Optimizing the Design of Bradykinin B1 Receptor-targeting PET Tracers via the Selection of Radiolabel Chelate

Objectives The bradykinin B1 receptor (B1R), a G-protein-coupled receptor, is up-regulated in many types of cancer. Due to its low expression in normal tissues, B1R is an attractive target for cancer imaging. Previously, we identified a promising B1R-targeting tracer ¹⁸F-L08060 (¹⁸F-AmBF₃-Mta-Pip-B9958). It contains a potent sequence B9958 (Lys-Lys-Arg-Pro-Hyp-Gly-Cpg-Ser-D-Tic-Cpg) and a preferable linker Pip (4-amino-(1-carboxymethyl)piperidine). ¹⁸F labeling was achieved via ¹⁸F-¹⁹F isotope exchange on the trifluoroborate moiety of AmBF₃-Mta (4-(N-trifluoroborylmethyl-N,N-dimethylammonio)methyl-1,​2,​3-​triazole-​1-​acetic acid). In this study, we investigated if tumor uptake and tumor-to-background contrast could be further improved by the selection of various radiolabel chelates.

Methods DOTA and NODA-Mpaa(4-methylphenylacetic acid) conjugated Pip-B9958 were synthesized using solid phase approach. Ga complexation was performed by incubating DOTA- and NODA-Mpaa-conjugated peptides with GaCl₃. AlOH complexation was achieved by incubating NODA-Mpaa-conjugated peptides with AlCl₃. Al-F cold standard was obtained by the reaction of AlOH-NODA-conjugated peptide with NaF. Binding affinity was measured by in vitro competition binding assays. ⁶⁸Ga and ¹⁸F labeling experiments were performed in acidic buffer using ⁶⁸GaCl₃ and ¹⁸F-fluoride, respectively. Imaging/biodistribution studies were performed in mice bearing both HEK293T wild-type (B1R-) and HEK293T::hB1R (B1R+) tumors.

Results The binding affinities (K_i) of Z02176 (Ga-DOTA-Pip-B9958), Z02137 (Ga-NODA-Mpaa-Pip-B9958) and Z04139 (AlF-NODA-Mpaa-Pip-B9958) towards hB1R were 2.5, 2.6, and 14.0 nM, respectively. ¹⁸F-Z04139, ⁶⁸Ga-Z02176 and ⁶⁸Ga-Z02137 were prepared with decay-corrected radiochemical yields of 46 ± 2, 47 ± 11 and 32 ± 11 %, respectively. The radiochemical purities were > 99% and the specific activities of ¹⁸F-Z04139, ⁶⁸Ga-Z02176 and ⁶⁸Ga-Z02137 were 210 ± 72, 261 ± 74, and 100 ± 8 GBq/μmol, respectively. Biodistribution/imaging studies (at 1-h post-injection) showed that all three tracers cleared rapidly from background 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 for 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) and previously identified ¹⁸F-L08060 (4.20 ± 0.98 %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) and ¹⁸F-L08060 (14.7 ± 3.56 and 48.6 ± 10.7).

Conclusions ⁶⁸Ga-Z02176 and ¹⁸F-Z04139 outperformed ⁶⁸Ga-Z02137, ¹⁸F-L08060, and previously reported ⁶⁸Ga and ¹⁸F-labeled B1R-targeting tracers. With superior tumor uptake and target-to-background contrast ratios, ⁶⁸Ga-Z02176 and ¹⁸F-Z04139 are promising B1R-trageting PET tracers, and warrant further investigation for cancer imaging.