Imidazolium‐methylene‐trifluoroborate: A novel radioprosthetic group validated with preclinical 18F‐Positron Emission Tomography imaging of Prostate Specific Membrane Antigen in mice

Organotrifluoroborates have gained acceptance as radioprosthetic groups for radiofluorination. Of these, the zwitterionic prosthetic group “AMBF3” with a quaternary dimethylammonium ion dominates the trifluoroborate space. Herein, we report on imidazolium‐methylene trifluoroborate (ImMBF3) as an alternative radioprosthetic group and report on its properties in the context of a PSMA‐targeting EUK ligand that was previously been conjugated to AMBF3. The ImMBF3 is readily synthesized from imidazole and conjugated via CuAAC “click” chemistry to give a structure similar to PSMA‐617. 18F‐labeling proceeded in one step per our previous reports and imaged in LNCaP‐xenograft bearing mice. The [18F]‐PSMA‐617‐ImMBF3 tracer proved to be less polar (LogP7.4 = −2.95 ± 0.03) while showing a significantly lower solvolytic rate (t1/2 = 8100 min) and slightly higher molar activity (Am) at 174 ± 38 GBq/μmol. Tumor uptake was measured at 13.7 ± 4.8%ID/g and a tumor:muscle ratio of 74.2 ± 35.0, tumor:blood ratio of 21.4 ± 7.0, tumor:kidney ratio of 0.29 ± 0.14, and tumor:bone ratio of 23.5 ± 9.5. In comparison with previously reported PSMA‐targeting EUK‐AMBF3 conjugates, we have altered the LogP7.4 value, tuned the solvolytic half‐life of the prosthetic, and increased radiochemical conversion while achieving similar tumor uptake, contrast ratios, and molar activities compared with AMBF3 bioconjugates. A new organotrifluoroborate radioprosthetic group, imidazolinium‐methylene‐trifluoroborate, is synthesized and appended to a ligand for aqueous one‐step radiofluorination. It is validated in the context of imaging PSMA on LNCaP xenografts in mice. By slightly weakening the B‐F bonds to solvolysis, radiochemical conversion by isotope exchange is increased, yet no significant bone uptake is observed in vivo, thereby validating this prosthetic for potential use in PET imaging.