Therapeutic targeting of tumor‐associated macrophages in pancreatic neuroendocrine tumors

Abstract

Pancreatic neuroendocrine tumors (PNETs) represent a heterogeneous group of neuroendocrine neoplasms with varying biological behaviour and response to treatment. Although targeted therapies have been shown to improve the survival for patients at advanced stage, resistance to current therapies frequently occurs during the course of therapy. Previous reports indicate that the infiltration of tumor-associated macrophages (TAMs) in PNETs might correlate with tumor progression and metastasis formation. We aimed to evaluate the prognostic and functional impact of TAMs in human PNETs in vitro and in vivo and to investigate the effect of therapeutic targeting TAMs in a genetic PNET mouse model. TAM expression pattern was assessed immunohistochemically in human PNET tissue sections and a tissue-micro-array of PNET tumors with different functionality, stage and grading. The effect of liposomal clodronate on TAM cell viability was analysed in myeloid cell lines and isolated murine bone macrophages (mBMM). In vivo, RIP1Tag2 mice developing insulinomas were treated with liposomal clodronate or PBS-Liposomes. Tumor progression, angiogenesis and immune cell infiltration were assessed by immunohistochemistry. In human insulinomas TAM density was correlated with invasiveness and malignant behaviour. Moreover, TAM infiltration in liver metastases was significantly increased compared to primary tumors. In vitro, liposomal clodronate selectively inhibited the viability of myeloid cells and murine bone macrophages, leaving PNET tumor cell lines largely unaffected. In vivo, repeated application of liposomal clodronate to RIP1Tag2 mice significantly diminished the malignant transformation of insulinomas, which was accompanied by a reduced infiltration of F4/80 positive TAM cells and simultaneously by a decreased microvessel density, suggesting a pronounced effect of clodronate-induced myeloid depletion on tumor angiogenesis. Concomitant treatment with the antiangiogenic TKI sunitinib, however, did not show any synergistic effects with liposomal clodronate. TAMs are crucial for malignant transformation in human PNET in particular in insulinomas and correlate with metastatic behaviour. Pharmacological targeting of TAMs via liposomal clodronate disrupts tumor progression in the RIP1Tag2 neuroendocrine tumor model and was associated with reduced tumor angiogenesis Based on these results, using liposomal clodronate to target proangiogenic myeloid cells could be employed as novel therapeutic avenue in highly angiogenic tumors such as PNET. This article is protected by copyright. All rights reserved.

Gastrointestinal neuroendocrine tumors in 2020

“…Different immune cells (T cell, B cells, macrophages, dendritic cells, NK cells and mast cells) infiltrate the NETs making the TME immunosuppressed; this is more pronounced in P-NETs than SI-NETs probably due to a higher mutation rate in P-NETs[ 21 ]. CD+FoxP3+ T regulatory (Treg) and tumor-associated macrophage infiltration have been associated with high-grade NET[ 22 , 23 ] and poor prognosis. Soluble inhibitory factors secreted by NETs impair the maturation and function of dendritic cells, and as a result, antigen presentation to dendritic cells becomes impaired.…”

Overview of recent advances in liposomal nanoparticle-based cancer immunotherapy

“…In pancreatic cancer, many studies have confirmed that infiltrating macrophages mediate tumor progression in invasive tumors and early preinvasive pancreatic intraepithelial precursor lesions. Further studies have revealed that the depletion of TAMs by liposomal clodronate can dramatically reduce tumor metastasis [58,59].…”

Angiogenesis in Pancreatic Cancer: Pre-Clinical and Clinical Studies

“…The infiltration of tumor associated macrophages in pancreatic neuroendocrine tumors might correlate with tumor progression and metastasis formation. Krug et al investigated the effect of targeted tumor associated macrophage therapy in vitro and in vivo using liposomal clodronate [67]. By immunohistochemistry and tissue-micro-array, they assessed that clodronate arrests tumor progression and reduce tumor angiogenesis in the RIP1-Tag2 mice [67].…”

“…Krug et al investigated the effect of targeted tumor associated macrophage therapy in vitro and in vivo using liposomal clodronate [67]. By immunohistochemistry and tissue-micro-array, they assessed that clodronate arrests tumor progression and reduce tumor angiogenesis in the RIP1-Tag2 mice [67]. However, combined therapy with liposomal Clodronate and the anti-angiogenic tyrosine kinase inhibitor sunitinib did not show any synergistic effects [67].…”