Genmab’s acquisition of Synaffix brought under one roof a set of conjugation technologies designed to solve the core engineering problems that have limited antibody-drug conjugates since their earliest clinical use. ADCs work on a simple premise: attach a potent cell-killing drug to an antibody that recognizes a cancer cell, and you get targeted chemotherapy with less collateral damage. In practice, building an ADC that is potent enough to shrink tumors without causing unacceptable side effects has proven far harder than the concept suggests, and much of the difficulty traces back to how the drug is physically attached to the antibody. Synaffix’s platform, anchored by a technology called GlycoConnect, approaches that attachment point differently from most competitors, and the implications ripple through stability, safety, manufacturability, and the kinds of next-generation ADC formats that are now entering the clinic.
The Heterogeneity Problem in Traditional ADCs
When an ADC is manufactured using conventional methods, the cytotoxic drug is typically linked to whichever amino acids happen to be accessible on the antibody’s surface. Early approaches attached drugs to exposed lysine residues or to the sulfur atoms in interchain disulfide bonds. Because an antibody has dozens of lysines and several disulfide sites, this “random conjugation” produces a mixture of molecules carrying different numbers of drug molecules in different positions. A single batch can contain antibodies carrying anywhere from zero to eight drug copies, each with its own pharmacological behavior.1PubMed Central. Site-specific antibody drug conjugates for cancer therapy That heterogeneity is not just a manufacturing nuisance. Molecules loaded with too many drug copies tend to aggregate, get cleared from the bloodstream faster, and cause more off-target toxicity. Molecules carrying too few are essentially expensive, inert antibodies. The net result is a narrow therapeutic window, where the dose that works against the tumor is uncomfortably close to the dose that harms the patient.
Site-specific conjugation technologies were developed precisely to overcome this problem, aiming to attach drugs at defined positions on the antibody so that every molecule in the batch carries the same number of payloads in the same locations.2PubMed Central. Site-specific antibody-drug conjugate heterogeneity characterization and heterogeneity root cause analysis Several approaches now exist. Some engineer new cysteine residues into the antibody sequence to create custom attachment points. Others use enzymes to place drugs at specific spots. Synaffix’s GlycoConnect takes a distinct route by exploiting a sugar chain that is already present on virtually every therapeutic antibody.
How GlycoConnect Works
Every immunoglobulin G antibody carries a sugar chain, or glycan, attached to a conserved asparagine residue at position 297 in the antibody’s constant region. This glycan is not just decorative; it influences the antibody’s structure and how immune cells interact with it. GlycoConnect uses this native glycan as the conjugation site through a two-step chemoenzymatic process. First, enzymes trim and remodel the glycan, introducing a small chemical handle, an azide-tagged sugar. Second, the cytotoxic payload is clicked onto that handle using copper-free click chemistry, a reaction that proceeds cleanly without metal catalysts that could damage the antibody.3PubMed. Chemoenzymatic Conjugation of Toxic Payloads to the Globally Conserved N-Glycan of Native mAbs Provides Homogeneous and Highly Efficacious Antibody-Drug Conjugates
A key advantage here is that the antibody itself does not need to be genetically engineered. Because the asparagine-297 glycan is present on every IgG subtype regardless of its original glycosylation pattern, GlycoConnect can be applied to essentially any therapeutic antibody as a plug-and-play platform.3PubMed. Chemoenzymatic Conjugation of Toxic Payloads to the Globally Conserved N-Glycan of Native mAbs Provides Homogeneous and Highly Efficacious Antibody-Drug Conjugates This modularity matters commercially: a partner company can bring its own antibody and its own drug, and Synaffix’s technology handles the connection without requiring the antibody to be re-engineered from scratch.
The enzymatic remodeling step has been refined to work as a one-pot reaction, where an engineered endoglycosidase and a glycosyltransferase cooperate to install the azide-bearing sugar in a single vessel.4PubMed Central. Enzymatic glycan remodeling–metal free click (GlycoConnect™) provides homogenous antibody-drug conjugates with improved stability and therapeutic index without sequence engineering That streamlining reduces manufacturing complexity and helps keep the process scalable for clinical-grade production.
