Antibody-Based Degraders: AbTACs, DACs & BiDACs
Introduction
Classical PROTACs operate inside the cell, recruiting cytoplasmic E3 ligases to degrade intracellular targets via the proteasome. But roughly 25–30% of the human proteome consists of integral membrane proteins — receptors, adhesion molecules, immune checkpoints — many of which sit on the cell surface beyond the reach of cell-permeable small molecules. Antibody-based degraders solve this by using bispecific antibodies to bridge membrane-bound E3 ligases (or other internalizing receptors) directly to cell-surface targets, routing them to the lysosome for degradation without ever needing to cross the plasma membrane.
Three related modalities occupy this space: AbTACs (antibody-based targeting chimeras) that recruit membrane E3 ligases like RNF43, DACs (degrader-antibody conjugates) that deliver PROTAC payloads via antibody targeting, and BiDACs (bispecific degrader antibody conjugates) that combine elements of both approaches. Each demands distinct SPR/BLI characterization strategies.
AbTAC Mechanism
AbTACs are bispecific antibodies where one arm binds a cell-surface E3 ubiquitin ligase (most commonly RNF43, with its paralog ZNRF3 as a closely related alternative) and the other binds a cell-surface target protein (e.g., PD-L1). Co-engagement brings the E3 ligase into proximity with the target and is presumed to trigger ubiquitination of accessible lysines on the target's intracellular domain — though direct ubiquitination of the target by AbTAC-recruited RNF43 was not demonstrated in the founding Cotton 2021 study, and the STTT 2022 review notes the mechanism remains inferred. The prevailing model is that the ubiquitin tag serves as a sorting signal driving clathrin-mediated endocytosis, with subsequent lysosomal proteolysis. For Cotton 2021's PD-L1 AbTAC, bafilomycin blocked degradation while MG132 did not — clearly lysosomal. However, Marei 2022 reported that proteasome inhibition also impaired IGF1R PROTAB degradation, so pathway involvement appears target-dependent rather than universally “lysosomal only.”
Note: because RNF43's catalytic RING domain faces the cytoplasm, AbTAC targets must present accessible intracellular lysines within geometric reach of the bridged E3. Targets with very short cytoplasmic tails (e.g., PD-L1, ~30 aa) impose real geometric constraints on linker design and arm geometry.
The AbTAC Degradation Cycle
RNF43: The Membrane E3 Ligase
RNF43 is a single-pass transmembrane RING-type E3 ubiquitin ligase that normally regulates Wnt signaling by ubiquitinating Frizzled receptors. AbTACs co-opt RNF43 to ubiquitinate non-native substrates on the cell surface.
- Broadly expressed transmembrane E3 ligase
- Relatively small ectodomain (~25–30 kDa unglycosylated) — accessible for antibody binding
- RING domain faces the cytoplasm — presumed to ubiquitinate accessible intracellular lysines on bridged targets; the resulting fate is predominantly lysosomal, though proteasomal contribution is target-dependent (Marei 2022)
- Paralog ZNRF3 shares the same Wnt- regulating role and is an analogous AbTAC E3 partner
- First demonstrated targeting PD-L1 degradation (Cotton et al., 2021)
Degrader-Antibody Conjugates (DACs)
DACs take a fundamentally different approach: an antibody targeting a cell-surface antigen is chemically conjugated to a small-molecule PROTAC payload via a cleavable linker. The antibody provides cell-type selectivity (like an ADC), and upon internalization and linker cleavage, the PROTAC is released intracellularly to degrade its target via the ubiquitin-proteasome system.
