HTP PET-CT

β-[¹¹C]-5-hydroxy-L-tryptophan (¹¹ C-5-HTP) is a PET tracer used for imaging neuroendocrine tumors which are derived from neuroendocrine cells. Neuroendocrine cells regulate a variety of body functions through paracrine stimulation. This is achieved via the production of a large variety of hormones, of which serotonin and cate-cholamines are examples. The production of these hormones is accomplished via amine precursor uptake and decarboxylation. In many differentiated neuroendocrine tumors, the capability to synthesize hormonal products has remained. This makes imaging with amine precursors interesting in these tumors. The serotonine pathway is active in many neuroendocrine tumors. In this pathway, the naturally occurring amino acid tryptophan and 5-HTP are both precursors. Serotonin is most abundantly present in blood platelets and the intestinal wall. In the brain serotonin acts as a neurotransmitter. Both tryptophan and 5-HTP are taken up via system L large amino acid transporters (LAT). After entering the cell, decarboxylation to serotonin takes place via the enzyme aromatic amino acid decarboxylase (AADC). The resulting end-product serotonin is then transported into storage vesicles through the vesicular monoamine transporter (VMAT). From these vesicles, serotonin can be released in the extracellular environment. Serotonine is thereafter degraded and eventually excreted as urinary 5-hydroxyindole acetic acid (5-HIAA). In the catecholamine pathway, the same LAT and VMAT transporter systems, and the enzyme AADC play a crucial role, resulting in the end-products, dop-amine, adrenaline and noradrenaline. Although the exact uptake mechanism and intracellular fate of these amines and their metabolites are not precisely understood, it appears that increased LAT activity plays a role to satisfy a high precursor turnover due to an increased metabolic pathway, e.g., serotonin or catecholamine, or at least increased AADC activity in neuroendocrine tumors.