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PURPOSE Pembrolizumab significantly improves clinical outcomes in advanced/metastatic microsatellite instability high (MSI-H)/deficient mismatch repair (dMMR) solid tumors but is not well studied in the neoadjuvant space. METHODS This is a phase II open-label, single-center trial of localized unresectable or high-risk resectable MSI-H/dMMR tumors. Treatment is pembrolizumab 200 mg once every 3 weeks for 6 months followed by surgical resection with an option to continue therapy for 1 year followed by observation. To continue on study, patients are required to have radiographic or clinical benefit. The coprimary end points are safety and pathologic complete response. Key secondary end points are response rate and organ-sparing at one year for patients who declined surgery. Exploratory analyses include interrogation of the tumor immune microenvironment using imaging mass cytometry. RESULTS A total of 35 patients were enrolled, including 27 patients with colorectal cancer and eight patients with noncolorectal cancer. Among 33 evaluable patients, best overall response rate was 82%. Among 17 (49%) patients who underwent surgery, the pathologic complete response rate was 65%. Ten patients elected to receive one year of pembrolizumab followed by surveillance without surgical resection (median follow-up of 23 weeks [range, 0-54 weeks]). An additional eight did not undergo surgical resection and received less than 1 year of pembrolizumab. During the study course of the trial and subsequent follow-up, progression events were seen in six patients (four of whom underwent salvage surgery). There were no new safety signals. Spatial immune profiling with imaging mass cytometry noted a significantly closer proximity between granulocytic cells and cytotoxic T cells in patients with progressive events compared with those without progression. CONCLUSION Neoadjuvant pembrolizumab in dMMR/MSI-H cancers is safe and resulted in high rates of pathologic, radiographic, and endoscopic response, which has implications for organ-sparing strategies.

BACKGROUND AND AIMS: The treatment of hepatocellular carcinoma (HCC) has been transformed by the use of immune checkpoint inhibitors. However, most patients with HCC do not benefit from treatment with immunotherapy. There is an urgent need to understand the mechanisms that underlie response or resistance to immunotherapy for patients with HCC. The use of syngeneic mouse models that closely recapitulate the heterogeneity of human HCC will provide opportunities to examine the complex interactions between cancer cells and nonmalignant cells in the tumor microenvironment. APPROACH AND RESULTS: We leverage a multifaceted approach that includes imaging mass cytometry and suspension cytometry by time of flight to profile the tumor microenvironments of the Hep53.4, Hepa 1–6, RIL-175, and TIBx (derivative of TIB-75) syngeneic mouse HCC models. The immune tumor microenvironments vary across these four models, and various immunosuppressive pathways exist at baseline in orthotopic liver tumors derived from these models. For instance, TIBx, which is resistant to anti–programmed cell death protein 1 therapy, contains a high proportion of “M2-like” tumor-associated macrophages with the potential to diminish antitumor immunity. Investigation of The Cancer Genome Atlas reveals that the baseline immunologic profiles of Hep53.4, RIL-175, and TIBx are broadly representative of human HCCs; however, Hepa 1–6 does not recapitulate the immune tumor microenvironment of the vast majority of human HCCs. CONCLUSIONS: There is a wide diversity in the immune tumor microenvironments in preclinical models and in human HCC, highlighting the need to use multiple syngeneic HCC models to improve the understanding of how to treat HCC through immune modulation.

Techniques for robust immune profiling of mouse tumor and blood are key to understanding immunological responses in mouse models of cancer. Here, we describe mass cytometry (cytometry by time-of-flight) procedures to facilitate high-parameter profiling of low-volume survival blood samples and end-of-study tumor samples. We employ live-cell barcoding systems to mark all cells from each tumor and blood to improve cost-effectiveness and minimize batch effects. For complete details on the use and execution of this protocol, please refer to Charmsaz et al. (2021).

