Abstract GS1-02: NSABP B-47 (NRG oncology): Phase III randomized trial comparing adjuvant chemotherapy with adriamycin (A) and cyclophosphamide (C) → weekly paclitaxel (WP), or docetaxel (T) and C with or without a year of trastuzumab (H) in women with node-positive or high-risk node-negative invasive breast cancer (IBC) expressing HER2 staining intensity of IHC 1+ or 2+ with negative FISH (HER2-Low IBC)

Background: Adjuvant trastuzumab (H) reduces cancer recurrence and improves survival in patients (pts) with HER2-amplified or overexpressing (IHC 3+ staining intensity) IBC. Two of the landmark trials that demonstrated the efficacy of H-based eligibility on HER2 testing performed at local site laboratories were found to contain a cohort of pts without amplification or IHC overexpression on tissue submitted for central testing. These HER2-low cohorts appeared to benefit from the addition of H, and efforts at external HER2 testing validation and laboratory explorations did not negate these findings. NSABP B-47 was performed to determine if these findings would be confirmed in a large prospective randomized trial. The primary aim was to determine whether the addition of H to chemotherapy (CT) regimens of AC→WP or TC (choice per investigator discretion) would improve invasive disease-free survival (IDFS). Methods: From 2/8/2011 to 2/10/2015, 3270 women were enrolled with 1630 pts randomly assigned to Arm 1 [TC: docetaxel 75mg/m2, cyclophosphamide 600 mg/m2 every 3 weeks x 6 cycles; or AC→WP: doxorubicin 60 mg/m2 and cyclophosphamide 600 mg/m2 every 2 or 3 weeks x 4 cycles followed by paclitaxel 80 mg/m2 every week x 12], and 1640 pts to Arm 2 [same CT regimens + 12 months of H]. Pts were stratified by IHC score (1+ vs 2+), number of positive nodes (0-3, 4-9, ≥10), hormone receptor status (ER or PgR positive vs both negative), and CT (TC vs AC→WP). Overall 58.5% were ≥50 years, 57% had tumors with IHC 1+, 17.3% were ER- and PgR-, 19.9% were node negative, and 27.4% had ≥4 positive nodes. TC was the intended CT regimen for 44.2%. Results: As of 7/31/2017, the median follow-up time was 46.1 months. We observed 264 IDFS events, which triggered the definitive analysis for the primary endpoint. The addition of H to CT showed a 5-year IDFS of 89.6% compared to 89.2% for CT alone (HR 0.98; 95%CI 0.77-1.26; P=0.90). The findings did not differ by level of HER2 IHC expression, level of lymph node involvement, or hormone receptor status. 5-year point estimates for RFI were 92.0% for CT+H compared to 92.2% for CT alone (HR 0.995; 95%CI 0.75-1.32; P=0.97). 5-year estimates for DRFI were 92.7% for CT+H and 93.5% for CT alone (HR 1.10; 95%CI 0.81-1.49; P=0.55). The addition of H did not change OS significantly with 5-year point estimates of 94.8% in CT+H vs 96.2% in CT alone (HR 1.33; 95%CI 0.91-1.94; P=0.14). 4.3% of women in the CT arm experienced Grade 4 or 5 toxicities compared to 5.0% in CT+H. Conclusion: The addition of H to CT did not demonstrate a reduction in IDFS, RFI, or DRFI in women with non-overexpressing but IHC measurable HER2 IBC. This prospective study did not confirm the retrospective findings in NSABP B-31 or N9831. The threshold of HER2 expression or genetic amplification for H benefit remains unchanged. Support: NCI U10-180868, -180822, -44066, UG1-189867, and Genentech, Inc. Citation Format: Fehrenbacher L, Cecchini RS, Geyer CE, Rastogi P, Costantino JP, Atkins JN, Polikoff J, Boileau J-F, Provencher L, Stokoe C, Moore TD, Robidoux A, Borges V, Albain KS, Swain SM, Paik S, Mamounas EP, Wolmark N. NSABP B-47 (NRG oncology): Phase III randomized trial comparing adjuvant chemotherapy with adriamycin (A) and cyclophosphamide (C) → weekly paclitaxel (WP), or docetaxel (T) and C with or without a year of trastuzumab (H) in women with node-positive or high-risk node-negative invasive breast cancer (IBC) expressing HER2 staining intensity of IHC 1+ or 2+ with negative FISH (HER2-Low IBC) [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr GS1-02.