A phase Ib study of safety and clinical activity of atezolizumab (A) and cobimetinib (C) in patients (pts) with metastatic colorectal cancer (mCRC).

560Background: The majority of pts with mCRC have microsatellite stable (MSS) tumors with minimal response to PD-L1/PD-1 blockade. MEK inhibition upregulates tumor major histocompatibility complex-I expression, promotes intratumoral T-cell accumulation and improves anti–PD-L1 responses (Ebert, Immunity. 2016), supporting clinical evaluation of combined anti–PD-L1 (A) plus MEKi (C) in pts with mCRC. Methods: Pts with chemotherapy-refractory or locally advanced mCRC were evaluated. A was administered IV q2w at 800 mg. C was dosed PO daily at 20 to 60 mg during dose escalation and 60 mg during dose expansion (14/14 or 21/7 d on/off schedule). Primary endpoints were safety and tolerability. Investigator-assessed ORR and PFS by RECIST v1.1 and OS were secondary endpoints. Microsatellite instability (MSI) status was locally reported and centrally confirmed by NGS-based scoring. Results: As of May 10, 2017, 84 pts (57 KRASmt, 25 KRASwt, 2 unknown) were enrolled and evaluable. 66 pts had received 5+ prior therapi...