Michael R. Kilbourn

Photosensitive Steroids as Probes of Estrogen Receptor Sites

Receptors for steroid hormones are those proteins found in the cytoplasm of target tissue cells that have the ability to bind steroids with high affinity and great selectivity. The initial steroid-receptor complex, which forms in the cytoplasm, undergoes an activation process, after which it moves into the nucleus and interacts with chromatin. This chromatin interaction, while still poorly understood, is thought to result in changes in gene expression that are ultimately reflected in the biochemical and physiological alterations that are characteristic of the tissue’s response to the hormone.’ Affinity labeling stands to make a substantial contribution to our understanding of the intricacies of both the interaction between steroid hormones and receptors and the actions of the receptors themselves.’ Steroid receptors, even in the richest tissue sources, are minor cellular constituents, whose purification is complicated by their thermal lability and their tendency to aggregate. Thus, a radiolabeled ligand that could be covalently attached to the receptor would be of great assistance to efforts directed toward the purification and characterization of receptors. Steroid receptors also exist in different states (unliganded, liganded and unactivated, liganded and activated), and move between different subcellular compartments. A process such as photoaffinity labeling, whereby a steroid derivative could be induced to attach covalently to a receptor a t some time subsequent to its initial binding to the receptor, would provide a unique way of following the dynamic features of steroid receptor interaction: activation, subcellular redistribution, chromatin interaction, receptor recycling, and receptor degradation. Finally, steroid receptors have a great tendency towards association, with the result that their apparent physicochemical properties may change upon purification. This causes one to doubt that attempts a t reconstituting the cellular interactions between steroids, receptors, and chromatin binding sites in vitro, using purified elements, will result in accurate representations of the in vivo interaction. Photoactivated steroid analogs capable of covalent labeling may prove to be an important avenue for the exploration of receptor action in certain intact systems (e.g., cells in culture). Thus, the ultimate goal of efforts to develop affinity labeling agents for steroid receptors should be to obtain agents with sufficient selectivity and efficiency to provide adequate means for covalent labeling of receptors in systems that are

From Cyclotron to Patient via HPLC

The use of HPLC (high pressure liquid chromatography, also called high performance liquid chromatography) has become an increasingly useful tool in radiopharmaceutical chemistry. Initially a quick method for analysis of radiochemical products, it has evolved into a method now used in many institutions for the preparation of radiopharmaceuticals for human administration.