Citalopram: labelling with carbon-11 and evaluation in rat as a potential radioligand for in vivo PET studies of 5-HT re-uptake sites.
In vivo autoradiography of [N-methyl-3H]citalopram in rat brain shows a differential regional localization which correlates with the localization of 5-HT re-uptake binding sites defined in vitro. A comparison of the biodistribution of [N-methyl-3H]citalopram over 2 h after i.v. injection in (1) control rats (2) rats pre-dosed with either citalopram or paroxetine and (3) rats chemically-lesioned with p-chloroamphetamine provides an estimate of specific binding relative to total binding in vivo. The ratio of binding in certain regions (e.g. cingulate) to binding in a reference tissue (e.g. cerebellum) at 30-120 min post injection is c. 1.4. In view of these results a method was developed for labelling citalopram with carbon-11 (t1/2 = 20.3 min, beta + = 99.8%) to provide a potential radioligand for studies using positron emission tomography. Thus, reaction of nca [11C]iodomethane, prepared from cyclotron-produced [11C]carbon dioxide, with norcitalopram in ethanol containing 2,2,6,6-tetramethyl-piperidine for 5 min at 95 degrees C gives crude [N-methyl-11C]citalopram in 60% radiochemical yield, decay-corrected. HPLC on silica gel provides radiochemically and chemically pure [N-methyl-11C]citalopram, as assessed by TLC, HPLC and MS. This product (isolated radiochemical yield, 49%) is easily formulated for i.v. injection. Up to 2 GBq of formulated product with a specific activity of c. 15 GBq/mumol have been prepared within 40 min from the end of radionuclide production. The described radiosynthesis has also been applied to give the single biologically active (+)-enantiomer of [N-methyl-11C]citalopram rather than the racemate. This product gives enhanced specific signal in the rat following i.v. injection, the ratio of uptake in regions of interest relative to cerebellum approaching 2 at 90 min.
MRC Cyclotron Unit, Hammersmith Hospital, London, England.
S P
SP
Hume
C
C
Pascali
V W
VW
Pike
D R
DR
Turton
R G
RG
Ahier
R
R
Myers
D M
DM
Bateman
J E
JE
Cremer
L G
LG
Manjil
R
R
Dolan
D000818:Animals; D001345:Autoradiography; D001923:Brain Chemistry; D002245:Carbon Dioxide / Q000378:metabolism; D002247:Carbon Isotopes; D002851:Chromatography, High Pressure Liquid; D002855:Chromatography, Thin Layer; D015283:Citalopram / Q000493:pharmacokinetics*; D003956:Dialysis; D007553:Isotope Labeling; D008297:Male; D013058:Mass Spectrometry; D018377:Neurotransmitter Agents / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D014018:Tissue Distribution; D014055:Tomography, Emission-Computed
D002247:Carbon Isotopes; D018377:Neurotransmitter Agents; D015283:Citalopram; D002245:Carbon Dioxide