The pharmacologic characteristics of ^(99)Tc^m-N-ethyl-N_2S_2-memantine,an NMDA receptor imaging agent,was investigated.It was prepared by a one-step reaction from N-ethyl-N_2S_2-memantine.The affinity and specificity were determined by radio-ligand receptor binding assay(RRA).Biodistribution in vivo in mice was performed.The results showed that ^(99)Tc^m-N-ethyl-N_2S_2-memantine bound to a single site on NMDA receptor with a K_d of 584.32 nmol/L and a B_(max)of 267.05 nmol/mg.A competitive analysis showed that such specific binding could be inhibited by specific inhibitors of NMDA receptor,such as ketamine and(+)-MK-801.The biodistribution exhibited rapid uptake and favorable retention in mice brains.The major radioactivity was metabolized by the hepatic system.A two-compartment model of C=4.49e^(-0.083t)+ 1.42e^(-0.0016t)was established,and the half life was 8.35 min in blood.In conclusion,the new radio-ligand ^(99)Tc^m-N-ethyl-N_2S_2-Memantine has a moderate affinity and specific binding to NMDA receptor,and can easily cross the blood-brain barrier(BBB).Therefore,it may be a potential NMDA receptor imaging agent.
LIU YingtaoZHOU XingqinCAO GuoxianZHANG JiankangQIN XiaofengXU XijieZOU Meifen
The 4,5-dioxo-4,5-dihydro-1H-pyrrolo(2,3-f)quinoline-2,7,9-tricarboxylic acid 2-ethyl ester 7,9-dimethyl ester (PQQE) was synthesized on the basis of Pyrroloquinoline quinine (PQQ). 99m Tc-PQQE was prepared using stannous fluoride (SnF 2 ) as reducing agent. Biological characteristics of 99m Tc-PQQE include lipophilic and the charge properties were compared to 99m Tc-PQQ. The biodistributions of 99m Tc-PQQE in mice and brain regional distribution were performed. In vivo distribution of 99m Tc-PQQE in mice indicates that the concentration ratio of drug and blood increases steadily over time. The major radioactivity may be metabolized by the hepatic and renal system. The elimination-phase half-time (t1/2β) results indicate that the residence time of 99m Tc-PQQE (203.92) in the body is twice as long as 99m Tc-PQQ (100.45). The uptake of 99m Tc-PQQE in brain was improved due to the ameliorating of charge and lipophilicity. The highest total regional brain uptake of 99m Tc-PQQE was in the frontal lobe and hippocampus, where the NMDA receptor is very abundant. 99m Tc-PQQE had a good target to nontarget ratio (hippocampus/cerebellum) which preserved a higher value (peak 4.0 at 120 min) from 60 min to 180 min after injection. In vitro autoradiographic results are in close agreement with the regional brain map. The enrichment can be blocked by N-methyl-D-aspartate receptor (NMDAR) redox modulatory site antagonists-ebselen (EB). This work suggests that 99m Tc-PQQE has some specific targeting to the NMDA receptor.
In this work,a calculation method of chemical kinetics was established for labeling reaction of 99mTc-N-ethyl-N2S2-memantine,a potential NMDA receptor imaging agent prepared in our laboratory.Four groups of vials (3 vials per group) were added with 0.02 mL (1 mg/mL) N-ethyl-N2S2Memantine,0.08 mL (40 mg/mL) GH,0.05 mL (10 mg/mL) EDTA-2Na,0.035 mL (2 mg/mL) SnF2,0.8 mL phosphate buffer(1mol/L,pH 6.5) and 37 MBq Na99mTcO4.The vials were incubated at 70℃,80℃,90℃ or 100℃.Samples were taken with capillary from the vials at 2,5,10,20,30,40 and 60min.Labeling yields were determined by TLC.Order of reaction n,rate constant k,activation energy Ea and half life t1/2 of labeling reaction were calculated with the kinetics software we compiled.Mean labeling yields of 99m Tc-N-ethyl-N2S2-memantine at 2,5,10,20,30,40 and 60min were (1) 13.5,15.7,34.0,64.8,81.9,91.4 and 95.4 at 70℃;(2) 13.2,20.5,40.1,70.0,88.2,94.5 and 95.6 at 80℃;(3) 15.6,22.9,43.7,74.3,87.2,93.4 and 96.1 at 90℃;and (4) 20.5,25.8,45.3,81.1,92.2,95.6 and 96.0 at 100℃.The other parameters were;n =1;k=0.053,0.061,0.063 and 0.076 L/min at 70℃,80℃,90℃ and 100℃,respectively;Ea=12.38 kJ/L;t1/2=13.11,11.45,11.05 and 9.07min at 70℃,80℃,90℃ and 100℃,respectively.The mean labeling yield increased with temperature and time,optimized at 100℃ and 40-60min.The concentration of 99mTc-N-ethyl-N2S2-Memantine was larger than that of Na99mTcO4,so n=1.The k increased with reaction,hence the accelerated reaction rate at higher temperatures.The labeling reaction was not so difficult because of the low Ea.The t1/2 decreased with increasing reaction temperature,hence the acceleration of labeling reaction.
CAO GuoxianZHOU XingqinLIU YingtaoKONG YanyanZHANG Jiankang
