Method for stabilizing non carrier added 2-[¹⁸F]fluoromethyl-l-phenylalanine, a new tumour tracer, during radiosynthesis and radiopharmaceutical formulation☆

Abstract 

Introduction

Non carrier added (NCA) 2-[¹⁸F]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 [¹⁸F⁻] occurred in acetonitrile using K₂CO₃/K_2.2.2 (120°C, 5 min). NCA 3-(2-[¹⁸F⁻]Fluoromethyl-phenyl)-2-tert-butoxycarbonylamino-propionic acid tert-butyl ester recovered from HPLC was submitted to deprotection in TFA/CH₂Cl₂ in the presence of CaCl₂. After evaporation and adsorption on a mini C18 column, the tracer was recovered in 4 ml of H₂O. Appropriate amounts of CaCl₂ and NaCl solutions were added for isotonic formulation, and this solution was sterilised by a 0.2-μm Cathivex filter. Shelf-life stability in the presence of Ca²⁺ and Mg²⁺ 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 Ca²⁺ ions during deprotection, minimizing the loss of [¹⁸F⁻] 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 CaCl₂ allows a shelf-life of about 6 h without significant radiodefluorination.

Conclusion

The described synthetic route yields 40% of radiochemical pure NCA 2-[¹⁸F]fluoromethyl-l-phenylalanine within 105 min. A solution was found to reduce considerably the radiodefluorination. Addition of CaCl₂ 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.

Keywords: 2-[¹⁸F]Fluoromethyl-l-phenylalanine, Nucleophilic ¹⁸F-fluorination, Ca²⁺ ions, Stability, Shelf-life