[Lys40(Ahx-DTPA-111In)NH2]exendin-4, a very promising ligand for glucagon-like peptide-1 (GLP-1) receptor targeting.
High levels of glucagon-like peptide-1 (GLP-1) receptor expression in human insulinomas and gastrinomas provide an attractive target for imaging, therapy, and intraoperative tumor localization, using receptor-avid radioligands. The goal of this study was to establish a tumor model for GLP-1 receptor targeting and to use a newly designed exendin-4-DTPA (DTPA is diethylenetriaminepentaacetic acid) conjugate for GLP-1 receptor targeting.


METHODS
Exendin-4 was modified C-terminally with Lys(40)-NH(2), whereby the lysine side chain was conjugated with Ahx-DTPA (Ahx is aminohexanoic acid). The GLP-1 receptor affinity (50% inhibitory concentration [IC(50)] value) of [Lys(40)(Ahx-DTPA)NH(2)]exendin-4 as well as the GLP-1 receptor density in tumors and different organs of Rip1Tag2 mice were determined. Rip1Tag2 mice are transgenic mice that develop insulinomas in a well-defined multistage tumorigenesis pathway. This animal model was used for biodistribution studies, pinhole SPECT/MRI, and SPECT/CT. Peptide stability, internalization, and efflux studies were performed in cultured beta-tumor cells established from tumors of Rip1Tag2 mice.


RESULTS
The GLP-1 receptor affinity of [Lys(40)(Ahx-DTPA)NH(2)]exendin-4 was found to be 2.1 +/- 1.1 nmol/L (mean +/- SEM). Because the GLP-1 receptor density in tumors of Rip1Tag2 mice was very high, a remarkably high tumor uptake of 287 +/- 62 %IA/g (% injected activity per gram tissue) was found 4 h after injection. This resulted in excellent tumor visualization by pinhole SPECT/MRI and SPECT/CT. In accordance with in vitro data, [Lys(40)(Ahx-DTPA-(111)In)NH(2)]exendin-4 uptake in Rip1Tag2 mice was also found in nonneoplastic tissues such as pancreas and lung. However, lung and pancreas uptake was distinctly lower compared with that of tumors, resulting in a tumor-to-pancreas ratio of 13.6 and in a tumor-to-lung ratio of 4.4 at 4 h after injection. Furthermore, in vitro studies in cultured beta-tumor cells demonstrated a specific internalization of [Lys(40)(Ahx-DTPA-(111)In)NH(2)]exendin-4, whereas peptide stability studies indicated a high metabolic stability of the radiopeptide in beta-tumor cells and human blood serum.


CONCLUSIONS
The high density of GLP-1 receptors in insulinomas as well as the high specific uptake of [Lys(40)(Ahx-DTPA-(111)In)NH(2)]exendin-4 in the tumor of Rip1Tag2 mice indicate that targeting of GLP-1 receptors in insulinomas may become a useful imaging method to localize insulinomas in patients, either preoperatively or intraoperatively. In addition, Rip1Tag2 transgenic mice represent a suitable animal tumor model for GLP-1 receptor targeting.
Wild
Damian
D
Clinic and Institute of Nuclear Medicine, University Hospital Basel, Basel, Switzerland.
Béhé
Martin
M
Wicki
Andreas
A
Storch
Daniel
D
Waser
Beatrice
B
Gotthardt
Martin
M
Keil
Boris
B
Christofori
Gerhard
G
Reubi
Jean Claude
JC
Mäcke
Helmut R
HR
D000818:Animals; D045744:Cell Line, Tumor; D000067757:Glucagon-Like Peptide-1 Receptor; D007340:Insulinoma / Q000000981:diagnostic imaging* / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D051379:Mice; D008810:Mice, Inbred C57BL; D009928:Organ Specificity; D009942:Organometallic Compounds / Q000493:pharmacokinetics* / Q000627:therapeutic use; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism*; D010455:Peptides / Q000493:pharmacokinetics* / Q000627:therapeutic use; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics / Q000627:therapeutic use; D018027:Receptors, Glucagon / Q000378:metabolism*; D014018:Tissue Distribution
C517541:(Lys(40)(Ahx-DTPA-111In)NH2)exendin-14; C000593833:GLP1R protein, human; C000593834:Glp1r protein, mouse; D000067757:Glucagon-Like Peptide-1 Receptor; D009942:Organometallic Compounds; D010455:Peptides; D019275:Radiopharmaceuticals; D018027:Receptors, Glucagon