Factors associated with prolonged time to treatment failure with fulvestrant 500 mg in patients with post-menopausal estrogen receptor-positive advanced breast cancer: a sub-group analysis of the JBCRG-C06 Safari study

<p><b>Objective:</b> The JBCRG-C06 Safari study showed that earlier fulvestrant 500 mg (F500) use, a longer time from diagnosis to F500 use, and no prior palliative chemotherapy were associated with significantly longer time to treatment failure (TTF) among Japanese patients with estrogen receptor-positive (ER+) advanced breast cancer (ABC). The objective of this sub-group analysis was to further examine data from the Safari study, focusing on ER + and human epidermal growth factor receptor-negative (HER2−) cases.</p> <p><b>Methods:</b> The Safari study (UMIN000015168) was a retrospective, multi-center cohort study, conducted in 1,072 patients in Japan taking F500 for ER + ABC. The sub-analysis included only patients administered F500 as second-line or later therapy (<i>n</i> = 960). Of these, 828 patients were HER2−.</p> <p><b>Results</b> Multivariate analysis showed that advanced age (≥65 years; <i>p</i> = .035), longer time (≥3 years) from ABC diagnosis to F500 use (<i>p</i> < .001), no prior chemotherapy (<i>p</i> < .001), and F500 treatment line (<i>p</i> < .001) were correlated with prolonged TTF (median = 5.39 months).</p> <p><b>Conclusions:</b> In ER+/HER2− patients receiving F500 as a second-line or later therapy, treatment line, advanced age, no prior palliative chemotherapy use, and a longer period from ABC diagnosis to F500 use were associated with longer TTF.</p>