Same underlying principle. Very different questions.
Here’s how to decide which one belongs in your program.

The target engagement space just got more interesting.

Promega’s launch of TarSeer™ BRETSA™ (BRETSA) at SLAS 2026 introduced a new approach to cellular target engagement measurement – and it’s already prompting questions from drug discovery teams about how it relates to CETSA®.

Both technologies are thermal shift-based assays that measure target engagement, and they both address the same longstanding problem: the gap between biochemical assays and the cellular reality your compound will eventually face.

However, CETSA measures endogenous, unmodified protein, while BRETSA works via an engineered luciferase-tagged construct. This distinction has significant implications for how each technology is used and what its data actually tells you.

Which one belongs in your program depends on the question you’re trying to answer.

What CETSA and BRETSA have in common

Both CETSA and BRETSA are built on the same underlying principle: thermal shift.

When a compound binds to a target protein, it changes that protein’s thermal stability. Both technologies detect that change in live cells, giving you direct cellular evidence of target engagement rather than just inference from biochemical binding assays.

Crucially, neither approach requires a tracer molecule, a displacement assay, or a previously identified binding site. Any compound that alters a protein’s thermal stability can potentially be detected – including allosteric binders and protein-protein interaction disruptors – making both technologies applicable to targets where no ligand has previously been identified.

Both are compatible with 96- and 384-well plate formats, and both deliver cellular evidence of target engagement that purely biochemical methods cannot.

Understanding the differences between CETSA and BRETSA

BRETSA (TarSeer™ BRETSA™, Promega)

BRETSA (Bioluminescence Resonance Energy Transfer Shift Assay) works by expressing your target protein in cells as a luciferase fusion construct. As cells are heated the target protein unfolds, exposing hydrophobic regions. These can then be bound by a cell-permeable fluorescent probe, generating a BRET signal (Capener et al., 2026).

When a test compound binds the protein, it changes how and when that unfolding happens, shifting the BRET signal profile. The result is a quantitative, cell-based readout of compound-protein interaction.

Sensitivity is a notable characteristic of the platform. Because BRETSA measures protein denaturation, it enables lower challenge temperatures. Promega reports potency measurement across a dynamic range of more than five orders of magnitude, making the platform well-suited for weak binders and early chemical matter.

Promega suggests applicability across a broad range of target classes and cellular compartments, though as an Early Access product the full breadth of its track record is still emerging. The workflow is addition-only and compatible with high-throughput formats.

The key constraint is that BRETSA requires an engineered system. Your target must be tagged with a ~170 amino acid luciferase fusion tag for every cell line you want to study it in, taking time and resource to produce.

A fusion tag of that size also has the potential to affect protein folding, localization, or binding behavior, adding an extra variable to your data. The assay typically relies on an overexpression system unless the tag has been introduced at the endogenous locus using CRISPR-Cas9. There are also open questions about whether tagging alters expression levels relative to the native protein, and whether the tagged construct reflects the correct protein variant, which is particularly relevant where splice variants or isoforms are biologically significant. In addition, the assay cannot be transferred to a new cell context without repeating the engineering process and validation, and cannot translate to primary tissue samples. There is also the question of whether the tagged construct represents the biologically relevant protein variant or isoform.

CETSA® (Pelago Bioscience)

CETSA (CEllular Thermal Shift Assay) measures the endogenous, unmodified target protein in its native cellular environment.

Compound binding stabilizes the protein structure. When the sample is heated, unbound protein denatures and falls out of solution, meaning it is no longer detectable in the soluble fraction. No tag. No engineering. No modification to the biology you’re trying to understand (Caballero & Lundgren, 2023).

Because CETSA works without tagging, the same assay moves from cell lines to animal tissue to patient samples without needing to rebuild the system from scratch. For programs heading toward the clinic, that continuity of translatable evidence – the same question asked in progressively more relevant biology – is difficult to replicate with an engineered approach.

