Comparison of four 64Cu-labeled somatostatin analogues in vitro and in a tumor-bearing rat model: evaluation of new derivatives for positron emission tomography imaging and targeted radiotherapy

J Med Chem. 1999 Apr 22;42(8):1341-7. doi: 10.1021/jm980602h.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adrenal Glands / metabolism
  • Animals
  • Ligands
  • Liver / metabolism
  • Male
  • Neoplasm Transplantation
  • Organometallic Compounds / chemical synthesis*
  • Organometallic Compounds / chemistry
  • Pancreas / metabolism
  • Pancreatic Neoplasms / metabolism
  • Radiopharmaceuticals / chemical synthesis*
  • Radiopharmaceuticals / chemistry
  • Radiopharmaceuticals / pharmacokinetics
  • Rats
  • Rats, Inbred Lew
  • Somatostatin / analogs & derivatives*
  • Somatostatin / chemical synthesis*
  • Somatostatin / chemistry
  • Somatostatin / pharmacokinetics
  • Structure-Activity Relationship
  • Tissue Distribution
  • Tumor Cells, Cultured


  • Ligands
  • Organometallic Compounds
  • Radiopharmaceuticals
  • copper(64)TETA-phenyalanyl-tyrosyl(3)-octreotate
  • Somatostatin