Background: Metastatic gastroesophageal adenocarcinoma (GEA) has poor prognosis. Overall survival (OS) remains around 12 months (mo) with current therapies. Pembrolizumab is approved for advanced GEA that has progressed on at least 2 prior lines of systemic therapy. However, the majority of patients progress on this treatment, and less than 15% of patients experience objective response (OR). This study will evaluate efficacy of pembrolizumab in combination with cyclin-dependent kinase 4/6 (CDK4/6) inhibitor, abemaciclib, in patients with advanced GEA. Preclinical studies have demonstrated that CDK4/6 inhibitors can increase anti-tumor immunity and can synergize with immune checkpoint inhibitors. Based on these data, we hypothesize that abemaciclib will augment response to pembrolizumab in GEA. Methods: This is a multi-institutional, single arm, open label, phase II study of abemaciclib in combination with pembrolizumab in patients with advanced GEA who have progressed or were intolerant to at least 2 prior lines of therapy. Patients previously treated with immune checkpoint inhibitors or with microsattelite unstable tumors will be excluded. Treatments will be given on a 21 day cycle until disease progression or intolerable toxicities. Pembrolizumab, 200 mg intravenously, will be given on day 1, and abemaciclib, 150 mg, will be taken orally twice a day on days 1-21. Primary endpoint is progression free survival (PFS). Secondary endpoints include PFS rate at 6 mo, disease control rate, OS and OR rate. Correlative endpoints will examine relationship between PDL1 status, genomic signature and treatment response. Saliva samples will be collected for microbiome analysis. Archival tumor tissue and blood samples will be banked for future studies. A total of 31 evaluable subjects will be enrolled to detect an anticipated increase in the median PFS from 2 months (null hypothesis) to 4 months with 80% power at the one-sided 0.05 significance level. The trial is open to enrollment. Clinical trial information: NCT03997448