CETSA also offers something no other thermal shift method can: a proteome-wide, unbiased readout. Rather than asking about a single target, by combining CETSA with mass spectrometry (MS), it’s possible to profile proteins across the proteome, revealing on-target engagement, off-target effects, and mechanism of action information in one experiment.

For selectivity profiling and de-risking, this information can be the difference between knowing early on whether a drug is likely to have unacceptable toxicity, or having to make a much more expensive no-go decision further down the road.

Readout options include Western blot, mass spectrometry, AlphaLISA, ELISA, MSD, FRET-based assays, and other suitable detection formats. CETSA can also be applied to novel targets without requiring tagged constructs and can measure endogenous target engagement directly in live cells.

Choosing the right target engagement assay for your drug development program

To sum up:

BRETSA asks: does this compound bind to this engineered version of my target in a cell?

CETSA asks: does this compound engage the native target in the biology that is relevant to my program?

To choose which approach is best for your program, start by asking what the data needs to do. The table below maps the key differences at a glance.

Both CETSA and BRETSA are applicable to early-stage programs and challenging targets with no known ligand. The decision comes down to what you need the data to do.

BRETSA is more applicable when:

• Potency ranking of early chemical matter is the primary goal – fragments, weak binders, or large compound sets where sensitivity to weak binding signals matters.
• You want an in-house, kit-based workflow your team can run and scale without external support.
• Speed and throughput are the priority and the program is not yet at a translational decision point (only if the engineered cell system is established)
• You’re screening in engineered cell lines and tissue translatability is not yet the critical question.

CETSA is likely the stronger choice when:

• Biological relevance is paramount – you need confidence that you’re measuring native protein behavior, not an artefact of an engineered system.
• You’re making go/no-go or program-shaping decisions that need to be defensible as the program advances.
• Your program is approaching translational decision points and you need continuity of evidence from cell lines through to tissue and primary cells.
• You want to understand selectivity and off-target effects across the proteome, not just binding at a single target.
• You’re working in tissue or primary cell systems where engineering is not feasible.
• You want expert scientific interpretation alongside the data – not just a readout, but a recommendation on next steps.

The bottom line

More tools in target engagement is good news for drug discovery. It means more teams can get cellular evidence earlier, and more challenging targets are becoming tractable. BRETSA addresses a gap in the target engagement toolkit, and its luminescence-based readout offers a well-defined signal for early hit validation in engineered systems.

So, which do you need?

If the priority is early quantitative ranking of compounds in an engineered cellular system, then BRETSA will deliver that data. For biological relevance, translational confidence, and evidence that travels from cells to tissue to the clinic, choose CETSA.

Where BRETSA requires re-engineering for each new context, the same CETSA assay can travel with the program from early cell-line work through to tissue and primary cell studies, providing a continuous thread of translatable evidence across every stage of drug discovery.

For some program, the answer may be both – though each might be doing a different job. BRETSA’s specific advantage at early stages is potency ranking of compounds in an engineered system. CETSA, including via high-throughput screening formats, provides compound ranking in native biology at early stages.

Both technologies solve real problems, and each brings something different to the table. The one that belongs in your program is the one that answers the question you’re asking.

Not sure what approach fits your program?

Our scientific team works with drug discovery teams at every stage from early discovery to preclinical studies. If you’re weighing up your target engagement strategy, we’re happy to think it through with you.

Book a conversation.

References: Caballero, I. M., & Lundgren, S. (2023). A Shift in Thinking: Cellular Thermal Shift Assay-Enabled Drug Discovery. ACS Medicinal Chemistry Letters, 14(4), 369-375. https://doi.org/10.1021/acsmedchemlett.2c00545

Capener, J. L., Schwalm, M. P., Vasta, J. D., Michaud, A., Teske, K. A., Marsiglia, W. M., Huber, K. V. M., Dar, A. C., Knapp, S., Axtman, A. D., & Robers, M. B. (2026). Advances in BRET probes for intracellular target engagement studies. Nature Chemical Biology. https://doi.org/10.1038/s41589-025-02103-y