Abstract 414: Efficacy of Balixafortide (POL6326) and Paclitaxel alone and in combination in humanized breast cancer PDX

Despite availability of new therapies, outcomes for patients with relapsed metastatic breast cancer is poor with moderate median overall survival. High CXCR4 levels correlate with aggressive metastatic phenotypes and poor prognosis in breast cancer (BC). Anti-cancer mechanisms of CXCR4 antagonists include inhibition of angiogenesis and metastasis, activation of immunosurveillance and chemo sensitization. Balixafortide (POL6326) is a potent, selective inhibitor of the chemokine receptor CXCR4 in PhIII for metastatic HER2-negative breast cancer (BC) in combination with tubulin-binding eribulin (NCT03786094). Clinical proof-of-concept in combination with eribulin was demonstrated in a PhI single arm dose-escalation trial in patients with metastatic HER2-negative BC (NCT01837095). The objective response rate for a dose of 5.5mg/kg balixafortide and 1.4mg/m2 eribulin) was 38% (median duration 4.4 months), and the clinical benefit rate was 63% (median duration 8.1 months). Tolerability and safety of eribulin in combination with balixafortide was comparable to that of eribulin monotherapy. Balixafortide was combined with paclitaxel in a humanized BC patient-derived xenograft (PDX) model to explore the efficacy in combination with another tubulin-binding drugs. Human immunophenotyping confirmed successful human CD34+ cell engraftment and expansion of human immune cells prior implantation of an invasive ductal carcinoma (ypT1a ypN1a R0 G3 L1 V1), ER/PR/Her-2 negative tumor. 10mg/kg paclitaxel was given 2x weekly iv. Dosing of balixafortide was 20mg/kg bid sc for 5 days with 2 days break. Total treatment duration was 28 days. Partial remission was achieved in the combination arm balixafortide+paclitaxel (T/C 13%, 87% inhibition of tumor volume vs vehicle control) which was stat. significantly better than paclitaxel monotherapy (T/C 42%). There was no decrease in tolerability in balixafortide combination vs paclitaxel alone. This data suggests combination with balixafortide is more efficacious and equally tolerated to paclitaxel single agent treatment. Citation Format: Daniel Obrecht, Johann Zimmermann. Efficacy of Balixafortide (POL6326) and Paclitaxel alone and in combination in humanized breast cancer PDX [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 414.