Synthesis and evaluation of fluorine-18 labeled ipomeanol as a CYP4B1 expression imaging probe

Objectives 4-Ipomeanol (1-(3-furyl)-4-hydroxy-1-pentanone, 4-IPO) is naturally occurring pulmonary toxin. The major metabolic pathway of 4-IPO by cytochrome P450 enzymes involves furan ring opening leading to the formation of unsaturated α,β-aldehyde/ketone of dialdehyde reactive metabolite. Among of cytochrome P450 isozyme, CYP4B1, efficiently converts the prodrug, 4-IPO, to the alkylating metabolite which was highly reactive and binding covanently with tissues immediately at its site of formation. The present study is the development of F-18 labeled 4-IPO which has efficient targeting of CYP4B1 gene transfected cells.

Methods The target compound, 5-fluoro-4-hydroxy-1-furan-3-yl-1-pentanone (F-4-IPO), was synthesized in twelve steps from 3-furaldehyde. [¹⁸F]F-4-IPO was synthesized from mesylate precursor through three-consecutive steps - displacement of [¹⁸F]fluoride, reduction and acidic-deprotection. C6-CYP4B1 cells were established using plasmid transfection. Evaluation of CYP4B1 expression level was confirmed by RT-PCR and MTT assay. Biodistribution study of [¹⁸F]F-4-IPO was investigated in normal rats. The cellular uptake of [¹⁸F]F-4-IPO and [³H]-4-IPO was performed in parental and CYP4B1-transfected cells and compared.

Results The radiochemical yield of [¹⁸F]F-4-IPO was 20-35% with >95% of radiochemical purity. [¹⁸F]F-4-IPO showed over 96% stability until 120 min in human serum. Biodistribution of [¹⁸F]F-4-IPO in normal rats showed similar uptake patterns to [³H]-4-IPO. The cellular uptake ratios of [¹⁸F]F-4-IPO and [³H]-4-IPO in C6-CYP4B1 cells increased up to about 1.8 times at 30 min for [¹⁸F]F-4-IPO and 2.3 times at 360 min for [³H]-4-IPO higher than that of parental cells.

Conclusions Our results show that [¹⁸F]F-4-IPO is an excellent analog of 4-IPO, and deserves further studies to evaluate its utility for PET imaging of CYP4B1 transfected cancer cells