Loss of HER2 after HER2-targeted treatment.

HER2 expression has been reported to be discordant between primary tumor and metastatic tissue. HER2 discordance and relation to HER2-targeted treatment was investigated in 227 patients with primary breast cancer. HER2 discordance between primary biopsy and second biopsy after neoadjuvant or adjuvant treatment was observed in 20.7%. This discordance was related only to the use of HER2-targeted treatment: 30 of 33 (90.9%) women with downgraded HER2 expression underwent a HER2-targeted therapy, whereas in the group of patients with concordant HER2 expression, only 32 of 180 (17.8%) received HER2-targeted treatment (p < 0.0001). HER2 discordance was associated with reduced disease-free survival but not overall survival. In a second cohort, including patients with HER2 overexpressing tumors, trastuzumab treatment was associated with change of HER2 expression from positive to negative in 47.3% of cases. Addition of pertuzumab increased the rate of HER2 loss up to 63.2%. Notably, the interval between last HER2-targeted treatment and the time of surgical excision of the tumor after neoadjuvant chemotherapy (NACT) or the biopsy of the metachronous metastasis was associated with a significant change in HER2 expression. The median time between NACT and the time of surgical excision was 23 days (range 5-81 days) for tumors with decreased HER2 expression and 51 days (range 10-179 days) for tumors with concordant HER2 expression. Furthermore, median time between the end of adjuvant treatment and second histology of the metachronous metastases accounted for 15 days (range 2-165 days) and 478 days (range 7-2739 days) was observed in the group of patients with decreased or unchanged HER2 expression, respectively. The interval between anti-HER2 treatment and the determination of HER2 in second histology is strongly associated with HER2 expression.