Background: O and C have single agent activity in recurrent platinum-sensitive high-grade serous ovarian cancer (HGSOC). A multi-institutional phase 2 open label study evaluated the efficacy of O with or without C in HGSOC pts. We hypothesized O+C combination may yield greater inhibition in tumor vascularity and VEGF pathways than O alone and these changes may correlate with response rate (RR) and progression-free survival (PFS). Methods: A self-selected subset of eligible pts were randomized 1:1 to O (400mg capsules BID) or O+C (O 200 mg capsules BID; C 30 mg daily). No prior anti-angiogenic therapy in the recurrent setting or prior PARPi was allowed. Blood samples were collected at baseline and day 3 to measure circulating endothelial cells (CEC: nucleated CD133-CD146+CD31+CD45-), circulating endothelial progenitor cells (CPC: viable nucleated CD133+, CD146-, CD31+ CD45 - or dim), plasma for cytokine concentrations of IL-6, IL-8, VEGF, and sVEGFR-2. DCE-MRI obtained at baseline and day 3 on therapy were evaluated for changes of K^trans and K^epas a result of decreases in angiogenesis. Results: The clinical cohort and results of the full trial will be presented independently. 13 pts (median age 53 yr [32-70]; 7pts on O, 6pts on O+C) had paired correlative studies including DCE-MRI (10pts), CEC/CPC (10pts), and cytokine measurements (12pts). Median PFS with RR for O and O+C were 11mo, 57% and 14mo, 83%, respectively for this subset. Pts on O+C had a greater decrease in IL-8 and a median 3.5 fold increase in CEC compared to O alone (p=0.016 and 0.013, respectively). The increase of CEC pretreatment to day 3 was correlated with PFS>6mo in 6 pts on O+C (p=0.011, 95%CI 0.47-0.99, R²=0.91). Pre-day3 K^trans and K^ep did not correlate with RR or PFS in either arm. Conclusions: Significant changes of IL-8 concentration and CEC number with O+C suggest greater inhibition in angiogenesis compared with O alone. Further studies of this combination with prospectively planned validation of these potential predictive biomarkers may yield information to focus therapy to HGSOC pts who may best respond. Clinical trial information: NCT01116648.