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Manipulation of [¹¹C]-5-Hydroxytryptophan and 6-[¹⁸F]Fluoro-3,4-Dihydroxy-L-Phenylalanine Accumulation in Neuroendocrine Tumor Cells

Departments of ¹ Nuclear Medicine and Molecular Imaging, ² Medical Oncology, and ³ Pathology and Laboratory Medicine, University Medical Center Groningen and University of Groningen, Groningen, the Netherlands; and ⁴ Department of Pharmacy, University of Ghent, Ghent, Belgium

Requests for reprints: Philip H. Elsinga, Department of Nuclear Medicine and Molecular Imaging, University Medical Center Groningen and University of Groningen, P.O. Box 30.001, 9700 RB Groningen, the Netherlands. Phone: 31-50-3613247; Fax: 31-50-3611687; E-mail: p.h.elsinga{at}ngmb.umcg.nl.

Key Words: FDOPA • HTP • PET

[¹¹C]-5-Hydroxytryptophan ([¹¹C]HTP) and 6-[¹⁸F]fluoro-3,4-dihydroxy-L-phenylalanine ([¹⁸F]FDOPA) are used to image neuroendocrine tumors with positron emission tomography. The precise mechanism by which these tracers accumulate in tumor cells is unknown. We aimed to study tracer uptake via large amino acid transporters, peripheral decarboxylation (inhibited by carbidopa), and intracellular breakdown by monoamine oxidase (MAO). [¹¹C]HTP and [¹⁸F]FDOPA tracer accumulation was assessed in a human neuroendocrine tumor cell line, BON. The carbidopa experiments were done in a tumor-bearing mouse model. Intracellular [¹¹C]HTP accumulation was 2-fold higher than that of [¹⁸F]FDOPA. Cellular transport of both tracers was inhibited by amino-2-norbornanecarboxylic acid. The MAO inhibitors clorgyline and pargyline increased tracer accumulation in vitro. Carbidopa did not influence tracer accumulation in vitro but improved tumor imaging in vivo. Despite lower accumulation in vitro, visualization of [¹⁸F]FDOPA is superior to [¹¹C]HTP in the neuroendocrine pancreatic tumor xenograft model. This could be a consequence of the serotonin saturation of BON cells in the in vivo model. [Cancer Res 2008;68(17):7183–90]

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