Complete pathological response of hormone receptor positive invasive breast cancer in a patient with multiple myeloma treated with ixazomib

Multiple myeloma is a hematological cancer characterized by relapse after treatment and poor prognosis. Ixazomib, a second-generation protease inhibitor, is one of the most recently available treatments for relapsed or refractory multiple myeloma, while it has also shown good potential as antitumoral agent in preclinical solid tumor models such as breast cancer cell lines. Here we report the case of a 68-year-old female with multiple myeloma and an incidental cT1b (9 mm) hormone receptor positive breast cancer lesion that showed a complete pathological response to a three-month combination therapy with Ixazomib, bendamustine and dexamethasone and no signs of disease relapse during the later follow-up. This is the first case report describing such clinical outcome in breast cancer following Ixazomib, bendamustine and dexamethasone combination therapy. To investigate the potential antitumoral activity of Ixazomib in breast cancer, we performed in vitro experiments using two hormone receptor positive breast cancer cell lines. We assessed the synergism between Ixazomib and bendamustine and the antiproliferative effect of Ixazomib. We found no synergistic interaction between the two drugs, while Ixazomib alone showed an antiproliferative effect against tumoral cells, suggesting that this drug has been responsible for tumor regression in our case.