11C-LY2428703, a positron emission tomographic radioligand for the metabotropic glutamate receptor 1, is unsuitable for imaging in monkey and human brains

Background: A recent study from our laboratory demonstrated that C-11-LY2428703, a new positron emission tomographic radioligand for metabotropic glutamate receptor 1 (mGluR1), has promising in vitro properties and excellent in vivo performance for imaging rat brain. The present study evaluated C-11-LY2428703 for imaging mGluR1 in monkey and human brains. Methods: Rhesus monkeys were imaged at baseline and after administration of an mGluR1 blocking agent to calculate nonspecific binding, as well as after the administration of permeability glycoprotein (P-gp) and breast cancer resistance protein (BCRP) blockers to assess whether C-11-LY2428703 is a substrate for efflux transporters at the blood-brain barrier. Human imaging was performed at baseline in three healthy volunteers, and arterial input function was measured. Results: Overall brain uptake was low in monkeys, though slightly higher in the cerebellum, where mGluR1s are concentrated. However, the uptake was not clearly displaceable in the scans after mGluR1 blockade. Brain penetration of the ligand did not increase after P-gp and BCRP blockade. Brain uptake was similarly low in all human subjects (mean V-T with a two-tissue compartment model, 0.093 +/- 0.012 mL/cm(3)) and for all regions, including the cerebellum. Conclusions: Despite promising in vitro and in vivo results in rodents, C-11-LY2428703 was unsuitable for imaging mGluR1s in monkey or human brain because of low brain uptake, which was likely caused by high binding to plasma proteins.