A new and rapid method of labeling peptides with F-18

Objectives A new and rapid one-pot, two-step method for labeling peptides with F-18, based on a metal chelator, was optimized to prepare ¹⁸F-labeled octreotide, bombesin and a pretargeting peptide.

Methods ¹⁸F was bound to aluminum and incubated with NOTA-conjugated peptides. The labeling was optimized with respect to the amount of aluminum, peptide concentration and labeling buffer. Radiochemical yield, specific activity, in vitro stability and receptor affinity were determined. ¹⁸F-octreotide was studied in vivo in AR42J tumor-bearing mice and compared to ⁶⁸Ga-octreotide.

Results The pretargeting peptide was labeled with Al¹⁸F in a single step with 20% yield. NOTA-bombesin and NOTA octreotide were labeled with 40% and 50% yield, respectively. The labeled products were purified by HPLC to remove unbound ¹⁸F and unlabeled peptide. Radiolabeling, including purification, was performed within 45 min. The specific activity of ¹⁸F-NOTA-octreotide was 45 GBq/µmol and was stable in serum for 4 h at 37° C. Labeling was optimal in Na-acetate buffer at pH 4.1. Biodistribution studies at 2 h pi showed rapid tumor uptake of the ¹⁸F-octreotide (28.3 ± 5.2 %ID/g), which could be blocked by an excess of unlabeled peptide (8.6 ± 0.7 %ID/g). Biodistribution of ⁶⁸Ga-NOTA-octreotide was similar to that of ¹⁸F-NOTA-octreotide. Moreover, ¹⁸F-octreotide was stable in vivo, since bone uptake was only 0.4 ± 0.2 %ID/g, whereas non-chelated Al¹⁸F rapidly accumulated in bone (36.9 ± 5.0 %ID/g). MicroPET/CT scans showed excellent tumor delineation, as well as some retention in the kidney cortex.

Conclusions By labeling NOTA-peptides with ¹⁸F, a simple and rapid peptide fluorination method has been developed. High specific activity could be obtained and the Al¹⁸F-NOTA complex proved to be stable in vivo. This method will be applicable for various NOTA-conjugated peptides.

Research Support Supported in part by NIH grant 1R43EB003751 to WJMcB