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

Using mass photometry and kinetic modeling, this study characterizes the binding dynamics of the bispecific antibody ivonescimab, revealing that it predominantly forms stable 2:2 dimers with VEGF and ternary complexes presenting two PD-1 molecules, with distinct dissociation constants for each interaction.

The Gist

Imagine you are trying to stop a tumor from growing. Tumors are like weeds in a garden that need two things to thrive: a water supply (blood vessels) and a "do not disturb" sign that tells the immune system to ignore them.

To fight this, scientists created a special "super-tool" called Ivonescimab. Think of it as a high-tech, two-pronged grappling hook. One prong grabs the water supply (VEGF), and the other prong grabs the "do not disturb" sign (PD-1) to wake up the immune system.

This paper is like a behind-the-scenes look at how this grappling hook actually works when it meets the weeds. The researchers used a special camera called Mass Photometry (think of it as a super-precise scale that can weigh individual molecules as they float in a drop of water) to see exactly how the pieces fit together.

Here is the story of what they found, broken down into simple concepts:

1. The "Solo" vs. The "Team"

Before mixing things up, the researchers looked at the grappling hook (Ivonescimab) on its own.

• The Finding: Most of the time, the hook floats around alone as a single unit. It's not naturally sticky to itself.
• The Analogy: Imagine a group of people at a party. Most are standing alone, chatting. Only a few are holding hands in pairs or groups.

2. The Water Supply (VEGF) Creates a Chain Reaction

Next, they added the "water supply" (VEGF) to the mix.

• The Finding: As soon as the grappling hook touched the water supply, it started grabbing onto other hooks. They didn't just form a small group; they formed a specific, stable "handshake" between two hooks and two water molecules.
• The Analogy: Imagine the water supply is a magnet. When the hooks touch it, they don't just stick to the magnet; they grab onto each other through the magnet, forming a perfect, tight circle of two hooks and two magnets.
• The Surprise: Scientists thought these hooks would form giant, messy chains (like a long train of cars). But the camera showed that the two-by-two circle is the most stable and common shape. Anything bigger than that is rare and falls apart easily.

3. The "Do Not Disturb" Sign (PD-1)

Then, they added the "do not disturb" sign (PD-1).

• The Finding: The grappling hook is designed to grab two of these signs. The researchers watched this happen in real-time. They saw the hook grab the first sign, and then quickly grab a second one.
• The Analogy: It's like a person putting on a pair of gloves. They put on the left glove, and then immediately the right glove. Once both are on, the person is "fully dressed" and ready for action.
• The Result: The hook binds very tightly to the signs, especially when it has both of them.

4. The Grand Finale: The Perfect Trio

Finally, they mixed everything together: The Hook, the Water, and the Signs.

• The Finding: The water supply (VEGF) is the boss. It forces the hooks to link up into those stable two-by-two circles. Once those circles are formed, they grab two "do not disturb" signs each.
• The Final Structure: The most common, stable, and effective shape is a complex of 2 Hooks + 2 Water Molecules + 4 Signs.
• The Analogy: Think of it like a dance floor. The water molecules are the DJ. They get the hooks (dancers) to pair up. Once the pairs are formed, they each grab two partners (the signs) to complete the dance formation. The researchers found that this specific dance formation is the "champion" of the party.

Why Does This Matter?

For a long time, scientists guessed that these drugs might form giant, messy chains. This paper proves that guess wrong.

• The "Sweet Spot": The drug works best when it forms these specific, tight pairs. If there are too many water molecules or too few hooks, the perfect pairs don't form, and the drug is less effective.
• The Tool: The "Mass Photometry" camera used here is like a high-definition security camera that can see exactly who is holding hands with whom in a crowded room. Traditional methods (like SPR or BLI) are like looking at the crowd through a foggy window; they can tell you people are interacting, but they can't see the specific shapes they are making.

The Takeaway

This study shows us that the "super-tool" (Ivonescimab) is a master of precision. It doesn't just randomly grab things; it builds a very specific, stable machine (the 2:2:4 complex) to fight cancer. Understanding this exact shape helps doctors figure out the perfect dose to give patients, ensuring the drug forms the right "dance partners" to win the battle against the tumor.

Technical Summary

1. Problem Statement

Ivonescimab is a first-in-class bispecific antibody targeting both PD-1 (an immune checkpoint) and VEGF (a pro-angiogenic factor). It is engineered to bind simultaneously to these targets, with a proposed mechanism of action involving "daisy chaining"—where binding to VEGF induces the formation of higher-order oligomeric structures that subsequently bind PD-1 with increased affinity.

Despite promising clinical results (superior progression-free survival compared to pembrolizumab in NSCLC), the biochemical characterization of ivonescimab has been limited. Key gaps include:

• Lack of detailed understanding of binding stoichiometries and interaction dynamics.
• Inadequacy of conventional techniques like Surface Plasmon Resonance (SPR) and Biolayer Interferometry (BLI) for analyzing multi-specific antibodies. These methods are optimized for 1:1 interactions, often suffer from avidity artifacts due to surface immobilization, and struggle to resolve complex mixtures of varying oligomeric states.
• Uncertainty regarding whether the "daisy chain" mechanism results in stable high-order oligomers or specific discrete complexes.

2. Methodology

The authors employed Mass Photometry (MP), a label-free, single-molecule technique that measures the interference of light scattered by particles on a glass surface to determine their molecular mass.

