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In Vivo Demonstration of Enzyme Activity in Endocrine Pancreatic Tumors: Decarboxylation of Carbon-11-DOPA to Carbon-11-Dopamine

Uppsala University PET Center and Department of Internal Medicine, Uppsala University Hospital, Uppsala, Sweden

Correspondence: For correspondence or reprints contact: Mats Bergström, PhD, Uppsala University PET Center, S-751 85 Uppsala, Sweden.

ABSTRACT

Methods: We used PET to characterize the uptake and decarboxylation of ¹¹C-L-DOPA in vivo in two patients with endocrine pancreatic tumors: one glucagonoma and one gastrinoma. Results: With L-DOPA labeled with ¹¹C in the beta position, in which the radioactive label follows the molecule through decarboxylation to dopamine, significant uptake was observed in the tumors. With L-DOPA labeled in the carboxyl group, in which the label is rapidly eliminated from the tissue as ¹¹CO₂ if decarboxylation takes place, an almost complete lack of uptake is noted. Conclusion: This study shows that, using selective position labeling, an in vivo action of enzymatic activity can be observed with PET and that significant decarboxylation occurs in the tested endocrine pancreatic tumors. Also, marked retention of radioactivity occurs after treatment with somatostatin analogs. It is hypothesized that this is a reflection of a reduction of exocytosis which is induced by this treatment.

Key Words: PET • endocrine pancreatic tumors • aromatic amino acid decarboxylase • carbon-11-DOPA • somatostatin analogs

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