Synthesis, Biodistribution, and Radiation Dosimetry of a Novel mGluR5 Radioligand: 18F-AZD9272

Abstract

The metabotropic glutamate receptor subtype mGluR5 has been proposed as a potential drug target for CNS disorders such as anxiety, depression, Parkinson's disease, and epilepsy. The AstraZeneca compound AZD9272 has previously been labeled with carbon-11 and used as a PET radioligand for mGluR5 receptor binding. The molecular structure of AZD9272 allows one to label the molecule with fluorine-18 without altering the structure. The aim of this study was to develop a fluorine-18 analogue of AZD9272 and to examine its binding distribution in the nonhuman primate brain in vivo as well as to obtain whole body radiation dosimetry. 18F-AZD9272 was successfully synthesized from a nitro precursor. The radioligand was stable, with a radiochemical purity of >99% at 2 h after formulation in a sterile phosphate buffered solution (pH = 7.4). After injection of 18F-AZD9272 in two cynomolgus monkeys, the maximum whole brain radioactivity concentration was 4.9-6.7% of the injected dose (n = 2) and PET images showed a pattern of regional radioactivity consistent with that previously obtained for 11C-AZD9272. The percentage of parent radioligand in plasma was 59 and 64% (n = 2) at 120 min after injection of 18F-AZD9272, consistent with high metabolic stability. Two whole body PET scans were performed in nonhuman primates for a total of 231 min after injection of 18F-AZD9272. Highest uptakes were seen in liver and small intestine, followed by brain and kidney. The estimated effective dose was around 0.017 mSv/MBq. 18F-AZD9272 shows suitable properties as a PET radioligand for in vivo imaging of binding in the primate brain. 18F-labeled AZD9272 offers advantages over 11C-AZD9272 in terms of higher image resolution, combined with a longer half-life. Moreover, based on the distribution and the estimated radiation burden, imaging of 18F-AZD9272 could be used as an improved tool for quantitative assessment and characterization of AZD9272 binding sites in the human brain by using PET.