What We Observe In Vivo Is Not Always What We See In Vitro: Development and Validation of ¹¹C-JNJ-42491293, A Novel Radioligand for mGluR2

Abstract

Positive allosteric modulators (PAM) of metabotropic glutamate receptor 2 (mGluR2) are a potential therapy for anxiety, schizophrenia, and addiction. Aside from pathophysiologic imaging studies, an mGluR2 PET tracer would enable confirmation of sufficient central target engagement and assist dose selection for proof-of-concept studies of PAM compounds. 11 C-JNJ-42491293, a novel high-affinity radioligand (human 50% inhibitory concentration 5 9.6 nM) for the PAM site of mGluR2, was evaluated as a selective mGluR2 PAM PET tracer. Methods: In vitro and ex vivo autoradiography binding experiments in Wistar and in mGluR2 knockout and wildtype rats as well as in vivo biodistribution and brain PET imaging studies in wildtype and mGluR2 knockout rats in a primate and in humans were performed. Results: In vitro binding studies and in vivo imaging studies in Wistar rats showed moderate brain uptake, with a distribution pattern fully consistent with the reported intracerebral distribution of mGluR2. Given these promising findings, biodistribution, dosimetry, and brain kinetic modeling of 11 C-JNJ-42491293 were determined in humans. Because of an unexpected high myocardial retention, additional 11 C-JNJ-42491293 imaging studies were performed in recently available mGluR2 knockout and wildtype rats and in a monkey using a structurally distinct mGluR2 PAM ligand with affinity for the same site, demonstrating off-target binding in vivo that could not have been anticipated from previous in vitro experiments. To date, the target of this non-mGluR2 tracer binding remains unknown. Conclusion: On the basis of in vivo selectivity issues suggested by human distribution and demonstrated in knockout rat models, 11 C-JNJ-42491293 was considered unsuitable as a specific PET ligand for in vivo imaging of mGluR2. These results emphasize the importance of elaborated in vitro/in vivo comparative studies and, when available, validation with knockout animal models or structurally distinct ligands with affinity for the same site, in radiotracer development.

Synthesis and Preliminary Studies of a Novel Negative Allosteric Modulator, 7-((2,5-Dioxopyrrolidin-1-yl)methyl)-4-(2-fluoro-4-[11C]methoxyphenyl) quinoline-2-carboxamide, for Imaging of Metabotropic Glutamate Receptor 2

“…The radioactivity was efficiently cleared from blood (1 min/60 min ratio of 2.4) and high radioactivity in the liver, kidneys and small intestine indicated urinary and hepatobiliary excretion, as well as a possible intestinal reuptake pathway. The distribution of [ 11 C]QCA in the peripheral organs was similar to prior reports with other compounds, 51,56,69,70 in which rapid clearance of radioactivity from heart, lungs, spleen and muscle was observed. Limited brain uptake (peak value 0.42% ID/g at 1 min post injection) was observed, and thus the regional brain distribution was further studied by in vitro autoradiography.…”

Imaging metabotropic glutamate receptor system: Application of positron emission tomography technology in drug development

“…JNJ‐42491293 is a potent mGlu 2 PAM ( EC 50 = 7.5 nM; E max = 282%) with high affinity ( IC 50 = 9.23 nM) to the human mGlu 2 receptors . [ 11 C]JNJ‐42491293 was developed as a potential mGlu 2 PET tracer .…”

“…JNJ‐42491293 is a potent mGlu 2 PAM ( EC 50 = 7.5 nM; E max = 282%) with high affinity ( IC 50 = 9.23 nM) to the human mGlu 2 receptors . [ 11 C]JNJ‐42491293 was developed as a potential mGlu 2 PET tracer . The PET data showed that [ 11 C]JNJ‐42491293 uptake reached at the plateau from 12 to 27 minutes in the rat brain after injection followed by a slow washout.…”

“…In vitro specific binding to WT rat brain sections only is observed, whereas in vivo tracer binding in both WT and mGlu 2 KO rat brains is observed. This study was originally published in The Journal of Nuclear Medicine by Leurquin‐Sterk et al IV, intravenous; KO, knockout; WT, wild type…”

“…190 Thus, the effective binding potential (V s = f p × 63 in the AD patient could be much lower than the healthy subjects. 191,192 [ 11 C]JNJ-42491293 was developed as a potential mGlu 2 PET tracer. [192][193][194] The PET data 193 showed that [ 11 C]JNJ-42491293 uptake reached at the plateau from 12 to 27 minutes in the rat brain after injection followed by a slow washout.…”