Through an effort to develop novel ligands that have subtype selectivity for the estrogen
receptors alpha (ERalpha) and beta (ERbeta), we have found that 2,3-bis(4-hydroxyphenyl)propionitrile
(DPN) acts as an agonist on both ER subtypes, but has a 70-fold higher relative binding
affinity and 170-fold higher relative potency in transcription assays with ERbeta
than with ERalpha. To investigate the ERbeta affinity- and potency-selective character
of this DPN further, we prepared a series of DPN analogues in which both the ligand
core and the aromatic rings were modified by the repositioning of phenolic hydroxy
groups and by the addition of alkyl substituents and nitrile groups. We also prepared
other series of DPN analogues in which the nitrile functionality was replaced with
acetylene groups or polar functions, to mimic the linear geometry or polarity of the
nitrile, respectively. To varying degrees, all of the analogues show preferential
binding affinity for ERbeta (i.e., they are ERbeta affinity-selective), and many,
but not all of them, are also more potent in activating transcription through ERbeta
than through ERalpha (i.e., they are ERbeta potency-selective). meso-2,3-Bis(4-hydroxyphenyl)succinonitrile
and dl-2,3-bis(4-hydroxyphenyl)succinonitrile are among the highest ERbeta affinity-selective
ligands, and they have an ERbeta potency selectivity that is equivalent to that of
DPN. The acetylene analogues have higher binding affinities but somewhat lower selectivities
than their nitrile counterparts. The polar analogues have lower affinities, and only
the fluorinated polar analogues have substantial affinity selectivities. This study
suggests that, in this series of ligands, the nitrile functionality is critical to
ERbeta selectivity because it provides the optimal combination of linear geometry
and polarity. Furthermore, the addition of a second nitrile group beta to the nitrile
in DPN or the addition of a methyl substitutent at an ortho position on the beta-aromatic
ring increases the affinity and selectivity of these compounds for ERbeta. These ERbeta-selective
compounds may prove to be valuable tools in understanding the differences in structure
and biological function of ERalpha and ERbeta.