The first nonxanthine 11C PET ligand of A1 R was [11C]FR194921, an analog of a potent A1R antagonist FR194921 (Ki = 2.9 nM).89,90 The PET imaging with [11C]FR194921 showed selective accumulation of A1Rs in the hippocampus, cerebral cortex, striatum, thalamus, and cerebellum of the rat brain.89 However, the specific binding of [11C]FR194921 was not as high as expected.89 Recently, a highly potent partial A1R agonist 2-amino-4-(3-methoxyphenyl)-6-(((6-methylpyridine-2-yl)methyl)thio)pyridine-3,5-dicarbonitrile was labeled with 11C to produce [11C]MMPD and showed brain uptake that was consistent with A1R.91 [11C]MMPD is currently under further evaluation for participation of A1R in sleep mechanisms.91
