Preparation of ^99mTc-PQQ and preliminary biological evaluation for the NMDA receptor

Pyrroloquinoline quinone (PQQ), an essential nutrient, antioxidant, redox modulator and nerve growth factor found in a class of enzymes called quinoproteins, was labeled with Tc-99m by using stannous fluoride (SnF2) method. Radiolabeling qualification, quality control and characterization of Tc-99m-PQQ and its biodistribution studies in mice were performed and discussed. Effects of pH values, temperature, time and reducing agents concentration on the radiolabeling yield were investigated. The quality control procedure of Tc-99m-PQQ was determined by thin layer chromatography (TLC), radio high-performance liquid chromatography (RHPLC) and paper electrophoresis methods. The average radiolabeling yield was 94 +/- 1% under optimum conditions of 0.99 mg of PQQ, 30 mu g of SnF2, 0.5 mg of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and 18.5 MBq of (NaTcO4)-Tc-99m at pH 6 and 25 degrees C with a response volume of 1 +/- 0.1 mL. Tc-99m-PQQ was stable and anionic. Lipid-water partition coefficient of Tc-99m-PQQ was -1.49 +/- 0.16. The pharmacokinetics parameters of Tc-99m-PQQ were t(1/2 alpha) = 18.16 min, t(1/2 beta) = 100.45 min, K-12 = 0.013 min(-1), K-21 = 0.017 min(-1), K-e = 0.016 min(-1), AUC (area under the curve) = 1040.78 ID% g(-1) min and CL (plasma clearance) = 0.096 mL min(-1). The dual-exponential equation was Y = 10.88e(-0.038t) + 5.21e(-0.0069t). The biodistribution of Tc-99m-PQQ was studied in ICR (Institute for Cancer Research 7701 Burhelme Are., Fox Chase, Philadelphia, PA 1911 USA) mice. In vitro autoradiographic studies clearly showed that the Tc-99m-PQQ radioactivity accumulated predominantly in the hippocampus and cortex, which had a high density of N-methyl-D-aspartate Receptor (NMDAR). The enrichment can be blocked by NMDAR redox modulatory site antagonists-ebselen (EB) and Tc-99m-PQQ is therefore a promising candidate for the molecular imaging of NMDAR. To date, however, there have been no studies characterizing Tc-99m-PQQ.