Abstract P1-12-08: Bortezomib enhances the efficacy of fulvestrant by promoting the aggregation of the ER in the cytoplasm
Authors: Germain Doris , Adelson Kerin , Raptis George , Waxman Samuel 
Published: May 2015

Background: Aromatase inhibitors (AIs) are standard treatment for Estrogen Receptor (ER)+ breast cancers in post-menopausal women where the main source of estradiol comes from adipose tissue when the aromatase enzyme converts androgens to estrogens. However, in premenopausal women, functioning ovaries flood the body with estrogens, and aromatase inhibitors used alone offer no therapeutic benefit. In addition to tamoxifen and aromatase inhibitors, estrogen receptor down-regulators, are a third type of anti-estrogen. The first of this class to be FDA approved is Fulvestrant, which acts by promoting the proteosomal degradation of the ER. Like tamoxifen, fulvestrant binds directly to the ER but while tamoxifen has both antagonist and agonist effects on the ER, fulvestrant is a pure antagonist. Other important advantages of fulvestrant over tamoxifen are that 1) fulvestrant prevents the ER from binding DNA, 2) fulvestrant is not linked to increased risk of endometrial cancer, 3) fulvestrant promotes permanent degradation of the ER.

The molecular machinery leading to the degradation of the ER in the nucleus following fulvestrant treatment is well described and correlates with its ubiquitination in the nucleus. A less well-recognized mechanism, is fulvestrant’s ability to promote the aggregation of the newly synthesized ER in the cytoplasm. Understanding that protein aggregates are toxic when not eliminated by the proteasome, we took advantage of this effect of fulvestrant to ask whether combining fulvestrant with the proteasome inhibitor Bortezomib could enhance the efficacy of Fulvestrant.

Results: We found that bortezomib enhances the aggregation of the ER in the cytoplasm following treatment with fulvestrant. Further, these aggregates were found to be insoluble and to activate the unfolded protein response (UPR), a stress response that leads to cell death. Further, bortezomib is able to prevent the activation of cytoprotective responses linked to the acquisition of fulvestrant resistance. Furthermore, in a breast cancer mouse model of tamoxifen resistance, the combination induced tumor regression. We currently are testing new generation proteasome inhibitors and the results will be presented at the meeting.

Conclusion: We conclude that adding bortezomib to fulvestrant enhances its efficacy by taking advantage of a previously poorly recognized mechanism–fulvestrant’s induction of ER aggregation in the cytoplasm. Further, our data suggest that this strategy will block the ability of cells to acquired resistance to fulvestrant. Our group has developed a clinical trial that has tested this combination and the results of this trial presented at the meeting.

Citation Format: Doris Germain, Kerin Adelson, George Raptis, Samuel Waxman. Bortezomib enhances the efficacy of fulvestrant by promoting the aggregation of the ER in the cytoplasm [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-12-08.