Mass photometry reveals stoichiometry and binding dynamics of bispecific tetravalent anti-VEGF-PD-1 antibody ivonescimab

BackgroundThe bispecific antibody ivonescimab targets programmed cell death protein 1 (PD-1) and vascular endothelial growth factor (VEGF). Recent clinical trials have shown it has greater efficacy against PD-L1 positive non-small cell lung cancer than pembrolizumab (Keytruda), a frequently prescribed anti-PD-1 monoclonal antibody. Ivonescimab binds to two VEGF and two PD-1 molecules, with complex formation through higher-order structure formation (or daisy chain binding). However, the binding stoichiometries and interaction dynamics of ivonescimab with VEGF and PD-1 have not been characterized in depth. MethodsWe used mass photometry (MP) and kinetic modelling to analyze these interactions, quantifying the complexes formed and their affinities. Dissociation constants (KD) for ivonescimabs binding to VEGF and PD-1 were calculated from equilibrium counts and real-time measurements, respectively. ResultsVEGF drove oligomerization of ivonescimab, which bound VEGF predominantly in a 2:2 stoichiometry, with KD=0.89 nM. Higher-order oligomeric complexes, present only at low abundance, displayed markedly weaker affinities (12.18 nM; 30.06 nM). Ternary complexes of ivonescimab with its two targets consistently presented two PD-1 antigens for each ivonescimab molecule, with a 1.66 nM KD for the binding of the first PD-1 and the slightly stronger 0.89 nM for the second PD-1 molecule. ConclusionsMP confirmed VEGF-induced ivonescimab oligomerization and revealed that dimers, not higher-order structures, were the most stable stoichiometry. MP enables detailed analysis of antibody-antigen interactions, even for bispecific antibodies that interact with antigens with complex stoichiometries.