Tuning Drug Load With Precision
One of the more interesting capabilities of the GlycoConnect platform is the ability to control exactly how many drug molecules end up on each antibody. The standard configuration produces ADCs with a drug-to-antibody ratio (DAR) of 2 or 4, meaning each antibody carries either two or four payload molecules. Both configurations yield homogeneous products, a sharp contrast with random conjugation’s messy distribution. The platform has also been extended to produce DAR1 ADCs for use with ultrapotent payloads, drugs so toxic that even two copies per antibody would be too much.5PubMed Central. Generation of DAR1 Antibody-Drug Conjugates for Ultrapotent Payloads Using Tailored GlycoConnect Technology In those cases, a specially designed bifunctional linker attaches a single drug molecule to the glycan, keeping the potency high while limiting the payload burden on each antibody.
At the other extreme, higher DAR values like 8 have traditionally been problematic. Loading more drug onto an antibody tends to make the molecule more hydrophobic, causing it to aggregate and get cleared from the bloodstream faster, which defeats the purpose of adding more drug. Research using compact branched PEG architectures has shown that you can reach a homogeneous DAR of 8 without increasing clearance rates, by introducing hydrophilicity right at the conjugation site to counteract the drug’s greasiness.6PubMed Central. Compact hydrophilic electrophiles enable highly efficacious high DAR ADCs with excellent in vivo PK profile This kind of DAR flexibility lets drug designers match the payload loading to the specific tumor biology and toxicity profile they are working with, rather than being stuck with a one-size-fits-all ratio.
HydraSpace and the Stability Challenge
The linker connecting drug to antibody is not just a passive tether. Its chemical properties affect how the ADC behaves in the bloodstream, how stable it is during storage, and how efficiently the drug gets released inside the cancer cell. Many ADC linkers are hydrophobic, and when combined with hydrophobic payloads, they can drag the whole molecule toward aggregation. Synaffix’s HydraSpace technology addresses this with a short, polar sulfamide spacer that sits between the antibody and the linker-drug construct.
Head-to-head comparisons of HydraSpace-containing ADCs against marketed drugs built from the same antibody and payload components showed improvements in both efficacy and safety across multiple animal models, backed by detailed pharmacokinetic analysis.7PubMed Central. A Polar Sulfamide Spacer Significantly Enhances the Manufacturability, Stability, and Therapeutic Index of Antibody–Drug Conjugates The spacer also improves the efficiency of the conjugation reaction itself and the stability of the final product. In practical terms, better stability can translate to longer shelf life and more predictable dosing, because the drug stays attached to the antibody instead of falling off prematurely in the bloodstream.
What makes HydraSpace particularly useful in the context of GlycoConnect is the stacking effect. GlycoConnect provides the site-specific attachment point, HydraSpace provides the hydrophilic spacer, and together they produce an ADC that is both homogeneous and well-behaved pharmacokinetically. A biparatopic HER2-targeting ADC constructed with site-specific glycan conjugation, for example, demonstrated superior structural homogeneity, optimized hydrophilicity, and reduced aggregation compared to conventional thiol-maleimide chemistry, with an acceptable safety profile in primate studies.8PubMed Central. A biparatopic HER2-targeting ADC constructed via site-specific glycan conjugation exhibits superior stability, safety, and efficacy
The Bystander Effect and Why Linker Design Matters
Tumors are not uniform masses of identical cells. A given tumor typically contains a mixture of cells expressing different levels of the target antigen, plus stromal cells and other non-cancerous tissue. An ADC that only kills cells it directly binds to will miss antigen-negative neighbors, potentially allowing resistant clones to survive and regrow. The bystander effect, where released payload diffuses out of the killed cell and takes out nearby cells too, is one of the most important mechanisms for dealing with this heterogeneity.
