PET Imaging of Brain mGluR5 Receptors Using [18F]SP203 and [11C]SP203

Metabotropic glutamate receptors are G-protein coupled receptors that respond to glutamate by activating proteins inside nerve cells that affect cell metabolism, thereby fine-tuning the signals sent between cells to maintain balance in neuronal activity. Metabotropic Glutamate receptors (mGluR5) are Group I receptors localized post-synaptically and found in several regions of the brain including the striatum, hippocampus, amygdala, and cortex. Activation of mGluR5 stimulates phospholipase C, resulting in phosphoinositide hydrolysis and increase of intracellular Ca2+ levels. Several potent antagonists for mGluR5 have been developed, including 6 methyl-2-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4yl)ethynyl] pyridine (MTEP) however, no simple derivatives of MPEP or MTEP had proven to be useful for in vivo imaging.

In the present protocol, we will use a new PET ligand [(18)F]SP203 for two reasons: Part 1.) we will perform kinetic brain imaging to quantify mGluR5 binding parameters in brain and determine the reliability and reproducibility of these measures in 15 healthy controls Part 2.) if the tracer is proved successful in the part 1, we plan to estimate radiation-absorbed doses of [(18)F]SP203 in healthy human subjects by performing whole body imaging.

We will also use another new ligand, [11C]SP203, which has the same structure as [(18)F]SP203 but labeled with (11)C instead of (18)F. Part 3) the same purpose as part 1 but with [(11)C]SP203 instead of [(18)F]SP203. Part 4) if [(11)C]SP203 is proved successful in the part 3, we plan to estimate radiation absorbed doses of [(11)C]SP203 in healthy human subjects. At this moment, we do not plan to perform additional scans of [(18)F]SP203 under the current protocol. All future subjects will participate in scans of [(11)C]SP203.

Additionally, Part 5) we will measure the difference in [(18)F]SP203 concentration between the artery and the vein without PET scanning to assess whether the venous blood is a valid and less invasive substitute of arterial blood. In some of [(18)F]SP203 scans performed in the current protocol, we compared [(18)F]SP203 levels between radial artery and antecubital vein and observed 30% - 60% lower levels in vein. We plan to test whether sampling from a vein in the hand and / or warming up makes [(18)F]SP203 levels in vein similar to artery.

Successful development of a PET ligand to image mGluR5 will have a strong impact on clinical management of brain disorders with disruptions in glutamatergic transmission such as schizophrenia, anxiety, and neurodegenerative disorders including Alzheimer s and Parkinson s disease.