Background: Despite initial response to conventional platinum-containing therapies, the majority of high grade serous ovarian cancer (HGSOC) patients will eventually develop treatment resistance. Despite interest for immune checkpoint inhibitor alternatives, pembrolizumab monotherapy is poor with objective response rates (ORR) ranging between 4.1 - 10%. Poor response may be linked to higher expression of immunosuppressive factors (e.g. VEGF, TGF-β) in the tumor microenvironment (TME). In an immunosuppressive TME, monotherapy with PD-1 inhibitors can induce T-cell dysfunction and apoptosis, leading to innate resistance to pembrolizumab. Preclinical studies of other tumors have demonstrated that lenvatinib can improve T-cell activation and reduce the tumor-associated macrophage population and TGF-β/VEGF/FGF-signaling. In HGSOC, mechanisms of resistance to pembrolizumab and whether this resistance can be overcome by the addition of lenvatinib remain unknown. The objective of this investigation is to uncover the immunologic basis and synergistic mechanisms of action underlying clinical efficacy of pembrolizumab and lenvatinib in HGSOC. Specifically, we will evaluate the dynamic changes in proliferative and dysfunctional T-cell populations in the peritoneal TME (p-TME) of HGSOC patients receiving monotherapy with pembrolizumab or lenvatinib followed by combination therapy. Methods: All patients will receive a cycle of monotherapy (pembrolizumab 200 mg IV every 3 weeks or lenvatinib 20 mg po daily) followed by combination therapy (both) up to 35 cycles. Enrolled patients will receive intra-peritoneal port placement for serial peritoneal fluid evaluations (cycle 0 day 1, cycle 0 day 8, cycle 1 day 8, cycle 2 day 1, cycle 3 day 1, cycle 5 day 1, then day 1 of every 3rd cycle) for multichannel flow cytometry and single-cell RNA sequencing. Our primary translational objective is to estimate both the individual and combined effects of pembrolizumab and lenvatinib on T-cell dysfunction (PD1+ CD38+) and proliferation (ki67+) in the p-TME using multichannel flow cytometry on the peritoneal fluid. Secondary clinical objective is ORR. Secondary translational objectives include individual and synergistic effects of monotherapy and combination therapy on T-cell effector function, T-cell memory, and myeloid cell subpopulations. Exploratory objectives include evaluating the dynamic changes in immune and non-immune cells in the p-TME for exceptional responders and non-responders using single cell RNA sequencing. Key eligibility criteria include 1) histologic diagnosis of high grade serous ovarian, peritoneal, or fallopian tube cancer 2) platinum-resistant disease and 3) no prior lenvatinib therapy. The trial is open with 10 patients enrolled at the time of submission. Clinical trial information: NCT05114421.