913 Figure 1IMAge of SH4 co culture with NK cell showing death(red). Combination of ADU & Vemurafinib increase NK mediated cytotoxicity. Above shows co culture of SH4 tumor cells (Blue) with NK cells( green) 1:1 Ratio at 48 h time point with ethidium homodimer red dye The synergistic increase in NK cytotoxic effect was seenAbstract 913 Figure 2Favorable shift in cytokines after combination therapy in different BRAFV6OOE Mutants. Full change of cytokines in SKMEL24,HTB66 &SH4 in different condition of control, ADU monotherapy, Vemurafinib Monotherapy & combination therapy. B2M, a key cytokine that low level is known to correlates with Immune checkpoint inhibitor resistance, is upregulated in combination treatment compared to control in all cell lines. IL8: key cytokine in melanoma metastasis level decreased in combination group significantly when compared to control. VEGF level reduction compared to control observed across all cell lines the favorable change in CXCL13/CXCL5 (increase/decrease) which is associated with melanoma progression and responsiveness to targeted immune therapy was observed across all cell line when compared to base levels