Radiosynthesis and Biological Evaluation of [F-18]Triacoxib: A New Radiotracer for PET Imaging of COX-2

摘要

Inducible isozyme cyclooxygenase-2 (COX-2) is upregulated under acute and chronic inflammatory conditions, including cancer, wherein it promotes angiogenesis, tissue invasion, and resistance to apoptosis. Due to its high expression in various cancers, COX-2 has become an important biomarker for molecular imaging and therapy of cancer. Recently, our group applied in situ click chemistry for the identification of the highly potent and selective COX-2 inhibitor triacoxib. In this study, we present the radiosynthesis in vitro and in vivo radiopharmacological validation of [F-18]triacoxib, a novel radiotracer for PET imaging of COX-2. Radiosynthesis of [F-18]triacoxib was accomplished using copper-mediated late-stage radiofluorination chemistry. The radiosynthesis, including radio-HPLC purification, of [F-18]triacoxib was accomplished within 90 mM in decay-corrected radiochemical yields of 72% (n = 7) at molar activities exceeding 90 GBq/itmol. Cellular uptake and inhibition studies with [F-18]triacoxib were carried out in COX-2 expressing HCA-7 cells. Cellular uptake of [F-18]triacoxib in HCA-7 cells reached 25% radioactivity/mg protein after 60 min. Cellular uptake was reduced by 63% upon pretreatment with 0.1 mM celecoxib, and 90% of the radiotracer remained intact in vivo after 60 mM p.i. in mice. [F-18]Triacoxib was further evaluated in HCA-7 tumor-bearing mice using dynamic PET imaging, radiometabolite analysis, autoradiography, and immunohistochemistry. PET imaging revealed a favorable baseline radiotracer uptake in HCA-7 tumors (SUV60min = 0.76 +/- 0.02 (n = 4)), which could be blocked by 20% through i.p. pretreatment with 2 mg of celecoxib. Autoradiography and immunohistochemistry experiments further the confirmed blocking of COX-2 in vivo. [F-18]Triacoxib, whose nonradioactive analogue was identified through in situ click chemistry, is a novel radiotracer for PET imaging of COX-2 in cancer. Despite a substantial amount of nonspecific uptake in vivo, [F-18]triacoxib displayed specific binding to COX-2 in vivo and reinforced the feasibility of optimal structure selection by in situ click chemistry. It remains to be elucidated how this novel radiotracer would perform in first-in-human studies to detect COX-2 with PET.