All of the 168 patients who contributed metastatic tumor biopsy samples to the study had developed resistance to estrogen receptor treatments, including aromatase inhibitors, tamoxifen, and fulvestrant. Of these biopsies, 12 had HER2 mutations, 8 of which had been previously characterized as activating.
Dr. Nayar and colleagues examined the untreated primary tumors in five of these patients; there was no mutation in four, suggesting that the mutations were a response to treatment. “In these 80%, the mutations were acquired as tumors were exposed to treatment and not present in the original tumor,” Dr. Nayar said.
These acquired HER2 mutations were mutually exclusive with ER mutations, which suggested a different mechanism of resistance to ER-directed therapies, she noted in her abstract. The mutations conferred resistance to tamoxifen, fulvestrant, and palbociclib.
However, the combination of fulvestrant and neratinib, an irreversible HER2 kinase inhibitor, overcame resistance in these cells.