Supplementary Material for: Effect of the Tryptophan Hydroxylase Inhibitor Telotristat on Growth and Serotonin Secretion in 2D and 3D Cultured Pancreatic Neuroendocrine Tumor Cells

Serotonin, a biologically active amine, is related to carcinoid syndrome in functioning neuroendocrine tumors (NETs). Telotristat ethyl is a novel inhibitor of the tryptophan hydroxylase (TPH), a key enzyme in the production of serotonin. While its use in patients with carcinoid syndrome and uncontrolled diarrhea under somatostatin analogs (SSAs) has been recently approved, in vitro data evaluating its effectiveness are lacking. For this reason, we aimed to evaluate the effect of telotristat as monotherapy, and in combination with SSAs, on proliferation and secretion in a NET cell line model. The human pancreatic NET cell lines BON-1/QGP-1 were used as 2D and 3D cultured models; somatostatin receptor and TPH mRNA expression, as well as the potential autocrine effect of serotonin on tumor cell proliferation using a 3D culture system were evaluated. Telotristat decreased serotonin production in a dose-dependent manner at a clinically feasible concentration, without affecting cell proliferation. Its combination with pasireotide, but not with octreotide, had an additive inhibitory effect on serotonin secretion. The effect of telotristat was slightly less potent, when BON-1 cells were co-treated with octreotide. Octreotide and pasireotide had no effect on the expression of TPH. Telotristat did not have an effect on mRNA expression of somatostatin receptor subtypes. Finally, we showed that serotonin did not have an autocrine effect on NET cell proliferation on the 3D cell model. These results suggest that telotristat is an effective drug for serotonin inhibition, but the effectiveness of its combination with SST₂ (somatostatin receptor subtype 2)-preferring SSA should be evaluated in more detail.

血清素（Serotonin）是一种具有生物活性的胺类物质，与功能性神经内分泌肿瘤（NETs）患者的类癌综合征密切相关。依洛司他乙酯（Telotristat ethyl）是色氨酸羟化酶（TPH）的新型抑制剂，而色氨酸羟化酶是血清素合成过程中的关键酶。尽管其在类癌综合征且经生长抑素类似物（SSAs）治疗后腹泻仍未得到控制的患者中的适应证已获批准，但目前仍缺乏评估其临床疗效的体外实验数据。基于此，本研究旨在以神经内分泌肿瘤细胞系模型为研究对象，评估依洛司他乙酯单药及与生长抑素类似物联合给药时，对肿瘤细胞增殖与分泌功能的影响。本研究采用人胰腺神经内分泌肿瘤细胞系BON-1/QGP-1作为二维（2D）与三维（3D）培养模型，同时通过三维培养体系，评估生长抑素受体、色氨酸羟化酶的mRNA表达水平，以及血清素对肿瘤细胞增殖的潜在自分泌效应。实验结果显示，在临床可行的浓度范围内，依洛司他乙酯可呈剂量依赖性降低血清素的产生，且不会对细胞增殖产生影响。依洛司他乙酯与帕瑞肽（pasireotide）联合给药时，可对血清素分泌产生叠加抑制效应，但与奥曲肽（octreotide）联合使用时则无此效果。当BON-1细胞与奥曲肽共处理时，依洛司他乙酯的抑制效果略有减弱。奥曲肽与帕瑞肽均不会影响色氨酸羟化酶的表达。依洛司他乙酯对生长抑素受体亚型的mRNA表达亦无显著影响。最终，本研究证实，在三维细胞模型中，血清素对神经内分泌肿瘤细胞增殖并无自分泌效应。上述结果表明，依洛司他乙酯是一种可有效抑制血清素产生的药物，但与偏好生长抑素受体亚型2（SST2）的生长抑素类似物联合使用的疗效，仍需进一步详细评估。