Background: AMP-224, a PD-L2 Fc fusion protein, binds to PD-1, an inhibitory receptor that is present on the cell surface of exhausted, activated, effector, and memory T cells. AMP-224 has a unique mechanism of action in that it binds specifically to PD-1^HI T cells (chronically stimulated / exhausted T cells) but not PD-1^LO cells which represent the normal activated T cell population. Preclinical studies have documented an increase in antitumor immunity following radiation therapy (RT), but also tumor PD-L1 expression as an escape mechanism. The aim of the study is to evaluate whether inhibition of PD-1/PDL-1 axis could improve anti-tumor immunity effects of RT. Methods: Patients with histologically confirmed metastatic colorectal cancer to liver were treated with SBRT to a site of liver metastasis at 8Gy in a single fraction (DL1) or 8Gy in 3 daily fractions (DL2). All patients received AMP-224 10mg/kg IV beginning Day 1 after SBRT preceded by cyclophosphamide 200mg/m2 (D0). Primary objective was to determine the safety and feasibility of AMP-224 in combination with stereotactic body radiation therapy (SBRT) to metastatic hepatic metastasis in patients with advanced colorectal cancer. Mandatory pre- and post-treatment biopsies were attempted on all patients. Results: N = 17 patients with refractory metastatic CRC were enrolled. N = 2 pts were unevaluable. 6pts were treated at DL1 (8Gy x 1fraction SBRT, AMP-224 10mg/kg q2-weekly) and 9pts were treated at DL2 (8Gy x 3fractions SBRT, AMP-224 10mg/kg q2-weekly) No DLT was encountered. The most common toxicity was fatigue (G1/2) in all patients in DL2. N = 3 patients experienced G2 infusion reaction which responded to standard interventions. 5/15 pts did not complete treatment due to rapid PD. No objective responses have been seen, although N = 6 pts remain on study. Pre- and post-tumor biopsies were performed on 7 of first 11 pts. Conclusions: AMP-224 in combination with SBRT to site of colorectal hepatic metastases is safe and feasible. Preliminarily no objective responses have been seen. Full clinical and correlative data including post-therapeutic radiated and non-radiated tumor biopsies will be presented. Clinical trial information: NCT02298946