MicroPET Imaging of a Gastrin-Releasing Peptide Receptor-Positive Tumor in a Mouse Model of Human Prostate Cancer Using a ⁶⁴Cu-Labeled Bombesin Analogue

Abstract

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a variety of carcinomas and has been the target for detection and treatment of these neoplasms in animals. In particular, analogues of the tetradecapeptide bombesin (BN) have been radiolabeled with (99m)Tc and (111)In for detection of GRPR-positive tumors by gamma ray scintigraphy. The goal of this study was to evaluate the potential of the bombesin analogue, DOTA-Aoc-BN(7-14), for positron-emission tomographic (PET) imaging after radiolabeling with the positron-emitter (64)Cu. A saturation binding assay on PC-3 human prostate cancer cells showed that (64)Cu-DOTA-Aoc-BN(7-14) had an equilibrium binding constant (K(d)) of 6.1 +/- 2.5 nM and a receptor concentration (B(max)) of 2.7 +/- 0.6 x 10(5) receptors/cell. The radiolabeled analogue also showed rapid internalization with 18.2% internalized into 10(5) PC-3 cells by 2 h. The tumor localization of (64)Cu-DOTA-Aoc-BN(7-14) was 5.5% injected dose per gram in athymic nude mice bearing PC-3 xenografts at 2 h postinjection. The tumor retention with respect to the 2 h value was 76% and 45% at 4 and 24 h, respectively, and was GRPR-mediated as shown by inhibition with a coinjection of excess peptide. MicroPET imaging of (64)Cu-DOTA-Aoc-BN(7-14) in athymic nude mice bearing subcutaneous PC-3 tumors showed good tumor localization. Further studies with (64)Cu-pyruvaldehyde-bis(N(4)-methylthiosemicarbazone) ((64)Cu-PTSM) suggested that low blood flow to the PC-3 tumors may have limited the localization of (64)Cu-DOTA-Aoc-BN(7-14). This study demonstrates that (64)Cu-DOTA-Aoc-BN(7-14) can be used to detect GRPR-positive tumors by PET imaging.

[ 64 Cu-NOTA-8-Aoc-BBN(7-14)NH 2 ] targeting vector for positron-emission tomography imaging of gastrin-releasing peptide receptor-expressing tissues

“…Studies show that the size of the parent macrocycle has a significant effect on the in vivo stability of 64 Cu conjugates (28). The more compact, neutral 64 Cu-NOTA complex of [ 64 Cu-NOTA-8-Aoc-BBN (7)(8)(9)(10)(11)(12)(13)(14)NH 2 ] appears to overcome demetallation and uptake of tracer by hepatobiliary proteins and accumulation and retention of conjugate in renal tissue in vivo, producing microPET images that are clearly superior to other conjugates described herein (23)(24)(25)(26)(27)(28)(29). Furthermore, ease of ligand synthesis, conjugation protocols, and radiolabeling techniques satisfies nearly all of the inherent require-ments for production of site-directed radiopharmaceuticals of this type.…”

“…However, ligands of this general type still suffer from difficult synthetic protocols and renal accumulation and retention of 64 Cu radionuclide (27)(28)(29). Thus, there is some impetus to improve the in vivo kinetic stability of 64 Cu-macrocyclic bioconjugates to reduce accumulation in collateral tissues such as liver (22)(23)(24)(25)(26). Furthermore, reduction of uptake in normal kidney would do much to improve the inherent renal toxicities of many peptide-based therapeutic agents (27)(28)(29).…”

“…Receptor-specific peptide conjugates containing the chelating agents DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and TETA (1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid) have shown some promise for production of Cu-64-labeled targeting vectors (22)(23)(24)(25)(26). However, 64 Cu 2ϩ complexes of these specific chelating ligands are only moderately stable under in vivo conditions, resulting in demetallation and subsequent accumulation in nontarget tissues such as liver.…”

64Cu-Labeled Alpha-Melanocyte-Stimulating Hormone Analog for MicroPET Imaging of Melanocortin 1 Receptor Expression

“…64 Cu is a positron emitter that can be readily produced using a medical cyclotron. Its intermediate half-life makes it suitable for small peptide radiolabeling (29)(30)(31)(32)(33). Considering all of these advantages, we hypothesized that radiolabeling DOTA-NAPamide with 64 Cu would generate a novel molecular probe for routine PET imaging of melanoma and MC1R expression.…”

Synthesis of bombesin-functionalized iron oxide nanoparticles and their specific uptake in prostate cancer cells

“…The human prostate cancer cell line, PC-3, has been shown to express high levels of GRP receptors (Rogers et al 2003), so this cell line was used to evaluate the specificity of our GRP receptor-targeted nanoparticles. Nanoparticles 2 and 4, incorporating the red fluorescent rhodamine dye, were incubated with GFP expressing PC-3 cells and were evaluated by multi-channel fluorescence microscopy.…”