Theranostic potential of a new [64Cu]-radiolabeled R-954 conjugate for kinin B1R positive tumors

1164 Objectives The inducible GPCR B1 receptor (B1R) is a molecular signature of several types of solid cancers, including brain cancer. It is thought to be involved in tumor growth, invasion and angiogenesis. Thus, the B1R may represent a highly promising new target for the development of cancer-targeted theranostic agents. Methods We developed a new 64Cu/NOTA-conjugated analog of our potent, selective B1R peptide antagonist, R-954 (AcOrn[Oic2,(αMe)Phe5,DβNal7,Ile8]-desArg9-bradykinin). Stability of the resulting peptide conjugate was evaluated ex vivo in undiluted rat blood samples and in vivo within systemic circulation in healthy rats. Intracellular uptake/nuclear localization and radiocytotoxicity of the conjugate were studied in cultured F98 glioma cells. In vivo biodistribution profile and efficacy of detection of malignant brain tumors by PET, were evaluated using the syngeneic, orthotopic F98 glioma rat model. Results 64Cu/NOTA-βAla-R-954 showed remarkable stability in rat plasma after a 24h incubation. In vivo stability study showed mainly intact radiolabeled peptide (no occurrence of radiotracer metabolites) in rat blood circulation and urine, 90min after tail-vein injection. In vitro, the conjugate supressed, in a concentration-dependent manner, clonogenic growth of F98 cells possibly through an intracellular radiation mechanism (i.e. via cellular/nuclear uptake). Intravenous 64Cu/NOTA-βAla-R-954 properly detected relatively small brain tumors by PET at 1.5h and 20h after injection. Moreover, µPET signals coincided perfectly with those of contrast-enhanced MRI. The conjugate exhibited high tumor-to-contralateral background uptake and tumor-to-muscle uptake ratios (18:1 and 6:1, respectively) at 20h after injection. µPET signals generated by the radiotracer were i) inhibitable with excess of the unlabeled B1R antagonist R-954, ii) substantially reduced in B1R-knockdown F98 tumors, and iii) primarily absent in sham-operated animals, thus confirming the B1R-targeted tumor uptake specificity of the conjugate. Conclusions Our findings provide initial proof-of-concept for the potential use of 64Cu/NOTA-βAla-R-954 radiotracer in both diagnosis and treatment of brain cancer, and possibly other types of B1R-positive solid cancers.