In 2008, researchers led by a team at Columbia University showed that, by turning on or off production of serotonin in the gut, they could control bone formation. Serotonin signals to cells in the skeleton to slow production of new bone and, by turning off the intestine’s release of serotonin, the team was able to prevent osteoporosis in mice undergoing menopause. The team have now shown that daily oral administration of LP-533401 for 6 weeks is effective both prophylactically and therapeutically against osteoporosis in ovariectomized mice.
LP-533401 inhibits tryptophan hydroxylase-1 (TPH-1), the first enzyme in gut-derived serotonin biosynthesis. TPH-1 is mostly expressed in peripheral tissues such as the gut, whilst TPH-2 is the major isoform in the central nervous system. Although LP-533401 inhibits human TPH-1 and TPH-2 with similar potency (Ki ~ 0.7µM) in vitro, it selectively lowers serotonin levels in the gut whilst leaving levels in the brain unchanged, likely because the compound does not cross the blood-brain barrier. LP-533401 and an ethyl ester pro-drug were originally developed to treat gastrointestinal diseases such as irritable bowel syndrome and to reduce chemotherapy-induced vomiting and nausea.
Although much work will need to be done before trials can be carried out in patients, the present study, which is published in Nature Medicine, demonstrates that pharmacological inhibition of synthesis of gut-derived serotonin could become a new anabolic treatment for osteoporosis. Most osteoporosis drugs only prevent the breakdown of old bone and are not able to stimulate the growth of new bone.