Abstract The combination of anti-CTLA-4 and anti-PD-1 antibodies has demonstrated significant clinical efficacy in metastatic melanoma, but also significant toxicities. Pharmacodynamic (PD) markers are important tools in optimizing the safety and efficacy of anti-CTLA-4/anti-PD-1 combinations, but have remained elusive to date. To test the hypothesis that blockade of both CTLA-4 and PD-1 together results in increased expression of the proliferation marker Ki67 in CD4+ T cells, we studied two dose regimens of ipilimumab in combination with nivolumab in patients with advanced or metastatic solid tumors, using a validated assay. Subjects received either 1.0 mg or 0.3 mg /kg of ipilimumab every 6 weeks in combination with nivolumab 3mg/kg every 2 weeks for a maximum of 12 weeks (2 cycles). A total of 9 subjects were enrolled at the 1.0 mg/kg of ipilimumab cohort and 12 subjects in the 0.3 mg/kg cohort. Highly statistically significant increases of 4.56 fold (range 3.35-6.20) from baseline at cycle 1 day 8 post-treatment (p < 0.001) in cohort 1, and 3.13 fold (range 2.31-4.33) from baseline at cycle 1 day 8 post-treatment (p < 0.001) in cohort 2, in Ki67 expression were observed in CD4+ T cells. The absolute increase in Ki67 expression was also observed to be dose dependent numerically. Significant increases in Ki67 expression were also observed in CD8+ T cells, regulatory T cells, and natural killer T (NKT) cells following treatment. These results identify and support Ki67 expression as a PD biomarker in the context of CTLA-4 and PD-1 blockade. More work is required to correlate these finding with clinical outcomes in order to establish a benchmark biomarker assay for anti-CTLA-4 and anti-PD-1 combinations. (ACTRN12617001059358) Citation Format: Paul De Souza, Agnieszka Malczewski, Igor Proscurshim, Guojun Yuan, Elisabeth Coart, Christopher Dupont, Mariana Manrique, Elise Drouin, Ana Gonzalez, Meg Carini, Olga Shebanova, Jean-Marie Cuillerot. Evaluation of peripheral T cell subset proliferation as a pharmacodynamic assay to guide the development of anti-CTLA-4 and PD-1 antibody combinations in patients with solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT104.