Radiosynthesis of [¹⁸F]cabozantinib and [¹⁸F]fluoroethyl-sunitinib: two RTK-inhibitors of VEGFR-2

Objectives: Radiolabeled inhibitors of the angiokinase VEGFR-2 might be suitable probes for monitoring induction of angiogenesis and anti-angiogenic therapy response in vivo with PET. [1,2]. We selected two VEGFR-2 inhibitors, cabozantinib (IC₅₀, 0.03 nM) and sunitinib (IC₅₀, 9.0 nM), both bearing a fluorine substituent, as lead structures for ¹⁸F-radiolabeled PET tracers.
Methods: [¹⁸F]Cabozantinib is synthesized by a 3-step radiosynthesis with final condensation of 4-[¹⁸F]fluoroaniline with an acyl chloride precursor. 4-[¹⁸F]Fluoroaniline is formed by substitution of 1,4-dinitrobenzene with [¹⁸F]fluoride, subsequent reduction of the intermediate 4-[¹⁸F]fluoro-nitrobenzene with Pd/C and NaBH₄. Since [¹⁸F]sunitinib is not accessible via direct nucleophilic ¹⁸F-substitution, we developed the 5-fluoroethylated derivative (IC₅₀, 9 nM) as well the corresponding radiolabeled analogue.
Results: 4-[¹⁸F]fluoroaniline was obtained in >60% rcy starting from [ 18F]fluoride after SPE purification. [¹⁸F]Cabozantinib was formed by reaction of 4-[¹⁸F]fluoroaniline with 10 mg of acyl precursor in THF at rt in >90% rcy. HPLC purification delivered [¹⁸F]cabozantinib in 95% purity and specific activity >20 GBq/μmol. Reaction of the methanesulfonyl-substituted sunitinib precursor with [¹⁸F]fluoride resulted in 8% ¹⁸F-incorporation. HPLC purification yielded [¹⁸F]fluoroethyl-sunitinib in 100 MBq scale. First in vitro investigations on VEGFR-2 expressing human A 2058 melanoma cell line showed cellular uptake of [¹⁸F]cabozantinib up to 790±100 %ID/mg protein at 60 min that could be significantly blocked by 46±3% by its non-radioactive counterpart (10 μM). For [¹⁸F]fluoroethyl-sunitinib the uptake reached 340±48 %ID/mg protein at 60 min. Stability tests in rat blood over 60 min revealed almost no metabolism for both radiotracers.
Conclusions: With the reliable radiosynthesis of [¹⁸F]cabozantinib and [¹⁸F]fluoroethyl-sunitinib two radiolabeled VEGFR-2 inhibitors with nano- and sub-nanomolar affinity and high in vivo stability are available.
Acknowledgements
References [1] Slobbe P. et al (2012) Drug Discov. Today, 17, 1175-1187
[2] Kniess T. (2012) Curr Pharm Des, 18, 2867-2874