Crucial role for somatostatin receptor subtype 2 in determining the uptake of [111In-DTPA-D-Phe1]octreotide in somatostatin receptor-positive organs.
Human somatostatin (SS) receptor (sst)-positive tumors can be visualized by gamma camera scintigraphy after the injection of [(111)In-diethylenetriaminepentaacetic acid (DTPA)-D-Phe(1)] octreotide. Uptake of [(111)In-DTPA-D-Phe(1)]octreotide is dependent on sst-mediated internalization of the radioligand by the tumor cells. Human sst-positive tumors frequently express multiple sst subtypes. In vitro studies have demonstrated that the 5 sst subtypes (sst(1-5)) differentially internalize sst-bound ligand. The present study was performed to evaluate the role of sst(2) in vivo in determining the uptake of [(111)In-DTPA-D-Phe(1)]octreotide, as well as of the more "universal" ligand [(111)In-DTPA]SS-14, by sst-positive organs expressing multiple sst subtypes.


METHODS
Wild-type and sst(2) knockout mice (n = 4 per treatment group) were injected intravenously with 1 MBq (0.1 micro g) [(111)In-DTPA-D-Phe(1)]octreotide or [(111)In-DTPA]SS-14. After 24 h, the animals were sacrificed and radioactivity in the organs under investigation was determined. In addition, the sst subtype messenger RNA (mRNA) expression pattern in these organs was determined by reverse transcriptase polymerase chain reaction (RT-PCR) analysis.


RESULTS
RT-PCR analysis demonstrated the presence of all 5 sst subtype mRNAs in the adrenals and pituitary of wild-type mice but no sst(2) in the knockout mice. The thymus expressed mRNA for sst(2) and sst(4) mRNA in wild-type mice, whereas no sst(2) was detected in knockout mice. In wild-type mice, the in vivo uptake values (in percentage injected dose per gram of tissue) of [(111)In-DTPA-D-Phe(1)]octreotide for the pituitary, adrenals, pancreas, and thymus amounted to 1.2 +/- 0.2, 0.26 +/- 0.03, 0.18 +/- 0.03, and 0.30 +/- 0.05, respectively, in wild-type mice. Compared with wild-type mice, sst(2) knockout mice had dramatically lower uptake values in these organs-lower by 97%, 83%, 96%, and 94%, respectively (P < 0.01 vs. wild type). Comparable differences in the uptake of radioactivity between wild-type and knockout mice were found using [(111)In-DTPA]SS-14 as the radiotracer. Interestingly, in some organs expressing sst(2) mRNA (liver, muscle, and peripheral blood mononuclear cells), no specific binding of [(111)In-DTPA-D-Phe(1)]octreotide or [(111)In-DTPA]SS-14 to sst in vivo was found, suggesting that the sst(2) protein expression level was very low in these tissues.


CONCLUSIONS
The uptake of [(111)In-DTPA-D-Phe(1)]octreotide and [(111)In-DTPA]SS-14 in sst-positive organs is determined predominantly by sst(2).
Hofland
Leo J
LJ
Department of Internal Medicine (Room Bd240), Erasmus Medical Center, Dr. Molewaterplein 40, 3015 GD Rotterdam, The Netherlands. L.hofland@erasmusmc.nl
Lamberts
Steven W J
SW
van Hagen
P Martin
PM
Reubi
Jean-Claude
JC
Schaeffer
James
J
Waaijers
Marlijn
M
van Koetsveld
Peter M
PM
Srinivasan
Ananth
A
Krenning
Eric P
EP
Breeman
Wout A P
WA
D000818:Animals; D007205:Indium Radioisotopes / Q000493:pharmacokinetics; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D015282:Octreotide / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D009928:Organ Specificity; D004369:Pentetic Acid / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D017481:Receptors, Somatostatin / Q000145:classification* / Q000172:deficiency / Q000378:metabolism*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013004:Somatostatin / Q000493:pharmacokinetics; D014018:Tissue Distribution; D014915:Whole-Body Counting
D007205:Indium Radioisotopes; D019275:Radiopharmaceuticals; D017481:Receptors, Somatostatin; C071632:SDZ 215-811; D013004:Somatostatin; D004369:Pentetic Acid; D015282:Octreotide