This study identified and confirmed angiotensin II (ATII) as a strong activator of signaling in neuroendocrine neoplasm (NEN) cells. Expression analyses of the ATII receptor type 1 (AGTR1) revealed an upregulation of mRNA levels (RT-qPCR) and radioligand binding (autoradiography) in small-intestinal (n = 71) NEN tissues compared to controls (n = 25). NEN cells with high AGTR1 expression exhibited concentration-dependent calcium mobilization and chromogranin A secretion upon stimulation with ATII, blocked by AGTR1 antagonism and Gαq inhibition. ATII also stimulated serotonin secretion from BON cells. AGTR1 ligand saralasin was coupled to a near-infrared fluorescent (NIRF) dye and tested for its biodistribution in a nude mouse model bearing AGTR1-positive BON and negative QGP-1 xenograft tumors. NIRF imaging showed significantly higher uptake in BON tumors. This proof of concept establishes AGTR1 as a novel target in NEN, paving the way for translational chelator-based probes for diagnostic PET imaging and radioligand therapy.

Peptide receptor targeting has proven to be a pivotal tool for diagnostic imaging and radioligand therapy of neuroendocrine neoplasms (NENs), which frequently express somatostatin receptors (SSTRs) on their cell surface. However, up to 30 % of NEN patients do not benefit from SSTR-based approaches, others develop a resistance. Consequently, alternative cell surface targets need to be identified. In this study, cell-based dynamic mass redistribution and calcium mobilization screening using a 998-compound library identified and confirmed angiotensin II (ATII) as a strong activator of cellular signaling in NEN cells. Expression analyses of the ATII receptor type 1 (AGTR1) revealed an upregulation of both mRNA levels (RT-qPCR) and radioligand binding (autoradiography) in pancreatic (n=42) and small-intestinal (n=71) NEN tissues compared to healthy controls (n=25). The two NEN cell lines BON (pancreas) and H727 (lung) with elevated AGTR1 expression exhibited concentration-dependent calcium mobilization and chromogranin A secretion upon stimulation with ATII, blocked by AGTR1 antagonism and Gαq inhibition. To assess the applicability of AGTR1 for optical in vivo imaging, the receptor ligand saralasin was coupled to the near-infrared dye indotricarbocyanine and tested for its biodistribution in a NMRI Foxn1nu/Foxn1nu mouse model bearing AGTR1-positive BON and negative QGP-1 xenograft tumors. Near-infrared fluorescent imaging showed a significantly higher uptake in BON tumors 3-6 hours after injection. This successful targeting in an NEN model establishes AGTR1 as an interesting target in this tumor entity, paving the way for the development of translational chelator-based probes for diagnostic PET imaging and peptide receptor radioligand therapy.