Serotonin biosynthesis as a predictive marker of serotonin pharmacodynamics and disease-induced dysregulation

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.

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“…PCPA created depression 30 could be due to inhibition of MAO specifically 5-HTP which is the intermediate precursor of serotonin in synapse due to its selective and irreversible inhibitor of tryptophan hydroxylase. This is the rate-limiting enzyme in the biosynthesis of serotonin 31 .The Graph II indicates no alteration of blood glucose level in case of PCPA treated groups, whereas STZ treated groups increased the fasting blood sugar level and confirmed diabetes.…”

Assessment of Peripheral Serotonin Synthesis Using Stable Isotope‐Labeled Tryptophan

“…For the dog study normal grade h‐Trp from Cambridge Institute Laboratories was used. Bioanalytical standards were from previously described sources …”

“…[ 13 C]‐tracers are currently accepted to have negligible biological side effects, which renders them ethically acceptable also for use in children . Stable isotope‐labeled Trp can be administered, and the conversion to labeled 5‐HT monitored in sampled body fluids by mass spectrometry . Earlier clinical studies with stable isotope‐labeled Trp were performed with one or two subjects and did not have the appropriate sensitivity or methodology to measure labeled 5‐HT production .…”

“…Earlier clinical studies with stable isotope‐labeled Trp were performed with one or two subjects and did not have the appropriate sensitivity or methodology to measure labeled 5‐HT production . We recently demonstrated the utility of universally [ 13 C 11 ]‐labeled Trp (h‐Trp) as a tracer in rats to monitor TPH inhibition and disease‐induced alterations in 5‐HT synthesis . The sensitivity of the methodology enabled measurement of altered 5‐HT synthesis long before the delayed changes in 5‐HT levels.…”

Cutting back on the essentials: Can manipulating intake of specific amino acids modulate health and lifespan?

“…Tryptophan may thus influence memory, mood and learning through this serotonin pathway as well as affect stress responses and aging (Markus, 2008; Ruddick et al, 2006). In excess, tryptophan can be both toxic and carcinogenic, traits attributed to the effects of excess serotonin and its modulation of cell growth, fibrosis and inflammation and its direct effects on pituitary function, in particular the adrenocorticotrophin and thyroid axes (Hiraku et al, 1995; King et al, 1997; Welford et al, 2016). …”