• Experimental Design:
  • Components: Ivonescimab, recombinant human VEGF165A, and human PD-1 (His-tag).
  • Conditions: Reactions were performed in solution at near-physiological concentrations (nanomolar range) to avoid surface-induced artifacts.
  • Time-Course Analysis: Samples were incubated for varying durations (10 seconds to 40 minutes) to track real-time assembly and reach equilibrium.
  • Stoichiometry Variations: Experiments were conducted at different molar ratios (e.g., 1:1, 1:2, 1:4 antibody:antigen) to map assembly pathways.
  • Data Analysis:
    • Equilibrium Analysis: Gaussian fitting of mass histograms to quantify the relative abundance of different species (monomers, dimers, trimers, and complexes). Dissociation constants ($K_D$) were calculated using mass balance equations and equilibrium counts.
    • Kinetic Modeling: Real-time data was fitted to a system of ordinary differential equations (ODEs) using a two-step sequential binding model to extract kinetic rate constants ($k_{on}$, $k_{off}$) and $K_D$ values for PD-1 binding.
    • Validation: Results were cross-referenced with known masses and compared to previous BLI data.

3. Key Contributions

• First MP Characterization: This is the first study to apply mass photometry to characterize the binding of ivonescimab to its dual targets.
• Resolution of Complex Stoichiometry: The study successfully resolved a mixture of multiple co-existing complexes with different stoichiometries, which is difficult for ensemble techniques.
• Solution-Phase Kinetics: By avoiding surface immobilization, the study provided $K_D$ values that reflect true solution-phase avidity and binding events without the rebinding artifacts common in SPR/BLI.
• Mechanistic Insight: The data clarified that while VEGF drives oligomerization, the most stable and predominant species is a specific dimeric complex, not the infinite "daisy chains" previously hypothesized.

4. Key Results

A. Ivonescimab and VEGF Interaction

• Oligomerization: VEGF drives the oligomerization of ivonescimab. While ivonescimab exists primarily as a monomer in isolation, VEGF induces the formation of complexes.
• Dominant Species: The most stable and abundant complex is a 2:2 stoichiometry (2 ivonescimab molecules : 2 VEGF dimers).
  • Affinity: This 2:2 complex exhibits an exceptionally high affinity with a $K_D$ of 0.08 nM.
• Higher-Order Structures: Complexes with higher stoichiometries (3:3, 4:4, 5:5) were detected but were present in low abundance and displayed significantly weaker affinities ($K_D$ ranging from 1.29 nM to 3.17 nM).
• Assembly Pathway: The data suggests a stepwise assembly where free ivonescimab binds to 1:2 complexes to form the stable 2:2 structure. The formation of higher-order chains is thermodynamically less favorable, likely due to steric hindrance or entropic costs.
• Concentration Dependence: The formation of higher-order structures is dependent on the molar ratio; excess VEGF saturates binding sites, preventing the cross-linking necessary for oligomerization.

B. Ivonescimab and PD-1 Interaction

• Stoichiometry: Ivonescimab binds PD-1 in a 1:2 ratio (one antibody binds two PD-1 molecules).
• Kinetics: The binding occurs via two sequential steps with similar affinities:
  • First PD-1 binding: $K_D \approx 1.66$ nM.
  • Second PD-1 binding: $K_D \approx 0.89$ nM.
• Heterogeneity: The study noted significant heterogeneity in PD-1 (likely due to glycosylation), which complicated absolute quantification of free PD-1. However, mass balance calculations allowed for accurate kinetic modeling.
• Comparison: These values align well with previous BLI data, validating the MP approach.

C. Ternary Complexes (Ivonescimab + VEGF + PD-1)

• Structure: When all three components were mixed, the system formed a 2:2:4 ternary complex (2 ivonescimab : 2 VEGF : 4 PD-1).
• Mechanism: VEGF drives the formation of the 2:2 antibody-antigen core. PD-1 subsequently binds to the available sites on the ivonescimab molecules without disrupting the VEGF-induced oligomerization.
• Cooperativity: The study confirmed that PD-1 does not promote or disrupt ivonescimab oligomerization; the oligomerization is strictly VEGF-driven. The presence of VEGF does not appear to alter the stoichiometry of PD-1 binding.

5. Significance and Conclusion

• Therapeutic Implications: The finding that the 2:2 dimer is the most stable species, rather than large oligomers, refines the understanding of ivonescimab's mechanism of action. It suggests that the therapeutic efficacy relies on the formation of this specific high-avidity dimeric complex in the tumor microenvironment (TME).
• Dosage Optimization: The study highlights that a critical concentration of ivonescimab is required in the TME to drive the formation of these active complexes. If VEGF is in vast excess, the antibody may be saturated in 1:1 or 1:2 states, preventing the formation of the potent 2:2 cross-linked species. This has direct implications for dosing strategies.
• Analytical Advancement: The paper demonstrates that Mass Photometry is a superior tool for characterizing complex, multi-specific biologics. It provides single-molecule resolution of stoichiometry and affinity in solution, overcoming the limitations of surface-based methods (SPR/BLI) which often obscure oligomerization pathways or introduce avidity artifacts.
• Future Impact: This approach offers a robust framework for the development and quality control of next-generation antibody modalities, enabling more precise protein engineering and better prediction of in vivo behavior.