D1.343 - Unveiling the Underlying Mechanism of Differentiation for STAR-0310, an Anti-OX40 Antibody for Atopic Dermatitis
Background
Inhibition of OX40 is a validated mechanism for the treatment of Atopic Dermatitis. OX40 trimerization is critical for enabling the OX40/OX40L interaction, a key event that initiates downstream signaling, including nuclear factor kappa B (NFκB) pathway activation and the promotion of T-cell survival and proliferation. STAR-0310 is a novel, potent and selective long-acting monoclonal anti-OX40 antibody inhibitor currently in clinical development. To further characterize the STAR-0310 inhibitory mechanism of action, we determined the structure of STAR-0310 in complex with OX40 by utilizing Cryogenic Electron Microscopy (Cryo-EM).
Method
The STAR-0310 Fab/OX40 complex was initially screened to evaluate the best conditions for imaging, followed by vitrification and ThermoFisher Krios G3i electron microscope imaging. Image data were analyzed to obtain a high-resolution structure of the complex. The ability of STAR-0310 to induce agonism was assessed through activating the NFκB pathway in a Jurkat-NFκB-OX40 reporter cell line. The assay evaluated the potential for STAR-0310 to stimulate the OX40 receptor, in turn triggering the NFκB signaling cascade in Jurkat cells, which are genetically engineered to express OX40 and a luciferase reporter gene driven by the NFκB transcription factor.
Results
The high-resolution Cryo-EM structure of the STAR-0310 Fab/OX40 complex uncovered a novel allosteric inhibition mechanism of the OX40/OX40L interaction. STAR-0310 binds to the CRD2 domain of OX40, on the opposite side of the OX40L binding site. Notably, STAR-0310 engages a region critical for OX40 trimerization, thereby preventing its proper assembly and subsequent activation. STAR-0310 was shown to be an antagonistic antibody and did not induce agonistic activity on Jurkat-NFκB-OX40 cells.
Conclusion
These findings demonstrate that by engaging a unique region within the CRD2 domain, STAR-0310 inhibits OX40 activation through disruption of OX40 trimerization. Together with data indicating its pure antagonistic properties, STAR-0310 appears to function as a full OX40 antagonist. Clinical trials are currently ongoing.
