Cerebral 6-[F]fluoro-l-DOPA uptake in rhesus monkey: pharmacological influence of aromatic amino acid decarboxylase (AAAD) and catechol-O-methyltransferase (COMT) inhibition

FDOPA/PET scans were performed in one rhesus monkey to study the influence of three catechol-O-methyltransferase (COMT) inhibitors (CGP 28014, OR-611 and Ro 40-7592) on FDOPA pharmacokinetics. COMT inhibitors were administered in combination with carbidopa, a peripherally acting inhibitor of the aromatic amino acid decarboxylase (AAAD). FDOPA was administered intravenously and its metabolic fate in plasma was determined using an HPLC system with an on-line γ-γ coincidence detector. Cerebral tracer uptake was assessed in the striatum and in a non-dopaminergic brain region (occipital cortex). In the periphery, the pharmacokinetic efficiency of FDOPA was increased due to the combined inhibition of COMT and AAAD activity. All three COMT inhibitors reduced the FDOPA methylation rate constant in plasma, with complete suppression obtained in the case of Ro 40-7592. In the brain, specific ¹⁸F radioactivity (striatal minus brain reference radioactivity) increased as a result of the increase in FDOPA plasma availability following the administration of COMT and AAAD inhibitors. We established a significant linear correlation between striatal radioactivity and FDOPA plasma levels (r=0.924±0.048, P<0.0001 for total striatal and r=0.948±0.054, P<0.0001 for specific striatal radioactivity). Using plasma FDOPA radioactivity as input, we found that the striatal FDOPA uptake rate constant K_i^FD was not changed by any of the inhibitors. Thus, the enhancement of striatal radioactivity after application of enzyme inhibitors is a consequence of the increase in plasma FDOPA that becomes available for conversion to fluorodopamine in the striatal dopaminergic nerve terminals. By contrast, using the radioactivity in a non-dopaminergic region (cortex) as input, we found that the striatal FDOPA uptake rate constant K_i^ref was significantly (P<0.0001) increased following pretreatment with COMT inhibitors. Our analysis demonstrated that K_i^ref and the 3-OMFD contribution to the cerebral radioactivity were inversely correlated.