Internalization of [DOTA degrees,125I-Tyr3]Octreotide by somatostatin receptor-positive cells in vitro and in vivo: implications for somatostatin receptor-targeted radio-guided surgery.
We compared internalization of three radioiodinated octreotide (OCT) somatostatin (SS) analogs-[125I-Tyr3]OCT, [DTPA degrees, 125I-Tyr3]OCT, and [DOTA degrees,125I-Tyr3]OCT-by somatostatin receptor (SSR)-positive mouse AtT20 pituitary tumor cells and human insulinoma cells. The three SS analogs were internalized in a specific, time-dependent manner. Internalization was significantly inhibited by pertussis toxin (100 microg/l) by 38%, 43%, and 31%, and by an inhibitor of receptor-mediated endocytosis (phenyl arsine oxide; 10 microM) by 98%, 94%, and 92%, respectively. Binding affinities of the three radioligands were comparable (0.2, 0.2, and 0.3 nM, respectively). However, [DOTA degrees,125I-Tyr3]OCT was internalized in a five-fold higher amount in comparison with the two other radioligands. A comparably high uptake of [DOTA degrees, 125I-Tyr3]OCT was found in SSR-positive organs (pituitary, pancreas, and adrenals) in vivo in rats (a ten-fold, five-fold, and eight-fold higher uptake 4 hr post injection, respectively, compared with the two other radioligands). This resulted in very high target-background ratios for [DOTA degrees,125I-Tyr3]OCT 4 hr post injection amounting to 274, 566, and 623 in the pituitary, adrenals, and pancreas, respectively. Both in vivo and in vitro there was a rapid dissociation of radioactivity from the SSR-positive cells. Main conclusions are that: 1) coupling of chelating groups like DTPA or DOTA to the SS analog [Tyr3]OCT does not prevent the internalization of OCT after binding to SSRs; 2) [DOTA degrees, 125I-Tyr3]OCT is internalized in a significantly higher amount by AtT20 and human insulinoma cells and in vivo in rats in SSR-positive organs, in comparison with [DTPA degrees,125I-Tyr3]OCT and [125I-Tyr3]OCT; and 3) the very high target-background ratios in vivo make radioiodinated [DOTA degrees,Tyr3]OCT a very suitable ligand for SSR-targeted radioguided surgery of SSR-positive human neuroendocrine tumors.
Hofland
L J
LJ
Department of Internal Medicine III, Erasmus University, Rotterdam, The Netherlands.
Breeman
W A
WA
Krenning
E P
EP
de Jong
M
M
Waaijers
M
M
van Koetsveld
P M
PM
Mäcke
H R
HR
Lamberts
S W
SW
D000818:Animals; D006801:Humans; D007457:Iodine Radioisotopes; D051379:Mice; D015282:Octreotide / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D011869:Radioligand Assay; D019275:Radiopharmaceuticals; D016634:Radiosurgery; D051381:Rats; D017481:Receptors, Somatostatin / Q000819:agonists / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals; D017481:Receptors, Somatostatin; D015282:Octreotide