16-alpha-ethyl-21-hydroxy-19-nor-4-pregnene-3-20-dione and 21-fluoro-16-ethyl-19-norprogesterone

We have synthesized 21-[18F]fluoro-16 alpha-ethyl-19-norprogesterone (FENP), a high affinity ligand for the progesterone receptor, labeled with the positron-emitting radionuclide fluorine-18 (t1/2 = 110 min). The synthesis proceeds in two steps from 21-hydroxy-16 alpha-ethyl-19-norprogesterone and involves [18F]fluoride ion displacement of the 21-trifluoromethanesulfonate (21-triflate). This material is purified by HPLC and is obtained in 4-30% overall yield (decay corrected) within 40 min after the end of bombardment to produce [18F]fluoride ion. The effective specific activity, determined by competitive radioreceptor binding assays, is 700-1400 Ci/mmol. In vivo, [18F]FENP demonstrates highly selective, receptor-mediated uptake by the uterus of estrogen-primed rats; the uterus to blood and uterus to muscle ratios were respectively 26 and 16 at 1 h and 71 and 41 at 3 h after injection. The high target tissue selectivity of this uptake suggests that this compound may be useful for the in vivo imaging of progestin target tissues and receptor-rich tumors (such as human breast tumors) by positron emission tomography.

Topics: Animals; Binding, Competitive; Chemical Phenomena; Chemistry; Chromatography, High Pressure Liquid; Estrogens; Female; Fluorine Radioisotopes; Norpregnenes; Norprogesterones; Pregnenediones; Progesterone; Progesterone Congeners; Promegestone; Rats; Rats, Inbred Strains; Receptors, Progesterone; Tomography, Emission-Computed; Uterus