Toward [<sup>18</sup>F]-Labeled Aryltrifluoroborate Radiotracers: In Vivo Positron Emission Tomography Imaging of Stable Aryltrifluoroborate Clearance in Mice

The use of a boronic ester as a captor of aqueous [<sup>18</sup>F]-fluoride has been previously suggested as a means of labeling biomolecules in one step for positron emission tomography (PET) imaging. For this approach to be seriously considered, the [<sup>18</sup>F]-labeled trifluoroborate should be humorally stable such that it neither leaches free [<sup>18</sup>F]-fluoride to the bone nor accumulates therein. Herein, we have synthesized a biotinylated boronic ester that is converted to the corresponding trifluoroborate salt in the presence of aqueous [<sup>18</sup>F]-fluoride. In keeping with its in vitro aqueous kinetic stability at pH 7.5, the trifluoroborate appears to clear in vivo quite rapidly to the bladder as the stable trifluoroborate salt with no detectable leaching of free [<sup>18</sup>F]-fluoride to the bone. When this labeled biotin is preincubated with avidin, the pharmacokinetic clearance of the resulting complex is visibly altered. This work validates initial claims that boronic esters are potentially useful as readily labeled precursors to [<sup>18</sup>F]-PET reagents.