Challenges of the copper-mediated radiofluorination

Ziel: Copper-mediated radiofluorination (CMRF) was a breakthrough of the last decade in the development of non-activated 18F-aryl-bearing radiopharmaceuticals.1 Despite extensive studies and improvements of the radiolabelling conditions, the formation of H-side product 3 and OH-side product 4 still possess a challenge in these Suzuki/Stille type reactions. In our work, we faced similar problems and additionally, the continuous hydrolysis of the boronic ester precursor 1 during the semi-preparative HPLC purification resulted in another impurity 5. In this study, we tried to address and overcome these challenges.

Methoden: The CMRF of 1 bearing a non-activating aromatic substituent at para position was optimized by varying the following parameters: solvent (DMA and DMI), reaction time (5 - 20 min), temperature (110 - 130 °C) and molar ratio of precursor 1 to Cu-complex (1:3, 1:4. 2:3, 1:8). Compounds 3 and 4 were synthesized as references for identification of the side-products in the final radiotracer formulation. Various stationary phases (pentafluorophenyl, cyano, phenyl, C18) and mobile phases were tested to separate unwanted side products 3 and 4 by HPLC. Solid phase extraction (SPE) was performed with the C18 plus cartridge before the semi-preparative HPLC purification of [18F]2.

Ergebnisse: [18F]2 was achieved with a high radiochemical conversion of 85 % using 2 mg of 1, 10 mg of [Cu(OTf)2(py)4] (molar ratio of 1:4) in n-BuOH/DMI (1:2, v/v) at 110 °C for 5 min. SPE under acidic conditions (pH 2) resulted in around 90 % recovery of [18F]2 compared to only 20 % under neutral conditions. Due to hydrolysis of residual precursor 1 with TFA prior to semi-preparative HPLC purification, the content of 5 in the formulated final product could be reduced from 25 % (without TFA hydrolysis) to 0.5-1 %. The side-products 3 and 4 were successfully separated using a ReproSil C18AQ column (250 x 20 mm) and 48 % THF/ACN (1:1, v/v) buffered with 20 mM ammonium acetate. Compound [18F]2 was isolated with a radiochemical purity of >95 % and molar activities in the range of 60 GBq/µmol were achieved.

Schlussfolgerungen: Despite a number of hurdles, the CMRF reactions are currently being widely employed for the production of radiopharmaceuticals embodying non-activated 18F-aryl scaffolds. To overcome the occasional purification difficulties of the resulting radioligands, further improvements and mechanistic studies need to be undertaken.

Referenzen:

[1] Preshlock, S., et al. Chemical Reviews 2016, 116(2), 719-766.