The synthesis of 7α-methyl-substituted estrogens labeled with fluorine-18: potential breast tumor imaging agents

Abstract

The 7α-methyl substituent is reported to increase the binding affinity of estradiol for the estrogen receptor (ER). In order to evaluate whether this substituent would improve the in vitro binding characteristics and the in vivo tissue distribution of 18F labeled estrogens that we are developing as positron emission tomographic (PET) imaging agents for ER-positive breast tumors, we have prepared four 18F labeled analogs of 7α-methylestradiol. These ligands were labeled in the 16α or 16β position with 18F by nucleophilic displacement of the corresponding epimeric estrone trifluoromethanesulfonates with 18F by fluoride ion. Lithium aluminum hybride reduction afforded the estradiol (E 2) series, while lithium trimethylsilylacetylide addition provided the 17α-ethynylestradiol (EE 2) series. The decay-corrected yields were 2–35% for a synthesis time of 85 minutes for the E 2 series, and 120 minutes for the EE 2 series, and the effective specific activities were 158–1213 Ci/mmol. In nearly every case, the 7α-methyl substituent increases ER binding affinity (measured at 25 C) and decreases binding to high affinity serum steroid binding proteins, alphafetoprotein, and sex steroid binding protein; this substituent, however, increases the lipophilicity and the predicted non-specific binding (estimated from octanol-water partition coefficients determined by reverse-phase high-pressure liquid chromatography/[HPLC]), with the result that the ratio of ER binding to non-specific binding is nearly the same for the 7α-methyl substituted analogs as for the corresponding unsubstituted analogs. In vivo distribution studies demonstrated a high level of receptor-mediated uptake in receptor-rich target tissues (uterus, ovaries), and in some cases, other tissues with low ER titers (secondary target tissues, e.g., muscle, thymus) showed significant displaceable uptake, presumed to be receptor-mediated. The uterus to blood ratios were higher for the EE 2 series, reflecting mainly a diminished level of blood activity in the EE 2 series. These analogs exhibited in vitro and in vivo characteristics comparable to the clinically useful 16α-fluoroestradiol; however, in some cases, the uptake of the 7α-methyl substituted analogs, by low ER-titer secondary target tissues, is more specific.