Axl is a receptor tyrosine kinase of the TAM family, which is structurally characterized by the presence of two extracellular immunoglobulin-like domains and two fibronectin type III domains, a single-pass transmembrane region and an intracellular tyrosine kinase domain. Axl is aberrantly expressed and activated in both solid and hematological malignancies and has been associated with poor clinical prognosis in a number of cancers, including non-small cell lung cancer, breast cancer, pancreatic cancer and acute myeloid lymphoma. Functionally, Axl has been implicated in multiple pathological processes, such as tumor cell motility, adherence and migration, epithelial-to-mesenchymal transition, metastasis, angiogenesis and resistance to tyrosine kinase inhibitors.

With the aim to target Axl expressing cancers, a panel of Axl-specific antibody-drug conjugates (ADC) was generated. First, high affinity human Axl antibodies were generated in HuMAb mice. Binding studies using Axl mutants, in which specific human Axl domains had been replaced with their murine analogs, demonstrated that the Axl antibody panel consisted of antibodies against different Axl extracellular domains. Second, the Axl antibodies were conjugated with the microtubule disrupting agent monomethyl auristatin E (MMAE) using the protease cleavable valine citrulline linker.

Axl ADCs showed cytotoxic activity in vitro, which was dependent on target expression and on conjugation with cytotoxic payload. The anti-tumor activity of Axl ADCs was confirmed in vivo. Axl ADCs induced tumor regression in cell line derived xenograft models for lung cancer and epidermoid carcinoma. Anti-tumor activity was especially efficient in the lung cancer xenograft model, in which inhibition of tumor growth was observed upon treatment with a single dose of 0.5 mg/kg Axl ADC. In the same model, 1 mg/kg Axl ADC was sufficient to induce complete tumor regression. Importantly, Axl ADCs also showed efficient anti-tumor activity in patient-derived xenograft (PDX) models that showed heterogeneous target expression.

In summary, Axl is a novel target for antibody-mediated delivery of cytotoxic agents into tumor cells. The promising results obtained in preclinical tumor models support further development of Axl-specific ADCs.

Citation Format: Esther C.W. Breij, Sandra Verploegen, Andreas Lingnau, Edward N. van den Brink, Maarten Janmaat, Mischa Houtkamp, Wim K. Bleeker, David Satijn, Paul W.H.I. Parren. Novel antibody-drug conjugates targeting Axl show anti-tumor activity in solid cancer xenograft models. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 634. doi:10.1158/1538-7445.AM2015-634