Radioiodination Techniques for Aromatic Amino Acids

Prevailing boron carriers for BNCT are disodium mercaptoundecahydro-closo-dodecaborate (borocaptate sodium, BSH) and 4-dihydroxyboryl phenylalanine (4-boronophenylalanine, BPA).1,2 For clinical patient studies BPA is administred as an anionic fructose complex (BPA-F) infusions. As an aromatic amino acid without the fructose moiety BPA is a structural analogue for natural aromatic amino acid tyrosine. In BPA the hydroxyl group of tyrosine is substituted by the dihydroxyboryl group (or borono group), —B(OH)2. The dihydroxyboryl group is known to be fragile in aromatic molecules and B(OH)2-group can be substituted by electrophiles.3,4 However, by direct electrophilic fluorination of BPA with [18F]AcOF or [18F]F2 followed by HPLC separation, 4-borono-2-[18F]fluorophenylalanine (18FBPA) was prepared with radiochemical yields of 25-35% and with a radiochemical purity of over 99%.5 Clinical patient studies have shown that positron emission tomography (PET) with fluorinated analogue of BPA is a valuable technique for prediction of the effectiveness of boron neutron capture therapy using BPA as a boron carrier.6,7