Improved Synthesis of [11C]Cou, a Selective MAO-B Substrate for Neurological Imaging

483 Objectives: The MAO-B selective substrate 4-methyl-7-((1-(methyl)-1,2,3,6-tetrahydropyridin-4-yl)oxy)-2H-chromen-2-one ([11C]Cou) was developed as a metabolic trapping imaging agent for MAO-B with brain uptake and MAO selectivity demonstrated in non-human primate brain studies[1]. It was recently demonstrated that Cou did not cause MPTP type neurotoxicity, with Cou treated animals being indistinguishable from saline control [2]. We report here the development of a one-step, high yielding, automated method for radiotracer synthesis with preliminary evaluation of [11C]Cou as an imaging agent in the peripheral nervous system. Methods: A two-pot procedure was previously required where pyridine 1 was first N-methylated with [11C]CH3OTf and then in a second reactor reduced by sodium borohydride to[11C]Cou. This method was not easily automated and resulted in low yields (1.7% yield non-decay corrected, n=10). To produce [11C]Cou via a one-pot automated methylation method, we required the desmethyl amine. The starting pyridine 1 was first alkylated with allyl bromide, which was subsequently reduced to tetrahydropyridine 2. The allyl group was then removed by palladium catalyzed reduction with 1,3-dimethylbarbituric acid to give the desired precursor 3. The precursor (1.0 mg) was dissolved in DMSO (100 µL) and sparged with [11C]CH3OTf at 15 mL/min for 3 minutes. Isolation and purification of [11C]Cou was accomplished using HPLC (Phenomenex Gemini C18, 250x10mm, 5µ column at 4 mL/min with 50% acetonitrile, 50mM NaHCO3, pH 10). The collected peak was diluted in milliQ H2O (50 mL) with a 1% solution of NH4OH (0.5 mL) added to it. The diluted product was passed through a Waters C18 solid phase extraction cartridge. The cartridge was then washed with H2O eluted with ethanol (0.5 mL) and formulated with saline (4.5 mL) for injection. Imaging studies were performed in Sprague-Dawley rats positioned to image the thorax of the amimal in a MicroPET P4. Blocking studies were performed by treatment of the animal with deprenyl (10 mg/Kg) i.p. 90 minutes prior to the scan. Results: The precursor was prepared in a three step, two pot synthesis in 22% overall yield. Using an automated radiosynthesis module, the precursor was methylated with [11C]CH3OTf and purified by HPLC to provide [11C]Cou (140 ± 19 mCi, 16.9% non-decay corrected yield, 35 min synthesis from end of beam, radiochemical purity >99%, n=6) in a saline solution. The microPET imaging studies showed high and rapid heart uptake in ventricle (peak at 90 sec, SUV of 4.6), a rapid washout and a plateau after approx. 10 min (SUV of 3.3). In deprenyl blocking studies the ventricle SUV plateuaued at 0.45. Conclusions: An improved one-step, automated synthesis of [11C]Cou has been developed. The radiotracer demonstrated MAO-B dependent imaging of the heart. [11C]Cou can now be prepared in high yield in an automated synthesis module by a standard methylation method. References: [1]Brooks, A. F.; et. al. ACS Chem. Neurosci. 2015, 6, 1965-1971. [2]Brooks, A. F.; Mufarreh, A.J.; Shao, X.; Kilbourn, M.R.; Scott, P.J.H. Journal of Nuclear Medicine. 2018, 59 (supplement 1), 618.