How DACs Work
- Antibody binds cell-surface antigen (e.g., HER2) for targeted delivery
- Cleavable linker releases PROTAC payload after internalization
- Free PROTAC degrades intracellular target (e.g., BRD4) via UPS
- Combines tissue selectivity of ADCs with catalytic degradation
Key Considerations
- Drug-antibody ratio (DAR) affects binding — high DAR can impair antibody kinetics
- Hydrophobic PROTAC payloads may cause antibody aggregation
- Dual characterization needed: whole conjugate + released payload
- High DAR can alter FcRn binding — impacts pharmacokinetics
BiDACs: Bispecific Degrader Antibody Conjugates
BiDACs represent a broader category encompassing AbTACs and related bispecific formats. One arm recruits an internalizing receptor (not necessarily an E3 ligase — could be ASGPR for liver targeting, TfR for broad distribution), while the other binds the target. Co-engagement triggers receptor-mediated endocytosis, routing the target to lysosomes.
BiDAC vs. AbTAC
| Feature | AbTAC | BiDAC (broader) |
|---|---|---|
| Recruited Partner | Membrane E3 ligase (RNF43, ZNRF3) | Any internalizing receptor (ASGPR, TfR, etc.) |
| Mechanism | Ubiquitin-tagged endocytosis → lysosomal proteolysis | Receptor-mediated endocytosis → lysosome |
| Pathway | Predominantly lysosomal (Cotton 2021, PD-L1); ubiquitin tag acts as sorting signal. Proteasomal contribution has been reported for some targets (Marei 2022, IGF1R) — pathway is target-dependent | Lysosomal; can be E3-independent (no ubiquitin tag required) |
| SPR Complexity | Binary + ternary + cooperativity | Binary + ternary (internalization rate is a cell assay readout, not SPR) |
Key Distinction: AbTACs recruit membrane-bound E3 ligases (RNF43/ZNRF3) so that the target is presumed to be ubiquitinated prior to endocytosis — with the ubiquitin tag driving predominantly lysosomal sorting (Cotton 2021, PD-L1), though proteasomal contribution has been reported for some targets (Marei 2022, IGF1R). The broader BiDAC category includes formats that route targets to lysosomes through E3-independent endocytosis. Both end primarily at the lysosome; the SPR characterization framework is similar, but the mechanistic interpretation differs. Note also that the “BiDAC ⊇ AbTAC” hierarchy used here is a convenient framing — in the literature these terms are sometimes used interchangeably.
SPR/BLI Characterization
Antibody-based degraders require characterization of each arm independently, both arms simultaneously (bridging), and the avidity effects inherent to bispecific formats.
1. Individual Arm Binding Kinetics
Each arm of the bispecific must be characterized independently to establish baseline affinity and kinetics.
| Interaction | KD Range | ka (M⁻¹s⁻¹) | kd (s⁻¹) | Notes |
|---|---|---|---|---|
| Anti-target arm → target ectodomain | 0.1–50 nM | 10⁵–10⁶ | 10⁻⁴–10⁻³ | Typical antibody kinetics |
| Anti-RNF43 arm → RNF43 ectodomain | 1–100 nM | 10⁴–10⁶ | 10⁻³–10⁻² | Some designs deliberately tune kd for sub-stoichiometric turnover, but optimal kinetics are target-dependent and not yet a settled design rule |
| Bispecific → bridged ternary complex | Low-nM apparent | — | — | Avidity-enhanced; context-dependent |
2. Simultaneous / Bridging Binding Assay
The critical experiment confirming that the bispecific can simultaneously engage both the E3 ligase and the target protein.
SPR Approach
Capture bispecific on anti-Fc surface (~200–500 RU)
Inject Arm 1 target at saturating concentration
Without regeneration, inject Arm 2 target — additional signal confirms dual engagement
BLI Approach (Preferred)
Load anti-Fc biosensor with bispecific antibody
Dip into Arm 1 target solution — measure association
Transfer to Arm 2 target — additional shift confirms bridging
BLI's dip-and-read format is convenient for sequential binding experiments, though large analytes can still suffer diffusion limitations at the static sensor tip. Always reverse the injection order as a control (Arm 2 first, then Arm 1) to confirm order-independence.
