Thus far, there are reports of 3 known 18F radioligands for evaluation of the P2X7R expression as shown in Figure 7. An analog of a potent P2X7R antagonist A-804598 was radiolabeled with 18F to yield [18F]EFB that showed high affinity for human and rat P2X7R.213 However, this PET tracer suffered from a low brain uptake in both healthy and LPS-treated rats that limited its application for brain imaging of the receptor.213 Another PET radioligand [18F]JNJ-64413739 was developed by 18F radiolabeling of a potent and selective P2X7 R antagonist JNJ-64413739 (Ki = 2.7 nM, rat cortex, Ki = 15.9 nM, hP2X7R). [18F]JNJ-64413739 has shown to be an effective PET ligand for mapping of P2X7R in human brain.214 The PET imaging studies with [18F]JNJ-64413739 in nonhuman primate showed engagement of the tracer with the P2X7R. In vitro blocking experiments of [18F]JNJ-64413739 with 2 known P2X7R antagonists demonstrated inhibition of the tracer binding to rat brain tissue sections in a dose-dependent manner.214-216 While [18F]JNJ-64413739 may be a useful tool for imaging of neuroinflammation, lack of a reference region in image analysis (ie, similar
