In addition, SSAs and TPH inhibitors inhibit serotonin secretion from the tumor and subsequently lower 5-HIAA levels, relieving carcinoid syndrome, a rare secretory syndrome mainly associated with small intestinal and bronchial NENs that becomes manifest when serotonin and other vasoactive substances from the tumor enter the systemic circulation escaping hepatic degradation [4]. Here, TPH1 is linked to neoplasm.