Synthesis, Radiolabeling and In vitro GRP Receptor Targeting Studies of 99mTc-Triaza-X-BBN[7-14]NH2 (X=Serylserylserine, Glycylglycylglycine, Glycylserylglycine, or Beta Alanine)

• Published in: Synthesis and reactivity in inorganic, metal-organic, and nano-metal chemistry Vol. 36; no. 6; pp. 481 - 491
• Main Authors: Veerendra, Bhadrasetty, Sieckman, Gary L., Hoffman, Timothy J., Rold, Tammy, Retzloff, Lauren, McCrate, Joseph, Prasanphanich, Adam, Smith, Charles J.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 01-06-2006
• Subjects: bombesin; gastrin-releasing peptide; prostate cancer
• ISSN: 1553-3174; 1553-3182
• DOI: 10.1080/15533170600778075

Gastrin-releasing peptide (GRP) receptors have been shown to be over-expressed on several types of human cancer cells including prostate, breast, small cell lung and pancreatic cancers. Bombesin (BBN) is a 14 amino acid peptide that is an analogue of human gastrin-releasing peptide, binding to GRP receptors (GRPr) with high affinity and specificity. The aim of these studies was to develop new
99m
Tc-labeled BBN analogs having high tumor uptake and optimal pharmacokinetics for specific targeting of human prostate cancers. In this study, a new tridentate bifunctional chelating agent, 2-(4,7-bis(tert-butoxycarbonyl)-1,4,7-triazonan-1-yl)-acetic acid, was synthesized by first reacting 2 equivalents of BOC-ON with 1,4,7-triazacyclanone (Triaza) in CHCl
3
at room temperature. The product, N, N′-bis-(tert-butoxycarbonyl)-1,4,7-triazacyclanone, was alkylated using BrCH
2
COOH in acetonitrile. This new ligand framework was characterized by
1
H and
13
C NMR and electrospray ionization mass spectrometry (ESI-MS). Solid-phase peptide synthesis (SPPS) was used to produce Triaza-X-BBN[7-14]NH
2
conjugates with the following structure: Triaza-X-Q-W-A-V-G-H-L-M-(NH
2
), where the spacer group X=SSS, GGG, GSG and β-Alanine (SSS=Serylserylserine, GGG=Glycylglycylglycine, and GSG=Glycylserylglycine). These conjugates were purified by reversed phase-HPLC (RP-HPLC) and characterized by ESI-MS. In vitro competitive binding assays, using
125
I-Tyr
4
-BBN as the radiolabelling gold standard, demonstrated IC
50
values in the nanomolar range for all the new non-metallated conjugates. For example, IC
50
s were 1.8±0.4, 3.9.±0.4, 1.9±0.3, and 1.3±0.2 nM for X=SSS, GGG, GSG and β-Alanine, respectively. The new BBN conjugates were radiolabeled with Tc-99m in moderate yield via the Isolink
®
radiolabeling kit available from Tyco Healthcare, St. Louis, MO. In vitro internalization and externalization analyses indicated receptor binding to be receptor specific in human, PC-3, prostate cancer cells. These studies indicate that future in vivo studies in tumor-bearing mouse models are justified.