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CETSA® and Tricky Targets: What Thermal Stability Can Tell You When Classical Assays Struggle

When a target is difficult to express, poorly behaved in vitro, or only functional in a native complex, how can we tell whether a compound truly engages it inside cells rather than driving an indirect phenotype?

“Tricky targets” are often difficult for overlapping reasons: low endogenous abundance, multiprotein assemblies, membrane dependence, conformational heterogeneity, or no clean catalytic readout. In these cases, even a good biochemical assay can answer the wrong question if it relies on an engineered or simplified system instead of the endogenous protein state in cells.

CETSA (Cellular Thermal Shift Assay) addresses a different question. It leverages the observation that ligand binding can change protein thermal stability (often shifting apparent aggregation/melting behaviour) and measures that change in cells or lysates using unmodified compounds and, when feasible, endogenous protein.

Why “label-free and endogenous” changes interpretation

Many engagement methods rely on labels-tagged proteins, engineered constructs, or derivatized compounds. These can be informative, but they can also change expression, localisation, complex formation, or binding geometry. CETSA is useful when the central uncertainty is whether such alterations obscure what happens in native biology. Practically, it shifts the question from “does the compound bind a construct?” to “is there evidence consistent with engagement of the native protein state under relevant conditions?”

Where CETSA helps with common classes of tricky targets

  1. Targets that are hard to express or purify
    For unstable proteins, obligate complexes, or targets requiring native partners, recombinant work can be slow and can alter the relevant state. An early decision question is therefore: does the compound engage the endogenous target in a relevant cellular background? CETSA can address this in intact cells (or lysate when appropriate) before major investment in an in vitro system. [1,2]
  2. Multipass membrane proteins and targets with solubility constraints
    Membrane targets are challenging because extraction and solubilization can shift conformation and ligand occupancy. CETSA can be applied in live cells, but interpretation needs care because the readout depends on aggregation/solubility behavior. Live-cell CETSA has been demonstrated on integral multipass targets including TSPO, SERCA2, and a GPCR (PAR2), and responses can be stabilizing or destabilizing depending on target class and compound mechanism. [3,4]
  3. “Stateful” proteins: complexes, PTMs, ligand occupancy, localization
    When engagement depends on assembly state, post-translational modifications (PTMs), endogenous ligands, or localization, comparing intact-cell CETSA with lysate CETSA can help: disappearance (or inversion) of a shift in lysate suggests context dependence, while persistence is more consistent with a direct interaction under simplified conditions. However, thermal profiles can also shift indirectly via pathway effects (metabolites, PTMs, protein-protein interactions), so CETSA shifts support changed protein state and require appropriate controls before concluding direct binding. [2,5,6]
  4. Degraders and other “beyond binding” modalities
    For degraders, phenotype reflects multiple steps beyond binding (ternary complex formation, ubiquitination, proteasome capacity, resynthesis). Engagement signals are typically necessary but not sufficient to explain degradation. CETSA, particularly mass spectrometry-based CETSA, can contribute by measuring engagement of the protein of interest and (when detectable) the recruited ligase in the same cellular context used for degradation readouts, helping distinguish “engaging but not degrading” from true lack of engagement. [7–9]

Practical experimental logic (not a protocol)

  • Start with a falsifiable question. Example: “The series binds a recombinant domain; do we see evidence consistent with engagement of endogenous full-length target in cells?”
  • Choose the decision-relevant matrix first. Prioritize models where expression and state are credible; if engineered expression is required for detectability, treat it as supportive rather than definitive.
  • Use concentration logic, not a single point. Dose-response designs help separate plausible engagement from high-concentration stress effects.
  • Use intact-cell vs lysate CETSA to test context dependence and strengthen interpretation.

What conclusions are immediately valid, and which need more data?

CETSA shifts support a change in protein state, not mechanism by themselves: shifts can be non-observable for technical/biological reasons, Stability changes do not guarantee functional outcome, and downstream biology can shift proteins. Controls and (when needed) orthogonal evidence often remain important.

