Abstract P2-04-08: BGB324, a selective small molecule inhibitor of the receptor tyrosine kinase AXL, enhances immune checkpoint inhibitor efficacy in mammary adenocarcinoma

The AXL receptor tyrosine kinase is associated with poor overall survival in breast cancer. Axl signaling is an important regulator of tumor plasticity related to epithelial-to-mesenchymal transition (EMT) and stem cell traits that drive metastasis and drug resistance. Signaling via AXL is also a key suppressor of the anti-tumor innate immune response. AXL is expressed on several cells associated with the tumor immune microenvironment including natural killer cells, dendritic cells and tumor-associated macrophages. AXL is required for tumor immune evasion in mammary adenocarcinoma models and EMT-mediated resistance to cytotoxic T cell and natural killer (NK)-cell mediated cell killing. Hence AXL signaling contributes uniquely to both tumor cell intrinsic and microenvironmental anti-tumor immune suppression mechanisms in breast cancer. We evaluated whether blocking AXL signaling with BGB324, a selective clinical-stage small molecule Axl kinase inhibitor, enhances the effect of immune checkpoint blockade in the aggressive mammary adenocarcinoma (4T1) syngeneic (Balb/C) mouse modelthat display limited immunogenicity.

Immune therapy with anti-CTLA-4/anti-PD-1 increased AXL and EMT-marker expression in 4T1 tumors, and correlated with lack of response to immune therapy. Combination treatment with BGB324 (50 mg/kg bid) significantly enhanced responsiveness to anti-CTLA-4/anti-PD-1 treatment (10 mg/kg of each, 4 doses) in Balb/C mice bearing established 4T1 tumors. The combination of BGB324 + anti-CTLA-4/anti-PD-1 resulted in durable primary tumor clearance in 23 % of treated mice versus 5.6% obtained with anti-CTLA-4/anti-PD-1 alone (p=0.0157). In a separate study, BGB324 + anti-CTLA-4 treated resulted in 22% long-term primary tumor clearance while no response was observed with anti-CTLA4 treatment alone. The extensive metastasis to the lung, liver and spleen characteristic of this model were concomitantly abrogated in the animals responding to the combination treatment. In addition, BGB324 + anti-CTLA-4/anti-PD-1 treated tumors displayed enhanced infiltration of cytotoxic T lymphocytes (CTLs). Enhanced presence of CTLs was also detected in spleens from animals responding to treatment. BGB324 + anti-CTLA-4/anti-PD-1 treatment increased the number of NK cells, macrophages and polymorphonuclear neutrophils, but decreased the number of mMDSC. Importantly, responding animals rejected orthotopic 4T1 tumor cell re-challenge, demonstrating sustained tumor immunity.

Together with recent results in other tumor types that support a prominent role for AXL in resistance to immune therapy and encouraging results from ongoing clinical trials with BGB324, support combining BGB324 with immune checkpoint inhibitors to improve treatment of breast cancer.

Citation Format: Lorens JB, Lipinska KW, Davidsen K, Blø M, Hodneland L, Engelsen A, Kang J, Lie MK, Bougnaud S, Aguilera K, Ahmed L, Rybicka A, Nævdal EM, Deyna P, Boniecka A, Straume O, Chouaib S, Brekken RA, Gausdal G. BGB324, a selective small molecule inhibitor of the receptor tyrosine kinase AXL, enhances immune checkpoint inhibitor efficacy in mammary adenocarcinoma [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-04-08.