11
C]Hydroxyurea has been successfully labelled using [ 11 C]carbon monoxide at low concentration. The decay-corrected radiochemical yield was 38 AE 3%, and the trapping efficiency of [ 11 C]carbon monoxide in the order of 90 AE 5%. This synthesis was performed by a rhodium-mediated carbonylation reaction starting with azidotrimethylsilane and the rhodium complex being made in situ by chloro(1,5cyclooctadiene)rhodium(I) dimer ([Rh(cod)Cl] 2 ) and 1,2-bis(diphenylphosphino)ethane (dppe). ( 13 C)Hydroxyurea was synthesized using this method and the position of the labelling was confirmed by 13 C-NMR. In order to perform accurate LC-MS identification, the derivative 1-hydroxy-3-phenyl[ 11 C]urea was synthesized in a 35 AE 4% decay-corrected radiochemical yield. After 13 mA h bombardment and 21 min synthesis, 1.6 GBq of pure 1-hydroxy-3-phenyl[ 11 C]urea was collected starting from 6.75 GBq of [ 11 C]carbon monoxide and the specific radioactivity of this compound was in the order of 686 GBq/mmol (3.47 nmol total mass). [ 11 C]Hydroxyurea could be used in conjunction with PET to evaluate the uptake of this anticancer agent into tumour tissue in individual patients.