Synthesis and evaluation of ¹⁸F-labeled ammoniomethyl-trifluoroborate derivatives of somatostatin in a human breast cancer xenograft model

Objectives High affinity somatostatin (SST) analogues labeled with ⁶⁸Ga are used in the diagnosis and staging of neuroendocrine tumors. Such tracers could benefit from radiolabeling with ¹⁸F. We previously reported an ammoniomethyl-trifluoroborate (AmBF₃) moiety that facilitates ¹⁸F radiolabeling by a simple ¹⁸F-¹⁹F isotope-exchange reaction. Here we apply this technique to three SST receptor 2a (SSTR2a) ligands to determine if binding affinity and uptake for the target are preserved. The agonist sequences tyr³-octreotide (TOC) and tyr³-octreotate (TATE) are used, along with the antagonist sequence Cpa-c[D-Cys-Aph(Hor)-D-Aph(Cbm)-Lys-Thr-Cys]-D-Tyr-NH₂ (JR11).

Methods The three peptides were synthesized by solid-phase peptide synthesis and modified with an azide at the N-terminus. Azidoacetyl-TOC (N₃-TOC), N₃-TATE and N₃-JR11 were click-reacted with N-propargyl-N,N-dimethylammoniomethyltrifluoroborate and purified by HPCL to obtain the cold-standards: AmBF₃-TOC, AmBF₃-TATE and AmBF₃-JR11, respectively. Their affinity for SSTR2a was analysed in a membrane-based competition binding assay. For radiolabeling, 100 nmol of AmBF₃-peptide was reacted with ¹⁸F-floride via ¹⁸F-¹⁹F isotope-exchange reaction, and purified by HPLC. The PET/CT imaging and biodistribution were evaluated in NOD scid gamma mice harbouring the SSTR-positive human breast cancer xenograft model ZR-75-1.

Results See Table 1 for summary of results. All tracers had low nanomolar binding affinity for SSTR2a. The ¹⁸F-fluorinated ligands were obtained in good radiochemical yield and purity, with specific activities > 1 Ci/μmol, and had high uptake in the tumor and pancreas at 1h post-injection. The excretion profile was predominantly renal, except for [¹⁸F]AmBF₃-JR11, which showed higher uptake in the liver (3.83 ± 0.50 %ID/g compared to 0.89 ± 0.08 %ID/g for [¹⁸F]AmBF₃-TOC and 0.59 ± 0.17 %ID/g for [¹⁸F]AmBF₃-TATE). Tumor-to-blood ratios for [¹⁸F]AmBF₃-TOC, [¹⁸F]AmBF₃-TATE, and [¹⁸F]AmBF₃-JR11 were 14.2 ± 6.42, 15.5 ± 7.45, and 14.8 ± 3.15 respectively, and tumor-to-muscle ratios were 61.4 ± 27.4, 60.0 ± 27.4, and 60.2 ± 16.4, respectively.

Conclusions Here we demonstrate successful radiolabeling of well-known SST analogues with ¹⁸F in good radiochemical yield, purity and specific activity. The resulting tracers retain their binding affinity for the target and show promising in vivo tumor uptake and biodistribution. This ¹⁸F-labeling strategy via ¹⁸F-¹⁹F isotope-exchange on the AmBF₃ moiety is fast, and easy to use. Furthermore, the half-life of ¹⁸F allows for the distribution of these ¹⁸F-labeled tracers to other PET facilities, thus making them more accessible to patients with SSTR2a-positive breast and neuroendocrine tumors.

• Download figure
• Open in new tab
• Download powerpoint