Discovery and Characterization of Novel Tryptophan Hydroxylase Inhibitors That Selectively Inhibit Serotonin Synthesis in the Gastrointestinal Tract

Abstract

5-Hydroxytryptamine (serotonin) (5-HT) is a neurotransmitter with both central and peripheral functions, including the modulation of mood, appetite, hemodynamics, gastrointestinal (GI) sensation, secretion, and motility. Its synthesis is initiated by the enzyme tryptophan hydroxylase (TPH). Two isoforms of TPH have been discovered: TPH1, primarily expressed in the enterochromaffin cells of the gastrointestinal tract, and TPH2, expressed exclusively in neuronal cells. Mice lacking Tph1 contain little to no 5-HT in the blood and GI tract while maintaining normal levels in the brain. Because GI 5-HT is known to play important roles in normal and pathophysiology, we set out to discover and characterize novel compounds that selectively inhibit biosynthesis of GI 5-HT. Here, we describe two of a series of these inhibitors that are potent for TPH activity both in biochemical and cell-based assays. This class of compounds has unique properties with respect to pharmacokinetic and pharmacodynamic effects on GI serotonin production. Similar to the Tph1 knockout results, these TPH inhibitors have the ability to selectively reduce 5-HT levels in the murine GI tract without affecting brain 5-HT levels. In addition, administration of these compounds in a ferret model of chemotherapy-induced emesis caused modest reductions of intestinal serotonin levels and a decreased emetic response. These findings suggest that GI-specific TPH inhibitors may provide novel treatments for various gastrointestinal disorders associated with dysregulation of the GI serotonergic system, such as chemotherapyinduced emesis and irritable bowel syndrome.5-Hydroxytryptamine (serotonin) (5-HT) is a neurotransmitter with both synaptic and paracrine activities that modulates central and peripheral functions through action on neurons, smooth muscle, and other cell types. It is synthesized from tryptophan by the sequential actions of tryptophan hydroxylase (TPH) and aromatic amino acid decarboxylase. Only one form of TPH was known to exist until Walther et al. (2003a) published a second distinct TPH-encoding gene. The originally identified isoform was renamed TPH1, and the newly discovered isoform was designated TPH2. The two enzymes share an overall identity of ϳ70%, and they are both ϳ50% identical to the other two members of the aromatic amino acid hydroxylase family, phenylalanine and tyrosine hydroxylases. Tph1 is primarily expressed in the pineal gland and non-neuronal tissues, such as enterochromaffin (EC) cells of the gastrointestinal (GI) tract (Côté et al., 2003;Patel et al., 2004). In contrast, Tph2 is expressed exclusively in neuronal cells, such as the dorsal raphe and myenteric plexus (Côté et al., 2003;Walther et al., 2003a;Patel et al., 2004).5-HT is involved in the control and modulation of multiple physiological and psychological processes. In the central nervous system (CNS), 5-HT regulates mood, appetite, and other Article, publication date, and citation information can be found at

Chemotherapy-induced Nausea and Vomiting

“…In fact, pre-clinical studies using an TPH inhibitor to selectively inhibit 5-HT in the gut using a ferret model of chemotherapy-induced emesis. 52 Substance P is another strong regulator of the emetic response; it binds to the neurokinin-1 receptor (NK-1). Both compound and receptor are found within the CNS and also within the gut.…”

Inhibition of Peripheral Synthesis of Serotonin as a New Target in Neuroendocrine Tumors

“…Illustration of biosynthesis of serotonin (blue arrows), the inhibitor action of telotristat etiprate on TPH (red X), conversion of serotonin to 5-hydroxyindole acetic acid and urinary excretion (green arrows), and activating and inhibitory pathways [20,[24][25][26][27][28].…”

“…Consequently, it does not diminish central serotonin levels, thus avoiding the side effects that it would entail otherwise [27,28,[52][53][54][55].…”

Gut-derived serotonin induced by depression promotes breast cancer bone metastasis through the RUNX2/PTHrP/RANKL pathway in mice

“…1C). LP533401 (Dalton Chemical Laboratories Inc., Toronto, ON, Canada), a small-molecule inhibitor of Tph-1 which is the initial enzyme in gut-derived serotonin (GDS) biosynthesis is currently being tested for the treatment of irritable bowel syndrome with no overt deleterious side effects (29). Oral administration of LP533401 (250 mg/kg/day) for a continuous 6 weeks significantly decreased osteolytic lesions in the CMS group compared to the PBS control (Fig.…”