Toru Iijima

Potent estrogen agonists based on carborane as a hydrophobic skeletal structure. A new medicinal application of boron clusters.

BACKGROUND Carboranes (dicarba-closo-dodecaboranes) are a class of carbon-containing polyhedral boron-cluster compounds having remarkable thermal stability and exceptional hydrophobicity. Applications of the unique structural and chemical properties offered by icosahedral carboranes in boron neutron capture therapy have received increasing attention over the past 30 years. However, these features of carboranes may allow another application as a hydrophobic pharmacophore in biologically active molecules that interact hydrophobically with receptors. RESULTS We have designed candidate estrogen-receptor-binding compounds having carborane as a hydrophobic skeletal structure and synthesized them. The most potent compound bearing a carborane cage exhibited activity at least 10-fold greater than that of 17beta-estradiol in the luciferase reporter gene assay. Estrogen receptor-alpha-binding data for the compound were consistent with the results of the luciferase reporter gene assay. The compound also showed potent in vivo effects on the recovery of uterine weight and bone loss in ovariectomized mice. CONCLUSION Further development of the potent carborane-containing estrogenic agonists described here, having a new skeletal structure and unique characteristics, should yield novel therapeutic agents, especially selective estrogen receptor modulators. Furthermore, the suitability of the spherical carborane cage for binding to the cavity of the estrogen receptor-alpha ligand-binding domain should provide a basis for a similar approach to developing novel ligands for other steroid receptors.

Estrogenic antagonists bearing dicarba-closo-dodecaborane as a hydrophobic pharmacophore.

Dicarba-closo-dodecaboranes (carboranes), which have spherical geometry and hydrophobicity, are applicable as a hydrophobic pharmacophore of biologically active molecules. We have designed and synthesized estrogenic antagonists based on the structure of the potent agonist 1-hydroxymethyl-12-(4-hydroxyphenyl)-1,12-dicarba-closo-d odecaborane, which we have developed. The compounds showed potent antagonistic activity in luciferase reporter gene assay using COS-1 cells transfected with rat ER alpha-expression plasmid and an appropriate reporter plasmid.

Structure-activity study of retinoid agonists bearing substituted dicarba-closo-dodecaborane. Relation between retinoidal activity and conformation of two aromatic nuclei

We have investigated the structure activity relationships of the potent retinoid agonist, 4-[4-(2-propyl-1,2-dicarba-closo-dodecaboran-l-yl)phenylamino]benzoic acid (BR403), which we have previously reported. Substitution of a methyl group on the aromatic nucleus or a methyl group on the nitrogen atom, or replacement of the amino group with ether, methylene, carboxyl or 1,1-ethylene greatly decreased the activity. The relatively planar conformation at the phenyl-N-phenyl moiety seems to play a critical role in the appearance of the biological activity.

Structure-activity study of estrogenic agonists bearing dicarba-closo-dodecaborane. Effect of geometry and separation distance of hydroxyl groups at the ends of molecules.

Dicarba-closo-dodecaboranes (carboranes), which have spherical geometry and hydrophobicity, are applicable as a hydrophobic pharmacophore of biologically active molecules. We have investigated structure-activity relations based on the structure of the potent estrogenic agonist, 1-hydroxymethyl-12-(4-hydroxyphenyl)-1,12-dicarba-closo-d odecaborane, which we have previously reported. The geometry and separation distance of the phenolic and alcoholic hydroxyl groups play a critical role in the appearance of biological activity.

Synthesis and biological evaluation of p-carborane bisphenols and their derivatives: structure-activity relationship for estrogenic activity.

p-Carborane bisphenols and their derivatives were prepared and evaluated for binding affinity to estrogen receptor alpha. Their estrogenic activity was evaluated by means of transcriptional assay and cell proliferation assay using MCF-7 cell lines. 1,12-Bis(4-hydroxyphenyl)-1,12-dicarba-closo-dodecaborane 4a showed potent estrogenic activity, approaching that of 17beta-estradiol, in transactivation assay. The activity of isomers 5a and 6a was drastically affected by the change in the position of one of the hydroxyl groups; 6a (ortho-OH in one ring) was about 1000 times less potent than 4a. Modification of this hydroxyl group with alkyl groups decreased the estrogenic activity in all isomers. Compound 4a also showed potent MCF-7 cell proliferation-enhancing activity.