New Gastrin Releasing Peptide Receptor-Directed [^99mTc]Demobesin 1 Mimics: Synthesis and Comparative Evaluation

Abstract

We have previously reported on the gastrin releasing peptide receptor (GRPR) antagonist [Tc]1, ([Tc]demobesin 1, Tc-[N'-diglycolate-dPhe,Leu-NHEt]BBN(6-13)). [Tc]1 has shown superior biological profile compared to analogous agonist-based Tc-radioligands. We herein present a small library of [Tc]1 mimics generated after structural modifications in (a) the linker ([Tc]2, [Tc]3, [Tc]4), (b) the peptide chain ([Tc]5, [Tc]6), and (c) the C-terminus ([Tc]7 or [Tc]8). The effects of above modifications on the biological properties of analogs were studied in PC-3 cells and tumor-bearing SCID mice. All analogs showed subnanomolar affinity for the human GRPR, while most receptor-affine 4 and 8 behaved as potent GRPR antagonists in a functional internalization assay. In mice bearing PC-3 tumors, [Tc]1-[Tc]6 exhibited GRPR-specific tumor uptake, rapidly clearing from normal tissues. [Tc]4 displayed the highest tumor uptake (28.8 ± 4.1%ID/g at 1 h pi), which remained high even after 24 h pi (16.3 ± 1.8%ID/g), well surpassing that of [Tc]1 (5.4 ± 0.7%ID/g at 24 h pi).

Comparative evaluation of the new GRPR‐antagonist 111In‐SB9 and 111In‐AMBA in prostate cancer models: Implications of in vivo stability

“…The forming sulfoxide (Met 14 =O) was reduced by dithiothreitol and [ 125 I‐Tyr 4 ]BBN was isolated in a highly pure form and free from excess nonradioiodinated [Tyr 4 ]BBN by HPLC at a molar activity of 74 GBq/μmol. Methionine was added to the purified radioligand solution to prevent oxidation of Met 14 to the corresponding sulfoxide and the resulting stock solution in 0.1% BSA‐PBS was kept at −20 °C; aliquots thereof were used in competition binding assays …”

“…Methionine was added to the purified radioligand solution to prevent oxidation of Met 14 to the corresponding sulfoxide and the resulting stock solution in 0.1% BSA-PBS was kept at −20°C; aliquots thereof were used in competition binding assays. 20 All manipulations with solutions of radionuclides were performed behind suitable shielding in dedicated laboratories in compliance to national and international radiation safety guidelines and supervised by the Greek Atomic Energy Commission.…”

“…A group of GRPR‐radioantagonists developed by us comprise analogs of the potent [DPhe 6 ,Leu 13 NHEt, des Met 14 ]BBN(6‐14) antagonist, modified with a series of linkers and chelators at the N‐terminus to accommodate theranostic radionuclides of interest . These studies have demonstrated a major impact of N‐terminal modifications on several biological responses of resulting radioligands, such as receptor binding affinity, cell uptake capabilities, metabolic stability, and pharmacokinetics in animal models.…”

“…[12][13][14][15][16][17] A group of GRPR-radioantagonists developed by us comprise analogs of the potent [DPhe 6 ,Leu 13 NHEt, desMet 14 ]BBN (6)(7)(8)(9)(10)(11)(12)(13)(14) antagonist, modified with a series of linkers and chelators at the N-terminus to accommodate theranostic radionuclides of interest. [18][19][20] These studies have demonstrated a major impact of N-terminal modifications on several biological responses of resulting radioligands, such as receptor binding affinity, cell uptake capabilities, metabolic stability, and pharmacokinetics in animal models. The SB3 member of this series (DOTAp-aminomethylaniline-diglycolic acid-DPhe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt) in particular after labeling with the PET radionuclide 68 Ga has shown excellent properties for targeting GRPR-expressing prostate cancer.…”

Tumor targeting with 99mTc radiolabeled peptides: Clinical application and recent development

“…[99][100][101][102] Demobesin-1 is a potent new GRPR-selective bombesin (BBN) antagonist containing a bombesin antagonist [(D) Phe 6 , Leu-NHEt 13 , des-Met 14 ]BBN (6)(7)(8)(9)(10)(11)(12)(13)(14) unit for receptor binding and tetraamine unit for 99m Tc binding. [105] One of the other promising BBN analogues is 99m Tc-Leu 13 -bombesin which was used for imaging tumor in patient with breast, prostate, colon, and SCLC cancer. Reaction between 99m Tc and tetraamine forms a polar 99m TcO2 + -tetraamine metal chelate with hydrophilic characteristic which is expected to show favorable pharmacokinetic properties.…”