Abstract CT028: Atezolizumab (atezo) + nab-paclitaxel (nab-pac) in metastatic triple-negative breast cancer (mTNBC): 2-year update from a ph Ib trial

Introduction : Outcomes for patients (pts) with mTNBC remain poor, with chemotherapy as a historic mainstay treatment. Atezo (anti-PD-L1) demonstrated single-agent activity in mTNBC, and initial data suggested the addition of chemotherapy was safe with encouraging activity (Adams, 2016). Here we report long-term outcomes, with OS, for atezo + nab-pac in mTNBC. Methods: Arm F of multi-cohort Ph Ib study (NCT01633970) enrolled pts with locally advanced unresectable/stage IV mTNBC and 0 to 2 prior regimens for advanced disease. Atezo 800 mg IV q2w and nab-pac 125 mg/m 2 IV qw (3 weeks on/1 week off) were given concurrently (with nab-pac run-in for biopsy cohort); maintenance atezo was permitted until loss of clinical benefit. In-tumor PD-L1 expression (VENTANA SP142 assay), CD8 and stromal TILs were assessed by IHC; blood-based CD8+Ki67+ via FACS, and CXCL10 cytokine RNA by NanoString. Results: Of 33 evaluable pts, 82% had ECOG PS 1, 58% had visceral mets, and median age was 55 years (data cutoff, April 30, 2017). Grade 3-4 treatment-related AEs (TRAE) occurred in 73%, with no Grade 5 TRAEs. Atezo-related AEs ≥ 20% included fatigue, pyrexia and decreased neutrophil count. Five pts discontinued nab-pac due to typical chemotherapy-related toxicities, and 3 withdrew from atezo due to TRAEs. Responses occurred across all subgroups (Table), with numerically higher ORR and longer OS/PFS in pts with less pre-treatment and higher PD-L1 status. Serial biopsy samples (n = 14) had elevated PD-L1+ TILs after combination treatment (but not after nab-pac alone). Peripheral blood analyses found transient on-treatment increases in CD8+Ki67+ and CXCL10, which were independent of efficacy. Conclusions: In this single-arm study cohort, atezo + nab-pac was well tolerated with promising efficacy, especially for treatment-naive pts. Ongoing randomized Ph III trial IMpassion130 (NCT02425891) is investigating this regimen in untreated mTNBC. Citation Format: Paula R. Pohlmann, Jennifer R. Diamond, Erika Hamilton, Sara M. Tolaney, Wei Zhang, Koho Iizuka, Paul Foster, Luciana Molinero, Roel Funke, Sylvia Adams. Atezolizumab (atezo) + nab-paclitaxel (nab-pac) in metastatic triple-negative breast cancer (mTNBC): 2-year update from a ph Ib trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT028.