Radioiodinated indomethacin amides as COX-2-specific SPECT tracers: Synthesis and initial in vitro study

Objectives Cycloxygenases (COX-1 and COX-2) catalyze oxidation of arachidonic acid to prostaglandines. COX-1 is characterized by constitutive and ubiquitary expression. In contrast, COX-2 is induced by different stimuli and overexpressed in inflamed tissues and different tumors. The aim of this work was synthesis and in vitro evaluation of the COX-2 specific tracers suitable for PET/SPECT imaging.

Methods 125I-labelled indomethacin derived diamides were prepared in one or two steps (iodination or iodination/deprotection, or iodination of the prostethic group/click reaction). Cellular uptake was evaluated in COX-2+ HEK cells (Tet-inducible COX-2 expression) and in COX-1+ HUVEC cells with and w/o co-incubation with Celecoxib. Tracer binding was investigated by phosphor imager and western blot analysis.

Results 125I-labelled indomethacin derivatives, Ind-NH-(CH2)4-NH-3-I-Bz (1), Ind-NH-(CH2)4-NH-5-I-Py-3-CO2H (2) and Ind-NH-(CH2)4-NH-3-CO2H-5-I-Bz (3) and Ind-NH-(CH2)4-NH-C(O)-triazole-1-(5-I-Bz-CO2H (4), were synthesized in 46-80% RCY. For preparation of 125I-4 new radioiodinating synthon, 3-azido-5-iodobenzoic acid, was used. Tet-induced COX-2 expression in HEK cells correlated with increased uptake of 125I-1 and 125I-2 (3.4±0.2 vs. 11.9±03 and 1.9 ± 0.3 vs. 9.8 ± 0.1, for 125I-1 and 125I-2, respectively). Co-incubation with Celecoxib completely inhibited COX-2 binding of both tracers. The binding of 125I-2 to COX-2 was visualized by phosphor imager analysis, which showed distinct target bands corresponding to tetrameric form of COX-2 (confirmed by western blot). Cellular uptake of 125I-3 and 125I-4 was low. The low uptake of 125I-1 and 125I-2 in HUVEC verified a good COX-2 selectivity of both tracers.

Conclusions 125I-1 and 125I-2 demonstrated COX-2 selective uptake and retention, and are feasible candidates for further evaluation in vivo.

Research Support This work was supported by ERFE-Ziel2 programm (NRW).