Abstract
Tumor-specific uptake of the radiolabeled norepinephrine analogue meta-iodobenzylguanidine
via norepinephrine transporter or radiolabeled somatostatin analogues octreotide/octreotate
via somatostatin receptors offers possibilities to diagnose and treat metastatic pheochromocytoma/paraganglioma.
High uptake of 123I-meta-iodobenzylguanidine is dependent on high expression of vesicular monoamine
transporters responsible for mediating uptake of biogenic amines into dense core granules.
A patient with metastatic paraganglioma (liver and bone metastases) underwent surgical
removal of the primary after injection of 131I-meta-iodobenzylguanidine and 111In-octreotide. Radioactivity was determined in biopsies from tumor and normal tissue
biopsies. The tumor/blood concentration value was high: 180 for 131I-meta-iodobenzylguanidine 3 h after injection and 590 for 111In-octreotide 27 h after injection. Studies of primary tumor cell cultures demonstrated
increased cell membrane binding and internalization over time for 131I-meta-iodobenzylguanidine. The vesicular monoamine transporter antagonist reserpine
and the norepinephrine transporter inhibitor clomipramine reduced internalization
by 90% and 70%, respectively, after 46 h of incubation. The results demonstrated increased
cell membrane binding and internalization over time also for 111In-octreotide. Internalization was highest for a low concentration of 111In-octreotide. Excess of octreotide reduced internalization of 111In-octreotide with 75% after 46 h of incubation. In conclusion, uptake and tumor/blood
concentration values of radiolabeled meta-iodobenzylguanidine and somatostatin analogues
can be determined for metastatic pheochromocytoma/paraganglioma to evaluate the possibility
to use one or both agents for therapy. For this patient, the high tumor/blood values
clearly demonstrated that therapy using both radiopharmaceuticals would be most beneficial.
In vitro studies verified specific cell-membrane binding and internalization in tumor
cells of both radiopharmaceuticals.
Key words
norepinephrine transporter - VMAT - clomipramine - reserpine - somatostatin receptors
- octreotate