A phase II study of the combination of endocrine treatment and bortezomib in patients with endocrine-resistant metastatic breast cancer.

e11072 Background: The majority of patients with hormone receptor-positive metastatic breast cancer die from disease progression despite different types of anti-hormonal treatments. Preclinical studies have indicated that resistance to anti-hormonal therapies may be the result of an activated NF-κB signalling pathway in breast cancer. Bortezomib is a proteasome inhibitor that blocks the NF-κB pathway. Recent pharmacodynamic and pharmacokinetic xenograft studies have shown that drug exposure may be a crucial factor for the efficacy of bortezomib in solid tumours. Methods: The aim was to investigate whether the addition of bortezomib to anti-hormonal therapy would result in antitumour activity in patients with both progressive and measurable disease on the identical endocrine agent. Clinical benefit was defined as patients obtaining SD, PR or CR after 2 cycles, lasting for at least another five weeks. Bortezomib was administered on days 1, 8, 15, 22 of a 5 week regimen (1.6 mg/sqm). Results: Eight patients ...