Combination of a STAT3 inhibitor with anti-PD-1 immunotherapy is an effective treatment regimen for a vemurafenib-resistant melanoma.

Patients with BRAF-mutant melanoma are effectively treated with the BRAF-inhibiting drug, vemurafenib, but soon develop drug resistance, limiting vemurafenib's therapeutic efficacy. Constitutive activation of STAT3 in cancer cells and immune cells in the tumor microenvironment (TME) is a crucial contributor to the development of drug resistance and immune evasion in most cancers. Here, we investigated the antitumor efficacy and TME remodeling by APT-9R, a cell-permeable STAT3 inhibitory peptide, as a strategy to treat vemurafenib-resistant melanoma. We found that vemurafenib-resistant melanoma remodels into immunosuppressive TME by increasing the expression of specific chemokines to facilitate the infiltration of immunosuppressive immune cells, such as myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs). Intratumoral treatment of APT-9R led to a reduction in the population of MDSCs and TAMs, while increasing infiltration of cytotoxic T lymphocytes in the TME. Moreover, combination therapy with APT-9R and an anti-PD-1 antibody enhanced significant suppression of tumor growth by decreasing infiltration of these immunosuppressive immune cells while increasing the infiltration and cytotoxicity of CD8 T cells. These findings suggest that combined blockade of STAT3 and PD-1 signaling pathways may be an effective treatment option for overcoming poor therapeutic outcomes associated with drug-resistant BRAF-mutant melanoma.