Abstract
The biogenic amine serotonin (5-HT) is a multi-faceted hormone that is synthesized from dietary tryptophan with the rate limiting step being catalyzed by the enzyme tryptophan hydroxylase (TPH). The therapeutic potential of peripheral 5-HT synthesis inhibitors has been demonstrated in a number of clinical and pre-clinical studies in diseases including carcinoid syndrome, lung fibrosis, ulcerative colitis and obesity. Due to the long half-life of 5-HT in blood and lung, changes in steady-state levels are slow to manifest themselves. Here, the administration of stable isotope labeled tryptophan (heavy “h-Trp”) and resultant in vivo conversion to h-5-HT is used to monitor 5-HT synthesis in rats. Dose responses for the blockade of h-5-HT appearance in blood with the TPH inhibitors L-para-chlorophenylalanine (30 and 100 mg/kg) and telotristat etiprate (6, 20 and 60 mg/kg), demonstrated that the method enables robust quantification of pharmacodynamic effects on a short time-scale, opening the possibility for rapid screening of TPH1 inhibitors in vivo. In the bleomycin-induced lung fibrosis rat model, the mechanism of lung 5-HT increase was investigated using a combination of synthesis and steady state 5-HT measurement. Elevated 5-HT synthesis measured in the injured lungs was an early predictor of disease induced increases in total 5-HT.
Highlights
Non-brain penetrating Tryptophan hydroxylase (TPH) inhibitor telotristat etiprate (LX-1032) has demonstrated clinical benefit in patients with carcinoid syndrome[20,21]
We show that de novo synthesized HT and new 5-HT synthesis (h-5-HT) can be used as a predictive biomarker for 5-HT formation with only negligible perturbation to the system studied
The ability to track changes of h-5-HT in accessible biofluids with time, combined with the opportunities provided by utilizing more than one isotope of h-Trp vastly expands the options for studying the kinetics of 5-HT production, inhibition via TPH inhibitors, and regulation in disease
Summary
Non-brain penetrating TPH inhibitor telotristat etiprate (LX-1032) has demonstrated clinical benefit in patients with carcinoid syndrome[20,21]. In particular stable isotope tracers, enable the collection of detailed kinetic data on flux through molecular systems in vivo, which can be described in mathematical models[22,23]. Previous in vivo studies investigating 5-HT synthesis employed tracers with some inherent disadvantages (vide infra), which we sought to overcome by using a 13C,15N stable isotope labeled Trp tracer ((13C11)Trp or (13C11,15N2)Trp) on collectively referred to as heavy-Trp (h-Trp)), accompanied by measurement with LC-MS/MS (Fig. 1b). During the process of completing the work described here, a single study using (15N2)Trp in rats with monitoring of labeled 5-HT by chemical derivatization and GCMS was published, only a single biological condition was tested[38]. Monitoring of 5-HT synthesis was demonstrated to enable medium through-put testing of TPH1 inhibitors in vivo and was used to explore the mechanism of 5-HT dysregulation in a bleomycin-induced model of lung fibrosis
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
