A New 18F-Labeled Folic Acid Derivative with Improved Properties for the PET Imaging of Folate Receptor–Positive Tumors

Abstract

The folate receptor is a proven target for folate-based diagnosis and treatment of cancer. Several folic acid conjugates have been developed as radiopharmaceuticals, but a suitable (18)F-labeled folic acid derivative for routine clinical use is still lacking. The purpose of this study was to investigate the potential of 2'-(18)F-fluorofolic acid as a PET agent for folate receptor-positive tumors. The binding affinity of the cold reference compound 2'-fluorofolic acid was determined by in vitro displacement assays using human folate receptor-positive KB cells and (3)H-folic acid. (18)F labeling of 2'-fluorofolic acid was accomplished via a direct nucleophilic aromatic substitution of N(2)-(N,N-dimethylamino-methylene)-2'-nitrofolic acid di-tert-butylester followed by acidic cleavage of the amino and carboxylic protecting groups. The new radiofolate was evaluated in nude mice bearing KB tumor xenografts under control and blocking conditions. Animals were either scanned from 75 to 105 min after injection of the radiotracer or sacrificed 75 min after injection for ex vivo biodistribution studies. 2'-fluorofolic acid showed a high binding affinity (inhibition constant, 1.8 ± 0.1 nM) for the folate receptor. Direct aromatic (18)F labeling of 2'-fluorofolic acid was achieved within 80 min via a convenient 2-step procedure in satisfactory radiochemical yields. The new radiotracer exhibited excellent pharmacokinetics with fast renal clearance and only moderate hepatobiliary elimination. Uptake of 2'-(18)F-fluorofolic acid in folate receptor-positive KB tumors was high and specific, allowing a clear-cut visualization by PET. 2'-(18)F-fluorofolic acid, obtained via an integrated approach, is a promising PET agent for folate receptor-positive tumors and outperforms previously reported (18)F-labeled folates.