Radiolabeled R954 Derivatives for Imaging Bradykinin B1 Receptor Expression with Positron Emission Tomography

Peptide receptors have emerged as promising targets for diagnosis and therapy. The aberrant overexpression of these receptors in different cancer subtypes allows for the adoption of new treatment strategies that complement conventional chemotherapies. Bradykinin B1 receptor (B1R) is a G protein-coupled receptor that is overexpressed in many cancers, with limited expression in healthy tissues. Previously, we developed Ga-68- and F-18-labeled derivatives of BIR antagonist peptides B9858 and B9958, and successfully targeted B1R-expressing tumor xenografts in vivo. R954 (Ac-Orn-Arg-Oic-ProGly-aMePhe-Ser-D-2-Nal-11e), a potent BIR antagonist, is reportedly more stable than B9858 against peptidase degradation. We evaluated two radiolabeled derivatives of R954 (Ga-68-HTK01083 and F-18-HTK01146) for B1R PET imaging. Peptides were synthesized via solid phase strategy. Nonradioactive standards were obtain by reacting GaCl3 with DOTA-dPEG2-R9S4 and by clicking N-propargyl-N,N-dimethylammoniomethyl-trifluoroborate with azidoacetyl-dPEG2-R954. Binding affinity for B1R was determined by an in vitro competition binding assay. Ga-68-HTK01083 was obtained by incubating DOTA-dPEG2-R954 with (GaCl3)-Ga-68 under acidic conditions, while F-18-HTK01146 was prepared via an F-18-F-19 isotope exchange reaction. Biodistribution and imaging studies were conducted at 1 h postinjection (p.i.) in mice inoculated with BIR-expressing (B1R+) and BIR-nonexpressing (B1R) cells. HTK01083 and HTK01146 bound BIR with good affinity (K-i = 30.5 and 24.8 nM, respectively). Ga-68/F-18-labeled R954 were obtained on average in decay-corrected radiochemical yield with >99% radiochemical purity and >= 52 GBq/mu mol specific activity. For both tracers, clearance was predominantly renal with minimal involvement of the hepatobiliary system. For PET images, BIR+ tumors, kidneys, and bladder were visible. At 1 h p.i., uptake in B1R+ tumor was comparable between Ga-68-HTK01083 (8.46 +/- 1.44%ID/g) and F-18-HTK01146 (9.25 +/- 0.69%ID/g). B1R+ tumor-to-blood and B1R+ tumor-to-muscle ratios were 6.32 +/- 1.44 and 20.7 +/- 3.58 for Ga-68-HTK01083, and 7.24 +/- 2.56 and 19.5 +/- 4.29 for F-18-HTK01146. Our results indicate R954 is a good lead sequence for optimization of B1R tracers for cancer imaging.