18F-Glutathione Conjugate as a PET Tracer for Imaging Tumors that Overexpress L-PGDS Enzyme

Abstract

Lipocalin-type prostaglandin D synthase (L-PGDS) has been correlated with the progression of neurological disorders. The present study aimed at evaluating the imaging potency of a glutathione conjugate of fluorine-18-labeled fluorobutyl ethacrynic amide ([18F]FBuEA-GS) for brain tumors. Preparation of [18F]FBuEA-GS has been modified from the -4-tosylate derivative via radiofluorination in 5% radiochemical yield. The mixture of nonradioactive FBuEA-GS derived from a parallel preparation has be resolved to two isomers in a ratio of 9∶1 using analytic chiral reversed phase high performance liquid chromatography (RP-HPLC). The two fluorine-18-labeled isomers purified through nonchiral semipreparative RP-HPLC as a mixture were studied by assessing the binding affinity toward L-PGDS through a gel filtration HPLC, by analyzing radiotracer accumulation in C6 glioma cells, and by evaluating the imaging of radiotracer in a C6 glioma rat with positron emission tomography. The inhibition percentage of the production of PGD2 from PGH2 at the presence of 200 µM of FBuEA-GS and 4-Dibenzo[a,d]cyclohepten-5-ylidene-1-[4-(2H-tetrazol-5-yl)butyl]piperidine (AT-56) were 74.1±4.8% and 97.6±16.0%, respectively. [18F]FBuEA-GS bound L-PGDS (16.3–21.7%) but not the isoform, microsomal prostaglandin E synthase 1. No binding to GST-alpha and GST-pi was observed. The binding strength between [18F]FBuEA-GS and L-PGDS has been evaluated using analytic gel filtration HPLC at the presence of various concentrations of the cold competitor FBuEA-GS. The contrasted images indicated that the radiotracer accumulation in tumor lesions is probably related to the overexpression of L-PGDS.

Synthesis and Biological Evaluation of an 18Fluorine-Labeled COX Inhibitor—[18F]Fluorooctyl Fenbufen Amide—For Imaging of Brain Tumors

“…The tumor loci was easily identified by MRI before PET scanning, as reported previously [26,27]. The animal was assessed using static PET imaging over a 60-min time frame as indicated in Figure 8.…”

“…The residue was purified by flash chromatography using a 2.5 cm-diameter column with 20 g of silica gel under eluting conditions using acetone-CHCl 3 = 1:49 to give the product FOFA (1) as a white solid in 75% yield (45 mg). After recrystallization from CH 3 CH 2 OH, off-white needle-shaped crystals were obtained in 67% yield (39 mg 3 CN (1 mL) was added to the vessel followed by heating at 100˝C under fumes of He gas according to a previously described procedure [26]. The formed Kryptofix-+ K-[ 18 F]´F complex was dried via repeated dissolution and distillation.…”

“…The procedure was performed as described previously [26] A rat glioma cell line C6 was obtained from the American Type Cell Collection (ATCC, Manassas, VA, USA). The C6 glioma cells were cultured with Dulbecco's modified Eagles medium (DMEM) containing 10% fetal calf serum (Gibco, Thermo Fisher Scientific, MA, USA) under 5% of CO 2 at 37˝C in a 96-well microtiterplate.…”

Investigation of brain tumors using 18F-fluorobutyl ethacrynic amide and its metabolite with positron emission tomography

“…It has been documented that the toxicity of EA analogs may be mediated via both apoptotic and necrotic pathways. 31 Additionally, the hydrophobicity of FBuEA may serve to increase cellular penetration, thereby promoting toxicity. The lack of significant toxicity shown for FBuEA may be due to either the general insensitivity of tumor cells owing to downregulation of GST enzymes or to very low concentrations of GSH available for detoxification.…”

Fluorine-18 labelled building blocks for PET tracer synthesis

“…F]FBuEA with glutathione was accomplished in aqueous medium at pH 8.2, followed by semi-preparative HPLC purification, providing the racemic product in an overall radiochemical yield of 5% (dc) and an overall synthesis time of 2 hours (including the synthesis of the building block (Scheme 62)). 237…”