Method for stabilizing non carrier added 2-[(18)F]fluoromethyl-l-phenylalanine, a new tumour tracer, during radiosynthesis and radiopharmaceutical formulation.
BACKGROUND
Non carrier added (NCA) 2-[(18)F]fluoromethyl-l-phenylalanine is currently used in a Phase I study. Improvement of the stability of the fluorobenzyl analogue, very sensitive to defluorination and hydrolysis, during the synthetic route and in the radiopharmaceutical formulation was devised.


METHODS
The protected brominated precursor was synthesised in three steps. The labelling with [(18)F(-)] occurred in acetonitrile using K(2)CO(3)/K(2.2.2) (120 degrees C, 5 min). NCA 3-(2-[(18)F(-)]Fluoromethyl-phenyl)-2-tert-butoxycarbonylamino-propionic acid tert-butyl ester recovered from HPLC was submitted to deprotection in TFA/CH(2)Cl(2) in the presence of CaCl(2). After evaporation and adsorption on a mini C18 column, the tracer was recovered in 4 ml of H(2)O. Appropriate amounts of CaCl(2) and NaCl solutions were added for isotonic formulation, and this solution was sterilised by a 0.2-microm Cathivex filter. Shelf-life stability in the presence of Ca(2+) and Mg(2+) ions was studied. Stereoisomeric purity was checked by chiral HPLC.


RESULTS
The labelling showed a reproducible labelling yield of at least 90%. The followed strategy and the presence of Ca(2+) ions during deprotection, minimizing the loss of [(18)F(-)] from the labile fluorobenzyl group, allowed to obtain a decay-corrected yield of 75%. No racemisation was observed. The radiopharmaceutical formulation containing 0.04 M CaCl(2) allows a shelf-life of about 6 h without significant radiodefluorination.


CONCLUSIONS
The described synthetic route yields 40% of radiochemical pure NCA 2-[(18)F]fluoromethyl-l-phenylalanine within 105 min. A solution was found to reduce considerably the radiodefluorination. Addition of CaCl(2) prior to deprotection limits the loss of radiofluoride to less than 10%. The calcium ions present in the final radiopharmaceutical formulation (0.04 M) assure a shelf-life of at least 6 h.
Kersemans
Ken
K
Radiopharmaceutical Chemistry, BEFY, Vrije Universiteit Brussel, 1090 Brussels, Belgium. kkersema@vub.ac.be
Bauwens
Matthias
M
Mertens
John
J
D002118:Calcium / Q000737:chemistry; D002626:Chemistry, Pharmaceutical / Q000379:methods*; D004337:Drug Carriers; D004355:Drug Stability; D005462:Fluorine Radioisotopes / Q000737:chemistry*; D005464:Fluorobenzenes / Q000737:chemistry*; D007553:Isotope Labeling / Q000379:methods; D010649:Phenylalanine / Q000031:analogs & derivatives* / Q000737:chemistry; D019275:Radiopharmaceuticals / Q000138:chemical synthesis
C531055:2-fluoromethylphenylalanine; D004337:Drug Carriers; D005462:Fluorine Radioisotopes; D005464:Fluorobenzenes; D019275:Radiopharmaceuticals; D010649:Phenylalanine; D002118:Calcium