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

Abstract

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.

Preparation and Biological Evaluation of Copper-64–Labeled Tyr3-Octreotate Using a Cross-Bridged Macrocyclic Chelator

“…In the current study we report lower target tissue uptake and greater nontarget organ uptake than was found previously (1). The CA20948 rat pancreatic carcinoma model uses older rats than required for AR42J tumors (1,37). Immature rats may metabolize 64 Cu-TETA-Y3-TATE differently from older rats, resulting in the observed differences in biodistribution.…”

“…Modifications to octreotide, a somatostatin analogue, have been evaluated to improve target tissue uptake in somatostatin receptor subtype 2 (SSTr2)-positive tissues; based on these, the octapeptide Tyr 3 -octreotate (Y3-TATE) was identified (1,2). Radiolabeled somatostatin analogues incorporating positron-emitting radionuclides such as 64 Cu and 68 Ga have been shown to be equivalent or improved SSTr2 tracers for positron emission tomography (PET) imaging of SSTr2-positive tumors compared with 111 Inlabeled somatostatin analogues for ␥ scintigraphy (3)(4)(5).…”

Nuclear Uptake and Dosimetry of 64Cu-Labeled Chelator Somatostatin Conjugates in an SSTr2-Transfected Human Tumor Cell Line

“…Here, we compare two 64 Cu radiopharmaceuticals, 64 Cu-TETA-Y3-TATE ( 64 Cu- [1]) and 64 Cu-CB-TE2A-Y3-TATE ( 64 Cu- [2]). Y3-TATE-based radiopharmaceuticals are highly specific for SSTr2, have a higher affinity (typically, IC 50 , 2 nM (19)), and are more rapidly internalized into cells expressing this receptor than octreotide-based analogs (6,20). 64 Cu-[1] demonstrated higher uptake into tumor and other somatostatin-receptor-rich tissues than 64 Cu-TETAoctreotide in CA20948 tumor-bearing rats and AR42J tumor-bearing severe combined immunodeficiency (SCID) mice (21).…”

Toxicity and dosimetry of177Lu-DOTA-Y3-octreotate in a rat model

“…12,13 It has been demonstrated that with somatostatin-based peptides, changing the C-terminus from an alcohol (OC) to a carboxylic acid (TATE) increases uptake of these peptides in receptor-rich tissues. 18,23,28 This can result in a higher dose to the tumor, while not dramatically affecting the absorbed doses to normal tissue. 18 As a consequence, the most recent developments in the use of radiolabeled somatostatin analogs have focused on the use of the TATE derivative.…”