Synthesis and characterization of boron fenbufen and its F-18 labeled homolog for boron neutron capture therapy of COX-2 overexpressed cholangiocarcinoma
Boron neutron capture therapy (BNCT) is a binary therapy that employs neutron irradiation on the boron agents to release high-energy helium and alpha particles to kill cancer cells. An optimal response to BNCT depends critically on the time point of maximal 10B accumulation and highest tumor to normal ratio (T/N) for performing the neutron irradiation. The aggressive cholangiocarcinoma (CCA) representing a liver cancer that overexpresses COX-2 enzyme is aimed to be targeted by COX-2 selective boron carrier, fenbufen boronopinacol (FBPin). Two main works were performed including: 1) chemical synthesis of FBPin as the boron carrier and 2) radiochemical labeling with F-18 to provide the radiofluoro congener, m-[18F]fluorofenbufen ester boronopinacol (m-[18F]FFBPin), to assess the binding affinity, cellular accumulation level and distribution profile in CCA rats. FBPin was prepared from bromofenbufen via 3 steps with 82% yield. The binding assay employed [18F]FFBPin to compete FBPin for binding to COX-1 (IC50 = 0.91 ± 0.68 μM) and COX-2 (IC50 = 0.33 ± 0.24 μM). [18F]FFBPin-derived 60-min dynamic PET scans predict the 10B-accumulation of 0.8–1.2 ppm in liver and 1.2–1.8 ppm in tumor and tumor to normal ratio = 1.38 ± 0.12. BNCT was performed 40–55 min post intravenous administration of FBPin (20–30 mg) in the CCA rats. CCA rats treated with BNCT display more tumor reduction than that by NCT with respect of 2-[18F]fluoro-2-deoxy glucose uptake in the tumor region of interest, 20.83 ± 3.00% (n = 12) vs. 12.83 ± 3.79% (n = 10), P = 0.05. The visualizing agent [18F]FFBPin resembles FBPin to generate the time-dependent boron concentration profile. Optimal neutron irradiation period is thus determinable for BNCT. A boron-substituted agent based on COX-2-binding features has been prepared. The moderate COX-2/COX-1 selectivity index of 2.78 allows a fair tumor selectivity index of 1.38 with a mild cardiovascular effect. The therapeutic effect from FBPin with BNCT warrants a proper COX-2 targeting of boron NSAIDs.