Automated ¹⁸F-fluoroethylation - a labeling method for new potential COX-2 inhibitors

Objectives: The enzyme cyclooxygenase-2 (COX-2) regulates inflammation-associated processes also in various cancer entities. Hence, the visualization of COX-2 expression in vivo by radiolabeled COX-2 inhibitors provides a promising approach for functional characterization of certain solid tumors and their metastases by PET. With the aim to get towards highly affine ¹⁸F-radiolabeled fluoroethoxy substituted COX-2 inhibitors we developed a versatile automated procedure for ¹⁸F-fluoroethylation of hydroxyl compounds with mono- as well as bicyclic core structure.
Methods: Starting from two potent COX-2 inhibitors A* (IC₅₀ COX-2: 3 nM, O-ethoxy (1)) and B* (IC₅₀ COX-2: 5 nM, O-methoxy (2)) two new fluoroethoxy substituted derivatives A and B were developed. An indirect radiolabeling approach was used starting from the corresponding hydroxy precursor and 2-[¹⁸F]fluoroethyltosylate in a one-pot reaction using an automated nucleophilic synthesizer TracerLABFXN (GE) with subsequent purification to yield [¹⁸F]A and [¹⁸F]B (Figure 1).
Results: By an enzymatic assay we could confirm that the COX-2 inhibition potency was only slightly altered when the ethoxy group of A* or the methoxy group of B* was replaced by the fluoroethoxy moiety. Optimization of labeling conditions in terms of equivalents of ethylen-1,2-ditosylate and Cs₂CO₃ resulted in complete consumption of [¹⁸F]fluoroethyltosylate in the second step and decreased amount of non-radioactive by-products; a prerequisite for a one-pot process. In this manner [¹⁸F]A was obtained starting from [¹⁸F]fluoride within 70 min in 7.5% radiochemical yield (d.c.) with >95% radiochemical purity and 27-71 GBq/μmol specific activity inclusive HPLC purification (n=5). Analogously, [¹⁸F]B was obtained after 80 min in 7.8% radiochemical yield (d.c.) with a radiochemical purity of 98% and a specific activity of 19-43 GBq/μmol (n=7).
Conclusions: The novel radiolabeled COX-2 inhibitors [¹⁸F]A and [¹⁸F]B were successfully synthesized with the developed automated ¹⁸F-fluoroethylation method. This strategy offers an effective approach to radiolabel further COX-2 inhibitors with hydroxy functionalities. The radiopharmacological evaluation of [¹⁸F]A and ¹⁸F]B is currently under way.
Acknowledgements: This work was part of research initiatives within the Radiation-Induced Vascular-Dysfunction (RIVAD) Research Network and the Helmholtz-Portfoliothema "Technologie und Medizin -Multimodale Bildgebung zur Aufklaerung des In-vivo-Verhaltens von polymeren Biomaterialien".
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