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A toxicity signal is on-target only when the intended degrader mechanism is causally connected to the injury: the compound reaches the relevant cells or tissue, engages the intended target, produces target depletion at the toxic exposure, and target loss explains the phenotype. Binding alone, a concentration-response curve, or a protein-abundance change cannot establish that chain.
The practical approach is to map exposure to engagement, degradation, functional change and toxicity in the same model and time window, then challenge competing explanations with orthogonal controls, rescue or genetic comparisons, quantitative protein profiling and species-appropriate toxicology.
What “on-target” means for a degrader
PROTACs and molecular glues are proximity-based drugs. Their proposed action is a sequence rather than a single binding event:
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- Free compound is available inside the relevant cells or tissue.
- The degrader engages the intended protein and the recruited E3 ubiquitin ligase.
- A productive ternary complex forms.
- The target is ubiquitinated and removed by the proteasome.
- Loss of the target changes a relevant function or pathway.
- That functional change produces the observed toxicity.
A claim of on-target toxicity should be matched to the step actually measured. A binary binding result supports engagement; it does not demonstrate productive ternary-complex formation, ubiquitination, degradation or a causal toxic phenotype. Charlotte Crowe and Alessio Ciulli summarized the principle in their 2026 Chemical Reviews methods article: “Monitoring each mechanistic step of a degraders’ mode of action is important to confirm its bona fide cellular activity and guide rational design and optimization.”
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Use a causal map for every study: relevant exposure → cellular target engagement → target depletion → functional change → toxicity phenotype. At each arrow, ask what else could produce the same observation. Alternatives include inadequate cellular availability, nonproductive binding, unintended degradation, downstream adaptation, high-exposure pharmacology, proteasome disturbance and model-specific biology.
Build the evidence in the context where toxicity occurs
Match exposure, tissue and time
Measure mechanistic events in the same cell type, tissue or animal context in which the phenotype is recorded whenever feasible. A degradation result in one engineered cell line does not automatically explain toxicity in a different lineage, organ or species. Record free or total exposure as appropriate for the question, intracellular availability when it is relevant, dosing interval and duration of treatment.
Time matters because target engagement can precede degradation, and depletion can persist after plasma concentrations fall. Collect time points that show the order of events: exposure and engagement first, target-protein loss next, functional change after that, and toxicity on the timescale at which it actually appears. If toxicity continues after the compound is removed, test whether target depletion also persists; if injury appears before measurable depletion, an on-target degradation explanation is weakened.
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Confirm that the intended target is engaged
Use a cellular engagement method suitable for the target and model, and demonstrate engagement at concentrations that produce the phenotype. Engagement at a concentration far above the toxic exposure is not evidence that the target mediates toxicity. Conversely, absence of a detectable signal can reflect assay sensitivity, subcellular compartmentalization or inadequate intracellular availability rather than absence of binding.
Demonstrate productive degradation
Measure the intended target protein after treatment, with a time course and exposure range that overlap the toxicity experiment. Distinguish a true reduction in target abundance from changes caused by cell loss, altered cell composition or generalized translational shutdown. A target should be depleted under the conditions in which the toxic phenotype is observed if the proposed mechanism requires target loss.
Where the mechanism is uncertain, examine the recruited ligase and proteasome dependence as well as target engagement. Productive degradation is a mechanistic claim; it requires more than a ligand-binding measurement.
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Use controls that challenge the mechanism
Matched inactive or non-degrading analogs
A matched analog that retains relevant physicochemical features but cannot form a productive degrader complex can reveal whether toxicity requires degradation. Interpret the result in light of permeability, stability and residual binding: an analog that fails to enter cells or loses all target affinity is not a clean mechanistic control.
Competition at the target or ligase
Compete the degrader with a target-binding or ligase-binding ligand when the system supports that experiment. Protection from both target depletion and toxicity strengthens the argument that the recruited complex is involved. However, the competitor can have pharmacology of its own, alter protein function or perturb the ligase pathway, so include vehicle and competitor-only controls.
Disrupt the degradation machinery carefully
Genetic or pharmacologic disruption of the recruited E3 pathway or proteasome can test whether the phenotype requires the intended degradation route. Such interventions may independently stress cells, change protein turnover or alter susceptibility to injury. Therefore, interpret them by comparing effects on target depletion and toxicity, not by treating any protective effect as definitive proof.
Compare a degrader with a non-degrading binder
If a high-affinity binder that occupies the target without inducing degradation is available, compare its cellular activity with the degrader at matched target occupancy or relevant exposure. Similar toxicity from both compounds suggests binding or downstream target modulation; toxicity unique to the degrader points toward degradation-dependent or degrader-specific pharmacology, but does not by itself distinguish intended from unintended degradation.
Test necessity and sufficiency with rescue or genetics
Target restoration or degradation-resistant variants
Restoring the target, or expressing a variant that remains functional but resists degrader-induced removal, can test necessity. Attenuation of toxicity while degrader exposure and other cellular conditions remain comparable strengthens an on-target interpretation. Verify that the rescue construct is expressed at a biologically meaningful level, retains the relevant function and does not create a new stress.
