Preclinical evaluation of technetium-99m-labeled somatostatin receptor-binding peptides.
We report here the results of studies on the in vitro receptor binding affinity, in vivo tumor uptake and biodistribution of two 99m-Tc-labeled peptides.


METHODS
Peptides P587 and P829 were synthesized by N-alpha-Fmoc peptide chemistry, purified by reversed-phase HPLC and characterized by fast-atom bombardment mass spectrometry. The peptides were labeled with 99mTc by ligand exchange from 99mTc-glucoheptonate. In vitro somatostatin receptors (SSTR)-binding affinities of P587, P829 and their oxorhenium complexes, [DTPA]octreotide and In-[DTPA]octreotide were determined in an inhibition assay using AR42J rat pancreatic tumor cell membranes and 125I-[Tyr3]somatostatin-14 as the probe. In vivo single- and dual-tracer studies of 99mTc peptides and 111In-[DTPA]octreotide were carried out using Lewis rats bearing CA20948 rat pancreatic tumor implants.


RESULTS
Technetium-99m-P587 and 99mTc-P829 of high-specific activity (>60 Ci (2.2 TBq)/mmole) were prepared in >90% radiochemical yield. P587 and P829 had a Ki = 2.5 nM and 10 nM, respectively. [ReO]P587 and [ReO]P829, representing the 99mTc complexes, had Ki = 0.15 nM and 0.32 nM, respectively. In comparison, [DTPA]octreotide and In-[DTPA]octreotide had Ki = 1.6 and 1.2 nM, respectively. In vivo tumor uptake of 99mTc-P587 and 99mTc-P829 was high (4.1 and 4.9%ID/g at 90 min postinjection compared to 2.9% for 111In-[DTPA]octreotide). Tumor/blood and tumor/muscle ratios at 90 min postinjection were 6 and 33 for 99mTc-P587, 21 and 68 for 99mTcP829, and 22 and 64 for 111In-[DTPA]octreotide.


CONCLUSIONS
The high SSTR-binding affinity and high receptor-specific and saturable in vivo tumor uptake indicate that 99mTc-P587 and 99mTc-P829 are promising radiotracers for the clinical detection of SSTR-expressing tumors and other tissues by 99mTc gamma scintigraphy.
Vallabhajosula
S
S
Department of Radiology, The Mount Sinai Medical Center, New York 10029, USA.
Moyer
B R
BR
Lister-James
J
J
McBride
B J
BJ
Lipszyc
H
H
Lee
H
H
Bastidas
D
D
Dean
R T
RT
D000818:Animals; D009368:Neoplasm Transplantation; D009374:Neoplasms, Experimental / Q000000981:diagnostic imaging* / Q000378:metabolism; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism; D010455:Peptides / Q000138:chemical synthesis / Q000378:metabolism; D011877:Radionuclide Imaging; D051381:Rats; D011917:Rats, Inbred Lew; D017481:Receptors, Somatostatin / Q000032:analysis*; D013667:Technetium / Q000493:pharmacokinetics; D014018:Tissue Distribution; D014407:Tumor Cells, Cultured
D010455:Peptides; D017481:Receptors, Somatostatin; D013667:Technetium