21-[¹⁸F]Fluoro-16α-ethyl-19-norprogesterone: Synthesis and Target Tissue Selective Uptake of a Progestin Receptor Based Radiotracer for Positron Emission Tomography

We have synthesized 21-[¹⁸F]Fluoro-16α-ethyl-19-norprogesterone (FENP), a high affinity ligand for the progesterone receptor, labeled with the positron-emitting radionuclide fluorine-18 (t_1/2 = 110 min). The synthesis proceeds in two steps from 21-hydroxy-16α-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.