⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine As A Potential NMDA Receptor Imaging Agent

Schizophrenia is a complex multi-gene genetic disease, and its pathogenesis has not yet been fully clarified. With the deeply study, more and more evidences confirmed that NMDA (N-methyl-D-aspartate) receptor, DA(dopamine) receptor, 5-HT(5-Hydyoxytryptamine) receptor and other receptors in the spirit of the occurrence and development of schizophrenia play an important roles. Brain nuclear imaging is a clinical diagnosis of schizophrenia, and it's used to demonstrate the brain's physiological and pathological states at the molecular level by radioactive imaging agents. The researchs of varieties of receptor-imaging agents provide good tools for schizophrenia study. The study of NMDA receptor imaging agent is one of the hottest domain in nuclear medicine. However, the precent been found radioactive receptor imaging agents have obvious deficiencies, such as the low-receptor selective and low progressive brain volume, which restricted the radioactive imaging agent in clinical application. Therefore, to find receptors with high-receptor selectivity, high progressive brain volume and specific brain concentrain is the focus of further work.1-N-[N-[2-(S-thioethyl)]-N-[2-[N-[2-(S-thioethyl)amino]ethyl]aminoethyl]amino-3,5- dimethyladamantane(N-ethyl-N₂S₂-Memantine), which was systhesized by ourself according memantine, was labeled with ⁹⁹Tc^m by ligand exchange under the action of SnF2, then optimizated the condition of reaction, got the ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine whose radiochemical purity (RCP) was more than 90%. Studies in vivo and in vitro discussed the compouds fitting the imaging agent of NMDA receptor or not.The studies in vitro included determining the stability, lipophilicity, electrical, plasma protein of ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine, analyzed the affinity binding to NMDA receptor by radioactive receptor binding assay, and research the mechanism of moleculer action of between N-ethyl-N₂S₂-Memantine and HS-HY5Y, to analog the mode of action between compound and nerve cells. The results showed that ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine had a RCP more than 90% after 6h placing at 25℃, had an oil/water partition coefficient 1.74 in octanol/water system, and was neutal in pH7.4 phosphate/ethanol (V/V=1:1). The plasma protein binding rates were 77.5%,74.1%,73.2% at three different concentrations. It would not make the obvious change of free fraction with pharmacological action that the protein binding of ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine is not dependent on the radio-doses. These indicated that ⁹⁹Tc^m-N- ethyl-N₂S₂-Memantine could through the blood-brain barrier. The results of radio-ligand receptor binding assay (RRA) showed that ⁹⁹Tc^m-N-ethyl-N₂S₂- Memantine can specifically binded with NMDA receptor and the binding was a single saturation, with an equilibrium dissociation constant Kd of 584.32nmol/L and a Bmax of 267.05nmol/mg. It was reported that Kd of memantine was 700nmol/L. They are in the same magnitude. Ketamine and MK-801 were the specific inhitors of NMDA receptor, while haloperidol is the inhitor of dopamine D2 receptor andσreceptor. Use these three compounds as inhibitors to do competition binding experiments. The results showed that such specific binding of ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine to NMDA receptor could be inhibited by ketamine and MK-801, but not by haloperidol. So the binding of ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine to NMDA receptor was targeting and specific. These provided good evidences for ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine as a good NMDA receptor imaging agents. Cell physiology experiments show that N-ethyl-N₂S₂-Memantine can increase the cell viability of SH-HY5Y which was affected by glutamate. The results showed that the binding of ligand to NMDA receptor was satisfactory targeting.The studies in vitro included biodistribution, preliminary pharmacokinetics and autoradiography reseachers. The biodistribution in mice indicated that ⁹⁹Tc^m-N-ethyl- N₂S₂-Memantine was most concentrated in the liver, kidney and lung, and was primarily metabolized by liver and kidney. The brain had some uptake, especially in the haypothalamus, hippocampus and frontal lobe where the NMDA receptors was very abundant. The distribution and elimination accorded with two-compartment model, and half life of absorption was 7.66min and half life of eliminatation is 271.00min. Brain nuclear imaging showed that ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine enriched at frontal lobe, and significant different to control.This study showed that ⁹⁹Tc^m-N-ethyl-N₂S₂-Memantine was a potential SPECT imaging agent of NMDA receptor.