Comparison of non-isotopic exchange methods for n.c.a. labelling of 2-iodophenyl-metyrapone

The Cu(I)-assisted aromatic halogen exchange proved useful for preparation of macroscopic amounts of 2-iodophenyl-metyrapone as well as for the n.c.a. radioiodinated analogue. Semi-preparative HPLC-isolation provided the compound with high purity for use as chromatographic standard and for the determination of the inhibition constant of the 2-iodo-analogue. The non-isotopic exchange led to a high specific activity (> 5000 GBq/mumol) of 2-[123I]iodophenyl-metyrapone. A reaction in acetic acid at elevated temperatures proved superior to an exchange in aqueous solution with in-situ reduction of Cu2+. Even without Cu+ high radiochemical yields of > 80% were obtained, while addition of Cu+ provided the radiotracer with 95% radiochemical yield within 5 minutes in acetic acid.