Comparative in vitro studies of MR imaging probes for metabotropic glutamate subtype-5 receptor targeting

A series of magnetic resonance imaging probes has been evaluated to target selectively the metabotropic glutamate receptor subtype 5 (mGluR₅). Eight imaging probes based on the contrast agent [Gd·DOTA], previously derived by linking it to a series of specific and selective mGluR₅ antagonists, have been extensively tested for their functionality in vitro. The Nuclear Magnetic Relaxation Dispersion (NMRD) profiles of selected probes have been examined via field-cycling relaxometry in the presence and absence of a model protein. The properties of the targeted contrast agents were evaluated using a primary astrocyte model, as these cells mimic the in vivo situation effectively. The probes were non-toxic (up to 200 μM) to these mGluR ₅ expressing primary cells. Cellular proton longitudinal relaxation rate enhancements of up to 35% were observed by MRI at 200 μM of probe concentration. The antagonistic effect of all compounds was tested using an assay measuring changes of intracellular calcium levels. Furthermore, treatment at two different temperatures (4 °C vs. 37°C) and of an mGluR ₅-negative cell line provided further insight into the selectivity and specificity of these probes towards cell surface mGluR₅. Finally, two out of eight probes demonstrated an antagonistic effect as well as significant enhancement of receptor mediated cellular relaxation rates, strongly suggesting that they would be viable probes for the mapping of mGluR ₅ by MRI in vivo.