Internalization of [DOTA°,125I-Tyr3]octreotide by somatostatin receptor-positive cells in vitro and in vivo:: Implications for somatostatin receptor-targeted radioguided surgery

We compared internalization of three radioiodinated octreotide (OCT) somatostatin (SS) analogs-[I-125-Tyr(3)]OCT, [DTPA degrees,I-125-Tyr(3)]OCT, and [DOTA degrees,I-125-Tyr(3)]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 mu g/l) by 38%, 43%, and 31%, and by an inhibitor of receptor-mediated endocytosis (phenyl arsine oxide; 10 mu M) 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,I-125-Tyr(3)]OCT was internalized in a five-fold higher amount in comparison with the two other radioligands. A comparably high uptake of [DOTA degrees,I-125-Tyr(3)]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,I-125-Tyr(3)]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: I) coupling of chelating groups like DTPA or DOTA to the SS analog [Tyr(3)]OCT does not prevent the internalization of OCT after binding to SSRs; 2) [DOTA degrees,I-125-Tyr(3)]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,I-125-Tyr(3)]OCT and [I-125-Tyr(3)]OCT; and 3) the very high target-background ratios in vivo make radioiodinated [DOTA degrees,Tyr(3)]OCT a very suitable ligand for SSR-targeted radioguided surgery of SSR-positive human neuroendocrine tumors.