Abstract 4187: Combined MEK and STAT3 inhibition reprograms the tumor microenvironment to overcome immunotherapy resistance in pancreatic cancer
Authors: Garrido Vanessa Tonin , Datta Jashodeep , Dai Xizi , Bianchi Anna , Silva Iago De Castro , Lamichhane Purushottam , Mehra Siddharth , Singh Samara P. , Dosch Austin R. , Umland Oliver , VanSaun Michael n N. , Hosein Peter J. , Nagathihalli Nagaraj , Merchant Nipun 
Published: June 2022

Abstract
Background: Pancreatic ductal adenocarcinoma (PDAC) is characterized by immune exclusion, stromal desmoplasia, and resistance to immune checkpoint inhibition (ICI). We have previously demonstrated that reciprocally activated RAS/RAF/MEK/ERK and JAK/STAT3 pathways mediate therapeutic resistance, while combined MEK and STAT3 inhibition (MEKi+STAT3i) overcomes this resistance in preclinical PDAC models. Given the dramatic decrease in tumor burden observed following MEKi+STAT3i, we investigated its effect on the stromal and immune microenvironment.
Methods: Ptf1aCre/+KrasG12D/+Tgfbr2flox/flox (PKT) mice were treated with vehicle, trametinib (MEKi, 3.3 mg/Kg, oral gavage three times weekly), ruxolitinib (STAT3i, 20 mg/Kg, oral gavage three times weekly), αPD-1 antibody (200 µg/mouse, i.p. injection twice weekly), combined MEKi+STAT3i or MEKi+STAT3i with αPD-1 beginning at 4-4.5 weeks of age. Mice were sacrificed after four weeks of treatment and tumors were harvested for single cell RNA sequencing, and high-dimensional immune-profiling by mass cytometry and flow cytometry.
Results: Single-cell transcriptomic analysis revealed that combined MEKi+STAT3i not only altered stromal architecture but also reprogramed tumor-resident CAFs from Il6/Cxcl1-expressing inflammatory phenotype towards Ly6a/Cd34-expressing fibroblast phenotype with both mesenchymal and hematopoietic progenitor-like properties. This stromal plasticity was associated with a striking attenuation and reprogramming of F4/80+ macrophages, M2-like macrophages (F4/80+CD206+), and MDSCs (CD11b+F4/80-Ly6G+/Ly6C+), as well as enhanced trafficking of CD4+ and CD8+ T-cells which exhibited a distinct effector and anti-apoptotic transcriptional program. The addition of MEKi+STAT3i to PD-1 blockade overcomes immune checkpoint resistance by significantly augmenting anti-tumor responses and dramatically improving survival in PKT mice, compared with vehicle treatment (median 181 vs. 44 days, p&lt;0.0001) or anti-PD-1 monotherapy (median 181 vs. 57 days, p=0.0005). The addition of MEKi+STAT3i to PD-1 blockade not only augmented the recruitment of activated and memory T-cell populations, but also improved their degranulating capacity and functional cytotoxicity compared to PD-1 blockade alone. Importantly, treatment of a patient with chemotherapy-refractory metastatic PDAC with MEKi (Trametinib), STAT3i (Ruxolitinib), and PD-1 inhibitor (Nivolumab) was well-tolerated and yielded significant clinical benefit.
Conclusion: These data uncover a novel paradigm in which combined MEKi+STAT3i reprograms stromal inflammation and immune tolerance to overcome immunotherapy resistance in PDAC. The clinical efficacy of combined MEKi+STAT3i and anti-PD1 treatment provides encouraging signals for its translatability and is currently being pursued in a clinical trial.
Citation Format: Vanessa Tonin Garrido, Jashodeep Datta, Xizi Dai, Anna Bianchi, Iago De Castro Silva, Purushottam Lamichhane, Siddharth Mehra, Samara P. Singh, Austin R. Dosch, Oliver Umland, Michael n N. VanSaun, Peter J. Hosein, Nagaraj Nagathihalli, Nipun Merchant. Combined MEK and STAT3 inhibition reprograms the tumor microenvironment to overcome immunotherapy resistance in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 4187.