16β-[¹⁸F]fluoromoxestrol: A potent, metabolically stable positron emission tomography imaging agent for estrogen receptor positive human breast tumors

16β-[¹⁸F]Fluoromoxestrol (βFMOX, 1) is a highly selective, metabolically stable estrogen with potential as a receptor imaging agent. It demonstrates receptor- mediated uptake in the immature rat in the estrogen receptor-rich primary target tissues, uterus and ovaries, as well as, in receptor-poor secondary target tissues, muscle, thymus and kidneys; uptake in the uterus is nearly four times that of the clinically useful 16α-[¹⁸F]fluoroestradiol (FES), most likely due to the extended lifetime of the labeled βFMOX in the blood afforded by its relatively slow metabolism. In vivo and in vitro studies demonstrate a nearly four-fold decrease in metabolism rate between βFMOX and FES. Dosimetry studies indicate radiation absorbed doses comparable to FES. βFMOX possesses desirable imaging characteristics and may prove to be a clinically useful imaging agent.