Caveat: Confirming bridging by SPR/BLI in solution does not predict cellular degradation potency. Many bispecifics that bridge cleanly with soluble ectodomains fail in cells due to membrane geometry, target/E3 surface stoichiometry, glycan shielding, or trafficking bottlenecks. Treat solution bridging as necessary but not sufficient.
3. Avidity vs. Affinity for Bispecific Formats
Bispecific antibody formats introduce avidity effects that can dramatically overestimate true affinity. Understanding and deconvoluting avidity from intrinsic affinity is essential for meaningful kinetic characterization.
Measure using Fab fragments or monovalent antigen presentation
Reflects true single-site binding. Use for medicinal chemistry optimization.
Full bispecific on surfaces presenting both targets
Avidity-dominated. Relevant to functional potency but not intrinsic KD.
Best Practice: Report both intrinsic (Fab fragment or surface-blocked) and apparent (full bispecific) KD values. For 2+1 formats with two identical arms, use monovalent antigen presentation to measure true single-site affinity.
4. Competition & Epitope Binning
For AbTACs, confirming that target and E3 ligase epitopes are sterically compatible is critical. If the bispecific cannot engage both proteins simultaneously, no degradation will occur.
- Tandem injection: Inject one target, then the other without regeneration — additional signal confirms non-competing epitopes
- Premixed analytes: Co-inject both targets simultaneously and compare response to individual injections
- Isotype controls: Monospecific parental antibodies should bind only one target
- Failure to detect simultaneous binding may indicate steric incompatibility rather than lack of dual specificity — test with different linker variants
Experimental Design Considerations
Immobilization Strategies
Orientation matters critically for bispecific antibody characterization — which arm faces up determines what you can measure.
Anti-Fc Capture (Recommended)
Anti-human Fc capture kit (~10,000 RU amine-coupled anti-Fc) for oriented, regenerable capture. Both Fab arms are accessible for analyte binding. For IgG-like bispecifics, Protein A capture works but may show differential affinity for different Fc variants (e.g., knobs-in-holes).
Antigen Capture (Individual Arms)
Immobilize the target ectodomain or RNF43 ectodomain and flow the bispecific as analyte. This measures individual arm kinetics directly. Caution: Large bispecific analytes (~150 kDa) may encounter mass transport limitations — keep antigen surface density ≤200 RU.
RNF43 Ectodomain (~30 kDa)
Capture via His-tag or biotinylation on SA surface. Be aware that the soluble ectodomain may not fully recapitulate membrane-proximal geometry of the intact receptor.
Measuring Both Arms Independently and Together
Individual Arm Kinetics
- Capture bispecific via anti-Fc (~100–300 RU)
- Flow target or E3 ectodomain as analyte
- Multi-cycle kinetics: 5–7 concentrations spanning 0.1× to 10× expected KD
- Standard 1:1 Langmuir fitting
Dual-Arm Bridging
- Capture bispecific, saturate with Arm 1 target
- Inject Arm 2 target without regeneration
- Report ΔRU as % of theoretical Rmax
- Reverse order to confirm independence
Regeneration Challenges
Antibody-based degraders present unique regeneration challenges compared to small-molecule TPD characterization.
- Harsh conditions needed: Anti-Fc capture requires 10 mM glycine pH 1.5–2.0 for regeneration
- Low pH may denature: Unlike small molecules, antibody antigen-binding sites can be damaged by aggressive regeneration
- Verify surface longevity: Confirm that anti-Fc capture surface retains function across >100 cycles
- Glycosylation considerations: RNF43 ectodomains require proper glycosylation when producing recombinant protein for SPR — bacterial expression may not suffice
Key Advantages Over Small-Molecule PROTACs
Antibody-based degraders address fundamental limitations of small-molecule approaches, opening entirely new target classes.
