Radiosynthesis of F-18-Crizotinib, a potential radiotracer for PET imaging of the P-glycoprotein transport function at the blood-brain barrier

Complete text of publication follows: Objective: Active transporters in the brain such as P-glycoprotein (ABCB1) effectively transport several drugs out of the brain, including anticancer agents thus leading to lowered efficacy of the treatment in the central nervous system. Crizotinib is a tyrosine kinase inhibitor and an approved treatment for non-small cell lung carcinoma, and its brain accumulation is mainly restricted by ABCB1.1 This selectivity for ABCB1 of Crizotinib makes it a promising radiotracer to image this transporter. Herein, we report the preparation of two different precursors and for the first time a two-step fluorine-18 radiolabeling of Crizotinib. Methods: A spirocyclic hypervalent iodine (III) complex and a pheno-fluor activated phenol were synthesized as precursors for radiosynthesis with [¹⁸F]fluoride (Figure). Radiofluorination of these precursors was realized using a TRACERlab FXFN module, and conditions were optimized while monitoring the conversion rate by radio-TLC. Removal of the Boc-protecting groups was also optimized for the spirocyclic hypervalent iodine precursor and monitored by radio-HPLC. Radiochemical yields (RCY) and radiochemical purity (RCP) were assessed by analytical radio-HPLC. Results: The spirocyclic hypervalent iodine precursor was synthesized from 2,4-dichloro-1-iodobenzene in 5 steps and 2% overall yield. The activated phenol precursor was synthesized from Crizotinib in 5 steps and 50% overall yield. Up to 52% conversion rate was obtained for the radiofluorination of the spirocyclic hypervalent iodine precursor using tetraethylammonium bicarbonate as a base in DMF at 160 C for 10 minutes. Deprotection with 3 M HCl(aq) at the same temperature for 10 minutes was accomplished with full conversion and afforded ¹⁸F-Crizotinib with >98% RCP and 7% non-isolated RCY. Final HPLC purification is ongoing. Radiofluorination of the activated phenol resulted in 60% conversion rate using KHCO₃ as base in butanone: EtOH (10:1) at 130 C for 10 minutes, further investigations of this method are in progress. Conclusion: Two original precursors for the radiofluorination of Crizotinib were successfully synthesized, and ¹⁸F-Crizotinib was radiolabeled from the spirocyclic hypervalent iodine precursor in good yields and high purity. Purification conditions are currently being explored to assess ready-to-inject ¹⁸F-Crizotinib, which will be used as a radiotracer to explore the ABCB1 function at the blood brain barrier in rodents with PET imaging