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Bradykinin B1 receptor (B1R), which is upregulated in a variety of malignancies, is an attractive cancer imaging biomarker. In this study we optimized the selection of radiolabel-chelator complex to improve tumor uptake and tumor-to-background contrast of radiolabeled analogues of B9958 (Lys-Lys-Arg-Pro-Hyp-Gly-Cpg-Ser-d-Tic-Cpg), a potent B1R antagonist. Peptide sequences were assembled on solid phase. Cold standards were prepared by incubating DOTA-/NODA-conjugated peptides with GaCl3, and by incubating AlOH-NODA-conjugated peptide with NaF. Binding affinities were measured via in vitro competition binding assays. 68Ga and 18F labeling experiments were performed in acidic buffer and purified by HPLC. Imaging/biodistribution studies were performed in mice bearing both B1R-positive (B1R+) HEK293T::hB1R and B1R-negative (B1R-) HEK293T tumors. Z02176 (Ga-DOTA-Pip-B9958; Pip: 4-amino-(1-carboxymethyl)piperidine), Z02137 (Ga-NODA-Mpaa-Pip-B9958; Mpaa: 4-methylphenylacetic acid), and Z04139 (AlF-NODA-Mpaa-Pip-B9958) bound hB1R with high affinity (Ki = 1.4-2.5 nM). 68Ga-/18F-labeled peptides were obtained on average in ?32% decay-corrected radiochemical yield with >99% radiochemical purity and 100-261 GBq/mumol specific activity. Biodistribution/imaging studies at 1 h postinjection showed that all tracers cleared rapidly from background tissues (except kidneys) and were excreted predominantly via the renal pathway. Only kidneys, bladders, and B1R+ tumors were clearly visualized in PET images. Uptake in B1R+ tumor was higher by using 68Ga-Z02176 (28.9 ± 6.21 %ID/g) and 18F-Z04139 (22.6 ± 3.41 %ID/g) than 68Ga-Z02137 (14.0 ± 4.86 %ID/g). The B1R+ tumor-to-blood and B1R+ tumor-to-muscle contrast ratios were also higher for 68Ga-Z02176 (56.1 ± 17.3 and 167 ± 57.6) and 18F-Z04139 (58.0 ± 20.9 and 173 ± 42.9) than 68Ga-Z02137 (34.3 ± 15.2 and 103 ± 30.2). With improved target-to-background contrast 68Ga-Z02176 and 18F-Z04139 are promising for imaging B1R expression in cancers with PET.
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Reference：
Metal catalyst and ligand design,
Ligand Template Strategies for Catalyst Encapsulation – NCBI