1060P HER2-XPAT, a novel protease-activatable pro-drug T-cell engager (TCE), engineered to address on-target, off tumour toxicity and provide large predicted safety margins in non-human primate (NHP)

Abstract

TCEs are effective in creating remissions in hematologic cancers but have been challenging in solid tumors due to on-target, off-tumor toxicity. Attempts to circumvent CRS include complex molecular designs, but these have been unsuccessful due to toxicity and/or enhanced immunogenicity. Amunix has developed a conditionally active TCE, XPAT or XTENylated Protease-Activated bispecific T Cell Engager, that exploits the protease activity present in tumors vs. healthy tissue, expanding the therapeutic index (TI). The core of the HER2-XPAT (XPAT) consists of 2 tandem scFVs targeting CD3 and HER2. Two unstructured polypeptide masks (XTEN) are attached to the core and sterically reduce target engagement and extend T1/2. Protease cleavage sites at the base of XTEN enable proteolytic activation of XPATs in the tumor microenvironment, unleashing a highly potent TCE with a short T1/2, further improving the TI. The activity of XPAT and PAT (cleaved XPAT) were characterized for cytotoxicity in vitro and in huPBMC/tumor-bearing mice. Stability for XPAT was studied ex-vivo from cancer pt plasma. Toxicology studies were conducted in NHPs. Cleaved PAT demonstrated potent in vitro T cell cytotoxicity against tumor cells (EC50 1-2pM), while XPAT had up to 14,000-fold protection against killing. XPAT demonstrated CRs in with both BT-474 and SK-OV-3 tumors. In NHP, XPAT has been dose escalated up to 50 mg/kg, with 42mg/kg as the MTD which was well tolerated. PAT given by continuous infusion was not tolerated due to CRS at 0.3mg/kg/d. XPAT demonstrated T-cell margination at 2mg/kg but absence of CRS up to 42mg/kg. XPAT was also stable with no evidence of cleavage in the plasma of cancer pts over 7 days. XPAT has demonstrated >1000x protection over PAT with little evidence of CRS, including a 20x margin of safety over dose needed for pharmacodynamic activity. HER2-XPAT is a novel T cell engager pro-drug with promising evidence of a wide TI in NHP. With XTEN’s prior clinical data of low immunogenicity, the XPAT TCEs provide a promising solution and will enter phase I in the near future. Additional PK, PD, cytokines, safety, and efficacy data will be presented.