Abstract P024: Antitumoral effect of telotristat ethyl as a single agent in neuroendocrine tumor cell lines and potential synergies
Authors: Ramirez Arantzazu Sierra , Méndez Marinela , Plaza Adrián , Pastor Andrés , Cerrillo Javier Molina , Marcos Pablo José Fernández , Gordoa Teresa Alonso , Pulido Enrique Grande 
Published: December 2021

Abstract
Neuroendocrine tumors (NET) comprise a rare and heterogeneous family of tumors arising from cells throughout the diffuse endocrine system. Carcinoid syndrome, a common gastrointestinal and lung NET complication, is mainly caused by serotonin release into the bloodstream, promoting diarrhea, flushing and carcinoid heat disease. By inhibiting the tryptophan hydroxylase, telotristat ethyl diminishes the synthesis of serotonin and improves carcinoid-related bowel movements. However, to our knowledge, there is no data showing the potential activity of telotristat ethyl as anti-tumoral agent and not only as a symptom-reliever . The purpose of this study is to analyze possible anti-tumoral effects of telotristat ethyl in NETs. For this, we used the pancreatic NET cell lines BON-1 and QGP1, and the gastrointestinal cell line HROC57. We treated them with telotristat ethyl as a single agent or in combination with common NET therapies, such as somatostatin analogues, mTOR inhibitors, standard chemotherapy and tyrosine kinase inhibitors. We analyzed the effects of these treatments on cellular viability, apoptosis markers, cell cycle and signaling pathways. Telotristal ethyl as single agent induced apoptosis measured by MTT in all NET cell lines after 72 hours of treatment, with IC50 in the µM range. Telotristat ethyl maintained this apoptotic effect even after serotonin addition. Surprisingly, neither the somatostatin analog octreotide nor the standard NET chemotherapy capecitabin/temozolomide induced apoptosis in BON-1, QGP-1 nor HROC57, suggesting that these cell lines are somatostatin analog- and chemotherapy-resistant. mTOR inhibition by everolimus treatment in NET cell lines induced apoptosis as monotherapy with IC50 in the µM range. Telotristat ethyl combination with everolimus produced a strong synergetic effect, dramatically decreasing cell viability in all NET cell lines. Mechanistically, combination of telotristat ethyl with everolimus promoted increasing levels of the apoptosis marker Anexin V with no major effects on cell cycle. mTOR inhibition by everolimus, alone and in combination with telotristat ethyl, decreased phosphorylated levels of the mTOR pathway readout protein S6 and elevated phosphorylated p42/44 MAPK levels, with no clear effects of telotristat ethyl. Our results show that telotristat ethyl has the capacity to inhibit pancreatic and GI NET cells tumor growth as a single agent and seems synergistic when combined with mTOR inhibitors but not with antiangiogenics or chemotherapy.
Citation Format: Arantzazu Sierra Ramirez, Marinela Méndez, Adrián Plaza, Andrés Pastor, Javier Molina Cerrillo, Pablo José Fernández Marcos, Teresa Alonso Gordoa, Enrique Grande Pulido. Antitumoral effect of telotristat ethyl as a single agent in neuroendocrine tumor cell lines and potential synergies [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P024.