GRP receptor-targeted PET of a rat pancreas carcinoma xenograft in nude mice with a ⁶⁸Ga-labeled bombesin(6-14) analog

Bombesin (BN), a 14-amino-acid peptide, shows high affinity for the human gastrin-releasing peptide receptor (GRP-r), which is overexpressed on several types of cancer, including prostate, breast, gastrointestinal, and small cell lung cancer. Thus, radiolabeled BN or BN analogs may prove to be specific tracers for diagnostic and therapeutic targeting of GRP-r-positive tumors in nuclear medicine. This study evaluated a novel BN analog labeled with the positron emitter ⁶⁸Ga for receptor imaging with PET. Methods: DOTA-PEG₂-[D-Tyr⁶,β-Ala¹¹,Thi ¹³,Nle¹⁴] BN(6-14) amide (BZH3) (DOTA is 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴,-tetraacetic acid; PEG is ethyleneglycol (2-aminoethyl)carboxymethyl ether) was synthetized using the Fmoc strategy and radiolabeled with either ⁶⁷Ga or ¹⁷⁷Lu for in vitro and biodistribution experiments. ⁶⁸Ga for PET was obtained from a ⁶⁸Ge/⁶⁸Ga generator. In vitro binding, internalization, and efflux were determined using the pancreatic tumor cell line AR42J. Biodistribution of the peptide as a function of time and dose was studied in AR42J tumor-bearing mice. Results: In vitro assays demonstrated a high affinity of ⁶⁷Ga-BZH3 (dissociation constant = 0.46 nmol/L), a rapid internalization (70% of total cell-associated activity was endocytosed after a 15-min incubation), and an intracellular retention half-life (t _1/2) of the 67Ga activity ^of 16.5 ± 2.4 h. Biodistribution indicated a dose-dependent uptake in the tumor and a prolonged tumor residence time (t_1/2 ∼ 16 h). Clearance from GRP-r-negative tissues was fast, resulting in high tumor-to-tissue ratios as early as 1 h after injection. Replacing ⁶⁷Ga by ¹⁷⁷Lu, a therapeutic radionuclide, for peptide labeling resulted in a slightly reduced (∼20%) tumor uptake and tumor residence time of ¹⁷⁷Lu-BZH3. In contrast, ¹⁷⁷Lu decline in the pancreas was significantly accelerated by a factor of ∼3 compared with that of ⁶⁷Ga. PET of mice with ⁶⁸Ga-BZH3 clearly delineated tumors in the mediastinal area. Conclusion: The promising in vivo data of ⁶⁸Ga-BZH3 indicate its potential for an improved localization of GRP-r-positive tumors and also suggest its application in patients. PET may also be favorably used for GRP-r density determination, a prerequisite for therapeutic applications.