Intercepting Cancer’s Survival Mechanism
Cancer can be subdivided into different histological subtypes at the phenotypic level, but in fact exhibits widespread heterogeneity at the molecular level. While primary tumors possess similar genetic alterations, little is known about the mutations and pathways that drive invasive and migratory processes.
The invasive and migratory ability of tumor cells is tightly associated with epithelial to mesenchymal transition (EMT). Tumor cells undergoing EMT exhibit a dramatic change in morphology as well as cell-to-cell and cell-to-matrix interactions. We have found that GAS6 through the AXL receptor tyrosine kinase is a critical driver of EMT, invasion, survival and metastasis. A growing number of publications also supports these roles for GAS6/AXL in many tumor types. In addition, inhibition of AXL signaling using small molecules has been reported to sensitize drug resistant mesenchymal cells to cytotoxic and targeted therapeutics.
Since cancer cells have survival mechanisms that let them flourish even under constant stress, the inhibition of GAS6/AXL has important therapeutic effects for the survival and expansion of tumor cells of different origins. Most importantly, loss of GAS6 & AXL function does not appear to harm normal tissues, and interestingly, increases with tumor grade and stage.
To date, other groups have pursued both small molecule kinase inhibitors and antibodies against AXL and GAS6. Prior anti-AXL and anti-GAS6 antibody programs have failed to produce an antibody of sufficient affinity to disrupt the interaction between AXL and GAS6. Selective target engagement for AXL kinase using small molecules has proven a difficult challenge as current anti-AXL compounds in development appear to inhibit other kinases more so than AXL and have significant off target activity limits their utility.
Affinity matters because the natural interaction between the ligand Gas6 and the AXL receptor is quite high (Kd~30 pM), and requires a decoy receptor with higher affinity to bind Gas6. Our approach is to target the GAS6/AXL signaling pathway using a Fc-fusion decoy receptor.
Preclinical data has demonstrated that our drug candidate, Aravive-S6, has better than 100-fold tighter affinity for GAS6 compared to the natural affinity of the ligand and the receptor, providing remarkable high specificity and selectivity for the AXL/GAS6 pathway that other anti-AXL and anti-GAS6 inhibitors have been unable to match.
Intercepting the GAS6-AXL signaling pathway with a high affinity decoy has been shown to inhibit the survival of tumor cells, especially in vivo, resulting in decreased tumor growth and metastasis as well as development of resistance to cytotoxic chemotherapeutic agents.
Therefore, by targeting the adaptive response in tumors through the GAS6-AXL pathway with a high-affinity biologic molecule, we believe we can interrupt the signals and turn off the “survival switch” that otherwise allows cancer to grow and acquire resistance to cancer therapy.