Evaluation of [18F]fluoroxanomeline {5-{4-[(6-[18F]fluorohexyl)oxy]-1,2,5-thiadiazol-3-yl}-1-methyl-1,2,3,6-tetrahydropyridine} in muscarinic knockout mice.
BACKGROUND
We set out to develop a muscarinic M1-selective agonist (based on the structure of the functionally M1-selective xanomeline) that could be radiolabeled with fluorine-18 for use as an imaging agent for positron emission tomography.


METHODS
The radiochemical synthesis was achieved, employing the arts of organic and radiochemical syntheses. Binding selectivity studies employed biodistribution studies, using autoradiography and/or tissue dissection, in wild-type or muscarinic receptor knockout mice.


RESULTS
[(18)F]Fluoroxanomeline shows rather uniform uptake in all mouse brain regions and high specific binding, with a brain-to-blood ratio of 32 at 60 min postinjection. In addition, the specific binding is demonstrated by a 58% to 75% decrease in brain uptake upon coinjection with 5 nmol of unlabeled fluoroxanomeline or xanomeline. Brain uptake studies with [(3)H]xanomeline in muscarinic knockout mice show decreased uptake in M1 (17-34%) and M2 (2-20%) knockout mice compared with control. However, statistical significance was observed in only a few regions. Comparison of [(18)F]fluoroxanomeline in knockout mice showed no difference in M1 or M4 knockout mice but a general decrease in M2 (2-24%) knockout mice. The decrease of [(18)F]fluoroxanomeline uptake in M2 knockout mice reached statistical significance in brain stem, cerebellum, frontal cortex, hippocampus, inferior colliculus and superior colliculus.


CONCLUSIONS
Although xanomeline displays highly selective M1 agonist activity in functional assays, little selectivity for muscarinic subtype binding was observed for xanomeline or its fluorine-containing analogue, fluoroxanomeline. This emphasizes the lack of correlation between functional selectivity and binding selectivity.
Kiesewetter
Dale O
DO
Positron Emission Tomography Radiochemistry Group, NIBIB, NIH, Bethesda, MD 20892, USA. dk7k@nih.gov
Jagoda
Elaine M
EM
Shimoji
Kazuaki
K
Ma
Ying
Y
Eckelman
William C
WC
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D022781:Hepatocytes / Q000000981:diagnostic imaging / Q000378:metabolism; D008297:Male; D008657:Metabolic Clearance Rate; D051379:Mice; D018345:Mice, Knockout; D009928:Organ Specificity; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D051381:Rats; D011976:Receptors, Muscarinic / Q000235:genetics*; D013830:Thiadiazoles / Q000493:pharmacokinetics*; D014018:Tissue Distribution
D019275:Radiopharmaceuticals; D011976:Receptors, Muscarinic; D013830:Thiadiazoles; C521164:fluoroxanomeline