Predictive value of tumor Ki-67 expression in two randomized trials of adjuvant chemoendocrine therapy for node-negative breast cancer

Several small studies have reported that having a high percentage of breast tumor
cells that express the proliferation antigen Ki-67 (ie, a high Ki-67 labeling index)
predicts better response to neoadjuvant chemotherapy. However, the predictive
value of a high Ki-67 labeling index for response to adjuvant chemotherapy is
unclear. To investigate whether Ki-67 labeling index predicts response to adjuvant
chemoendocrine therapy, we assessed Ki-67 expression in tumor tissue from 1924
(70%) of 2732 patients who were enrolled in two randomized International Breast
Cancer Study Group trials of adjuvant chemoendocrine therapy vs endocrine therapy
alone for node-negative breast cancer. A high Ki-67 labeling index was associated
with other factors that predict poor prognosis. Among the 1521 patients with
endocrine-responsive tumors, a high Ki-67 labeling index was associated with
worse disease-free survival but the Ki-67 labeling index did not predict the relative
efficacy of chemoendocrine therapy compared with endocrine therapy alone. Thus,
Ki-67 labeling index was an independent prognostic factor but was not predictive of
better response to adjuvant chemotherapy in these studies.