Metabolic stability of 6,7-dialkoxy-4-(2-, 3- and 4-[18F]fluoroanilino)quinazolines, potential EGFR imaging probes

Epidermal growth factor receptors (EGFR), upregulated in many tumor types, have been a target for therapeutic development and molecular imaging. The objective of this study was to evaluate the distribution and metabolic characteristics of fluorine-18 labeled anilinoquinazolines as potential imaging agents for EGFR tyrosine kinase expression. Fluorine-18 labeled fluoronitrobenzenes were prepared by reaction of potassium cryptand [18F]fluoride with 1,2- and 1,4-dinitrobenzenes, and 3-nitro-N,N,N-trimethylanilinium triflate in 5 min. Decay-corrected radiochemical yields of [18F]fluoride incorporation into the nitro-aromatic compounds were 81 ± 2%, 44 ± 4% and 77 ± 5% (n = 3–5) for the 2-, 3- and 4-fluoro isomers, respectively. Sodium borohydride reduction to the corresponding [18F]fluoroanilines was achieved with greater than 80% conversion in 5 min. Coupling of [18F]fluoroaniline-hydrochlorides to 6,7-dimethoxy-4-chloro-quinazoline gave the corresponding 6,7-dimethoxy-4-(2-, 3- and 4-[18F]fluoroanilino)quinazolines in 31 ± 5%, 17 ± 2% and 55 ± 2% radiochemical yield, respectively, while coupling to the 6,7-diethoxy-4-chloro-quinazoline produced 6,7-diethoxy-4-(2-, 3- and 4-[18F]fluoroanilino)quinazolines in 19 ± 6%, 9 ± 3% and 36 ± 6% radiochemical yield, respectively, in 90 min to end of synthesis from [18F]fluoride. Biodistribution of 2- and 4-[18F]fluoroanilinoquinazolines was conducted in tumor-bearing mice (MDA-MB-435 and MDA-MB-468 xenografts). Low tumor uptake (<1% injected dose per gram (ID/g) of tissue up to 3 h postinjection of the radiotracers) was observed. High bone uptake (5–15% ID/g) was noted with the 4-[18F]fluoroanilinoquinazolines. The metabolic stabilities of radiolabeled quinazolines were further evaluated by incubation with human female cryopreserved isolated hepatocytes. Rapid degeneration of the 4-fluoro-substituted compounds to baseline polar metabolites was observed by radio-TLC, whereas, the 2- and 3-[18F]fluoroaniline derivatives were significantly more stable, up to 2 h, corroborating the in vivo biodistribution studies. para-Substituted [18F]fluoroanilines, a common structural motif in radiopharmaceuticals, are highly susceptible to metabolic degradation.

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