Previous studies have shown that modification of the
somatostatin analogue
octreotide (OC), by substitution of
tyrosine for
phenylalanine at position 3 and of a C-terminal
carboxylic acid for an alcohol, to give Tyr3-octreotate (Y3-TATE) improved uptake of the
peptide in
somatostatin receptor-positive tissues. To determine which substitution best accounts for increased target tissue uptake, the
peptides containing single modifications, Tyr3-octreotide (Y3-OC) and octreotate (TATE), were synthesized. These
peptides were conjugated to the macrocyclic
chelating agent 1,4,8, 11-tetraazacyclotetradecane-N,N',N",N"'-tetraacetic
acid (TETA) and radiolabeled with 64Cu(II). The in vitro receptor binding, in vitro
tumor cell uptake, and in vivo distribution properties of 64Cu-labeled TETA-Y3-OC and TETA-TATE were compared to those of [64Cu]TETA-OC and [64Cu]TETA-Y3-TATE.
Cu-TETA-TATE (IC50 = 0.297 +/- 0.0055 nM) and Cu-TETA-Y3-TATE (IC50 = 0.308 +/- 0.0375 nM) displayed significantly higher binding affinity to
somatostatin receptors on CA20948 rat pancreatic
tumor membranes than Cu-TETA-Y3-OC (IC50 = 0.397 +/- 0.0206 nM) and
Cu-TETA-OC (IC50 = 0. 498 +/- 0.039 nM). Similarly, the uptakes of [64Cu]TETA-Y3-TATE (60. 75 +/- 1.21%) and [64Cu]TETA-TATE (55.62 +/- 0.16%) into AR42J rat pancreatic
tumor cells over a 2-h time period were higher than those of [64Cu]TETA-Y3-OC (47.20 +/- 1.20%) and [64Cu]TETA-OC (34.07 +/- 2. 24%). The in vitro results suggest that the C-terminal carboxylate may contribute more to enhanced receptor binding and
tumor cell uptake than the substitution at the 3-position. Biodistributions in CA20948
tumor-bearing rats showed receptor-mediated uptake of the 64Cu-labeled
peptides in
somatostatin-rich tissues, including the pituitary, adrenals, pancreas, and
tumor. The structure-activity relationships of the four 64Cu-labeled
peptides did not show consistent trends in all target tissues, but [64Cu]TETA-Y3-TATE exhibited
tumor uptake 1.75-3.5 times higher than the other derivatives at 4 h postinjection. The greater
tumor retention of [64Cu]TETA-Y3-TATE justifies the selection of this agent for future PET imaging and
targeted radiotherapy studies.