Development of fluorinated GluN2B-NMDA receptor ligands for positron emission tomography (PET)

• Applicant: Professor Dr. Bernhard Wünsch
• Subject Area: Pharmacy
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 269539669

The NMDA (N-methyl-D-aspartate) receptor belongs to the class of ionotropic glutamate receptors. It plays an important role in cognitive processes like learning and memory but also in acute and chronic events of neuronal cell damage including stroke, injury and neurodegenerative disorders (e.g. Parkinson’s, Alzheimer’s disease). Four subunits with different expression pattern in different regions of the central nervous system form the functional heterotetrameric NMDA receptor. This project is devoted to image GluN2B subunit containing NMDA receptors by labeling the ifenprodil binding site within the N-terminal domain of the GluN2B subunit. A PET tracer labeling selectively GluN2B subunit containing NMDA receptors will be used for target validation and quantification of this NMDA receptor subtype under healthy and diseased conditions.The development of a selective PET tracer requires multidimensional optimization taking into account affinity, selectivity, lipophilicity, stability, radiochemical availability and pharmacokinetic parameters including blood-brain barrier penetration and biotransformation of the potential radiotracer. In the first funding period, several fluorinated ligands interacting with GluN2B subunit containing NMDA receptors were synthesized and pharmacologically evaluated. The radiosynthesis of the first fluorine-18 labeled compound was performed and its biological properties were evaluated in vitro, in vivo and ex vivo. Due to the moderate binding affinity and the rapid clearance from the GluN2B-receptor expressing brain regions the development was not further pursued. However, the carbon-11 labeled tracer 7-methoxy-3-(4-phenylbutyl)-2,3,4,5-tetrahydro-1H-3-benzazepin-1-ol showed very promising biological properties. The enantiomers were separated and the (R)-configured enantiomer exhibited a heterogeneous and specific accumulation in GluN2B-rich brain regions such as cortex, hippocampus, thalamus and striatum. The interaction of this PET tracer with centrally located receptors could be selectively blocked. Based on these results a GMP compliant radiosynthesis is currently performed for the first-in-man study.The second funding period will focus on a high affinity fluorinated PET tracer for labeling of GluN2B receptors in the brain. For this purpose three types of ligands will be evaluated: (1) 3-benzazepines / benzo[7]annulen-7-mines with a fluorinated substituent at the N-atom, (2) 4-fluoromethyl substituted 3-benzazepines derived from amino acids and (3) benzoxazolone bioisosteres of phenols. Pharmacokinetic parameters of promising ligands will be determined in vitro to reduce the number of animal experiments. After radiosynthesis, the biological evaluation will be performed in vivo and ex vivo including radiometabolism, biodistribution and blocking studies.

• DFG Programme: Research Grants
• International Connection: Switzerland
• Partner Organisation: Schweizerischer Nationalfonds (SNF)
• Cooperation Partner: Professor Dr. Simon Mensah Ametamey