Two cyclized minigastrin analogues for gastrin receptor scintigraphy were synthesized and derivatized with HYNIC at the N-terminus for labeling with 99mTc. Radiolabeling efficiency, stability, cell internalization, and receptor binding on CCK-2 receptor expressing AR42J cells were studied and the biodistribution evaluated in tumor bearing nude mice, including NanoSPECT/CT imaging. Metabolites in urine, liver, and kidneys were analyzed by radio-HPLC. Radiolabeled cyclic MG showed high stability in vitro and receptor mediated uptake in AR42J cells. In the animal tumor model, fast renal clearance and low nonspecific uptake in most organs were observed. A tumor uptake >3% was calculated ex vivo 1 h p.i. for both 99mTc-EDDA-HYNIC-cyclo-MG1 and 99mTc-EDDA-HYNIC-cyclo-MG2. In an imaging study with 99mTc-EDDA-HYNIC-cyclo-MG1, the tumor was clearly visualized. The metabolite analysis indicated rapid enzymatic degradation in vivo.

Recent advances in oncology involve the use of diagnostic/therapeutic radionuclide-carrier pairs that target cancer cells, offering exciting opportunities for personalized patient treatment. Theranostic gastrin-releasing peptide receptor (GRPR)-directed radiopeptides have been proposed for the management of GRPR-expressing prostate and breast cancers. We have recently introduced the PET tracer Ga-SB3 (SB3, DOTA- p-aminomethylaniline-diglycolic acid-DPhe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt), a receptor-radioantagonist that enables the visualization of GRPR-positive lesions in humans. Aiming to fully assess the theranostic potential of SB3, we herein report on the impact of switchingGa to In/Lu-label on the biological properties of resulting radiopeptides. Notably, the bioavailability of In/Lu-SB3 in mice drastically deteriorated compared with metabolically robust Ga-SB3, and as a result led to poorerIn/Lu-SB3 uptake in GRPR-positive PC-3 xenografts. The peptide cleavage sites were identified by chromatographic comparison of blood samples from mice intravenously receiving In/Lu-SB3 with each of newly synthesized In/Lu-SB3-fragments. Coinjection of the radioconjugates with the neprilysin (NEP)-inhibitor phosphoramidon led to full stabilization of In/Lu-SB3 in peripheral mouse blood and resulted in markedly enhanced radiolabel uptake in the PC-3 tumors. In conclusion, in situ NEP-inhibition led to indistinguishable Ga/In/Lu-SB3 profiles in mice emphasizing the theranostic prospects of SB3 for clinical use.

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).

Gastrin-releasing peptide receptors (GRPRs) expressed on human tumors can serve as molecular targets for radiolabeled peptide analogs based on the frog tetradecapeptide bombesin (BBN). We have recently expanded this approach toward human GRP(18-27) sequences and introduced 99m Tc-demomedin C, our first radiotracer based on , showing favorable biologic characteristics during preclinical evaluation in rodents. We now present a series of 99m Tc-demomedin C analogs, generated by single-Gly 24 or double-Gly 24 /Met 27 substitutions in the peptide chain, and compare their performance in GRPR-positive in vitro and in vivo models. at 4 h after injection). 99m Tc-SARNC6 displayed the highest tumor-to-nontumor ratios followed by 99m Tc-SARNC2. Conclusion: This structure-activity relationship study has shown the impact of single-Gly 24 or double-Gly 24 /Met 27 substitutions in the 99m Tc-SARNC1 motif on key biologic parameters, including GRPR affinity, internalization efficiency, and in vivo stability, which eventually determine the pharmacokinetic profile of resulting radiopeptides. By revealing improved analogs, this study has strengthened the applicability perspectives of radioligands based on human GRP sequences in the detection and therapy of GRPRexpressing tumors in humans.

Radiolabeled analogs of the frog tetradecapeptide bombesin (BBN) have been proposed for diagnosis and therapy of gastrin releasing peptide receptor (GRPR)-expressing tumors. Following a different and yet unexplored approach, we have developed four novel (111)In-labeled truncated analogs of the human 27-mer GRP after conjugation of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) at the N-terminus of GRP(13/14/17/18-27) fragments. Analog affinities for the human GRPR determined against [(125)I-Tyr(4)]BBN were at the nanomolar level and dependent on truncation site. The respective (111)In radioligands specifically internalized in GRPR-expressing PC-3 cells. The shorter chain [(111)In-DOTA]GRP(17/18-27) analogs showed higher metabolic stability in mice. Radioligands specifically localized in human PC-3 xenografts in SCID mice, with [(111)In-DOTA]GRP(17-27) exhibiting the most favorable pharmacokinetic profile. This study has demonstrated the efficacy of human GRP-based radiopeptides to target GRPR-positive lesions in vivo and has revealed the impact of GRP chain length on key biological parameters of resulting radiotracers.