Synthesis and Biological Evaluation of [ 11C]MK-912 as an α 2-Adrenergic Receptor Radioligand for PET Studies

Abstract

ABSTRACT. In vitro studies showed that MK-912 ((2S, 12bS)1’,3’-dimethylspiro(1,3,4,5’,6,6’,7,12b-octahydro-2H-benzo[b]furo[2,3-a]quinolizine)-2,4’-pyrimidin-2’-one) is a potent α 2-adrenergic receptor antagonist with high affinity (K i = 0.42, 0.26 and 0.03 nM to α 2A, α 2B and α 2C, respectively) and high selectivity (α 2A/α 1A = 240; α 2A/D-1 = 3600; α 2A/D-2 = 3500; α 2A/5-HT 1 = 700; α 2A/5-HT 2= 4100). The compound was labeled with 11C and evaluated in rodents and monkey as a specific radioligand for studying α 2-adrenergic receptors using PET. [ 11C]MK-912 ( [2]) was synthesized by methylation of its desmethyl precursor, L-668,929 ( [1]), with [ 11C]CH 3I in (Bu 3O)P=O at 85°C for 8 min followed by purification with HPLC in 18% yield in a synthesis time of 45 min from end of bombardment (EOB). The specific activity was 0.83–0.93 Ci/μmol and the radiochemical purity was 97%. The initial uptake of [ 11C]MK-912 in mouse brain, heart, lung, liver and kidney was high (5%, 4%, 5%, 17% and 8% per gram of organ, respectively, at 5 min postinjection) and the activities were then slowly cleared from these organs. The uptake of [ 11C]MK-912 in rat olfactory tubercle, a brain region with high density of α 2-adrenergic receptors, was reduced by 30%, and the ratio of radioactivity in olfactory tubercle/cerebellum was reduced from 2:1 to 1:1 by coinjection of [ 11C]MK-912 with a potent α 2-adrenergic receptor antagonist, atipamezole (3 mg/kg), indicating that compound 2 binds to α 2-adrenergic receptors. However, a PET study in a rhesus monkey revealed that the initial influx of [ 11C]MK-912 into various brain regions (cerebellum, cortex, olfactory tubercle and striatum) was high (0.02%/cc), and the radioactivity was then washed out slowly and without significantly differential retention in these brain regions. This, coupled with the fact that none of the high-density α 2-adrenergic receptor brain regions exceeds a few millimeters in diameter, suggests that [ 11C]MK-912 is probably not an ideal radioligand for studying α 2-adrenergic receptors in humans using commercially available PET.