An improved synthesis of 1'-[18F]fluoroethyl-β-D-lactose ([18F]-FEL) for positron emission tomography imaging of pancreatic cancer.

Introduction: Earlier, we reported syntheses of ethyl-β-D-galactopyranosyl-(1,4')-2'-deoxy-2'-[(18)F]fluoro-β-D-glucopyranoside (Et-[(18)F]FDL) and 1'-[(18)F]fluoroethyl-β-D-lactose ([(18)F]-FEL) for positron emission tomography (PET) of pancreatic carcinoma. Et-[(18)F]FDL requires a precursor, which involves 11 steps to synthesize and produces overall low yields. Synthesis of precursors for [(18)F]-FEL requires four steps, but those precursors produced low radiochemical yields. Here, we report new precursors and an improved synthesis of [(18)F]-FEL.

Method: Two precursors, 1'-(methanesulfonyl)ethyl-2',3',6',2,3,4,6-hepta-O-acetyl-β-D-lactose 2a and 1'-(p-nitrophenyl-sulfonyl)ethyl-2',3',6',2,3,4,6-hepta-O-acetyl-β-D-lactose 2b, were synthesized from lactose in four steps. Radiofluorination reactions were performed using K(18)F/kryptofix and the crude product [(18)F]-3 was purified by HPLC. Basic hydrolysis of [(18)F]-3 produced 1'-[(18)F]fluoroethyl-β-D-lactose [(18)F]-4, which was neutralized, diluted with saline, filtered on a 0.22-µm filter, and analyzed by radio-TLC.

Results: The average radiochemical yields of [(18)F]-4 (d. c.) from 2a and 2b were 21% (n = 6) and 65% (n = 6), respectively, with >99% radiochemical purity and specific activity of 55.5 GBq/µmol. Synthesis time was 90-95 min from the end of bombardment.

Conclusion: An improved synthesis of [(18)F]FEL has been achieved in high yields, with high purity and specific activity. Precursor 2b with this method should be applicable for high yield automated production in a commercial synthesis module for clinical application.