Independent target loss
Reproduce target depletion with an independent genetic perturbation and compare the phenotype with degrader treatment. Similarity supports sufficiency of target loss, whereas a degrader-specific phenotype suggests additional pharmacology. Genetic depletion may be slower, incomplete or accompanied by adaptation, so compare degree and duration of target loss rather than relying on a binary match.
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Interpret rescue failures cautiously
Failure to rescue does not automatically prove an off-target mechanism. A rescue protein may not reach the right compartment, may be degraded through another route, or may not restore a function that depends on developmental history. Document expression, localization, activity and the extent of target protection before drawing a conclusion.
Use proteomics to separate direct degradation from downstream change
What global profiling can show
Quantitative protein profiling can identify proteins whose abundance decreases after treatment, reveal possible unintended direct degradation and show pathway-level changes that follow target loss. Compare treated and control samples over time and across concentrations that include both pharmacologically active and toxic conditions.
Why an abundance change is not proof of direct degradation
A lower protein signal can result from direct ubiquitin-proteasome degradation, reduced synthesis, altered trafficking, cell injury, selective loss of a cell subpopulation or a change in tissue composition. A pathway protein that rises or falls downstream of target depletion is not necessarily a degrader substrate. Confirm important candidates with targeted protein measurements and mechanistic controls, such as dependence on the recruited ligase or protection by a suitable competitor.
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For each candidate unintended substrate, ask whether the change occurs at the toxic exposure, precedes the phenotype, is reproducible in the relevant model and is biologically capable of causing the observed injury. A protein that changes only after widespread cell damage is more likely to be a consequence than a trigger. Conversely, an early, exposure-linked loss that is blocked when the degradation machinery is disrupted deserves direct follow-up.
Combine global and focused assays
Global profiling is a discovery tool, not a standalone attribution test. Pair it with target-protein measurements, functional assays, histopathology or other phenotype-specific readouts, and controls that distinguish direct degradation from transcriptional or translational regulation. The 2025 pharmacology review and recent degrader-proteomics literature support this combined approach.
Compare the main competing explanations
| Candidate mechanism | Findings that support it | Findings that weaken it | Key follow-up |
|---|---|---|---|
| Intended-target depletion | Engagement and target loss occur at toxic exposure; phenotype tracks depletion over time; rescue or independent target loss reproduces the effect. | Toxicity appears without measurable target depletion, or target protection does not alter the phenotype. | Repeat engagement and degradation measurements in the toxic tissue and test a degradation-resistant target. |
| Unintended direct degradation | One or more non-target proteins fall early, dose-dependently and at toxic exposure; changes depend on the recruited ligase or degrader architecture. | Protein changes occur only after cell injury or are explained by altered cell composition. | Validate candidates with targeted assays, ligase dependence and functional perturbation. |
| Binary off-target pharmacology | Toxicity occurs at concentrations above those needed for intended degradation and resembles effects of a non-degrading binder or a ligand-only control. | Phenotype is fully blocked by preventing productive degradation at matched exposure. | Review binding activities of both ligands and compare a non-degrading analog across the same exposure range. |
| Proteasome or general protein-homeostasis stress | Broad protein-turnover disturbance, stress responses or toxicity appear at high exposure, with no specific substrate pattern. | Effects occur at low exposure where selective target depletion is established and general proteostasis remains intact. | Measure proteostasis markers and test whether pathway disruption changes toxicity independently of target loss. |
| Prolonged intended depletion | Injury follows sustained target absence and persists in proportion to depletion duration rather than peak concentration. | Short exposure causes immediate toxicity before substantial depletion. | Use recovery studies and compare transient versus sustained target knockdown. |
| Model-specific biology | Phenotype depends on target, ligase or substrate expression found only in one species, tissue or cell state. | Mechanism is conserved across relevant models with comparable exposure and expression. | Measure expression, tissue distribution and species differences before extrapolating to humans. |
These are evidence axes, not numerical thresholds. No universal percentage of degradation, concentration ratio or proteomic fold change can settle attribution for every degrader.
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Account for exposure-dependent pharmacology
Separate the active window from the toxic window
Plot target engagement, target depletion, functional activity and toxicity against exposure rather than comparing nominal doses alone. Consider free concentration, intracellular concentration, accumulation and dosing schedule. If the phenotype begins only when exposure exceeds the range required for intended degradation, investigate lower-affinity interactions and binary binding before calling the effect on-target.
Consider duration, not just peak concentration
Degraders can produce durable protein loss. A transient plasma exposure may create prolonged target absence, and repeated dosing may prevent recovery. Include washout or recovery observations where practical. A phenotype that follows cumulative duration of depletion differs mechanistically from one that follows an acute concentration spike.
