Synthesis and biological evaluation of [¹¹C]MK-912 as an α₂- adrenergic receptor radioligand for PET studies

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 α₂-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-HT₁ = 700; α(2A)/5-HT₂ = 4100). The compound was labeled with ¹¹C and evaluated in rodents and monkey as a specific radioligand for studying α₂-adrenergic receptors using PET. [¹¹C]MK-912 (2) was synthesized by methylation of its desmethyl precursor, L-668,929 (1), with [¹¹C]CH₃I in (Bu₃O)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 α₂-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 α₂-adrenergic receptor antagonist, atipamezole (3 mg/kg), indicating that compound 2 binds to α₂-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 α₂- adrenergic receptor brain regions exceeds a few millimeters in diameter, suggests that [¹¹C]MK-912 is probably not an ideal radioligand for studying α₂-adrenergic receptors in humans using commercially available PET.