Abstract P5-11-13: Outcomes of fulvestrant therapy among Japanese women with ER-positive HER2-positive advanced or metastatic breast cancer: A subgroup analysis of the JBCRG-C06 safari study

Background: After the CONFIRM trial, fulvestrant 500 mg (F500) became a standard treatment for patients with estrogen receptor positive (ER+) advanced/metastatic breast cancer (AMBC). The JBCRG-C06 Safari study (UMIN 000015168) showed that earlier F500 use, a longer time from diagnosis to F500 use, and no prior chemotherapy were associated with significantly longer time to treatment failure (TTF) among Japanese patients with ER+ and HER2-negative AMBC (Kawaguchi H, et al. Breast Cancer Res Treat, 2017; Kawaguchi H, et al. Curr Med Res Opin, 2018). We had carried out subgroup analyses on real world data from the Safari study focusing on ER+ and HER2+ AMBC with the objective to examine further on potential interactions between TTF and the following: F500 single treatment or F500 with anti-HER2 therapy; presence or absence of visceral metastasis; progesterone receptor (PgR) status; and prior chemotherapy for AMBC. Material & methods: The Safari study was a retrospective, multicenter cohort study, conducted in 1072 patients at 16 sites in Japan for ER+ AMBC. Patients starting treatment with F500 between Nov 2011 and Dec 2014 were registered. After the data cleaning through the central review, 94 patients (9.6%) has been included in this sub-analysis among the 978 patients in which HER2 status was confirmed by the documentation. The relationship between baseline clinical/pathological factors, treatment line, and TTF (time from start to cessation of F500 treatment for any reason, including toxicity and death) were analyzed using Kaplan-Meier methods. Univariate analysis of TTF data was performed by Cox hazards model using: age, histological type, histological/nuclear grade, visceral-metastases, stage, PgR, period from AMBC diagnosis to F500 use, treatment line of F500, prior chemotherapy. Results: Median age at diagnosis of AMBC was 57.5yrs (33-85); 59(33-85) and 56.5(39-75) for F500 with/ without anti-HER2 therapy. Median age of fulvestrant start was 61yrs (35-96). 21.3% had de novo metastatic disease; 78.7% recurrent disease; 37.2% visceral metastases. All patients’ ER status was positive, and PgR status was positive in 65.9%, negative 25.5% respectively. HER2 IHC 3+ patients accounted for 44.7%, IHC 2+/1+ for 38.3%. In the case of IHC 2+/1+, HER2-positivity was confirmed by FISH. F500 alone was administered in 52 patients, while F500 with anti-HER2 therapy in 42 patients. At median follow up of 3.9 years, median TTF was 4.4m (95% CI: 3.3-5.7); 3.7m (95% CI: 2.9-5.9), 5.1m(95% CI: 3.6-6.4) for the F500 alone group, F500 with anti-HER2 therapy group respectively (p = 0.5973). F500 with/ without anti-HER2 therapy was administered in 1stor 2nd-line in 20 (21.3%), and ≥3rd-line in 74(78.7%) patients. Median TTF was 6.6m (95% CI: 4.6-8.5) and 3.7m (95% CI: 2.8-5.2) in the 1stor 2nd-line and ≥3rd-line respectively (p value:0.0145). Although no statistically significant difference was found, TTF tended to be shorter in the group receiving chemotherapy prior to F500 (median TTF: 3.7m vs 5.5m). No other clinical parameters including age, PgR status, metastatic sites were associated with the effectiveness. The median OS was 7.8-years (95% CI: 5.9-9.5), which had not been influenced by with/ without anti-HER2 therapy. The toxicity profile we observed was similar to that in the registration trial. Conclusions: Our findings suggest that F500 with/without anti-HER2 therapy appears to be effective also in the ER+ and HER2-positive ABMC setting which is consistent with the limited results previously published. Earlier line use of F500 use may be useful in HER2-positive patients as well as HER2-negative patients. Citation Format: Misato Hagi, Hidetoshi Kawaguchi, Norikazu Masuda, Shigehira Saji, Yutaka Yamamoto, Takahiro Nakayama, Kenjiro Aogi, Keisei Anan, Yoshinori Ito, Shoichiro Ohtani, Nobuaki Sato, Toshimi Takano, Eriko Tokunaga, Seigo Nakamura, Yoshie Hasegawa, Masaya Hattori, Tomomi Fujisawa, Miki Yamaguchi, Hiroko Yamashita, Toshinari Yamashita, Daisuke Yotsumoto, Satoshi Morita, Masakazu Toi, Shinji Ohno. Outcomes of fulvestrant therapy among Japanese women with ER-positive HER2-positive advanced or metastatic breast cancer: A subgroup analysis of the JBCRG-C06 safari study [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-11-13.