The antiestrogen tamoxifen is extensively metabolized in patients to form a series of compounds with altered affinity for estrogen receptors (ERs), the primary target of this drug. Furthermore, these metabolites exhibit a range of partial agonist and antagonist activities for ER mediated effects that do not depend directly on their absolute affinity for ERs. Thus, clinical response to tamoxifen therapy is likely to depend on the aggregate effect of these different metabolites resulting from their abundance in the patient, their affinity for the receptors, and their agonist/antagonist profile. A recent study has shown that plasma concentrations of the tamoxifen metabolite 4-hydroxy- N -desmethyl tamoxifen (endoxifen), in patents undergoing tamoxifen therapy, are dependent on the cytochrome p450 (CYP) 206 ge notype of the patient and that medications commonly prescribed to patients on tamoxifen therapy can also inhibit endoxifen production. In this study we characterized the properties of this metabolite with respect to binding to ERs, ability to inhibit estrogen stimulated breast cancer cell proliferation and the regulation of estrogen responsive genes. We demonstrate that endoxifen has essentially equivalent activity to the potent metabolite 4-hydroxy tamoxifen (4-OH-tam) often described as the active metabolite of this drug. Since plasma levels of endoxifen in patients with functional CYP2D6 frequently exceed the levels of 4-OH-tam, it seems likely that endoxifen is at least as important as 4-OH-tam to the overall activity of this drug and suggests that CYP2D6 status and concomitant administration of drugs that inhibit CYP2D6 activity have the potential to affect response to tamoxifen therapy.

The determination of several structures of nuclear receptor ligand binding domains (LBD) has led to new insights into the mechanism of action of this very important class of receptors. This review describes and compares the different LBD structures and their relationship to the function of the nuclear receptors. The role of the ligand in the LBD structures and the implications of ligand structure on receptor activity are also discussed. Structural information regarding interactions between the LBD and coactivator proteins and the potential role of these interactions in ligand agonism and antagonism is reviewed. Different pathways for nuclear receptor signaling and the use of new ligands to investigate these pathways are also described.

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The binding of the mannose/glucose specific lectins from Canavalia ensiformis (concanavalin A) and Dioclea grandiflora to a series of C-glucosides were studied by titration microcalorimetry and fluorescence anisotropy titration. These closely related lectins share a specificity for the trimannoside methyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, and are a useful model system for addressing the feasibility of differentiating between lectins with overlapping carbohydrate specificities. The ligands were designed to address two issues: (1) how the recognition properties of non-hydrolyzable C-glycoside analogues compare with those of the corresponding O-glycosides and (2) the effect of presentation of more than one saccharide recognition epitope on both affinity and specificity. Both lectins bind the C-glycosides with affinities comparable to those of the O-glycoside analogues; however, the ability of both lectins to differentiate between gluco and manno diastereomers was diminished in the C-glycoside series. Bivalent norbornyl C-glycoside esters were bound by the lectin from Canavalia but only weakly by the lectin from Dioclea. In addition to binding the bivalent ligands, concanavalin A discriminated between C-2 epimers, with the manno configuration binding more tightly than the gluco. The stoichiometry of binding of the bivalent ligands to both di- and tetrameric lectin was two binding sites per ligand, rather than the expected 1:1 stoichiometry. Together, these results suggest that concanavalin A may possess more than one class of carbohydrate binding sites and that these additional sites show stereochemical discrimination similar to that of the previously identified monosaccharide binding site. The implications of these findings for possible in vivo roles of plant lectins and for the use of concanavalin A as a research tool are discussed.