In vivo demonstration of enzyme activity in endocrine pancreatic tumors: decarboxylation of carbon-11-DOPA to carbon-11-dopamine.
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.


CONCLUSIONS
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.
Uppsala University PET Center, Sweden.
M
M
Bergström
B
B
Eriksson
K
K
Oberg
A
A
Sundin
H
H
Ahlström
K J
KJ
Lindner
P
P
Bjurling
B
B
Långström
D000368:Aged; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000378:metabolism*; D002250:Carbon Radioisotopes; D003653:Decarboxylation; D004298:Dopamine / Q000378:metabolism*; D005260:Female; D015408:Gastrinoma / Q000000981:diagnostic imaging* / Q000188:drug therapy / Q000201:enzymology*; D005935:Glucagonoma / Q000000981:diagnostic imaging* / Q000188:drug therapy / Q000201:enzymology*; D006801:Humans; D007553:Isotope Labeling; D007980:Levodopa / Q000378:metabolism*; D008875:Middle Aged; D015282:Octreotide / Q000031:analogs & derivatives / Q000627:therapeutic use; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging* / Q000188:drug therapy / Q000201:enzymology*; D010456:Peptides, Cyclic; D013004:Somatostatin / Q000031:analogs & derivatives*; D014055:Tomography, Emission-Computed
D002250:Carbon Radioisotopes; D010456:Peptides, Cyclic; C060347:lanreotide; D007980:Levodopa; D013004:Somatostatin; D001142:Aromatic-L-Amino-Acid Decarboxylases; D015282:Octreotide; D004298:Dopamine