Syntheses and in vitro evaluation of fluorinated naphthoxazines as dopamine D2/D3 receptor agonists: radiosynthesis, ex vivo biodistribution and autoradiography of [(18)F]F-PHNO.
BACKGROUND
Carbon-11-labeled (+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol ([(11)C]-(+)-PHNO) is a dopamine D2/D3 agonist radioligand that is currently used to image the high-affinity state of dopamine receptors in humans with positron emission tomography (PET). The present study reports the preparation and evaluation of fluorinated (+)-PHNO derivatives.


METHODS
Five fluorinated (+)-PHNO derivatives were synthesized and tested in vitro for inhibition of binding of [(3)H]domperidone in homogenates of rat striatum and inhibition of binding to [(3)H]-(+)-PHNO in homogenates of human-cloned D2Long receptors in Chinese hamster ovary cells and rat striatum. Radiolabeling with fluorine-18 was carried out for the most promising candidate, N-fluoropropyl-(+)-HNO (F-PHNO), and ex vivo biodistribution and autoradiography studies with this radiopharmaceutical were performed in rodents.


RESULTS
(+)-PHNO and the fluorinated analogs inhibited binding of [(3)H]domperidone and [(3)H]-(+)-PHNO to the high- and low-affinity states of dopamine D2 receptors, consistent with D2 agonist behavior. The average dissociation constant at the high-affinity state of D2, K(i)(High), was 0.4 nM for F-PHNO and proved to be equipotent with (+)-PHNO (0.7 nM). All other fluorinated derivatives were significantly less potent (K(i)(High)=2-102 nM). The most promising candidate, F-PHNO, was labeled with fluorine-18 in 5% uncorrected radiochemical yield, with respect to starting fluoride. Ex vivo biodistribution and autoradiography studies in rodents revealed that [(18)F]F-PHNO rapidly enters the rodent brain. However, this radiotracer does not reveal specific binding in the brain and is rapidly cleared.


CONCLUSIONS
Five novel dopamine D2/D3 agonists based on (+)-PHNO were synthesized and evaluated in vitro. F-PHNO was shown to behave as a potent D2 agonist in vitro and was therefore radiolabeled with fluorine-18. Despite the promising in vitro pharmacological profile, [(18)F]F-PHNO did not display in vivo behavior suitable to image dopaminergic receptor expression using PET.
Vasdev
Neil
N
PET Centre for Addiction and Mental Health, Toronto, Ontario, Canada, M5T-1R8. neil.vasdev@camhpet.ca
Seeman
Philip
P
Garcia
Armando
A
Stableford
Winston T
WT
Nobrega
José N
JN
Houle
Sylvain
S
Wilson
Alan A
AA
D000818:Animals; D001345:Autoradiography; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D005462:Fluorine Radioisotopes / Q000737:chemistry / Q000493:pharmacokinetics; D007553:Isotope Labeling / Q000379:methods; D008297:Male; D008657:Metabolic Clearance Rate; D009928:Organ Specificity; D010078:Oxazines / Q000737:chemistry / Q000493:pharmacokinetics*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D017448:Receptors, Dopamine D2 / Q000819:agonists*; D050637:Receptors, Dopamine D3 / Q000819:agonists*; D014018:Tissue Distribution
D005462:Fluorine Radioisotopes; D010078:Oxazines; D019275:Radiopharmaceuticals; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; C042631:naxagolide