Review both ligands and the complete molecule
Assess the binding activities of the target ligand, E3-ligase ligand and assembled degrader. At high concentrations, each component can create binary interactions that are not part of the intended ternary mechanism. Changes in solubility, aggregation, membrane partitioning or intracellular concentration can also make nominal dose a poor proxy for pharmacologic exposure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Special case: CRBN-recruiting degraders
Cereblon (CRBN)-recruiting PROTACs and molecular glues can degrade unintended CRBN neosubstrates. This is distinct from degradation of the intended target and can create liabilities in several organ systems, including developmental toxicity. Do not infer a specific neosubstrate liability without compound-specific evidence.
Profile relevant neosubstrates
Use quantitative protein profiling and targeted confirmation to determine whether known or newly identified CRBN-associated substrates change in the relevant cells or tissues. Establish whether the change is direct, exposure-linked and mechanistically dependent on CRBN recruitment.
Choose models that represent human biology
Neosubstrate expression, CRBN sequence and activity, developmental stage and tissue exposure can differ among species. A negative result in a model that does not express the relevant substrate, or cannot achieve the relevant tissue exposure, has limited negative predictive value. Conversely, a finding in a species with no human counterpart requires careful translation.
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Include developmental considerations when relevant
If the intended patient population or treatment setting raises developmental concerns, evaluate embryonic or juvenile model relevance, timing of exposure and tissue distribution. The 2026 CRBN safety review and a 2024 toxicologic pathology conference synopsis emphasize neosubstrate assessment, model selection and ADME interpretation rather than a universal CRBN safety cutoff.
Check species, tissue and substrate relevance
- Target expression: Is the intended protein present at a level and in a compartment where depletion can alter function?
- E3-ligase expression: Can the recruited ligase form the productive complex in the test tissue?
- Substrate repertoire: Are relevant intended and unintended substrates expressed in that species and cell state?
- Tissue exposure: Does the organ showing toxicity receive enough active compound to engage the mechanism?
- Species biology: Do sequence, binding, degradation and pathway responses translate between the test species and humans?
- Model sensitivity: Can the model reveal the proposed injury, or is it missing the developmental, metabolic or immune context required?
Interpret a safety finding only after checking these prerequisites. A model can be useful for detecting hazard while still being unsuitable for identifying the human molecular cause.
A practical attribution workflow
- Define the phenotype precisely. Specify the affected cell type, organ, severity, onset, reversibility and exposure schedule. Separate primary injury from secondary changes caused by cell loss.
- Measure exposure in the relevant context. Use tissue or intracellular measurements where distribution or accumulation could alter interpretation.
- Confirm engagement and degradation. Demonstrate intended-target engagement and protein depletion at the exposure and time points associated with toxicity.
- Map the sequence. Establish whether engagement precedes depletion, depletion precedes functional change and functional change precedes injury.
- Apply orthogonal controls. Use a matched non-degrading analog, target or ligase competition, and carefully justified pathway or proteasome perturbation.
- Test necessity and sufficiency. Attempt target rescue or a degradation-resistant variant, and compare with an independent genetic loss-of-function approach.
- Profile the proteome. Identify direct-degradation candidates and downstream pathway shifts across active, toxic and recovery conditions.
- Validate candidate off-targets. Confirm important protein changes with targeted assays and determine whether they precede and mechanistically explain the phenotype.
- Evaluate exposure escalation. Examine whether toxicity appears only at concentrations associated with low-affinity binding, binary ligand pharmacology, proteasome stress or prolonged depletion.
- Assess translation. Compare target, ligase, substrate expression, tissue exposure and species biology before assigning human relevance.
How to report the conclusion without overstating it
Use calibrated language tied to the evidence. “Consistent with on-target toxicity” is appropriate when engagement, depletion, temporal tracking and an orthogonal perturbation align but rescue is incomplete. “Strongly supports an on-target mechanism” is justified when target protection attenuates toxicity and independent target loss reproduces it under comparable conditions. “Evidence for an off-target contribution” is appropriate when an unintended substrate or binary interaction is demonstrated at toxic exposure and explains the phenotype better than intended depletion.
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Minimum data package for a defensible attribution
- Exposure measurements linked to the cells, tissue, species and dosing schedule used for toxicity.
- Cellular target-engagement data at pharmacologically active and toxic exposures.
- Target-protein time course showing depletion, persistence and recovery.
- Phenotype measurements aligned to the same time points.
- At least one mechanistically relevant orthogonal control with control-only effects documented.
- Rescue, degradation-resistant target or independent genetic comparison when technically feasible.
- Quantitative protein profiling with targeted confirmation of important direct-degradation candidates.
- Assessment of proteasome, proteostasis, binary-binding and prolonged-depletion alternatives.
- Expression, tissue-exposure and species-relevance analysis, including CRBN neosubstrates where applicable.
The central discipline is simple: do not call a phenotype on-target because the compound binds, and do not call it off-target because one assay failed. Attribute toxicity only after the measured exposure, mechanistic sequence, controls, protein changes and model relevance collectively support the same causal explanation.
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