Pseudo-hemorrhagic region formation in pancreatic neuroendocrine tumors is a result of blood vessel dilation followed by endothelial cell detachment

The Landscape and Clinical Application of the Tumor Microenvironment in Gastroenteropancreatic Neuroendocrine Neoplasms

Simple Summary

The tumor microenvironment (TME) plays a role in promoting tumor progression. Elucidating the relationship between the TME and tumor cells will benefit current therapies. Therefore, this review summarizes the most recent relationship between the TME and tumor characteristics, discusses the differences in the TME at various sites along the digestive tract, and compares the TMEs of neuroendocrine tumors and neuroendocrine carcinomas. Microbial ecological changes in the TME were reviewed. The clinical application of the TME was summarized from bench to bedside. The TME can be used as a tumor drug target for diagnostic value, prognosis prediction, and efficacy evaluation, further revealing the potential of immune checkpoints combined with antiangiogenic drugs. The clinical application prospects of adoptive cell therapy and oncolytic viruses were described. The potential therapeutic approaches and strategies for gastrointestinal neuroendocrine neoplasms are considered.

Abstract

Gastroenteropancreatic neuroendocrine neoplasms feature high heterogeneity. Neuroendocrine tumor cells are closely associated with the tumor microenvironment. Tumor-infiltrating immune cells are mutually educated by each other and by tumor cells. Immune cells have dual protumorigenic and antitumorigenic effects. The immune environment is conducive to the invasion and metastasis of the tumor; in turn, tumor cells can change the immune environment. These cells also form cytokines, immune checkpoint systems, and tertiary lymphoid structures to participate in the process of mutual adaptation. Additionally, the fibroblasts, vascular structure, and microbiota exhibit interactions with tumor cells. From bench to bedside, clinical practice related to the tumor microenvironment is also regarded as promising. Targeting immune components and angiogenic regulatory molecules has been shown to be effective. The clinical efficacy of immune checkpoint inhibitors, adoptive cell therapy, and oncolytic viruses remains to be further discussed in clinical trials. Moreover, combination therapy is feasible for advanced high-grade tumors. The regulation of the tumor microenvironment based on multiple omics results can suggest innovative therapeutic strategies to prevent tumors from succeeding in immune escape and to support antitumoral effects.

Apatinib inhibits tumor growth and angiogenesis in PNET models

Angiogenesis has a pivotal role in the growth and metastasis of pancreatic neuroendocrine tumors (PNETs). Apatinib inhibits angiogenesis as a highly selective KDR inhibitor and has been used to treat advanced gastric cancer and malignancies in clinical settings. However, the efficacy of apatinib in PNETs remains unclear. The aim of this study was to compare the antitumor efficacy of apatinib with that of the standard PNET drug sunitinib in our subcutaneous and liver metastasis models of insulinoma and non-functional PNET. Our results revealed that apatinib had a generally comparable or even superior antitumor effect to that of sunitinib on primary PNET, and it inhibited angiogenesis without directly causing tumor cell cytotoxicity. Apatinib inhibited the tumor in a dose-dependent manner, and the high dose was well tolerated in mice. We also found that the apatinib efficacy in liver metastasis models was cell-type (disease) selective. Although apatinib efficiently inhibited INR1G9-represented non-functional PNET liver metastasis, it led to the emergence of a hypoxic area in the INS-1-represented insulinoma and promoted liver metastasis. Our study demonstrated that apatinib has promise for clinical applications in certain malignant PNETs, and the application of anti-angiogenesis drugs to benign insulinomas may require careful consideration.