[11C]5-Hydroxy-tryptophan model for quantitative assessment of in vivo serotonin biosynthesis, retention and degradation in the endocrine pancreas.
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Lubberink M, Eriksson O
American journal of nuclear medicine and molecular imaging 10 (5) 226-234 [2020-10-15; online 2020-10-15]
[11C]5-Hydroxy-tryptophan ([11C]5-HTP) is a Positron Emission Tomography marker for serotonergic biosynthesis and degradation, with use in imaging of neuroendocrine tumors and recently also the endocrine pancreas in diabetes. In order to further develop [11C]5-HTP as a quantitative in vivo tool for understanding the mechanisms of serotonin signaling in human pancreas, we aimed to develop a kinetic modeling approach sensitive for changes in serotonin biosynthesis, retention and degradation. Cynomolgus monkeys were examined by [11C]5-HTP PET/CT, either at baseline (n=9) or following intravenous pretreatment with 3 mg/kg carbidopa (Dopa Decarboxylase inhibitor, n=3) or 2 mg/kg clorgyline (Monoamine Oxidase-A inhibitor, n=5). The dynamic tissue uptake was analysed by a 2-tissue compartment model including an efflux mechanism from the second tissue compartment (2TC kloss), which theoretically reproduces the known processing of 5-HTP in neuroendocrine cells. The 2TC kloss model could accurately describe all three modes of tissue kinetics depending on the pretreatment regiment. Rate constant k3 (corresponding to DDC activity) and the macro-parameter Flux (Ki) was decreased (P<0.05) by carbidopa pretreatment, while k2 (corresponding to cellular washout of intact [11C]5-HTP) was increased (P<0.05). The efflux parameter kloss (corresponding to MAO-A activity) was decreased (P<0.05) by pretreatment of clorgyline, while the macro-parameter Flux/Efflux ratio (Ki/kloss) was increased (P<0.0001). We present a compartment model analysis method that can quantitatively assess in vivo pharmacological interactions with several of the key enzymatic steps of the serotonergic biosynthesis in pancreas.
PubMed 33224618