Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Aim: Cyclooxygenase-2 (COX-2) is an enzyme induced during inflammation, but overexpression of COX-2 also has been observed in carcinogenic processes. Noninvasive monitoring and quantitative characterization of functional expression of
COX-2 by means of PET would substantially improve understanding of the role of this enzyme during manifestation and progression of inflammatory diseases and cancer. Herein we report first radiosynthesis and cellular uptake studies of two
highly affine COX-2 inhibitors having a 1,2-diaryl substituted indole scaffold.
Materials and methods: The highly affine COX-2 inhibitors 3-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-1H-indole 1 and 3-(4-methoxyphenyl)-2-(4-methylsulfonylphenyl)-1H-indole 2 (both IC50 COX-2 ~ 20 nM) [1] served as nonradioactive references. The 18F-radiolabeled tracer [18F]1 was synthesized by nucleophilic substitution of an appropriate trimethylammonium-substituted aromatic precursor with [18F]fluoride and subsequent McMurry cyclization. The 11C-radiolabeled probe [11C]2 was formed via a methylation reaction of the corresponding desmethyl precursor with [11C]CH3I. Different human tumor cell lines showing selectively high COX-2 (FaDu, HT-29, A2058) or COX-1 expression (A375) were used to study the overall uptake or cellular association of [18F]1 and [11C]2 in vitro. To further differentiate the specific contribution of COX-1 and COX-2 to overall tracer uptake, unstimulated human monocytic leukemia cells (THP-1) and phorbol ester stimulated THP-1 macrophages (THP-1M) were used as models. The cell tracer uptake experiments using compounds [18F]1 and [11C]2 (0.4 MBq/mL) were performed in quadruplicate at 37°C for 30, 60, and 120 min.Results:[18F]1 was synthesized from [18F]fluoride in 10% yield (d.c.) in 98% radiochemical purity with a specific activity of 74-91 GBq/μmol. The 11Cradiolabeled compound [11C]2 was obtained in 23% yield (d.c.) in 99% radiochemical purity with a specific activity of 79-89 GBq/μmol. The radiotracer cellular uptake in each model used correlated well with the observed COX protein synthesis. Cell models with prominent COX-2 overexpression showed a substantially higher uptake of both [18F]1 and [11C]2 in the order FaDu>HT29>THP-1M>A2058 when compared to COX-1 overexpressing A375 cells. The lowest cellular uptake was observed in THP-1 showing no or very low baseline expression of both COX-1 and COX-2.
Conclusion:The new radiolabeled COX-2 inhibitors were synthesized in good radiochemical yield and high purity. Cellular studies demonstrated well correlation of the overall radiotracer uptake with COX expression/protein synthesis rate. Further exploration of the new COX-2 inhibitors [18F]1 and [11C]2 regarding in vivo metabolic stability and biodistribution, including dynamic small animal PET investigations in tumor bearing mice, is currently under the way.[1] Hu W., Bioorg. Med. Chem. 11, (2003), 1153