Purpose: Fulvestrant (F) is a selective estrogen receptor downregulator (SERD) with activity in aromatase-inhibitor (AI) resistant estrogen receptor (ER)-positive metastatic breast cancer (MBC). In preclinical studies, the proteasome inhibitor bortezomib (B) enhances the antineoplastic effects of F, in part by promoting accumulation of large ER-aggregates that lead to cell death (Ishii et al. Clin Cancer Res 2011 17:2292). The objective of this study was to determine if the combination of F+B was more efficacious than F alone in MBC after AI progression.

Patients and Methods: Postmenopausal women with ER-positive MBC who had progressive disease after prior AI therapy were eligible. They were randomized to F alone (500 mg IM days -15, 1, 15 in cycle 1, and day 1 of each subsequent cycle) or in combination with B (1.6 mg/m2 IV on days 1, 8, 15). The primary endpoint was progression free survival (PFS), measured from cycle 1, day 1 of starting F. A sample size of 118 was pre-specified in order to provide sufficient power to detect an improvement in median PFS from 5.4 to 9.0 months, and compare PFS rates after 6 and 12 months (1-sided alpha=0.10, beta=0.10). Patients with progression on F could cross over to the F+B combination.

Results: Of 118 patients enrolled, 59 received F alone (arm A), 57 received F+B (arm B), and 2 assigned to arm B never initiated protocol therapy. There were no significant differences in patient characteristics between arms with regard to median age (57 vs. 59 years), ECOG performance status (0 and 1, 64% and 36%, respectively), prior chemotherapy for metastasis (25%), or liver metastases (37%), although patients in arm A had longer median interval between diagnosis and metastasis (49 vs. 28 months) and were more likely to present with metastasis (32% vs. 26%). Patients in arm B had more adverse events (all grades), including nausea (63% vs. 29%), diarrhea (47% vs. 8%), sensory neuropathy (46% vs. 29%), and limb edema (37% vs. 19%), although grade 3-4 events were uncommon, and only 11% discontinued B due to toxicity. At 12 months, the PFS proportion in Arm A and Arm B was 13.6% vs. 28.1%, respectively (P=0.03, 1-sided chi-square test) (95% CI for difference [14.5%] = -0.06%, 29.1%). Although median PFS was similar in the two arms (2.69 vs. 2.73 months, respectively), the hazard ratio for Arm B vs. Arm A (referent) was 0.73 (95% CI = 0.49, 1.09, P=0.06, 1-sided log rank test). Both results were significant at the pre-specified 1-sided 0.10 alpha level. Of 27 patients on arm A who crossed over to F+B at progression, 4 (15%) were progression-free for at least 24 weeks and had periods of disease control that were longer than when treated with F alone.

Conclusion: Adding bortezomib to fulvestrant in AI-resistant ER-positive MBC enhances its effectiveness by delaying acquired fulvestrant resistance. These results support additional evaluation of proteasome inhibitors in combination with SERDs.

Acknowledgement: Supported by contract N01-CM-62204 to the New York Cancer Consortium (P.I. J. Sparano) and grant P30 CA013330 (P.I. D. Goldman) from the National Institutes of Health, and by a grant from Millennium, Inc.

Citation Format: Kerin B Adelson, Bhuvaneswari Ramaswamy, Joseph A Sparano, Paul J Christos, John J Wright, George Raptis, Miguel C Villalona, Cynthia X Ma, Dawn Hershman, Joseph Baar, Paula Klein, Tessa Cigler, G Thomas Budd, Yelena Novik, Antoinette R Tan, Susan Tannenbaum, Anupama Goel, Ellis Levine, Charles L Shapiro, Eleni Andreopoulou, Michael Naughton, Kevin Kalinsky, Samuel Waxman, Doris Germain. Randomized phase II trial of fulvestrant alone or in combination with bortezomib in hormone receptor-positive metastatic breast cancer resistant to aromatase inhibitors: A New York cancer consortium trial [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 S6-03.