Background: Ang-2 is produced by endothelial cells, predominantly in tissues undergoing vascular remodeling. Current studies suggest that Ang-2 partially suppresses T cell activation by increasing PD-L1 expression and decreasing activation of monocytes. We demonstrated that high pre-treatment serum Ang-2 is associated with reduced overall survival in patients treated with PD-1 blockade. We therefore hypothesized that the combination of Ang-2 and PD-1 blockade may be effective for treatment of patients with advanced cancer. Methods: We initiated a phase 1b trial of the combination of pembrolizumab and trebananib, an Ang-1/2 neutralizing peptibody in advanced solid tumors. Treatment consisted of an induction phase of pembrolizumab 200 mg every 3 weeks and trebananib weekly (with an initial run-in dose escalation of 15-30 mg/kg) for 12 weeks followed by pembrolizumab alone for 2 years. Here we present the updated data in the fully enrolled CRC expansion cohort. Results: The study enrolled 18 microsatellite stable (MSS) CRC patients. There were no DLTs in the dose escalation phase, and 30mg/kg was deemed to be the MTD. This summary is based on 15 CRC patients treated with 30 mg/kg trebananib plus pembrolizumab. The most common treatment-related adverse events (AEs) were abdominal distension, diarrhea, limb edema, transaminitis, and proteinuria, each reported in 40% of the patients. As of September 2019 (median follow up of 10 months), 13 patients were off treatment and two were continuing. Eleven patients were off treatment due to disease progression and 2 due to unacceptable toxicities (grade 4 pneumonitis and grade 3 transaminitis). The response rate was 7% (1 partial response for 23.8+ months) and the disease control rate was 27%, with 4 stable disease for a median of 10 months (4-18 months). Median time to progression and overall survival were 2.8 months (90% CI: 1.5 to 8.1 months) and 9.0 months (90% CI: 2.6 months to ∞), respectively. Conclusions: The combination of pembrolizumab and trebananib is well tolerated and demonstrated promising activity in patients with heavily treated MSS CRC. Clinical trial information: NCT03239145