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.

5-羟色胺（Serotonin）是一种生物活性胺，与功能性神经内分泌肿瘤（functioning neuroendocrine tumors, NETs）引发的类癌综合征（carcinoid syndrome）密切相关。替洛司他乙酯（Telotristat ethyl）是一种新型色氨酸羟化酶（tryptophan hydroxylase, TPH）抑制剂，而TPH正是5-羟色胺合成过程中的关键酶。尽管其联合生长抑素类似物（somatostatin analogs, SSAs）治疗类癌综合征且腹泻难以控制的患者的适应症已获批准，但目前仍缺乏评估其疗效的体外研究数据。为此，本研究旨在以神经内分泌肿瘤细胞系模型为对象，评估替洛司他单用及联合生长抑素类似物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与QGP-1构建二维（2D）及三维（3D）培养模型，检测了生长抑素受体与色氨酸羟化酶的mRNA表达水平，并通过三维培养体系评估了5-羟色胺对肿瘤细胞增殖的潜在自分泌效应。实验结果显示，替洛司他在临床可行的浓度范围内可呈剂量依赖性降低5-羟色胺的产生，且未对细胞增殖产生影响。其与帕瑞肽（pasireotide）联合使用时，可对5-羟色胺分泌产生累加抑制效应，但与奥曲肽（octreotide）联合时则无此效果；当BON-1细胞与奥曲肽联合给药时，替洛司他的药效略有减弱。奥曲肽与帕瑞肽均未对色氨酸羟化酶的表达产生影响，替洛司他也未改变生长抑素受体各亚型的mRNA表达水平。最后，本研究证实，在三维细胞模型中，5-羟色胺并未对神经内分泌肿瘤细胞增殖产生自分泌效应。上述结果表明，替洛司他是一种有效的5-羟色胺合成抑制剂，但针对其与偏好生长抑素受体亚型2（somatostatin receptor subtype 2, SST₂）的生长抑素类似物联合使用的疗效，仍需开展更深入的研究加以评估。