Cyclooxygenase-1 (COX-1) is constitutively expressed in brain and plays a major role in the biosynthesis of proinflammatory mediators in the CNS. COX-1 expression is upregulated in activated microglia in monkey neuroinflammatory models, suggesting COX-1 could be a useful biomarker for neuroinflammation. We have developed a highly selective COX-1 radioligand, [¹¹C]PS13, (hCOX-1 IC₅₀= 1 nM and hCOX-2 >1000 nM), which is now used with positron emission tomography (PET) to quantify COX-1 in monkey and human brain. A COX-1 PET radioligand labeled with longer-lived fluorine-18 would be more appealing because of the potential for wider use through distribution to PET imaging centers that are considerably distant from central radiosynthesis sites. We have labeled PS13 with fluorine-18 in the trifluoroethoxy position in modest yield and with low molar activity and have shown that this tracer can well image COX-1 in monkey brain. However, it is desirable to suppress occupancy of COX-1 by carrier from low molar activity [¹⁸F]PS13 to better comply with the tracer principle. We therefore embarked on the design and synthesis of a next generation of ¹⁸F-labeled COX-1 PET ligands, where the new candidates retain or have improved COX-1 affinity and selectivity, and also provide a possibility for labeling with fluorine-18 in high yield and molar activity.
Systematic SAR was performed on the 1,2,4-triazole core of PS13 with modifications exploring rational alterations to one or more of the two pendant aryl groups and/or alkoxy side-arm, resulting in 31 new COX-1 ligands. These ligands were assayed for COX-1 binding affinity with a method developed in-house, in which recombinant human COX-1 was immobilized onto 96-well plates and the new ligands competed with [³H]PS13 binding. Initial screening revealed two promising candidates, ZQC7 and ZQC17 with COX-1 K_i values of 6.6 nM and 6.1 nM, respectively (c.f. a K_Dvalue of 7.9 nM for PS13). Both compounds feature readily accessible sites for straightforward labeling with either carbon-11 or fluorine-18 at high molar activity. ZQC17 was labeled in the fluoroethoxy position with fluorine-18 in high yields and with high molar activity (241 GBq/µmol). The lipophilicity (logD_7.4) of [¹⁸F]ZQC17 was also measured to be desirably less than that for [¹¹C]PS13 (3.26 vs 4.26) with potential for less unwanted non-specific binding in vivo. Evaluation of [¹⁸F]ZQC7 and [¹⁸F]ZQC17 for PET imaging of COX-1 in monkey brain is now being pursued.

Joo Eun Jee

Appearances