A Novel Facile Method of Labeling Octreotide with 18F-Fluorine

Abstract

Several methods have been developed to label peptides with ¹⁸F. However, in general these are laborious and require a multistep synthesis. We present a facile method based on the chelation of ¹⁸F-aluminum fluoride (Al¹⁸F) by 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). The method is characterized by the labeling of NOTA-octreotide (NOTA-d-Phe-cyclo[Cys-Phe-d-Trp-Lys-Thr-Cys]-Throl (MH⁺ 1305) [IMP466]) with ¹⁸F. Methods: Octreotide was conjugated with the NOTA chelate and labeled with ¹⁸F in a 2-step, 1-pot method. The labeling procedure was optimized with regard to the labeling buffer, peptide, and aluminum concentration. Radiochemical yield, specific activity, in vitro stability, and receptor affinity were determined. Biodistribution of ¹⁸F-IMP466 was studied in AR42J tumor–bearing mice and compared with that of ⁶⁸Ga-labeled IMP466. In addition, small-animal PET/CT images were acquired. Results: IMP466 was labeled with Al¹⁸F in a single step with 50% yield. The labeled product was purified by high-performance liquid chromatography to remove unbound Al¹⁸F and unlabeled peptide. The radiolabeling, including purification, was performed in 45 min. The specific activity was 45,000 GBq/mmol, and the peptide was stable in serum for 4 h at 37°C. Labeling was performed at pH 4.1 in sodium citrate, sodium acetate, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, and 2-(N-morpholino)ethanesulfonic acid buffer and was optimal in sodium acetate buffer. The apparent 50% inhibitory concentration of the ¹⁹F-labeled IMP466 determined on AR42J cells was 3.6 nM. Biodistribution studies at 2 h after injection showed a high tumor uptake of ¹⁸F-IMP466 (28.3 ± 5.2 percentage injected dose per gram [%ID/g]; tumor-to-blood ratio, 300 ± 90), which could be blocked by an excess of unlabeled peptide (8.6 ± 0.7 %ID/g), indicating that the accumulation in the tumor was receptor-mediated. Biodistribution of ⁶⁸Ga-IMP466 was similar to that of ¹⁸F-IMP466. ¹⁸F-IMP466 was stable in vivo, because bone uptake was only 0.4 ± 0.2 %ID/g, whereas free Al¹⁸F accumulated rapidly in the bone (36.9 ± 5.0 %ID/g at 2 h after injection). Small-animal PET/CT scans showed excellent tumor delineation and high preferential accumulation in the tumor. Conclusion: NOTA-octreotide could be labeled rapidly and efficiently with ¹⁸F using a 2-step, 1-pot method. The compound was stable in vivo and showed rapid accretion in somatostatin receptor subtype 2–expressing AR42J tumors in nude mice. This method can be used to label other NOTA-conjugated compounds with ¹⁸F.