The metabolism of 16-fluoroestradiols in vivo: chemical strategies for restricting the oxidative biotransformations of an estrogen-receptor imaging agent.

16 alpha-Fluoro-17 beta-, 16 alpha-fluoro-17 alpha-, and 16 beta-fluoro-17 beta-[6,7-3H]estradiol were prepared from [6,7-3H]estrone via fluorination of 3,17-bis(tert-butyldimethylsilyloxy)-[6,7-3H]estratetraene with N-fluoropyridinium triflate and reduction of 16 alpha/beta-fluoro[6,7-3H]estrone with NaBH4. The three isomers were separated by silica-phase high-performance liquid chromatography. They were administered intravenously (4 mumol/kg to anaesthetized male rats. Their biliary metabolites (90-97% of dose over 6 h) were characterized by high performance liquid chromatography-mass spectrometry and compared with those of [6,7-3H]17 beta-estradiol. The four estrogens and their hydroxylated and methoxylated metabolites were excreted as glucuronides. C-16 fluorination blocked C-16 hydroxylated and also the dehydrogenation of the C-17 hydroxyl group. The 16 alpha-17 beta isomer was extensively glucuronylated at C(O)3 but also underwent aromatic hydroxylation and methoxylation before conjugation. Its C-17 epimer was subject to much greater aromatic hydroxylation but the catecholestrogen was O-methylated to a greater relative extent. The 16 beta-17 beta derivative underwent alicyclic as well as substantial aromatic hydroxylation and yielded numerous isomeric glucuronides of O-methylated catechols. Thus, the fluorine exerted complex effects (inhibitory and enhancing) on both localized (D-ring) and distal (A-ring) biotransformations of the estradiol molecule; the direction and magnitude of the effects being dependent upon the stereochemistry at C-16 and C-17. These findings provide structural guidelines for restricting the metabolism of tumor-imaging fluoroestrogens and thereby enhancing their delivery to the target tissue.