11.beta.-Methoxy-, 11.beta.-ethyl, and 17.alpha.-ethynyl-substituted 16.alpha.-fluoroestradiols: receptor-based imaging agents with enhanced uptake efficiency and selectivity

Abstract

We have prepared three analogues of 16 alpha-fluoroestradiol (FES) substituted either with an 11 beta-methoxy group (1, 11 beta-MeO-FES), an 11 beta-ethyl group (2, 11 beta-Et-FES), or a 17 alpha-ethynyl group (3, 17 alpha-ethynyl-FES). These substituents all lower the binding of FES to the serum proteins alphafetoprotein and sex steroid binding protein, but their effect on estrogen receptor binding varies: Receptor binding is increased by the 11 beta-ethyl and 17 alpha-ethynyl groups, but decreased by the 11 beta-methoxy group. These substituents also have a parallel effect on the lipophilicity, and hence the nonspecific binding estimated for these compounds. All three compounds were prepared in fluorine-18 labeled form, at effective specific activities of 90-1600 Ci/mmol, by fluoride ion displacement reactions as done previously with FES. Tissue distribution studies in immature rats show high uptake selectivity by target tissue (uterus) and effective competition by an excess of unlabeled estradiol. Percent injected dose per gram values (% ID/g) at 1 h are 6% for 11 beta-MeO-FES and 11-13% for 11 beta-Et-FES and 17 alpha-ethynyl-FES (FES itself has a % ID/g of 9%). Uptake selectivity in terms of uterus to blood or muscle ratios at 1 h is highest for 11 beta-MeO-FES and 17 alpha-ethynyl-FES (43-149). Metabolic consumption studies show that most activity in uterus is unmetabolized and in blood is rapidly and nearly completely metabolized. In muscle, FES and the substituted estrogens show intermediate levels of metabolic consumption; in some cases activity in muscle extracts is nearly unmetabolized. Thus, the substituents on FES cause major alterations in receptor and nonreceptor binding affinity, uptake efficiency and selectivity, and extent of metabolism. It is not readily clear, however, whether the alterations in uptake efficiency and selectivity are the result of differences in receptor or nonreceptor binding or lipophilicity, or altered patterns of metabolism. Nevertheless, these compounds should be useful in providing a spectrum of uptake properties that could be used for imaging different estrogen-receptor-containing structures.

Synthesis and biodistribution of fluorine-18-labeled fluorocyclofenils for imaging the estrogen receptor

“…Their binding affinities for the estrogen receptors, ERα and ERβ, are of the same order of many other fluorine-18 labeled estrogens that do show ERmediated uptake in the uterus of immature female rats [10,12]. Their in vivo tissue distribution also did not reveal any obvious unusual pharmacokinetic or metabolism liabilities.…”

“…Their level of non-specific binding, which generally tracks with compound lipophilicity (as characterized by cLogP values [1]: estradiol (3. The most distinguishing feature of these compounds is that they are non-steroidal, whereas most fluorine-18 labeled estrogens studied that have shown clear evidence of ER-mediated uterine uptake have been steroidal [10,12]. Perhaps by following more closely the structure of "nature's preferred ligand" -namely, a steroid -one can obtain compounds with pharmacokinetic and pharmacodynamic properties that are better suited for selective receptormediated uptake and retention, as are required for imaging steroid receptors with PET.…”

“…A number of estradiol derivatives labeled with bromine [7,8], iodine [9], and fluorine [10][11][12] have been developed as tumor imaging agents for this purpose. While most of these agents have been based on steroidal estrogens, a few investigations were focused on non-steroidal estrogens or antiestrogens including selective estrogen receptor modulators [13][14][15][16][17][18].…”

Evaluation of a bromine-76-labeled progestin 16α,17α-dioxolane for breast tumor imaging and radiotherapy: in vivo biodistribution and metabolic stability studies

“…At later times, metabolites also began to appear in muscle and uterus, although there was substantial retention of unconverted [ 76 Br]3 remained in the uterus, where it is presumably protected from metabolism by its binding to PR. The target tissue protection of receptor binding radiopharmaceuticals has been noted before with ER ligands [43][44][45]. The rapid metabolism we have observed is not surprising, because liver is known to be the major site of steroid metabolism [46] and the C-21 hydroxy group in [ 76 Br]3 might facilitate the reduction of the C-20 ketone.…”

“…Metabolic stability is an issue facing the development of PR-based steroid ligands [41][42][43]. The C-20 ketone in progestins is at risk for reduction to a C-20 hydoxy group, giving a compound with very low affinity for PR that is inactive in vivo and would thus be unsuitable as a progestin radiotracer imaging agent.…”