Radiosynthesis and evaluation of [18F]FMTP: A potential COX-2 PET ligand

1106 Objectives: Cyclooxigenase-2 (COX-2) enzyme is expressed under normal physiologic conditions and has a major role in fundamental brain functions. Upregulation of COX-2 is involved in neuroinflammation associated with many neurological diseases and cancers of the brain. Also, neuroinflammation and COX-2 induction contributes significantly to the pathogenesis of pain, arthritis, and acute allograft rejection. Therefore, targeting COX-2 may be a potential neuroprotective treatment strategy aiming to reduce the progression of neurodegenerative and other diseases. In this presentation, we report the radiochemical synthesis and evaluation of high affinity COX-2 ligand (IC50 = 2 nM) [18F]6-fluoro-2-(4-(methylsulfonyl)- phenyl)-N-(thiophen-2-ylmethyl)pyrimidin-4-amine ([18F]FMTP), as potential PET ligand for COX-2 imaging. Methods: Synthesis of nonradioactive FMTP was achieved in five steps from 4-(methylthio)- benzimidamide. The radiolabeling precursor 6-chloro-2-(4-(methylsulfonyl)phenyl)-N-(thiophen-2-ylmethyl)pyrimidin-4-amine was obtained from 4-(methylthio)benzimidamide in four steps. The radiochemical synthesis of [18F]FMTP was optimized by reacting [18F]fluoride/K222/K2CO3 using a chlorine to fluorine displacement reaction with the precursor molecule. The cell uptake of [18F]FMTP was performed in COX-2 negative PANC-1 and COX-2 positive BxPC3 cells with unlabeled FMTP and celecoxib as nonspecific binding agents. Dynamic microPET image acquisition was performed in anesthetized nude mice (n=3) with Trifoil mPET/CT for 60 minute. Results: Synthesis of FMTP and its chloro-precursor were accomplished with 65% and 75% yields respectively. [18F]FMTP was produced in high radiochemical purity (>98%) and specific activity (2.5+0.5 Ci/μmol) in 30+5% radiochemical yield, decay corrected to EOS. [18F]FMTP did not show significant specific binding to COX-2 negative PANC-1 cells. Whereas, a 2-fold higher binding of the radioligand was found in COX-2 positive BxPC3 cells with up to 50% specific binding using unlabeled FMTP as blocking agent. Under identical conditions, the COX-2 based prescription drug celecoxib exhibited a much lower specific binding in BxPC3 cells. PET studies in normal mice indicated blood-brain barrier (BBB) penetration of [18F]FMPT and a faster washout of radioactivity in brain was observed presumably due to the lack of low constitutive COX-2 in normal brain. Conclusions: We report a facile radiolabeling procedure for the COX-2 ligand [18F]FMTP, in excellent purity and molar activity. [18F]FMTP showed moderate uptake and specific binding in COX-2 positive BxPC3 cells. MicroPET imaging in mice showed BBB penetration and fast washout in brain due to low concentration of COX-2 in normal brain. In vitro binding to COX-2 and the BBB permeability of [18F]FMTP indicate that it may be a useful PET tracer for in vivo imaging of COX-2 in neuroinflammation. Research support: Diane Goldberg Foundation (NYSPI/CUMC), NCATS UL1TR001873 (Reilly) Irving Institute/CTSA Translational Therapeutics Accelerator