BACKGROUND: Biotin-labeled trastuzumab (BiotHER) can be used to test for HER2 by 
immunohistochemistry. We previously showed that BiotHER immunoreactivity is 
highly correlated with HER2 amplification and indicated that it could be 
associated with better clinical outcome in advanced breast cancer patients 
receiving trastuzumab.
PATIENTS AND METHODS: Tumor specimens and clinical information from 234 patients 
who received trastuzumab-based treatments were collected from 10 institutions. 
HER2 amplification and BiotHER immunoreactivity were assessed centrally. The 
effect of BiotHER positivity on response rate (RR), time to progression and 
survival were studied by univariate and multivariate analysis in patients 
presenting HER2-amplified breast cancer. The pathologic reviews of the assays 
were blinded to patient outcomes.
RESULTS: BiotHER was positive in 109/194 (56%) HER2-amplified breast cancers and 
in one not amplified tumor. RRs were 74% [95% (confidence interval) CI 64%-81%] 
and 47% (95% CI 36%-58%) in BiotHER-positive and -negative tumors, respectively 
(P < 0.001). BiotHER immunoreactivity was independently associated with 
increased probability of tumor response (odds ratio 3.848; 95% CI 1.952-7.582), 
with reduced risk of disease progression [hazard ratio (HR) 0.438; 95% CI 
0.303-0.633] and with reduced risk of death (HR 0.566; 95% CI 0.368-0.870) by 
multivariate analysis.
CONCLUSION: The results support a role for BiotHER testing in better tailoring 
trastuzumab-based treatments in patients with advanced HER2-amplified breast 
cancers.