An indirect fluorine-18 labeling of biomolecules: a method as good as direct fluorination

Objectives: Our objective is to develop an improved, optimized, simple, reproducible and clinically translatable method to label biomolecules with fluorine-18.

Methods: Fluorine-18 labeling of biomolecules was achieved in two steps using 6-fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, a useful synthon first reported by Olberg et al. [1]. Fluorine-18 radiolabeled 6-fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester for this study was prepared quickly by passing its quaternary ammonium triflate precursor through a fluorine-18 trapped Sep-Pak (PS-HCO₃) as reported by us recently [2]. This process takes less than 5 min. The conjugation efficiency of the labeled ester prepared by this method was evaluated with small molecule (DCFPyL), peptide [c(RGDfK)] and protein (albumin) substrates. As proof of principle the quality of [¹⁸F]DCFPyL was tested in vitro (binding assays) and in vivo (biodistribution/imaging).

Results: This is a straight forward approach requiring no azeotropic drying of fluorine-18 with conjugation reactions completed in 10 min at 40-50°C. The overall radiochemical yield was 25-43% (n= 9, uncorrected) in 30-45 min with a specific activity of 1000-2600 Ci/mmol. In vitro binding studies with [¹⁸F]DCFPyL exhibited high affinity (nM) for prostate-specific membrane antigen (PSMA) in human prostate cancer cells with known high PSMA expression. In vivo [¹⁸F]DCFPyL biodistributions and PET images with xenograft mouse models using this same tumor cell line were comparable with previously reported results indicating that the biological activity had been retained[3].

Conclusion: We have successfully prepared [¹⁸F]c(RGDfK), [¹⁸F]DCFPyL and [¹⁸F]albumin in short synthesis times (30-50 min) with moderate to high radiochemical yields. This method is comparable with direct fluorine-18 labeling approaches. Due to the simplicity of the method, it could easily be automated for routine clinical production. References: [1] Olberg DE, Arukwe JM, Grace D, Hjelstuen OK, Solbakken M, Kindberg GM, et al. J. Med. Chem., 2010, 53, 1732-1740. [2] Basuli F, Zhang X, Jagoda EM, Choyke PL, and Swenson RE. Nucl Med Biol., 2016;43:770-772. [3] Chen Y, Pullambhatla M, Foss CA, Byun Y, Nimmagadda S, Srinivasan S, et al. Clin Cancer Res 2011;17:7645-7653. Research Support: This study was funded by the intramural program of the National Institutes of Health.