Whether an ADC produces a meaningful bystander effect depends heavily on the linker. Cleavable linkers, which break apart inside the cell to release a freely diffusible payload, tend to produce stronger bystander killing than non-cleavable linkers. Hydrophobic payloads can more easily cross cell membranes once released, further enhancing the effect. Clinically, this distinction shows up clearly: ADCs designed with bystander activity, such as trastuzumab deruxtecan, have demonstrated superior efficacy compared to non-bystander ADCs like trastuzumab emtansine.9PubMed. Bystander effect in antibody-drug conjugates: Navigating the fine line in tumor heterogeneity
Newer linker designs are pushing the concept further. Researchers have developed a caspase-3 cleavable peptide linker that triggers drug release not inside the target cell, but in the surrounding tumor environment after the cell dies and releases caspase-3. In breast cancer models, ADCs using this linker maintained their killing power even after antigen-positive cells were depleted, continuing to exert a bystander effect on remaining tumor cells.10Journal of Controlled Release. Enhancing the bystander effect of antibody-drug conjugate by using a novel caspase-3 cleavable peptide linker to overcome tumor heterogeneity For a platform like GlycoConnect, which provides precise control over where and how the linker-payload attaches, the ability to pair with these advanced linker chemistries without disrupting the antibody’s structure is a meaningful advantage.
Drug Resistance and the Payload Question
Even the best-designed ADC can eventually fail if the cancer cell finds a way to resist the payload. Resistance to ADCs arises through several routes: the target antigen can be downregulated so the antibody has nothing to bind, the cell can slow or block internalization of the ADC complex, or the cell can pump the released drug right back out through efflux transporters.11PubMed Central. Antibody-drug conjugates in breast cancer treatment: resistance mechanisms and the role of therapeutic sequencing
The efflux transporter problem is particularly relevant to conjugation technology. Many of the most commonly used ADC payloads, including monomethyl auristatin E (MMAE), mertansine (DM1), ravtansine (DM4), and the entire class of pyrrolobenzodiazepines, are substrates of the P-glycoprotein pump and other ABC transporters that cancer cells upregulate to expel drugs. Researchers testing a panel of payloads found that the efficacy of the FDA-approved ADC mirvetuximab soravtansine, which carries DM4, was reduced in cell lines expressing P-glycoprotein.12PubMed Central. Identification of antibody-drug conjugate payloads that are substrates of ATP-binding cassette drug efflux transporters Not all payloads are equally vulnerable, though. Modified anthracyclines like PNU-159682 were poorly transported by the major efflux pumps and retained extraordinarily high potency across a panel of nearly 100 cancer cell lines of varying origins.12PubMed Central. Identification of antibody-drug conjugate payloads that are substrates of ATP-binding cassette drug efflux transporters
This is where the modularity of a platform like GlycoConnect becomes strategically important. Because the technology does not depend on a specific payload chemistry, it can accommodate next-generation payloads that sidestep efflux resistance, as well as established warheads where resistance is less of a concern for a particular indication. The platform’s flexibility in DAR control adds another dimension: matching the payload potency to the appropriate loading ratio reduces the risk of both under-dosing the tumor and overdosing the patient.
Getting Drugs Into Solid Tumors
Solid tumors present a delivery challenge that goes beyond target-cell recognition. An ADC must leave the bloodstream, diffuse through dense extracellular matrix, and reach cancer cells that may be several cell layers away from the nearest blood vessel. Modeling studies show that when the target receptor is expressed at high levels, the ADC tends to bind and get stuck near the point of entry, creating a steep concentration gradient where cells close to the vessel are saturated while cells deeper in the tissue see almost no drug. Lower receptor density actually produces more uniform drug penetration, but of course you cannot easily control receptor density in a patient.13PLoS ONE. A Mechanistic Tumor Penetration Model to Guide Antibody Drug Conjugate Design
Systems pharmacology models that simulate ADC transport through realistic three-dimensional tumor blood vessel networks have been developed to account for diffusion, binding, internalization, intracellular processing, and payload release all at once.14PubMed Central. A Systems Pharmacology Model for Drug Delivery to Solid Tumors by Antibody-Drug Conjugates: Implications for Bystander Effects These models underscore how much the bystander effect matters for solid tumors: if the released payload can diffuse into neighboring cells, even imperfect penetration of the ADC itself can translate into broader tumor killing. Conjugation technology contributes to this equation indirectly. A more homogeneous ADC with predictable pharmacokinetics delivers a more consistent dose to the tumor vasculature, which in turn gives the penetration and bystander mechanisms a more reliable starting point.