Background: Oncogenic mutations in KRAS are expressed in up to 90% of pancreatic ductal adenocarcinomas (PDAC). Vaccination against mutant KRAS (mKRAS) is thus a promising approach as an off-the-shelf immunotherapeutic treatment for PDAC. We developed a mKRAS peptide vaccine targeting 6 common KRAS mutations (G12V, G12A, G12C, G12R, G12D, or G13D (NCT04117087). We evaluated the mKRAS specific T cell response induced by vaccination in combination with immune checkpoint inhibitors (ICIs) in patients with resected PDAC. Materials and Methods: This is an ongoing pilot study of a pooled mKRAS long peptide vaccine in combination with ICIs in patients with resected PDAC and mutations in one of 6 KRAS mutations in our vaccine. Patients with no evidence of disease on imaging within 6 months of completion of adjuvant chemotherapy were vaccinated with the mKRAS vaccine (0.3mg/peptide and 0.5mg poly-ICLC (Hiltonol: Oncovir)) weekly for 4 doses followed by booster vaccines every 8 weeks. Patients also received ipilimumab (1mg/kg, every 6 weeks for 2 doses) and nivolumab (3mg/kg, every 3 weeks in the priming phase) followed by nivolumab (480mg, flat dose in boost phase). To evaluate the expansion of mKRAS-specific T cells in the periphery, pre- and post-vaccination peripheral blood mononuclear cells (PBMCs) were restimulated with control or individual mKRAS peptides and assessed for IFNγ release by ELISPOT. To further phenotype the responding CD4 and CD8 T cell compartments, peptide-restimulated PBMCs were evaluated for T cell activation, proliferation, memory and exhaustion marker expression by CyTOF. Paired single-cell RNA and TCR sequencing are being performed to isolate mKRAS-specific TCRs and their corresponding transcriptional profiles. These TCRs are being functionally validated in vitro using CRISPR-Cas12a-based genome knock-in of human T cells. Results: At the time of data cut off (1/12/2023) 8/11 patients enrolled had a positive mKRAS-specific T cell response in post-vaccine sample timepoints defined by a >5 fold increase in IFNγ producing T cells after peptide restimulation. CyTOF analysis of peptide-restimulated PBMCs demonstrated expansion of polyfunctional (IFNγ+IL2+TNFα+) mKRAS-specific CD4 and CD8 T cells with both central and effector memory phenotypes after vaccination. mKRAS-specific CD4 T cells were induced at a greater proportion. Our single-cell analysis has identified and validated a novel CD4+ TCR that recognizes KRAS G12V in the context of HLA-DRB1*07:01. Conclusions: This study thus far indicates the induction of de novo, high quality mKRAS-specific T cells in the periphery post-vaccine. Ongoing studies will define TCR diversity and clonality of mKRAS-specific T cells to each mKRAS epitope. Overall, our data will be used to identify peripheral T cell-based biomarkers that may be able to predict response to mKRAS-targeted immunotherapy. Citation Format: Amanda L. Huff, Saurav D. Haldar, Emily Davis-Marcisak, Thatcher Heumann, Gabriella Longway, Alexei Hernandez, Maximillian F. Konig, Brian Mog, Ludmila Danilova, Luciane Kagohara, Julie M. Nauroth, Amy M. Thomas, Elana J. Fertig, Won Jin Ho, Elizabeth M. Jaffee, Nilo Azad, Neeha Zaidi. A pooled mutant KRAS peptide vaccine activates polyfunctional T cell responses in patients with resected pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB197.

Pancreatic adenocarcinoma (PDAC) is extremely lethal and resistant to checkpoint immunotherapy, characterized by an immunosuppressive tumor microenvironment consisting of stromal and myeloid cells. Prior studies have demonstrated the utility of targeting the chemokine signaling axis CXCR4-CXCL12 (SDF-1), a highlighted feature in PDACs, to overcome the CXCL12-driven immobilization of T cells and thus facilitate their antitumor role within the tumor. Based on these findings, we have conducted a phase 2 trial evaluating the effects of plerixafor, a CXCR4 antagonist, and cemiplimab, a PD1 inhibitor antibody, in patients with metastatic PDAC who have progressed after one line of systemic chemotherapy (NCT04177810). To determine the immunological responses to therapy, blood samples and tissue biopsies were obtained at baseline and during treatment. Hematological assessment confirmed the activity of CXCR4 antagonist in mobilizing hematopoietic precursors (CD34+), immature myeloid cells, and lymphoid cells, as well as monocytic and granulocytic cell populations. Suspension mass cytometry analysis of peripheral blood mononuclear cells revealed that mobilized monocytic subpopulations had high expressions of chemokine receptors CCR2, CCR5, and CXCR2. Histopathologic evaluation of the serial tissue biopsies from the liver metastatic PDAC revealed increased levels of inflammation upon treatment. To further characterize the cellular constituents of the observed inflammation, multiplexed immunohistochemistry by imaging mass cytometry was performed, demonstrating strong trends toward increased infiltration of not only effector T cells but also macrophages and granulocytic cells into the tumor microenvironment. Taken together, these findings suggest that mobilization of myeloid cells by CXCR4 antagonism results in the recruitment of additional myeloid cells from circulation and that alternative chemokine signaling pathways are sufficient for doing so. This implicates a potential mode of resistance against CXCR4-targeted therapies. Furthermore, these observations reinforce the value of ongoing research efforts in the field to subvert the recruitment or immunosuppressive function of myeloid cells, which would be particularly relevant in the setting of CXCR4 antagonism. Citation Format: Sarah M. Shin, Alexei Hernandez, Erin Coyne, Zhehao Zhang, Sarah Mitchell, Jennifer Durham, Xuan Yuan, Hongqui Yang, Elana J. Fertig, Elizabeth M. Jaffee, Katherine M. Bever, Dung T. Le, Won Jin Ho. Combination of CXCR4 antagonist and anti-PD1 therapy results in significant mobilization and increased infiltration of myeloid cells into the metastatic liver microenvironment of PDAC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2270.