Background: Metastatic TNBC (mTNBC) has a poor prognosis. Preclinical data suggests benefit for combination therapy of immune checkpoint inhibitor and anti-angiogenesis. Olinvacimab (O) is a fully humanised monoclonal antibody (MAB) which binds to Vascular Endothelial Growth Factor Receptor 2 (VEGFR2) and antiangiogenic and antitumor effects have been demonstrated. Pembrolizumab (P) is an anti-PD1 MAB. This study aimed to identify the safety and tolerability of O and P to establish a phase 2 recommended dose of O. Methods: From December 2018 to September 2020, we conducted a two-site, single arm, open-label study of O and P in pts with mTNBC. Eligible patients (pts) were >18 years, had ER, PR & HER2-negative MBC with at least one measurable lesion and adequate organ function. Pts with history of serious thromboembolism, gastrointestinal haemorrhage and prior anti-VEGF therapy were excluded. A modified Toxicity Probability Interval design was used. Pts received O 12 mg/kg q7d (dose level 1) or 16mg/kg day q7d (dose level 2) in combination with P 200mg flat dose day 1 q21d. Treatment continued until dose limiting toxicity (DLT), disease progression (PD) or treatment intolerance. Results: 11 pts, median age 62 (range 39-67) were recruited and received at least one dose of treatment. 8 (73%) had ECOG PS 1 and 3 (27%) had ECOG PS 2. 5 pts had previous chemotherapy for mTNBC (with 3 pts also having received immunotherapy), 6 pts were treated in the first-line metastatic setting (with all pts having received anthracycline and taxane in the adjuvant setting). 5 pts received O at 12mg/kg with P, completing a median of 6 cycles (range 1-18). As no DLTs were seen, 6 pts were treated with O at 16mg/kg with median of 8 cycles (range 2-21), with no DLTs being observed. Treatment was ceased due to PD in 6 pts, 3 pts are receiving treatment at data cut-off. Haemangiomas were seen in 8 pts accounting for 47 events of CTCAE grade 1 and 21 events of grade 2. Treatment emergent adverse events (TEAEs) of ≥grade 3 was seen in 6 pts (27 events), with 8 events being related to treatment. 4 pts (36%) had partial response (PR) as best overall response 5 pts (45%) had clinical benefit (PR+SD≥24weeks). Conclusions: Combination therapy of O and P was well tolerated with evidence of efficacy in mTNBC pts who had all previously received anthracycline and taxanes, with 3 pts having previously received immunotherapy. These results support the combination entering into a phase 2 study.

Citation Format: Arlene Chan, Eugene Moylan, Sally Jackson, Jeannette Devoto, Nicola Jones, Silvie Radmil, Kate Wilkinson, Aflah Roohullah, Tamiem Adam, Seon Young Lee, Weon Sup Lee, Jin-San Yoo. Phase Ib trial of olinvacimab and pembrolizumab in patients with metastatic triple-negative breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS10-18.