A Combination of the Tryptophan Hydroxylase Inhibitor Telotristat with the mTOR Inhibitor Everolimus as an Effective Strategy Against Neuroendocrine Tumors.

Abstract

Background and objectives: Neuroendocrine tumors are a rare and heterogeneous group of neoplasms that frequently cause carcinoid syndrome, characterized by diarrhea, flushing, and carcinoid heart disease. A recent therapy for carcinoid syndrome involves inhibition of tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, using telotristat ethyl. Telotristat has demonstrated clinical control of carcinoid syndrome; however, its potential antitumoral effects remain unclear. This study aimed to evaluate the antitumor activity of telotristat ethyl alone and in combination with everolimus in neuroendocrine tumor models.

Methods: Cell viability was assessed in three neuroendocrine tumor cell lines of pancreatic (BON-1, QGP-1) and intestinal (HROC57) origin following treatment with telotristat ethyl. Combination treatments with telotristat ethyl and everolimus or other antitumoral agents were evaluated for effects on cell viability, apoptosis, and cell cycle distribution. In vivo efficacy was examined in nude mice bearing BON-1-derived xenografts treated with telotristat ethyl, everolimus, or the combination. Tumor growth, toxicity, proliferation (Ki67), and apoptosis (active caspase-3) were analyzed.

Results: Telotristat ethyl reduced cell viability in all three neuroendocrine tumor cell lines. Combination with everolimus, but not with other antitumoral treatments, synergistically decreased cell viability, induced apoptosis, and reduced the proportion of cells in the G2/M phase. In nude mice bearing BON-1 xenografts, combined treatment with telotristat ethyl and everolimus arrested tumor growth without signs of toxicity compared with single treatments. At the end of treatment, tumors from combination-treated mice showed a reduction in proliferation (Ki67) comparable to single-treated groups and an increase in apoptosis as indicated by active caspase-3.

Conclusions: Telotristat ethyl exhibits antitumoral activity in neuroendocrine tumor models in vivo. Moreover, the combination of telotristat ethyl and everolimus, two therapies already used in clinical practice, may represent a safe and effective therapeutic strategy for neuroendocrine tumors.