Michael Morse

Gastroenteropancreatic neuroendocrine tumors (GEP-NETs), although curable when localized, frequently metastasize and require management with systemic therapies, including somatostatin analogues, peptide receptor radiotherapy, small-molecule targeted therapies, and chemotherapy. Although effective for disease control, these therapies eventually fail as a result of primary or secondary resistance. For small-molecule targeted therapies, the feedback activation of the targeted signaling pathways and activation of alternative pathways are prominent mechanisms, whereas the acquisition of additional genetic alterations only rarely occurs. For somatostatin receptor (SSTR)-targeted therapy, the heterogeneity of tumor SSTR expression and dedifferentiation with a downregulated expression of SSTR likely predominate. Hypoxia in the tumor microenvironment and stromal constituents contribute to resistance to all modalities. Current studies on mechanisms underlying therapeutic resistance and options for management in human GEP-NETs are scant; however, preclinical and early-phase human studies have suggested that combination therapy targeting multiple pathways or novel tyrosine kinase inhibitors with broader kinase inhibition may be promising.

This chapter provides an overview of intracellular cytokine assays. Intracellular cytokine assays are a relatively new method of identifying cytokine production by individual T cells and have the ability to correlate cytokine expression with cell surface phenotype without cell separation. In addition, this highly sensitive flow cytometric method allows for the rapid detection of low frequency T cells expressing cytokine in response to specific antigen stimulation. The unique capabilities of this method make it a model assay for clinical and research applications. The overall premise of intracellular cytokine assays is direct detection of intracellular cytokine expression in response to antigen stimulation. Intracellular cytokine assays can be performed using various sources of cells and antigen depending on the target(s) of interest. T cell stimulation can be performed directly on whole blood, peripheral blood mononuclear cells, in vitro manipulated lymphocytes, isolated cells, and lymph nodes; although using whole blood for these assays provides a more physiological environment and may have an effect on the T cell response to stimulation. Since these assays are most often used to detect very low frequency events, appropriate control antigens are particularly important to ensure clear antigen specific response.

XIAP, the most potent inhibitor of cell death pathways, is linked to chemotherapy resistance and tumor aggressiveness. Currently, multiple XIAP-targeting agents are in clinical trials. However, the characterization of XIAP expression in relation to clinicopathological variables in large clinical series of breast cancer is lacking. We retrospectively analyzed non-metastatic, non-inflammatory, primary, invasive breast cancer samples for XIAP mRNA (n = 2341) and protein (n = 367) expression. XIAP expression was analyzed as a continuous value and correlated with clinicopathological variables. XIAP mRNA expression was heterogeneous across samples and significantly associated with younger patients' age (≤50 years), pathological ductal type, lower tumor grade, node-positive status, HR+/HER2- status, and PAM50 luminal B subtype. Higher XIAP expression was associated with shorter DFS in uni- and multivariate analyses in 909 informative patients. Very similar correlations were observed at the protein level. This prognostic impact was significant in the HR+/HER2- but not in the TN subtype. Finally, XIAP mRNA expression was associated with lower pCR rate to anthracycline-based neoadjuvant chemotherapy in both uni- and multivariate analyses in 1203 informative patients. Higher XIAP expression in invasive breast cancer is independently associated with poorer prognosis and resistance to chemotherapy, suggesting the potential therapeutic benefit of targeting XIAP.

Immune checkpoint inhibition of program death protein-1 (PD-1) and its ligands PD-L1 and PD-L2 is an established therapeutic modality in melanoma, non-small cell lung cancer, renal cell carcinoma, and other tumor types. Unfortunately, 60 to 80% of all patients experience disease progression and become refractory to immune checkpoint therapies. Broadly, mechanisms of immune checkpoint inhibitor resistance can be categorized as presence of oncogenic driver mutations, severe T cell exhaustion, neoantigen burden, epigenetic alterations, or mutations involved in critical pathways including PTEN, JAK, or Wnt signaling. The dysregulation of inflammatory signaling pathways (namely, genes involved in angiogenesis, chemotaxis, matrix remodeling, wound healing, and mesenchymal transition) is of critical importance to response to immune checkpoint therapies. Inflammatory cytokine signaling pathways exert downstream effects on immunosuppressive elements such as regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) which inhibit the function of effector T cells, NK cells, and dendritic cells, promoting immune tolerance and tumor growth. We herein review three targets for inflammatory pathway modulation: indoleamine 2,3-dioxygenase (IDO), transforming growth factor β (TGFβ), and adenosine. Targeting these pathways may address the unmet need to develop novel therapeutic approaches to increase response rates to immune checkpoint inhibitors and improve clinical outcomes.