Comparison of agonist/antagonist versions of a synthetic tracer for human bradykinin B1 receptor imaging using positron emission tomography

Objectives Human Bradykinin (BK) B1 receptor (hB1R), a G protein-coupled receptor that is overexpressed in many cancers as well as acutely damaged tissues. B1R is not expressed under healthy conditions, hence making it a promising marker for cancer targeting. The goal of this study was to evaluate whether there is a difference between the tumor uptake of hB1R agonist and antagonist.

Methods We compared the hB1R agonist ⁶⁸Ga-DOTA-Ahx-Lys-[Hyp³,Cha⁵,D-Phe⁸,desArg⁹]BK (Z01115) with the antagonist ⁶⁸Ga-DOTA-Ahx-Lys-[Hyp³,Cha⁵,Leu⁸,desArg⁹]BK (SH01078). Binding affinity was measured by competitive binding assays using CHO:hB1R cell membrane. ⁶⁸Ga labeling was performed in NaOAc buffer (pH 4.1) with microwave heating for 1 min. PET imaging and biodistribution studies were performed in NSG mice bearing wild-type (HEK293T) and transfected (HEK293T:hB1R) tumors.

Results Z01115 and SH01078 bound hB1R with high affinity (Ki (nM): 27.8 ± 4.9 (SH01078), 25.4 ± 5.1 (Z01115)). [⁶⁸Ga]Z01115 and [⁶⁸Ga]SH01078 were obtained in high radiochemical yield (>70%) and purity (>99%). Both tracers were stable in mouse plasma with negligible degradation after 1h incubation. PET imaging and biodistribution studies showed rapid clearance of the radioactivity from blood and normal tissues, and excretion via the renal pathway. At 1h p.i., only kidneys, bladder and the hB1R positive HEK293T:hB1R tumor were visualized on PET images. Average uptake ratios of HEK293T:hB1R to HEK293T tumor, blood and muscle at 1h p.i. were higher for the agonist [⁶⁸Ga]Z01115 (29.4, 86.4, 86.2, respectively) compared to the antagonist [⁶⁸Ga]SH01078 (6.8, 20.6, 11.2).

Conclusions [⁶⁸Ga]Z01115 and [⁶⁸Ga]SH01078 generated specific, high contrast images of hB1R positive tumors xenografted in mice. Our results showed that in this experimental model, the agonist provided superior contrast to visualize hB1R-expressing lesions.