Generally supportable (with appropriate controls):

  • Evidence consistent with the compound reaching the relevant cellular environment and perturbing the target’s stability state in that context.
  • Relative ranking of engagement potency across a series (within the same assay context).
  • Proteome-scale thermal profiling can generate hypotheses about direct binders vs responders when combined with careful design and interpretation.

Bottom line: For tricky targets, CETSA is most useful as an inference tool: it anchors chemistry and biology to the same question—are we engaging what we think we are engaging, in a system that matters?

Contact

If your team is exploring targets where classical binding or activity assays are difficult to interpret, please contact us. We would be pleased to exchange views on experimental logic and interpretation, including when CETSA data is likely to be informative and when it is not.

Pelago Bioscience has a representative in Japan, Stefan Sandstrom, who would welcome the opportunity to support your scientific discussion and help connect you with the most relevant scientific expertise at the company.

Contact: stefan.sandstrom@pelagobio.com

References

  1. Martinez Molina D, Jafari R, Ignatushchenko M, Seki T, Larsson EA, Dan C, et al. Monitoring drug target engagement in cells and tissues using the cellular thermal shift assay. Science. 2013;341(6141):84–87. doi:10.1126/science.1233606. Available from: https://europepmc.org/article/MED/23828940 (Europe PMC)
  2. Jafari R, Almqvist H, Axelsson H, Ignatushchenko M, Lundbäck T, Nordlund P, et al. The cellular thermal shift assay for evaluating drug target interactions in cells. Nat Protoc. 2014;9(9):2100–2122. doi:10.1038/nprot.2014.138. Available from: https://www.nature.com/articles/nprot.2014.138 (Nature)
  3. Kawatkar A, Schefter M, Hermansson N-O, Snijder A, Dekker N, Brown DG, et al. CETSA beyond soluble targets: a broad application to multipass transmembrane proteins. ACS Chem Biol. 2019;14(9):1913–1920. doi:10.1021/acschembio.9b00399. Available from: https://europepmc.org/article/MED/31329413 (Europe PMC)
  4. Massey AJ. A high content, high throughput cellular thermal stability assay for measuring drug-target engagement in living cells. PLoS ONE. 2018;13(4):e0195050. doi:10.1371/journal.pone.0195050. Available from: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0195050 (PLOS)
  5. Savitski MM, Reinhard FBM, Franken H, Werner T, Savitski MF, Eberhard D, et al. Tracking cancer drugs in living cells by thermal profiling of the proteome. Science. 2014;346(6205):1255784. doi:10.1126/science.1255784. Available from: https://europepmc.org/article/MED/25278616 (Europe PMC)
  6. Mateus A, Määttä TA, Savitski MM. Thermal proteome profiling: unbiased assessment of protein state through heat-induced stability changes. Proteome Sci. 2017;15:13. doi:10.1186/s12953-017-0122-4. Available from: https://link.springer.com/content/pdf/10.1186/s12953-017-0122-4.pdf (Springer)
  7. Cromm PM, Crews CM. Targeted protein degradation: from chemical biology to drug discovery. Cell Chem Biol. 2017;24(9):1181–1190. doi:10.1016/j.chembiol.2017.05.024. Available from: https://europepmc.org/article/MED/28648379 (Europe PMC)
  8. Chernobrovkin AL, Cázares-Körner C, Friman T, Martin Caballero I, Amadio D, Martinez Molina D. A tale of two tails: efficient profiling of protein degraders by specific functional and target engagement readouts. SLAS Discov. 2021;26(4):534–546. doi:10.1177/2472555220984372. Available from: https://europepmc.org/article/MED/33445986 (Europe PMC)
  9. Békés M, Langley DR, Crews CM. PROTAC targeted protein degraders: the past is prologue. Nat Rev Drug Discov. 2022;21:181–200. doi:10.1038/s41573-021-00371-6. Available from: https://www.nature.com/articles/s41573-021-00371-6 (Nature)