2659 Background: Cancer of unknown primary (CUP) represents a heterogeneous group of cancers that present with metastatic epithelial disease with no identifiable primary lesion at the time of diagnosis. Patients with poor prognostic features have limited effective systemic therapy options. Researchers hypothesize that these primary lesions are occult, in part, due to anti-cancer immune surveillance. We hypothesize, pembrolizumab will have activity in this setting. Methods: This single arm phase 2 study evaluated the use of pembrolizumab in treatment naïve patients with poor prognosis CUP. Patients ≥18 years of age with ECOG PS 0-1 and histologically proven, measurable metastatic carcinoma with no primary site identified were eligible. Pembrolizumab was administered at 200 mg intravenously every 3-weeks for up to 24 months. The primary endpoint was safety and objective response rate (ORR) per RECIST 1.1. Secondary objectives included progression free survival (PFS) and overall survival (OS). The study used a Simon’s two stage design based on a one-sided test with a type 1 error rate of 0.1, null hypothesis for ORR ≤ 20% yielding a power of 0.8 with an alternate hypothesis of ≥40%. The null hypothesis will be rejected if 8 or more responses are observed in 25 patients. Mismatch repair (MMR) and microsatellite instability (MSI) testing was not mandatory for patients. Results: There were 27 evaluable and 6 unevaluable (no response assessment, included in adverse event analysis) patients. The median age was 64 (range 21-80) and the majority were male (57.6%) and ECOG 1 (84.8%). Four patients experienced grade 3 treatment related adverse events (pain, anorexia, fatigue, confusion, and anemia). There were no treatment related deaths or new signals of toxicity. Of the evaluable patients, the ORR was 18.5% (4 partial responses and 1 complete response), disease control rate was 51.9% (9 stable disease) and thus the null hypothesis could not be rejected. For evaluable patients, the median PFS and OS were 2.56 (95% CI 2.10 – 7.46) and 9.79 (95% CI 3.84 – 16.03) months respectively. The one patient who developed a CR had MMR deficiency and has completed 2 years of therapy. Of the 4 patients with a partial response, one had an MMR proficient tumour, while the remaining 3 had unknown MMR status. Conclusions: Although not meeting the primary endpoint, pembrolizumab has some activity in CUP patients. Patients with CUP should be screened for MMR or MSI. Future studies should evaluate combination therapies with the addition of pembrolizumab. Clinical trial information: NCT03391973 .