Bispecific ADCs and the Multi-Target Frontier
One of the more ambitious directions in ADC development is the bispecific ADC, which uses an antibody engineered to recognize two different targets on cancer cells. The rationale is straightforward: if a tumor can escape a single-target ADC by downregulating one antigen, binding two targets simultaneously makes escape harder. Bispecific antibodies can also improve tumor selectivity when the two targets are co-expressed on tumor cells but appear separately on normal tissues, reducing off-target toxicity.
Building bispecific ADCs adds a layer of complexity to conjugation. Attaching drugs randomly to a bispecific antibody risks interfering with one or both binding sites, since the drug could land anywhere. Site-specific and enzyme-based conjugation methods solve this by placing the drug at a defined location away from the antigen-binding regions, minimizing the chance that conjugation disrupts dual-target binding.15Acta Pharmaceutica Sinica B. Bispecific antibody drug conjugates: Making 1+1>2 GlycoConnect’s attachment at asparagine-297, which sits in the constant region far from the variable binding domains, is well suited for this application.
Computational modeling of bispecific ADC pharmacology reveals that tumor selectivity depends heavily on getting the binding affinities for both targets right. Simply making an antibody bispecific does not automatically improve selectivity; the on-rates and off-rates for each target must be tuned together, creating a four-dimensional optimization problem that benefits from computational guidance rather than trial and error.16Academia Drug Development and Pharmacotherapy. Model-optimized bispecific antibodies improve the selectivity of antibody–drug conjugates for tumors The reassuring finding from this modeling work is that improvements in tumor selectivity remain robust even when target expression levels vary by an order of magnitude, suggesting the approach can tolerate the kind of biological variability seen across patients.
Competing Approaches to Site-Specific Conjugation
GlycoConnect is not the only site-specific conjugation technology in the field. Engineered cysteine approaches insert new cysteine residues at chosen positions on the antibody, then use selective reducing agents to activate only those cysteines for drug attachment while leaving the antibody’s natural disulfide bonds intact.17PubMed Central. A Platform for the Generation of Site-Specific Antibody-Drug Conjugates That Allows for Selective Reduction of Engineered Cysteines Two ADCs built using engineered cysteine conjugation have already received regulatory approval, which has generated considerable interest in the approach.18Taylor & Francis Online / Expert Opinion on Biological Therapy. An overview of site-specific methods for achieving antibody drug conjugates with homogenous drug to antibody ratio
Enzyme-based methods beyond GlycoConnect also exist. Microbial transglutaminase, isolated from the bacterium Streptomyces mobaraensis, can attach drugs to specific glutamine residues on the antibody, producing homogeneous ADCs with defined drug loads.19PubMed. Site-Specific Antibody-Drug Conjugation Using Microbial Transglutaminase Each approach involves trade-offs. Engineered cysteines require genetic modification of the antibody sequence, which adds development time but gives precise positional control. Transglutaminase methods work on native or minimally modified antibodies but require the right glutamine residues to be accessible. GlycoConnect avoids antibody engineering entirely by using the native glycan, but the enzymatic remodeling step adds its own manufacturing considerations.
