Comparative Studies of Three 68Ga-Labeled [Des-Arg10] Kallidin Derivatives for Imaging Bradykinin B1 Receptor Expression with PET

Bradykinin B1 receptor (B1R) is a G-protein-coupled receptor that is overexpressed in a variety of cancers. B1R is not expressed in healthy tissues, making it an attractive cancer imaging marker. Previously, we reported selective uptake of Ga-68-P03034 (Ga-68-DOTA-dPEG2-Lys-Arg-Pro-Hyp-Gly-Cha-Ser-Pro-Leu) in B1R-positive (B1R+) HEK293T:: hB1R tumor xenografts in mice. In this study, we compare Ga-68-P03034 with Ga-68-labeled P04158 (Ga-68-DOTA-dPEG2-Lys-Lys-Arg-Pro-Hyp-Gly-Igl-Ser-D-Igl-Oic) and Z02090 (Ga-68-DOTA-dPEG2-Lys-Lys-Arg-Pro-Hyp-Gly-Cpg-Ser-D-Tic-Cpg) derived from 2 potent B1R antagonists, B9858 and B9958, respectively, for imaging B1R expression with PET. Methods: Peptide sequences were assembled on solid-phase. Cold standards were prepared by incubating DOTA-conjugated peptides with GaCl3. Binding affinity was measured via competition binding assays using hB1R-expre sing Chinese hamster ovary-K1 cell membranes. Ga-68 labeling was performed in N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) buffer with microwave heating and purified by high-performance liquid chromatography. Imaging/biodistribution studies were performed in mice bearing wild-type HEK293T (B1R-) and B1R+ HEK293T:: hB1R tumors. Results: P03034, P04158, and Z02090 bound B1R with high affinity, with K-i values at 16.0 +/- 2.9, 1.5 +/- 1.9, and 1.1 +/- 0.8 nM, respectively. Ga-68-labeled P03034, P04159, and Z02090 were obtained in greater than 50% decay-corrected radiochemical yields with more than 99% radiochemical purity. Biodistribution studies showed that all three Ga-68-labeled tracers cleared rapidly from the blood and normal tissues, with excretion mainly via the renal pathway. At 1 h after injection, only the kidneys, bladders, and B1R+ HEK293T::hB1R tumors were clearly visualized in PET images. Uptake values of Ga-68-labeled P03034, P04158, and Z02090 in B1R+ tumors were 2.17 +/- 0.49, 19.6 +/- 4.50, and 14.4 +/- 1.63 percentage injected dose per gram, respectively. Uptake ratios of B1R+ to B1R-tumor, blood, and muscle were 6.23 +/- 1.69, 5.72 +/- 2.20, and 25.5 +/- 13.1 for Ga-68-P03034; 34.5 +/- 10.5, 19.2 +/- 8.21, and 66.1 +/- 17.0 for Ga-68-P04158; and 29.3 +/- 9.68, 29.9 +/- 5.58, and 124 +/- 28.1 for Ga-68-Z02090, respectively. Conclusion: All three 68Ga-labeled B1R-targeting peptides generated specific and high-contrasted images of B1R+ tumors xenografted in mice. With significantly higher tumor uptake and target-to-nontarget ratios, Ga-68-labeled P04158 and Z02090 are superior to P03034 for imaging B1R expression with PET.