Target tissue uptake selectivity of three fluorine-substituted progestins: Potential imaging agents for receptor-positive breast tumors

We have studied three new fluorine-substituted progestins (1-3) as potential imaging agents for progesterone receptor (PgR)-positive human breast tumors. Two of these are fluorine-substituted analogs of the potent progestin R5020 (promegestone), derived from (21S)-hydroxy R 5020 (RU 27987) and (21R)-hydroxy R 5020 (RU 27988), known metabolites of R 5020, which have affinities for PgR that are 116 and 4%, respectively (relative to R 5020 = 100%). These precursors were protected as their 3,3-dioxolane derivatives and converted to the 21-trifluoromethanesulfonate derivatives. Fluoride ion displacement, followed by acid-catalyzed deprotection, furnished in good yield the epimeric fluoroanalogs, (21S)- and (21R)-fluoro R 5020 (1 and 2, affinities for PgR, 11 and 45%, respectively). These compounds were also prepared in ¹⁸F labeled form by the same route, in 14-32% overall radiochemical yield (decay corrected; synthesis time 90 min; sp. act. 370-1060 Ci/mmol). In tissue distribution studies in estrogen-primed immature rats, uterus-to-muscle ratios were 4.3 at 1 h for the 21S-epimer and 1.1 for the 21R-epimer, paralleling their relative binding affinities. Considerable metabolic defluorination was observed. The third fluorine-substituted progestin, DU 41165, has a novel retroprogesterone (9β, 10α) structure, substituted with fluorine at C-6; its binding affinity is 145% relative to R 5020, and it was prepared in tritium-labeled form by acetylation of DU 41231, the 17α-hydroxy precursor, with [³H]acetic anhydride. In estrogen-primed immature rats, this compound shows uterus-to-muscle ratios of 15 at 1 h, and 18-71 between 2 and 6 h, suggesting that compounds in this retroprogesterone series may be very promising candidates for selective imaging of PgR-positive tissues and tumors.