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Figure 8 Cyclic tamoxifen derivatives.

(Kd = 2.54 and 1.60nmolL \ respectively) and both compounds stimulated MCF-7 proliferation, with methyl ether (12) displaying greater activity at a 1 mmol L _ concentration; the EC50 of 12 was calculated to be 2.4 x 10 " 8 mol L " 1. The activity was inhibited by the antiestrogen ICI 182,780, thus confirming that the effect was estrogen receptor-mediated. In addition, molecular modeling studies comparing the benzothiazinones to E2 were consistent with the compounds acting as estrogen receptor ligands. In transient transfection studies using MCF-7 cells, 12 and 13 showed a modest capacity to activate transcription.

7.07.5.1.4 Triphenylethylenes and related structures

Tamoxifen and related structures containing a triphenylethene backbone continue to generate interest in the search for new estrogen receptor ligands with unique structural or biological properties. Many modifications to the backbone have been investigated, with heterocyclic replacements predominating. The process of designing new molecules which modulate estrogen receptor activity often begins by first synthesizing high-affinity ligands which possess agonist activity, and subsequently appending a basic side chain in an effort to produce a true SERM, with tissue-selective antagonism.

7.07.5.1.4.1 Triphenylethylenes without antagonist side chains

Kim and Katzenellenbogen have reported a series of ring-constrained tamoxifen analogs where the alkyl substituent is tethered via a five-, six-, or seven-membered ring to the distal phenyl ring (Figure 8).67 The compounds generally bound well to both ERa and ERb, with the six- and seven-membered ring being favored (14, n = 2 or 3). Interestingly, 14 (n = 2) displayed modest binding selectivity for ERb (RBA = 240%, purified commercial estrogen receptor) compared to ERa (RBA = 132%), unlike 4-hydroxytamoxifen, which exhibited a twofold selectivity for ERa, and 14 (n = 3), which exhibited approximately threefold selectivity for ERa. Reduction of the olefin which afforded the saturated analogs led to compounds with lower binding affinity and no ERa/b-selectivity. The affinities measured with purified receptor were much higher than those measured in a uterine cytosol preparation, an observation attributed to the high lipophilicity of the compounds and the likelihood of nonspecific protein binding in the cytosol preparation.

The construction of estrogen receptor ligands using solid-phase chemistry has been somewhat limited in scope, since nonsteroidal ligands tend to be relatively complex structures synthesized using a series of carbon-carbon bond-forming reactions which can be lowyielding and difficult to generalize for library preparation. Stauffer and Katzenellenbogen68 have successfully demonstrated the use of solid-phase, split-split chemistry to synthesize a series of tetrasubstituted pyrazole derivatives targeted at the estrogen receptor. Modifications at the N1 and C5 positions were explored most extensively with limited alkyl substitution at C4 (Figure 9). A 12-member pilot library was initially prepared, followed by a more comprehensive 96-member library, with the average purity of the compounds being 50%. The crude compounds were tested in a radioligand binding assay using lamb uterine cytosol at 0 °C and selected compounds were purified by chromatography before retesting. Hydroxy substitution was preferred at

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