TPH1 and carcinoid syndrome: Several TPH1 inhibitors have been developed for treating diseases associated with elevated 5-HT levels in peripheral tissues such as carcinoid syndrome, irritable bowel disease with diarrhea, inflammatory bowel disease, pulmonary arterial hypertension, obesity, and diabetes (Manocha and Khan, 2012; Margolis et al., 2014; Kim et al., 2015; Waloen et al., 2017; Matthes and Bader, 2018); e.g., p-chlorophenylalanine (fenclonine, pCPA) (Engelman et al., 1967), which was the first TPH inhibitor to reach clinical studies.