An orally bioavailable, small-molecule, tryptophan hydroxylase (TPH) inhibitor prodrug with potential antiserotonergic activity. Upon administration, TPH inhibitor LX1606 is converted to its active metabolite LP-778902, which binds to and blocks the activity of TPH. This may result in a reduction in peripheral serotonin (5-HT) production and improvement of serotonin-mediated gastrointestinal effects such as severe diarrhea. TPH, the rate-limiting enzyme in serotonin biosynthesis, is overexpressed in carcinoid tumor cells.