Phase Ib Clinical and Pharmacodynamic Study of the TIE2 Kinase Inhibitor Rebastinib with Paclitaxel or Eribulin in HER2-Negative Metastatic Breast Cancer

Abstract Purpose: Breast cancer cells disseminate to distant sites via tumor microenvironment of metastasis (TMEM) doorways. The TIE2 inhibitor rebastinib blocks TMEM doorway function in the PyMT mouse model of breast cancer. We aimed to assess the safety and pharmacodynamics of rebastinib plus paclitaxel or eribulin in patients with HER2-negative metastatic breast cancer (MBC). Patients and Methods: This phase Ib trial enrolled 27 patients with MBC who received 50 mg or 100 mg of rebastinib orally twice daily in combination with weekly paclitaxel 80 mg/m2 (if ≤2 prior non-taxane regimens) or eribulin 1.4 mg/m2 on days 1 and 8 (if ≥1 prior regimen). Safety, tolerability, and pharmacodynamic parameters indicating TIE2 kinase inhibition and TMEM doorway function were evaluated. Results: No dose-limiting toxicities in cycle 1 or 2 were observed among the first 12 patients at either rebastinib dose level. The most common treatment-emergent adverse events were anemia (85%), fatigue (78%), anorexia (67%), leukopenia (67%), increased alanine aminotransferase (59%), hyperglycemia (56%), nausea (52%), and neutropenia (52%). Adverse events attributed to rebastinib include muscular weakness and myalgias. Intraocular pressure increased at the 100-mg rebastinib dose level, whereas angiopoietin-2 levels increased at both dose levels, providing pharmacodynamic evidence for TIE2 blockade. Circulating tumor cells decreased significantly with the combined treatment. Objective response occurred in 5/23 (22%) evaluable patients. Conclusions: In patients with MBC, the recommended phase II dose of rebastinib associated with pharmacodynamic evidence of TIE2 inhibition is either 50 or 100 mg orally twice daily in combination with paclitaxel or eribulin.