Comparative studies of three 68Ga-labeled [Des-Arg10]kallidin derivatives for imaging bradykinin B1 receptor...

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 (68)Ga-P03034 ((68)Ga-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 (68)Ga-P03034 with (68)Ga-labeled P04158 ((68)Ga-DOTA-dPEG2-Lys-Lys-Arg-Pro-Hyp-Gly-Igl-Ser-D-Igl-Oic) and Z02090 ((68)Ga-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.

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-expressing Chinese hamster ovary-K1 cell membranes. (68)Ga 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.

P03034, P04158, and Z02090 bound B1R with high affinity, with Ki values at 16.0 ± 2.9, 1.5 ± 1.9, and 1.1 ± 0.8 nM, respectively. (68)Ga-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 (68)Ga-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 (68)Ga-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 (68)Ga-P03034; 34.5 ± 10.5, 19.2 ± 8.21, and 66.1 ± 17.0 for (68)Ga-P04158; and 29.3 ± 9.68, 29.9 ± 5.58, and 124 ± 28.1 for (68)Ga-Z02090, respectively.

All three (68)Ga-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, (68)Ga-labeled P04158 and Z02090 are superior to P03034 for imaging B1R expression with PET.