Abstract Text: We have discovered a novel anti-PSGL-1 monoclonal antibody, ALTB-168 (neihulizumab), that acts as an immune checkpoint enhancer (ICE) by down-regulating T effector function. With this unique mechanism of action, ALTB-168 has been advanced clinically for the treatment of T-cell mediated inflammatory diseases. ALTB-168 was found to induce inhibitory signaling upon binding to PSGL-1, which is enhanced by cross-linking with anti-human antibody in vitro. An Fv engineered tetravalent antibody, with four PSGL-1 binding sites, can potentially facilitate the clustering of cell surface PSGL-1 and the downstream signaling compared to a conventional bivalent antibody. Here we show that a tetravalent version of ALTB-168, named ALTB-268, demonstrated greater than 10-fold higher potency in in vitro T cell activation inhibition assays compared to ALTB-168. When compared in a human-mouse trans-vivo DTH study as well as in a non-human primate (NHP) DTH study, greater than 3-fold higher potency was observed for ALTB-268. The increased potency is likely related to differences in stoichiometry and increased avidity rather than increased affinity, as a single 268 molecule can bind to more PSGL-1 compared to a single 168 molecule, while similar affinity for both ALTB-168 and ALTB-268 was measured by SPR or ELISA. Most importantly, a similar safety profile as ALTB-168 was observed for ALTB-268 in NHP toxicology assessments, with a NOAEL of 120 mg/kg in a definitive 28-day weekly repeat-dose toxicity study, and a bioavailability of 70% by subcutaneous (sc) route. These data support the clinical development of ALTB-268, sc, for the treatment of T-cell mediated inflammatory diseases.