The competitive picture is less about one technology winning and more about which platform fits best for a given antibody-payload-indication combination. A company developing an ADC with a well-characterized antibody that already has engineered cysteines might stick with that route. A company wanting to rapidly screen multiple antibodies against the same payload may find GlycoConnect’s plug-and-play approach faster. The GlycoConnect platform has already been applied in several clinical programs, including ADCT-601, XMT-1660, and MRG004a, confirming that the technology translates from bench to clinic.5PubMed Central. Generation of DAR1 Antibody-Drug Conjugates for Ultrapotent Payloads Using Tailored GlycoConnect Technology
What Homogeneity Actually Changes for Patients
It is worth spelling out why all this engineering matters beyond the manufacturing floor. A heterogeneous ADC batch essentially delivers a range of different drugs to the patient in a single infusion. Some molecules are over-loaded and drive toxicity. Some are under-loaded and contribute nothing therapeutically. The effective dose is therefore a fuzzy average of these species, and the therapeutic window, the gap between “works” and “hurts,” shrinks as a result. Site-specific conjugation tightens that window by ensuring every molecule in the vial behaves the same way in the body.
The practical consequence for patients is a better chance of landing in the zone where the drug is effective without intolerable side effects. In animal studies, GlycoConnect-based ADCs have shown a significantly expanded therapeutic index compared to ADCs made with several other conjugation technologies.5PubMed Central. Generation of DAR1 Antibody-Drug Conjugates for Ultrapotent Payloads Using Tailored GlycoConnect Technology Whether those preclinical advantages translate proportionally into clinical benefit is still being tested across the programs using the platform, but the direction of the evidence is consistent with what you would expect: less product variability leads to more predictable pharmacology.
Immune Considerations and Glycan Manipulation
Modifying the glycan at asparagine-297 is not immunologically neutral. That glycan plays a well-known role in how the antibody’s Fc region interacts with Fc gamma receptors on immune cells, influencing processes like antibody-dependent cellular cytotoxicity (ADCC). Trimming or remodeling the glycan during GlycoConnect processing could in theory alter these immune effector functions. For some ADCs this may be acceptable or even desirable, since the therapeutic activity comes primarily from the cytotoxic payload rather than from immune-mediated killing. For antibodies where ADCC or complement activation is part of the therapeutic mechanism, though, any reduction in effector function would need to be evaluated carefully.
The question of whether the remodeled glycan or the conjugated drug triggers an immune response against the ADC itself, producing anti-drug antibodies, is another open area. Anti-drug antibodies can neutralize the therapeutic, accelerate its clearance, or cause infusion reactions. Site-specific conjugation in general is expected to produce a more consistent immunogenic profile than random conjugation, simply because the drug is always in the same position rather than scattered across the antibody surface. But clinical immunogenicity data from GlycoConnect-based ADCs remains limited and will accumulate as more programs progress through trials.
Manufacturing at Scale
ADC manufacturing is inherently more complex than standard antibody production because it adds conjugation chemistry to the already demanding process of producing a biological molecule. Every additional step introduces yield loss, quality control requirements, and potential failure modes. GlycoConnect’s one-pot enzymatic remodeling followed by click chemistry is designed to minimize the number of intermediate purification steps.4PubMed Central. Enzymatic glycan remodeling–metal free click (GlycoConnect™) provides homogenous antibody-drug conjugates with improved stability and therapeutic index without sequence engineering The click chemistry reaction itself is attractive for manufacturing because it is highly selective, meaning it proceeds cleanly without generating significant side products that would need to be removed.
Scalability remains a critical question for any conjugation technology moving from lab-scale proof of concept to commercial production serving thousands of patients. Enzymatic reactions can be sensitive to conditions like temperature, enzyme purity, and reaction time in ways that small-molecule chemistry is not. The fact that GlycoConnect has been used in multiple clinical-stage programs suggests that scalability has been at least partially validated, since clinical trial material must meet stringent quality standards. But full commercial-scale manufacturing, producing ADCs for marketed products at volumes that serve global patient populations, is a different challenge that will test the robustness of the enzymatic process further.