| Feature | Small-Molecule PROTACs | Antibody-Based Degraders |
|---|---|---|
| Target Space | Intracellular proteins only | Cell-surface proteins, immune checkpoints, receptors |
| Cell Permeability | Required (limits MW & polarity) | Not required — acts at cell surface |
| Molecular Weight | ~800–1200 Da (beyond Rule of 5) | ~150 kDa (standard IgG-like) |
| Hook Effect | Yes — bell-shaped dose-response | Less pronounced than small-molecule PROTACs due to avidity, but can still occur at saturating concentrations |
| Tissue Selectivity | Limited (systemic distribution) | Driven by differential E3 ligase expression (e.g., RNF43/ZNRF3 upregulated in CRC, Marei 2022) rather than antibody engineering per se |
| Half-Life | Hours (oral dosing) | Weeks (IgG format, FcRn recycling) |
Key Insight: Antibody-based degraders expand TPD to integral membrane and cell-surface proteins — roughly a quarter of the human proteome — that small-molecule PROTACs cannot reach. Cell-surface targets like PD-L1, HER2, and EGFR (critical oncology and immunology targets) become accessible to degradation without the cell-permeability constraints that limit small-molecule PROTACs.
Common Pitfalls & Solutions
| Pitfall | Description | Mitigation |
|---|---|---|
| Fc-Mediated Artifacts | Anti-Fc capture may preferentially orient one arm of asymmetric bispecifics (knobs-in-holes). | Use arm-specific capture (anti-idiotype or antigen capture) for individual arm kinetics. |
| Mass Transport | Flowing a 150 kDa bispecific over high-density antigen surfaces causes mass transport limitation. | Keep antigen surface density ≤200 RU. Alternatively, capture the bispecific and flow the smaller antigen. |
| Bivalent Binding Artifacts | 2+1 bispecific formats show avidity on surfaces presenting the target bivalently. | Use monovalent antigen presentation or Fab fragments for intrinsic affinity. |
| Steric Clash in Bridging | Failure to detect simultaneous binding may be steric incompatibility, not lack of dual specificity. | Test with different linker variants or engineered flexibility domains. |
| DAC Conjugation Effects | Hydrophobic PROTAC payloads cause antibody aggregation; high DAR alters FcRn binding. | Monitor by SEC before SPR. Measure FcRn kinetics to predict PK impact. |
Key References
Cotton AD, Nguyen DP, Gramespacher JA, Seiple IB, Wells JA. Development of antibody-based PROTACs for the degradation of the cell-surface immune checkpoint protein PD-L1. J Am Chem Soc. 2021;143(2):593–598. (First AbTAC demonstration; induces lysosomal — not proteasomal — degradation of PD-L1 via RNF43.)
Maneiro MA, Forte N, Shchepinova MM, Kounde CS, Chudasama V, Baker JR, Tate EW. Antibody-PROTAC conjugates enable HER2-dependent targeted protein degradation of BRD4. ACS Chem Biol. 2020;15(6):1306–1312. (Pioneering DAC work)
Dragovich PS, Pillow TH, Blake RA, Sadowsky JD, Adaligil E, et al. Antibody-mediated delivery of chimeric BRD4 degraders. J Med Chem. 2021;64(5):2576–2607. (Genentech DAC systematic exploration)
Marei H, Tsai WK, Kee YS, Ruiz K, He J, Cox C, et al. Antibody targeting of E3 ubiquitin ligases for receptor degradation. Nature. 2022;610(7930):182–189. (PROTAB concept; expanded cell-surface E3 recruitment beyond RNF43.)
Zebisch M, Xu Y, Krastev C, MacDonald BT, Chen M, Gilbert RJC, He X, Jones EY. Structural and molecular basis of ZNRF3/RNF43 transmembrane ubiquitin ligase inhibition by the Wnt agonist R-spondin. Nat Commun. 2013;4:2787. (Structural foundation for the RNF43/ZNRF3 ectodomain that informs AbTAC arm design.)