First visualization of adenosine A(2A) receptors in the human brain by positron emission tomography with [11C]TMSX.

[11C]TMSX is a new positron emission tomography (PET) radioligand that provides visualization of adenosine A(2A) receptors (A(2A)Rs) in the brain, heart and skeletal muscle. Here we report on the first visualization of the A(2A)Rs in the human brain by PET and [11C]TMSX in a male healthy volunteer, compared with the adenosine A1 receptors (A1Rs) and dopamine D2 receptors (D2Rs) which were measured by PET with [11C]MPDX and [11C]raclopride, respectively. The distribution volume (DV) of [11C]TMSX in the baseline was relatively high in the head of caudate nucleus, putamen, and thalamus and relatively low in the cortical regions. Infusion of theophylline, a nonselective A(2A)R antagonist (Ki for A(2A)Rs = 16000 nM for theophylline vs 5.9 nM for TMSX), slightly reduced the DVs in the head of caudate nucleus (8.0% reduction) and putamen (4.5% reduction), but not in the other regions having much lower levels of A(2A)Rs, demonstrating the A(2A)R-specific binding of [11C]TMSX. On the other hand, the A1Rs were widely distributed in the whole brain except for the cerebellum, while the binding potential of [11C]raclopride was predominantly high in the striatum. We concluded that [11C]TMSX is an applicable PET ligand for mapping the A(2A)Rs in the caudate nucleus and putamen in clinical studies because of no availability of other radioligands until now. The [11C]TMSX PET is of great interest for studying the pathophysiology of neurological and psychiatric disorders together with the [11C]raclopride PET for D2Rs evaluation and/or the [11C]MPDX PET for A1Rs evaluation.