One-step synthesis of [18 F]cabozantinib for use in positron emission tomography imaging of c-Met

Cabozantinib is an FDA approved kinase inhibitor for the treatment of medullary thyroid cancer and advanced renal cell carcinoma, which exerts its therapeutic effect by inhibiting, among others, the tyrosine kinase c-Met. Non-invasive imaging techniques are becoming increasingly important clinically to ensure drug efficacy, staging, monitoring and patient stratification. PET isotope labelled tyrosine kinase inhibitors have, for the same reason, potential as PET tracers for imaging of various cancers. Based on cabozantinib, we synthesized the novel boronic acid pinacol ester 4 as a labelling precursor, where the boronic ester moiety replaces the fluorine native to this kinase inhibitor. By this we wanted to explore whether recently developed Cu-mediated fluorination methods are adaptable to more complex substrates and thereby provide easy access to [18F]cabozantinib directly. Hydrolysis was implemented before preparative purification due to challenges with on-column hydrolysis of the precursor 4, and [18F]cabozantinib was obtained in ≥ 99 % radiochemical purity and in 2.8±0.05 % (n=4) isolated decay corrected yield in a synthesis time of 90 minutes. The molar activity of representative batches was determined to be 17±8 GBq/μmol.