Optimized labeling of NOTA-conjugated octreotide with F-18

Abstract

We recently reported a facile method based on the chelation of [(18)F]aluminum fluoride (Al(18)F) by NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid). Here, we present a further optimization of the (18)F labeling of NOTA-octreotide (IMP466). Octreotide was conjugated with the NOTA chelate and was labeled with (18)F in a two-step, one-pot method. The labeling procedure was optimized with regard to the labeling buffer, ionic strength, peptide concentration, and temperature. Radiochemical yield, specific activity, in vitro stability, and receptor affinity were determined. Biodistribution of (18)F-IMP466 was studied in AR42J tumor-bearing mice. In addition, microPET/CT images were acquired. IMP466 was labeled with Al(18)F in a single step with 97% yield in the presence of 80% (v/v) acetonitrile or ethanol. The labeled product was purified by HPLC to remove unlabeled peptide and unbound Al(18)F. The radiolabeling, including purification, was performed for 45 min. Specific activities of 48,000 GBq/mmol could be obtained. (18)F-IMP466 showed a high tumor uptake and excellent tumor-to-blood ratios at 2 h post-injection. In addition, the low bone uptake indicated that the Al(18)F-NOTA complex was stable in vivo. PET/CT scans revealed excellent tumor delineation and specific accumulation in the tumor. Uptake in receptor-negative organs was low. NOTA-octreotide could be labeled with (18)F in quantitative yields using a rapid two-step, one-pot, method. The compound was stable in vivo and showed rapid accretion in SSTR(2)-receptor-expressing AR42J tumors in nude mice. This method can be used to label other NOTA-conjugated compounds such as RGD peptides, GRPR-binding peptides, and Affibody molecules with (18)F.

Fig. 1

RP-HPLC chromatograms of the IMP466 ¹⁸F-labeling mix (a) and the purified ¹⁸F-IMP466 (b). Red traces represent radioactivity (left y-axis) and blue traces represent UV signal (right y-axis). In the HPLC chromatogram of the crude mixture, unbound Al¹⁸F eluted with the void volume (R _t = 0.8 min). Two radioactive peaks correspond to the stereoisomers of radiolabeled peptide (R _t = 17.4 and R _t = 19.8 min). Finally, the unlabeled IMP466 was present in the UV channel (R _t = 21.4 min). After purification, only two radioactive peptide peaks are observed, indicating the formation of two stereoisomers

Fig. 2

Competitive binding assay (apparent IC₅₀) of ¹⁹F-IMP466 and ¹¹⁵In-DTPA-octreotide determined on AR42J tumor cells. Values on the y-axis represent binding expressed as a percentage of the binding without competitor

Fig. 3

Biodistribution of ¹⁸F-IMP466 and unbound Al¹⁸F at 2 h p.i. in AR42J tumor-bearing mice (n = 5/group). Tumors weighed 0.07–0.36 g

Fig. 4

Anterior 3D volume-rendering projections of fused PET and CT scans of mice with a s.c. AR42J tumor on the right flank injected with ¹⁸F-IMP466 (a) and with ¹⁸F-IMP466 in the presence of an excess of unlabeled IMP466 (b). “T” indicates tumor, “K” indicates kidneys, and “I” indicate intestine. Scans were recorded at 2 h p.i