Phase Ib trial of atezolizumab in combination with nab-paclitaxel in patients with metastatic triple-negative breast cancer (mTNBC).

1009Background: Atezolizumab (atezo; MPDL3280A) is a humanized MAb that inhibits binding of PD-L1 to PD-1 and B7.1, thus restoring tumor-specific T-cell immunity. Atezo has single-agent activity in mTNBC. Combining atezo with chemotherapy is hypothesized to enhance tumor-specific T-cell immunity by exposing the immune system to high levels of tumor antigens and modulating T-cell and NK cell functions. This arm of a Phase Ib study evaluated atezo + nab-paclitaxel (pac) in patients (pts) with mTNBC treated with ≤ 3 prior lines of therapy. Methods: Primary endpoints were safety and tolerability; secondary endpoints included clinical activity. Pts received atezo 800 mg q2w (d1,15) and nab-pac 125 mg/m2 q1w (d1,8,15) q3 of 4 wks. After nab-pac discontinuation, maintenance atezo was allowed until loss of clinical benefit. ORR was assessed by RECIST v1.1. PD-L1 staining on tumor cells and tumor-infiltrating immune cells was scored using the SP142 IHC assay. Results: 32 pts were safety evaluable (median follow-up...