LP-533401 hydrochloride is a tryptophan hydroxylase 1 inhibitor that regulates serotonin production in the gut. IC50 & Target: Tryptophan hydroxylase 1^[1] In Vitro: LP-533401 hydrochloride is a tryptophan hydroxylase 1 inhibitor. LP-533401 (1 μM) completely inhibits serotonin production in Tph1-expressing cells (RBL2H3 cells) after treatment for 3 days^[1]. In Vivo: Oral administration once daily for up to 6 weeks of LP-533401 (25, 100 or 250 mg/kg) dose-dependently prevents the development of and fully rescues, osteoporosis in ovariectomized rodents because of an isolated increase in bone formation. Pharmacokinetic studies in rodents show that LP-533401 level in the brain is negligible following oral administration, indicating that it is virtually unable to cross the blood-brain barrier^[1]. Mice treated repeatedly with LP-533401 (30-250 mg/kg per day) exhibit marked 5-HT content reductions in the gut, lungs, and blood, but not in the brain. After a single LP-533401 dose (250 mg/kg), lung and gut 5-HT contents decrease by 50%, whereas blood 5-HT levels remain unchanged, suggesting gut and lung 5-HT synthesis^[2]. Adult, healthy mice treated with the Tph-1 inhibitor LP-533401 show 30% decrease in circulating serotonin levels, with a consequent 30% increase in osteoblast numbers. Administration of LP-533401 to mice injected with EL4 cells inhibits the decrement in osteoblast numbers and trabecular bone volume, prolongs survial, and decreases leukemic infiltration^[3].